Rapid cooling of planetesimal core-mantle reaction zones from Mn-Cr isotopes in pallasites
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Abstract
Figures and Tables
 Figure 1 LA-ICP-MS trace element concentrations traverses in olivine for (a) Brenham (left panels) and (b) Brahin (right panels). Traverses were conducted from core to rim; concentrations are in parts per million by weight (ppm) and horizontal axes represent distances from olivine rims (μm). Data are represented by grey points and traverse averages are traced by solid lines. Vertical dashed lines (blue and green for Brenham and Brahin respectively) represent limit of SIMS spatial resolution (~30 μm) when analysing near olivine margins. LA-ICP-MS tracks from this study and ablation pits from previous work (McKibbin et al., 2013c). |  Figure 2 Mn-Cr isotopic composition of olivine and chromite from Brenham and Brahin pallasite meteorites. (a) SIMS Mn-Cr systematics for olivine from Brenham (blue ellipses) and Brahin (green ellipses) (errors are 2-sigma), and (b) Brahin chromite (data near the origin expanded in subfigure; 2-sigma errors in grey). The terrestrial Cr-isotopic composition (no anomaly) and increasing initial 53Mn/55Mn correlations are illustrated by an unbroken horizontal and inclined dashed lines respectively. |  Figure 3 Model of the Mn-Cr evolution of a planetary (pallasite) reservoir as a function of 55Mn/52Cr and time of isolation from the Solar System. Hyperbolic lines indicate various 53Cr/52Cr compositions (in per mille). SIMS analyses for Brenham and Brahin olivine which yielded radiogenic Cr (in the 55Mn/52Cr range 20-40, blue and green respectively) and literature compositions for phosphate minerals (orange) are presented as vertical lines. A Solar System initial 53Mn/55Mn of 9.1 × 10-6 (Nyquist et al., 2009), along with a conservatively estimated 53Cr isotope enrichment in pallasite olivine rims of 3.5 ± 1.4 per mille and a bulk 55Mn/52Cr for late interstitial melts of 40-60 (bounded by olivine and high Mn/Cr phosphates) constrains the isolation of evolved silico-phosphate melt to before ~2.5 to 4 Myr (yellow field) followed by decay of 53Mn to extinction (white field bound by olivine rims and phosphate minerals). The evolution of a planetesimal isolated from the nebula sufficiently early (i.e. with cooling and core-mantle redox-reactions occurring in the yellow region, before 2.5-4 Myr) is given on the right. Chronological constraints are from Mn-Cr (this study) and Hf-W data (Kruijer et al., 2014). |
| Figure 1 | Figure 2 | Figure 3 |
Supplementary Figures and Tables
 Table S-1 Secondary standard reference materials for LA-ICP-MS. | Table S-2 LA-ICP-MS traverse data for Brenham olivine. | Table S-3 LA-ICP-MS traverse data for Brahin olivine. |
| Table S-1 | Table S-2 | Table S-3 |
| Table S-4 Cr-isotope data for San Carlos olivine and terrestrial spinel standards. |  Figure S-1 Primary beam and mean 52Cr+ counts per second against raw 53Cr+/52Cr+ ion count ratios, uncorrected for instrumental mass fractionation, for olivine from Brenham (a and b) and Brahin (c and d), with San Carlos terrestrial olivine measurements in each of their respective analytical sessions. | Table S-5 Cr-isotope data for meteoritic olivine and chromite. |
| Table S-4 | Figure S-1 | Table S-5 |
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Introduction
The accumulated oxygen isotopic evidence for five distinct parent bodies of olivine and metal-rich pallasite meteorites (summarised in Boesenberg et al., 2012
Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
) suggests that pallasitic material is the default end-product of planetesimal differentiation. Pallasites may represent samples of quiescent core-mantle boundaries (Boesenberg et al., 2012Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
), or violently formed mixtures of core and mantle materials (Yang et al., 2010Yang, J., Goldstein, J.I., Scott, E.R.D. (2010) Main-group pallasites: Thermal history, relationship to IIIAB irons, and origin. Geochimica et Cosmochimica Acta 74, 4471-4492.
; Tarduno et al., 2012Tarduno, J.A., Cottrell, R.D., Nimmo, F., Hopkins, J., Voronov, J., Erickson, A., Blackman, E., Scott, E.R.D., McKinley, R. (2012) Evidence for a dynamo in the main group pallasite parent body. Science 338, 939–942.
) and are often discussed in terms of trapping of olivine in a liquid Fe-Ni metal groundmass progressively becoming saturated in chromite, troilite, schreibersite, and phosphate minerals during cooling (Boesenberg et al., 2012Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
). Despite this diverse mineralogy, the short-lived isotope chronologies of pallasites remain poorly understood due to depletion in crustal components (plagioclase and pyroxene; Nyquist et al., 2009Nyquist, L.E., Kleine, T., Shih, C.-Y., Reese, Y.D. (2009) The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization. Geochimica et Cosmochimica Acta 73, 5115-5136.
). The 53Mn-53Cr decay scheme (t1/2 3.7 Myr) would seem ideal for such meteorites due to the fractionation of parent and daughter between olivine (generally with high Mn/Cr) and chromite (low Mn/Cr); however, pallasite chronologies have stalled since pioneering Thermal Ionisation Mass Spectrometry (TIMS) studies (Birck and Allègre, 1988Birck, J.-L., Allègre, C.J. (1998) Manganese-chromium isotope systematics and the development of the early Solar System. Nature 331, 579-584.
; Lugmair and Shukolyukov, 1998Lugmair, G.W., Shukolyukov, A. (1998) Early solar system timescales according to 53Mn-53Cr systematics. Geochimica et Cosmochimica Acta 62, 2863-2886.
), with conflicting Secondary Ion Mass Spectrometry (SIMS) results having been later retracted (Huss et al., 2011Huss, G.R., Ogliore, K., Nagashima, M., Telus, M., Jilly, C.E. (2011) Dangers of determining isotope ratios using means of individual ratios. Lunar and Planetary Science Conference 42, 2608.
; Telus et al., 2012Telus, M., Huss, G.R., Ogliore, R.C., Nagashima, K., Tachibana, S. (2012) Recalculation of data for short-lived radionuclide systems using less-biased ratio estimation. Meteoritics and Planetary Science 47, 2013-2030.
). In this study, we re-investigate pallasite olivine trace elements and Mn-Cr isotopes using improved Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS; Spandler and O’Neill, 2010Spandler, C., O’Neill, H.St.C. (2010) Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications. Contributions to Mineralogy and Petrology 159, 791-818.
) and SIMS (McKibbin et al., 2013aMcKibbin, S.J., Ireland, T.R., Amelin, Y., O’Neill, H.St.C., Holden, P. (2013a) Mn-Cr relative sensitivity Factors for Secondary Ion Mass Spectrometry analysis of Mg-Fe-Ca olivine and implications for the Mn-Cr chronology of meteorites. Geochimica et Cosmochimica Acta 110, 216-228.
) in two texturally different Main-Group pallasites: Brenham, with rounded olivines sharing equilibrated triple-junctions across a continuous network; and Brahin, with fragmental olivine of variable grain size (McKibbin et al., 2013bMcKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013b) LA-ICP-MS mapping of olivine from the Brahin and Brenham meteorites: Complex elemental distributions in the pallasite olivine precursor. Geochimica et Cosmochimica Acta 119, 1-17.
).top
Methods
Samples were cast in epoxy and polished for microbeam analysis. Trace element analyses of olivine were carried out following a previously developed LA-ICP-MS profiling method (Spandler and O’Neill, 2010
Spandler, C., O’Neill, H.St.C. (2010) Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications. Contributions to Mineralogy and Petrology 159, 791-818.
). Sampling was undertaken with a 193 nm wavelength ArF Excimer laser coupled to a custom-built two-volume vortex ablation cell at RSES-ANU. An aperture was used to shape the laser beam into a rectangle, enabling high spatial resolution sampling parallel to crystal margins. Sampling of meteoritic olivine was bracketed by NIST610 and 612 glass standards, with the gas being analysed using an Agilent 7500s ICP-MS. Data were reduced according to established methods (Longerich et al., 1996Longerich, H.P., Jackson, S.E., Günther, D. (1996) Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. Journal of Analytical and Atomic Spectrometry 11, 899–904.
; Spandler and O’Neill, 2010Spandler, C., O’Neill, H.St.C. (2010) Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications. Contributions to Mineralogy and Petrology 159, 791-818.
) with NIST 612 as the primary external standard (Pearce et al., 1997Pearce, N.J.G., Perkins, W.T., Westgate, J.A., Gorton, M.P., Jackson, S.E., Neal, C.R., Chenery, S.P. (1997) A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletter 21, 115–144.
). For SIMS Mn-Cr analysis, we modified previous Sensitive High-mass Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG) methodology (McKibbin et al., 2013aMcKibbin, S.J., Ireland, T.R., Amelin, Y., O’Neill, H.St.C., Holden, P. (2013a) Mn-Cr relative sensitivity Factors for Secondary Ion Mass Spectrometry analysis of Mg-Fe-Ca olivine and implications for the Mn-Cr chronology of meteorites. Geochimica et Cosmochimica Acta 110, 216-228.
,cMcKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P., Sugiura, N. (2013c) A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D’Orbigny using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 123, 181-194.
). Suitable areas of olivine ~30 μm in diameter were sputtered by a high current unfiltered primary ion beam (~16.9-56.6 nA mixed O- and O2-; McKibbin et al., 2015McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P. (2015) Mn-Cr dating of Fe- and Ca-rich olivine from ‘quenched’ and ‘plutonic’ angrite meteorites using Seconary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 157, 13-27.
). Ion beams of interest were directed sequentially to a single electron multiplier by cycling the magnet. For measurements in Brahin chromite (~4.0-5.5 nA filtered O2- primary ion beam), sequential collection of 52Cr+, 53Cr+, and 55Mn+ was achieved with a single Faraday Cup. Data were reduced according to the ‘first ratio estimator’ method (Ogliore et al., 2011Ogliore, R., Huss, G., Nakashima, K. (2011) Ratio estimation in SIMS analysis. Nuclear Instruments and Methods in Physics Research Section B 269, 1910-1918.
) which we earlier referred to as ‘ratio of total counts’ (McKibbin et al., 2013cMcKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P., Sugiura, N. (2013c) A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D’Orbigny using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 123, 181-194.
). Instrumental mass fractionation was corrected by sample-standard bracketing with San Carlos olivine or in-house reference spinels. To convert 55Mn+/52Cr+ to 55Mn/52Cr, we used a Relative Sensitivity Factor (RSF) of 0.58 (±0.02) as a multiplier on olivine data. No multiplier was applied to chromite.top
Results
LA-ICP-MS trace element profiles (Fig. 1 and Supplementary Information) indicate low Cr and high P and Mn concentrations near edges of pallasite olivine grains. Grain-to-grain and intra-grain variations in P and Cr are more pronounced in rounded than in fragmental olivine, whereas Mn is very consistent. Mn and Cr variations range over a factor of ~2-3 in olivine 55Mn/52Cr which ordinarily would be considered marginal for SIMS Mn-Cr dating. Nevertheless, this is well correlated with 53Cr/52Cr in the rounded olivines of Brenham (Fig. 2, blue data), while fragmental olivine exhibits more restricted 55Mn/52Cr and 53Cr/52Cr (Fig. 2, green data). Chromite has normal 53Cr/52Cr (Fig. 2 and Supplementary Information). 55Mn/52Cr and 53Cr/52Cr regression yields excessively high initial 53Mn/55Mn values, assuming in situ 53Mn decay and that the system was closed to disturbance (Brenham: initial 53Mn/55Mn of 4.0 (±1.2) × 10-5 with sub-terrestrial initial 53Cr/52Cr at 0.11285 (±0.00024) (2-sigma; MSWD = 0.63); Brahin: initial 53Mn/55Mn of 8.4 (±4.5) × 10-6 with initial 53Cr/52Cr of 0.11340 (±0.00009) (2-sigma, MSWD = 0.49)).
Figure 1 LA-ICP-MS trace element concentrations traverses in olivine for (a) Brenham (left panels) and (b) Brahin (right panels). Traverses were conducted from core to rim; concentrations are in parts per million by weight (ppm) and horizontal axes represent distances from olivine rims (μm). Data are represented by grey points and traverse averages are traced by solid lines. Vertical dashed lines (blue and green for Brenham and Brahin respectively) represent limit of SIMS spatial resolution (~30 μm) when analysing near olivine margins. LA-ICP-MS tracks from this study and ablation pits from previous work (McKibbin et al., 2013c
McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P., Sugiura, N. (2013c) A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D’Orbigny using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 123, 181-194.
).Figure 2 Mn-Cr isotopic composition of olivine and chromite from Brenham and Brahin pallasite meteorites. (a) SIMS Mn-Cr systematics for olivine from Brenham (blue ellipses) and Brahin (green ellipses) (errors are 2-sigma), and (b) Brahin chromite (data near the origin expanded in subfigure; 2-sigma errors in grey). The terrestrial Cr-isotopic composition (no anomaly) and increasing initial 53Mn/55Mn correlations are illustrated by an unbroken horizontal and inclined dashed lines respectively.
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Discussion and Conclusions
The cooling history of pallasites is well recorded by late crystallising, P2O5-rich accessory minerals, as well as our trace element and isotope results for olivine rims. Phosphoran olivine (with several wt. % P2O5) and Mg- and Si-phosphate minerals crystallised from an unusual P2O5-rich silicate melt (Boesenberg et al., 2012
Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
) which might ultimately be formed by oxidation of metallic P. High temperature merrillite and peritectic stanfieldite formed at or above ~1175 °C and have generally high rare earth element (REE) concentrations, while eutectic stanfieldite, farringtonite and silico-phosphate crystallised near the ~1120 °C eutectic after in situ fractional crystallisation (with generally lower REE concentrations; Ando, 1958Ando, J. (1958) Phase diagrams of Ca3(PO4)2–Mg3(PO4)2 and Ca3(PO4)2–CaNaPO4 systems. Bulletin of the Chemical Society of Japan 31, 201–205.
; Davis and Olsen, 1991Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies. Nature 353, 637-640.
; Hsu, 2003Hsu, W. (2003) Minor element zoning and trace element geochemistry of pallasites. Meteoritics and Planetary Science 38, 1217-1241.
). A parallel trend is recorded by olivine geochemistry: olivine cores exhibit variable Cr and P (Fig. 1) while divalent Mn varies over a shorter distance of several tens of micrometres, consistent with an overgrowth or rapid diffusion of divalent elements (Zhukova et al., 2014aZhukova, I., O’Neill, H.St.C., Cambell, I.H., Kilburn, M.R. (2014a) The effect of silica activity on the diffusion of Ni and Co in olivine. Contributions to Mineralogy and Petrology 168, 1-15.
) and more slowly diffusing trivalent Cr (Ito and Ganguly, 2006Ito, M., Ganguly, J. (2006) Diffusion kinetics of Cr in olivine and 53Mn-53Cr thermochronology of early solar system objects. Geochimica et Cosmochimica Acta 70, 799-809.
) and pentavalent P (Spandler and O’Neill, 2010Spandler, C., O’Neill, H.St.C. (2010) Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications. Contributions to Mineralogy and Petrology 159, 791-818.
). However, the outermost ~30 μm of olivine crystal margins have very low Cr and steeply increasing P, recording crystallisation of olivine after cooling to the silico-phosphate eutectic (~1120-980 °C; crystallising farringtonite, stanfieldite, olivine, and probably iron and troilite as well). The occurrence of genuinely ‘phosphoran’ olivine rims in some pallasites, with several wt. % P2O5 (Boesenberg et al., 2012Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
) also indicates rapid cooling at this stage and eventual saturation of this metastable mineral.The inferred initial 53Mn/55Mn of 4.0 (±1.2) × 10-5 for Brenham and 8.4 (±4.5) × 10-6 for Brahin are very high compared to other early Solar System materials (Birck and Allègre, 1988
Birck, J.-L., Allègre, C.J. (1998) Manganese-chromium isotope systematics and the development of the early Solar System. Nature 331, 579-584.
; Lugmair and Shukolyukov, 1998Lugmair, G.W., Shukolyukov, A. (1998) Early solar system timescales according to 53Mn-53Cr systematics. Geochimica et Cosmochimica Acta 62, 2863-2886.
) and cannot represent fossil isochrons because they exceed or are close to the initial 53Mn/55Mn for the Solar System (9.1 (±1.7) × 10-6; Nyquist et al., 2009Nyquist, L.E., Kleine, T., Shih, C.-Y., Reese, Y.D. (2009) The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization. Geochimica et Cosmochimica Acta 73, 5115-5136.
). Small variations (~0.1 per mille) in 53Cr/52Cr are likely present in Brenham pallasite olivine, with slightly higher (~2 per mille) excesses in coexisting metal (Hsu, 2005Hsu, W. (2005) Mn-Cr systematics of pallasites. Geochemical Journal 39, 311-316.
; Qin et al., 2010Qin, L., Alexander, C.M.O’D., Carlson, R.W., Horan, M.F., Yokohama, T. (2010) Contributors to chromium isotope variations of meteorites. Geochimica et Cosmochimica Acta 74, 1122-1145.
), due to cosmic ray exposure effects during transport to Earth. However, our 53Cr/52Cr in olivine rims are higher and inconsistent with the lower susceptibility of olivine to cosmogenic effects relative to metal (via lower Fe/Cr). Anomalies related to pre-solar, nucleosynthetic isotope components are even smaller and unlikely to survive planetary differentiation (Trinquier et al., 2008Trinquier, A., Birck, J.-L., Allègre, C.J., Ulfbeck, D. (2008) 53Mn-53Cr systematics of the early Solar System revisited. Geochimica et Cosmochimica Acta 72, 5146-5163.
; Qin et al., 2010Qin, L., Alexander, C.M.O’D., Carlson, R.W., Horan, M.F., Yokohama, T. (2010) Contributors to chromium isotope variations of meteorites. Geochimica et Cosmochimica Acta 74, 1122-1145.
). We therefore conclude that 53Cr excesses are radiogenic.Although relatively low, 55Mn/52Cr (~10-40) implies that radiogenic Cr is unsupported by Mn, the elevated 53Cr/52Cr nevertheless implicates a reservoir with high Mn/Cr and indicates fractionation of Mn from Cr after significant decay, i.e. at least several million years after establishment of an early reservoir. The unsupported nature of Cr is inconsistent with in situ decay of 53Mn followed by 55Mn loss since elemental profiles indicate that, if Mn was mobile, then it migrated into olivine rather than out. Rather, association of 53Cr/52Cr with P suggests that ingrowth of 53Cr* occurred in a high Mn/Cr silico-phosphate melt before crystallisation of olivine rims and phosphates.
High Mn/Cr in eutectic phosphates (~60-145 in Springwater and Eagle Station pallasites; Davis and Olsen, 1991
Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies. Nature 353, 637-640.
; Hutcheon and Olsen, 1991Hutcheon, I.D., Olsen, E. (1991) Cr isotopic composition of differentiated meteorites: a search for 53Mn. Lunar and Planetary Science Conference 22, 605-606.
), probably due primarily to early chromite saturation which keeps available Cr concentrations low, indicates that the melt from which they crystallised is a good candidate for the high Mn/Cr reservoir. Ingrowth of 53Cr* in this melt requires several million years, i.e. an interval similar to or longer than the half-life of 53Mn. Such a reservoir is modelled in Figure 3 as a function of 55Mn/52Cr and the timing of isolation from the parent body or nebula (Myr after Solar System formation) in order to find a model-dependent Mn-Cr date for this event. We assume a Solar System initial of 53Mn/55Mn of 9.1 × 10-6 (Nyquist et al., 2009Nyquist, L.E., Kleine, T., Shih, C.-Y., Reese, Y.D. (2009) The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization. Geochimica et Cosmochimica Acta 73, 5115-5136.
) and that 53Mn decays completely, implying that the reservoir persists for ~10-20 Myr. The difficulties of a lower Solar System initial, such as in Trinquier et al. (2008)Trinquier, A., Birck, J.-L., Allègre, C.J., Ulfbeck, D. (2008) 53Mn-53Cr systematics of the early Solar System revisited. Geochimica et Cosmochimica Acta 72, 5146-5163.
, or incomplete decay of 53Mn, are discussed below.Figure 3 Model of the Mn-Cr evolution of a planetary (pallasite) reservoir as a function of 55Mn/52Cr and time of isolation from the Solar System. Hyperbolic lines indicate various 53Cr/52Cr compositions (in per mille). SIMS analyses for Brenham and Brahin olivine which yielded radiogenic Cr (in the 55Mn/52Cr range 20-40, blue and green respectively) and literature compositions for phosphate minerals (orange) are presented as vertical lines. A Solar System initial 53Mn/55Mn of 9.1 × 10-6 (Nyquist et al., 2009
Nyquist, L.E., Kleine, T., Shih, C.-Y., Reese, Y.D. (2009) The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization. Geochimica et Cosmochimica Acta 73, 5115-5136.
), along with a conservatively estimated 53Cr isotope enrichment in pallasite olivine rims of 3.5 ± 1.4 per mille and a bulk 55Mn/52Cr for late interstitial melts of 40-60 (bounded by olivine and high Mn/Cr phosphates) constrains the isolation of evolved silico-phosphate melt to before ~2.5 to 4 Myr (yellow field) followed by decay of 53Mn to extinction (white field bound by olivine rims and phosphate minerals). The evolution of a planetesimal isolated from the nebula sufficiently early (i.e. with cooling and core-mantle redox-reactions occurring in the yellow region, before 2.5-4 Myr) is given on the right. Chronological constraints are from Mn-Cr (this study) and Hf-W data (Kruijer et al., 2014Kruijer, T.S., Touboul, M., Fischer-Gödde, M., Bermingham, K.R., Walker, R.J., Kleine, T. (2014) Protracted core formation and rapid accretion of protoplanets. Science 344, 1150-1154.
).The 55Mn/52Cr of phosphates (~60-145; Davis and Olsen, 1991
Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies. Nature 353, 637-640.
; Hutcheon and Olsen, 1991Hutcheon, I.D., Olsen, E. (1991) Cr isotopic composition of differentiated meteorites: a search for 53Mn. Lunar and Planetary Science Conference 22, 605-606.
) and olivine rims (~25-40; this study) can be used to bracket the parental silico-phosphate melt (vertical lines in Fig. 3). Mn and Cr are generally considered to be fairly neutrally partitioned between olivine and melt (e.g., Mallmann and O’Neill, 2013Mallmann, G., O’Neill, H.St.C. (2013) Calibration of an empirical thermometer and oxybarometer based on the partitioning of Sc, Y and V between olivine and silicate melt. Journal of Petrology 54, 933-949.
) although by comparison with other pallasites, Mn may have been more incompatible in olivine from the Brenham and Brahin environment (Boesenberg et al., 2012Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
). However, Mn is compatible in phosphates, while Cr is probably incompatible (Davis and Olsen, 1991Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies. Nature 353, 637-640.
; Hutcheon and Olsen, 1991Hutcheon, I.D., Olsen, E. (1991) Cr isotopic composition of differentiated meteorites: a search for 53Mn. Lunar and Planetary Science Conference 22, 605-606.
). Therefore, olivine and the various phosphates constrain their parental melt to 55Mn/52Cr ~40-60. This melt may have occasionally been quenched to ‘phosphoran olivine’ under favourable conditions (Boesenberg et al., 2012Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
) in the terminal pallasite cooling event (Fowler-Gerace and Tait, 2015Fowler-Gerace, N.A., Tait, K.T. (2015) Phosphoran olivine overgrowths: Implications for multiple impacts to the Main Group pallasite parent body. American Mineralogist 100, 2043-2052.
).Because olivine 55Mn/52Cr is not high enough to generate detectable anomalies in situ, Mn-Cr fractionation must occur after decay, as during crystallisation from such a silico-phosphate melt strongly enriched in Mn and hence eventually in 53Cr*. A weighted mean of the 53Cr/52Cr for olivine rims (taking points with 55Mn/52Cr >20) gives a value of 0.11380 ± 0.00016 (95 % conf., Mean Square Weighted Deviation [MSWD] 2.4), or an enrichment of 3.5 ± 1.4 per mille. These data give a model-dependent Mn-Cr date for isolation at ~1 Myr after Solar System formation, with absolute upper limits of 2.5-4 Myr for a melt with 55Mn/52Cr in the range 40-60. This timescale confirms early accretion (0.1-0.3 Myr) and protracted core-mantle differentiation (to ~1.1-1.3 Myr) suggested by Hf-W chronologies for iron meteorite parent bodies (Kruijer et al., 2014
Kruijer, T.S., Touboul, M., Fischer-Gödde, M., Bermingham, K.R., Walker, R.J., Kleine, T. (2014) Protracted core formation and rapid accretion of protoplanets. Science 344, 1150-1154.
). Incomplete decay of 53Mn, or low values of initial 53Mn/55Mn for the Solar System (Trinquier et al., 2008Trinquier, A., Birck, J.-L., Allègre, C.J., Ulfbeck, D. (2008) 53Mn-53Cr systematics of the early Solar System revisited. Geochimica et Cosmochimica Acta 72, 5146-5163.
), become difficult to accommodate. We therefore favour a higher Solar System initial 53Mn/55Mn (Nyquist et al., 2009Nyquist, L.E., Kleine, T., Shih, C.-Y., Reese, Y.D. (2009) The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization. Geochimica et Cosmochimica Acta 73, 5115-5136.
) and near complete decay of 53Mn (~10 Myr at eutectic temperatures) for 55Mn/52Cr in the range 40-60. If isolation occurred earlier than indicated by Hf-W, then such constraints could be relaxed.The history outlined above closely follows silicate-metal differentiation to form planetesimal cores at ~1.1-1.3 Myr after Solar System formation (Kruijer et al., 2014
Kruijer, T.S., Touboul, M., Fischer-Gödde, M., Bermingham, K.R., Walker, R.J., Kleine, T. (2014) Protracted core formation and rapid accretion of protoplanets. Science 344, 1150-1154.
). Elevated 53Cr/52Cr in Brenham olivine therefore indicates heating, core-mantle separation, and cooling-induced core-mantle back-reaction in the Main-Group pallasite parent body within ~2.5 to 4 Myr of Solar System formation. The latter requires cooling of olivine and liquid metal to the silico-phosphate and Fe-FeS eutectics around 1120-980 °C (Boesenberg et al., 2012Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
). Cooling to low-temperature eutectics therefore occurred rapidly. It suggests that even though cooling was initially fast, at perhaps 100-200 K Myr-1, it slowed considerably at the phosphate eutectic (~1120 °C) and stayed near this temperature for at least several million years, allowing ingrowth of 53Cr* before quench-crystallisation. Trace element variations and clear charge-balancing relationships between Cr and slow-diffusing Al in olivine (McKibbin et al., 2013bMcKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013b) LA-ICP-MS mapping of olivine from the Brahin and Brenham meteorites: Complex elemental distributions in the pallasite olivine precursor. Geochimica et Cosmochimica Acta 119, 1-17.
) indicates suppression of Cr diffusion. That Cr might move slower than divalent cations (Ito and Ganguly, 2006Ito, M., Ganguly, J. (2006) Diffusion kinetics of Cr in olivine and 53Mn-53Cr thermochronology of early solar system objects. Geochimica et Cosmochimica Acta 70, 799-809.
), and that low silica activity can suppress divalent (Zhukova et al., 2014aZhukova, I., O’Neill, H.St.C., Cambell, I.H., Kilburn, M.R. (2014a) The effect of silica activity on the diffusion of Ni and Co in olivine. Contributions to Mineralogy and Petrology 168, 1-15.
) and trivalent cation diffusivity (Zhukova et al., 2014bZhukova, I., O’Neill H., Capbell, I. (2014b) The effect of mineral paragenesis on Al diffusion in olivine. EGU General Assembly Conference Abstracts 16, 573.
) suggests that in pallasites, trace element variations might not be erased at 1000 °C even over millions of years. This timescale, similar to or longer than the lifetime of 53Mn, accommodates a cooling history comprising 1) rapid accretion, heating and core-mantle separation (Kruijer et al., 2014Kruijer, T.S., Touboul, M., Fischer-Gödde, M., Bermingham, K.R., Walker, R.J., Kleine, T. (2014) Protracted core formation and rapid accretion of protoplanets. Science 344, 1150-1154.
); 2) rapid cooling to near-solidus temperatures followed by thermal buffering due to removal of crust, formation of regolith (Warren, 2011Warren, P.H. (2011) Ejecta–megaregolith accumulation on planetesimals and large asteroids. Meteoritics and Planetary Science 46, 53-78.
) and, potentially, crystallisation reactions (Morse, 2011Morse, S.A. (2011) The fractional latent heat of crystallizing magmas. American Mineralogist 96, 682-689.
; Namur et al., 2014Namur, O., Humphrey, M.C.S., Holness, M.B. (2014) Crystallisation of interstitial liquid and latent heat buffering in solidifying gabbros: Skaergaard Intrusion, Greenland. Journal of Petrology 55, 1389-1427.
); and 3) final quenching to preserve disequilibrium assemblages (Boesenberg et al., 2012Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
; Fowler-Gerace and Tait, 2015Fowler-Gerace, N.A., Tait, K.T. (2015) Phosphoran olivine overgrowths: Implications for multiple impacts to the Main Group pallasite parent body. American Mineralogist 100, 2043-2052.
).Isolation of late silico-phosphate melt before ~2.5 to 4 Myr after Solar System formation implies that it represents one of the earliest silicate differentiation products, potentially predating almost all basaltic eucrites and angrites (Wadhwa et al., 2009
Wadhwa, M., Amelin, Y., Bogdanovski, O., Shukolyukov, A., Lugmair, G.W., Janney, P. (2009) Ancient relative and absolute ages for a basaltic meteorite: Implications for timescales of planetesimal accretion and differentiation. Geochimica et Cosmochimica Acta 73, 5189-5201.
; Schiller et al., 2010Schiller, M., Baker, J.A., Bizzarro, M. (2010) 26Al-26Mg dating of asteroidal magmatism in the young Solar System. Geochimica et Cosmochimica Acta 74, 4844-4864.
; Brennecka and Wadhwa, 2012Brennecka G.A., Wadhwa, M. (2012) Uranium isotope compositions of the basaltic angrite meteorites and the chronological implications for the early Solar System. Proceedings of the National Academy of Sciences 109, 9299-9303.
; McKibbin et al., 2015McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P. (2015) Mn-Cr dating of Fe- and Ca-rich olivine from ‘quenched’ and ‘plutonic’ angrite meteorites using Seconary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 157, 13-27.
; Schiller et al., 2015Schiller, M., Connelly, J.N., Glad, A.C., Mikouchi, T., Bizzarro, M. (2015) Early accretion of protoplanets inferred from a reduced inner solar system 26Al inventory. Earth and Planetary Science Letters 420, 45-54.
). Early cooling of the Main-group pallasite parent body to near solidus temperatures provides further evidence for early accretion of iron meteorite-type parent bodies, distinct from later forming ureilite- and chondrite-type parent bodies (e.g., Budde et al., 2015Budde, G., Kruijer, T.S., Fischer-Gödde, M., Irving, A.J., Kleine, T. (2015) Planetesimal differentiation revealed by the Hf-W systematics of ureilites. Earth and Planetary Science Letters 430, 316-325..
). Additionally, this process appears to have been reproducible: the presence of high 55Mn/52Cr phosphate in the Eagle Station pallasite (Davis and Olsen, 1991Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies. Nature 353, 637-640.
), which originated in a different parent body in a different part of the Solar System (Clayton and Mayeda, 1996Clayton, R.N., Mayeda, T.K. (1996) Oxygen isotope studies of achondrites. Geochimica et Cosmochimica Acta 60, 1999-2017.
) indicates that coexisting silico-phosphate and metal-sulphide-phosphide melts could be a general feature of planetesimal differentiation. Such phosphates and associated olivine rims (potentially phosphoran) should lend themselves to dating using Mn-Cr, enabling broad-scale mapping of the 53Mn extinct nuclide chronometer to other systems and constraining the timing of core-mantle separations in planetesimals.top
Acknowledgements
We extend thanks to the Centre for Advanced Microscopy at the Australian National University for use of scanning electron microscope facilities, and Peter Lanc and Charles Magee for assistance with collection and discussion of the data. Three anonymous reviewers are thanked for their constructive criticism and help in improving this manuscript. This research was supported by an Australian National University Research Scholarship to S.J. McKibbin, who is currently a postdoctoral fellow of the Research Foundation Flanders (FWO).
Editor: Bruce Watson
top
References
Ando, J. (1958) Phase diagrams of Ca3(PO4)2–Mg3(PO4)2 and Ca3(PO4)2–CaNaPO4 systems. Bulletin of the Chemical Society of Japan 31, 201–205.
Show in context High temperature merrillite and peritectic stanfieldite formed at or above ~1175 °C and have generally high rare earth element (REE) concentrations, while eutectic stanfieldite, farringtonite and silico-phosphate crystallised near the ~1120 °C eutectic after in situ fractional crystallisation (with generally lower REE concentrations; Ando, 1958; Davis and Olsen, 1991; Hsu, 2003).
View in article
Birck, J.-L., Allègre, C.J. (1998) Manganese-chromium isotope systematics and the development of the early Solar System. Nature 331, 579-584.
Show in context The 53Mn-53Cr decay scheme (t1/2 3.7 Myr) would seem ideal for such meteorites due to the fractionation of parent and daughter between olivine (generally with high Mn/Cr) and chromite (low Mn/Cr); however, pallasite chronologies have stalled since pioneering Thermal Ionisation Mass Spectrometry (TIMS) studies (Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998), with conflicting Secondary Ion Mass Spectrometry (SIMS) results having been later retracted (Huss et al., 2011; Telus et al., 2012).
View in article
The inferred initial 53Mn/55Mn of 4.0 (±1.2) × 10-5 for Brenham and 8.4 (±4.5) × 10-6 for Brahin are very high compared to other early Solar System materials (Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998) and cannot represent fossil isochrons because they exceed or are close to the initial 53Mn/55Mn for the Solar System (9.1 (±1.7) × 10-6; Nyquist et al., 2009).
View in article
Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
Show in context The accumulated oxygen isotopic evidence for five distinct parent bodies of olivine and metal-rich pallasite meteorites (summarised in Boesenberg et al., 2012) suggests that pallasitic material is the default end-product of planetesimal differentiation.
View in article
Pallasites may represent samples of quiescent core-mantle boundaries (Boesenberg et al., 2012), or violently formed mixtures of core and mantle materials (Yang et al., 2010; Tarduno et al., 2012) and are often discussed in terms of trapping of olivine in a liquid Fe-Ni metal groundmass progressively becoming saturated in chromite, troilite, schreibersite, and phosphate minerals during cooling (Boesenberg et al., 2012).
View in article
Phosphoran olivine (with several wt. % P2O5) and Mg- and Si-phosphate minerals crystallised from an unusual P2O5-rich silicate melt (Boesenberg et al., 2012) which might ultimately be formed by oxidation of metallic P.
View in article
The occurrence of genuinely ‘phosphoran’ olivine rims in some pallasites, with several wt. % P2O5 (Boesenberg et al., 2012) also indicates rapid cooling at this stage and eventual saturation of this metastable mineral.
View in article
Mn and Cr are generally considered to be fairly neutrally partitioned between olivine and melt (e.g., Mallmann and O’Neill, 2013) although by comparison with other pallasites, Mn may have been more incompatible in olivine from the Brenham and Brahin environment (Boesenberg et al., 2012).
View in article
This melt may have occasionally been quenched to ‘phosphoran olivine’ under favourable conditions (Boesenberg et al., 2012) in the terminal pallasite cooling event (Fowler-Gerace and Tait, 2015).
View in article
The latter requires cooling of olivine and liquid metal to the silico-phosphate and Fe-FeS eutectics around 1120-980 °C (Boesenberg et al., 2012).
View in article
This timescale, similar to or longer than the lifetime of 53Mn, accommodates a cooling history comprising 1) rapid accretion, heating and core-mantle separation (Kruijer et al., 2014); 2) rapid cooling to near-solidus temperatures followed by thermal buffering due to removal of crust, formation of regolith (Warren, 2011) and, potentially, crystallisation reactions (Morse, 2011; Namur et al., 2014); and 3) final quenching to preserve disequilibrium assemblages (Boesenberg et al., 2012; Fowler-Gerace and Tait, 2015).
View in article
Brennecka G.A., Wadhwa, M. (2012) Uranium isotope compositions of the basaltic angrite meteorites and the chronological implications for the early Solar System. Proceedings of the National Academy of Sciences 109, 9299-9303.
Show in context Isolation of late silico-phosphate melt before ~2.5 to 4 Myr after Solar System formation implies that it represents one of the earliest silicate differentiation products, potentially predating almost all basaltic eucrites and angrites (Wadhwa et al., 2009; Schiller et al., 2010; Brennecka and Wadhwa, 2012; McKibbin et al., 2015; Schiller et al., 2015).
View in article
Budde, G., Kruijer, T.S., Fischer-Gödde, M., Irving, A.J., Kleine, T. (2015) Planetesimal differentiation revealed by the Hf-W systematics of ureilites. Earth and Planetary Science Letters 430, 316-325.
Show in context Early cooling of the Main-group pallasite parent body to near solidus temperatures provides further evidence for early accretion of iron meteorite-type parent bodies, distinct from later forming ureilite- and chondrite-type parent bodies (e.g., Budde et al., 2015).
View in article
Clayton, R.N., Mayeda, T.K. (1996) Oxygen isotope studies of achondrites. Geochimica et Cosmochimica Acta 60, 1999-2017.
Show in context Additionally, this process appears to have been reproducible: the presence of high 55Mn/52Cr phosphate in the Eagle Station pallasite (Davis and Olsen, 1991), which originated in a different parent body in a different part of the Solar System (Clayton and Mayeda, 1996) indicates that coexisting silico-phosphate and metal-sulphide-phosphide melts could be a general feature of planetesimal differentiation.
View in article
Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasite meteorites as probes of mantle processes in small planetary bodies. Nature 353, 637-640.
Show in context High temperature merrillite and peritectic stanfieldite formed at or above ~1175 °C and have generally high rare earth element (REE) concentrations, while eutectic stanfieldite, farringtonite and silico-phosphate crystallised near the ~1120 °C eutectic after in situ fractional crystallisation (with generally lower REE concentrations; Ando, 1958; Davis and Olsen, 1991; Hsu, 2003).
View in article
High Mn/Cr in eutectic phosphates (~60-145 in Springwater and Eagle Station pallasites; Davis and Olsen, 1991; Hutcheon and Olsen, 1991), probably due primarily to early chromite saturation which keeps available Cr concentrations low, indicates that the melt from which they crystallised is a good candidate for the high Mn/Cr reservoir.
View in article
The 55Mn/52Cr of phosphates (~60-145; Davis and Olsen, 1991; Hutcheon and Olsen, 1991) and olivine rims (~25-40; this study) can be used to bracket the parental silico-phosphate melt (vertical lines in Fig. 3).
View in article
However, Mn is compatible in phosphates, while Cr is probably incompatible (Davis and Olsen, 1991; Hutcheon and Olsen, 1991).
View in article
Additionally, this process appears to have been reproducible: the presence of high 55Mn/52Cr phosphate in the Eagle Station pallasite (Davis and Olsen, 1991), which originated in a different parent body in a different part of the Solar System (Clayton and Mayeda, 1996) indicates that coexisting silico-phosphate and metal-sulphide-phosphide melts could be a general feature of planetesimal differentiation.
View in article
Fowler-Gerace, N.A., Tait, K.T. (2015) Phosphoran olivine overgrowths: Implications for multiple impacts to the Main Group pallasite parent body. American Mineralogist 100, 2043-2052.
Show in context This melt may have occasionally been quenched to ‘phosphoran olivine’ under favourable conditions (Boesenberg et al., 2012) in the terminal pallasite cooling event (Fowler-Gerace and Tait, 2015).
View in article
This timescale, similar to or longer than the lifetime of 53Mn, accommodates a cooling history comprising 1) rapid accretion, heating and core-mantle separation (Kruijer et al., 2014); 2) rapid cooling to near-solidus temperatures followed by thermal buffering due to removal of crust, formation of regolith (Warren, 2011) and, potentially, crystallisation reactions (Morse, 2011; Namur et al., 2014); and 3) final quenching to preserve disequilibrium assemblages (Boesenberg et al., 2012; Fowler-Gerace and Tait, 2015).
View in article
Kruijer, T.S., Touboul, M., Fischer-Gödde, M., Bermingham, K.R., Walker, R.J., Kleine, T. (2014) Protracted core formation and rapid accretion of protoplanets. Science 344, 1150-1154.
Show in context Figure 3 [...] The evolution of a planetesimal isolated from the nebula sufficiently early (i.e. with cooling and core-mantle redox-reactions occurring in the yellow region, before 2.5-4 Myr) is given on the right. Chronological constraints are from Mn-Cr (this study) and Hf-W data (Kruijer et al., 2014).
View in article
This timescale confirms early accretion (0.1-0.3 Myr) and protracted core-mantle differentiation (to ~1.1-1.3 Myr) suggested by Hf-W chronologies for iron meteorite parent bodies (Kruijer et al., 2014).
View in article
The history outlined above closely follows silicate-metal differentiation to form planetesimal cores at ~1.1-1.3 Myr after Solar System formation (Kruijer et al., 2014).
View in article
This timescale, similar to or longer than the lifetime of 53Mn, accommodates a cooling history comprising 1) rapid accretion, heating and core-mantle separation (Kruijer et al., 2014); 2) rapid cooling to near-solidus temperatures followed by thermal buffering due to removal of crust, formation of regolith (Warren, 2011) and, potentially, crystallisation reactions (Morse, 2011; Namur et al., 2014); and 3) final quenching to preserve disequilibrium assemblages (Boesenberg et al., 2012; Fowler-Gerace and Tait, 2015).
View in article
Huss, G.R., Ogliore, K., Nagashima, M., Telus, M., Jilly, C.E. (2011) Dangers of determining isotope ratios using means of individual ratios. Lunar and Planetary Science Conference 42, 2608.
Show in context The 53Mn-53Cr decay scheme (t1/2 3.7 Myr) would seem ideal for such meteorites due to the fractionation of parent and daughter between olivine (generally with high Mn/Cr) and chromite (low Mn/Cr); however, pallasite chronologies have stalled since pioneering Thermal Ionisation Mass Spectrometry (TIMS) studies (Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998), with conflicting Secondary Ion Mass Spectrometry (SIMS) results having been later retracted (Huss et al., 2011; Telus et al., 2012).
View in article
Hutcheon, I.D., Olsen, E. (1991) Cr isotopic composition of differentiated meteorites: a search for 53Mn. Lunar and Planetary Science Conference 22, 605-606.
Show in context High Mn/Cr in eutectic phosphates (~60-145 in Springwater and Eagle Station pallasites; Davis and Olsen, 1991; Hutcheon and Olsen, 1991), probably due primarily to early chromite saturation which keeps available Cr concentrations low, indicates that the melt from which they crystallised is a good candidate for the high Mn/Cr reservoir.
View in article
The 55Mn/52Cr of phosphates (~60-145; Davis and Olsen, 1991; Hutcheon and Olsen, 1991) and olivine rims (~25-40; this study) can be used to bracket the parental silico-phosphate melt (vertical lines in Fig. 3).
View in article
However, Mn is compatible in phosphates, while Cr is probably incompatible (Davis and Olsen, 1991; Hutcheon and Olsen, 1991).
View in article
Hsu, W. (2003) Minor element zoning and trace element geochemistry of pallasites. Meteoritics and Planetary Science 38, 1217-1241.
Show in context High temperature merrillite and peritectic stanfieldite formed at or above ~1175 °C and have generally high rare earth element (REE) concentrations, while eutectic stanfieldite, farringtonite and silico-phosphate crystallised near the ~1120 °C eutectic after in situ fractional crystallisation (with generally lower REE concentrations; Ando, 1958; Davis and Olsen, 1991; Hsu, 2003).
View in article
Hsu, W. (2005) Mn-Cr systematics of pallasites. Geochemical Journal 39, 311-316.
Show in context Small variations (~0.1 per mille) in 53Cr/52Cr are likely present in Brenham pallasite olivine, with slightly higher (~2 per mille) excesses in coexisting metal (Hsu, 2005; Qin et al., 2010), due to cosmic ray exposure effects during transport to Earth.
View in article
Ito, M., Ganguly, J. (2006) Diffusion kinetics of Cr in olivine and 53Mn-53Cr thermochronology of early solar system objects. Geochimica et Cosmochimica Acta 70, 799-809.
Show in context A parallel trend is recorded by olivine geochemistry: olivine cores exhibit variable Cr and P (Fig. 1) while divalent Mn varies over a shorter distance of several tens of micrometres, consistent with an overgrowth or rapid diffusion of divalent elements (Zhukova et al., 2014a) and more slowly diffusing trivalent Cr (Ito and Ganguly, 2006) and pentavalent P (Spandler and O’Neill, 2010).
View in article
That Cr might move slower than divalent cations (Ito and Ganguly, 2006), and that low silica activity can suppress divalent (Zhukova et al., 2014a) and trivalent cation diffusivity (Zhukova et al., 2014b) suggests that in pallasites, trace element variations might not be erased at 1000 °C even over millions of years.
View in article
Longerich, H.P., Jackson, S.E., Günther, D. (1996) Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. Journal of Analytical and Atomic Spectrometry 11, 899–904.
Show in context Data were reduced according to established methods (Longerich et al., 1996; Spandler and O’Neill, 2010) with NIST 612 as the primary external standard (Pearce et al., 1997).
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Lugmair, G.W., Shukolyukov, A. (1998) Early solar system timescales according to 53Mn-53Cr systematics. Geochimica et Cosmochimica Acta 62, 2863-2886.
Show in context The 53Mn-53Cr decay scheme (t1/2 3.7 Myr) would seem ideal for such meteorites due to the fractionation of parent and daughter between olivine (generally with high Mn/Cr) and chromite (low Mn/Cr); however, pallasite chronologies have stalled since pioneering Thermal Ionisation Mass Spectrometry (TIMS) studies (Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998), with conflicting Secondary Ion Mass Spectrometry (SIMS) results having been later retracted (Huss et al., 2011; Telus et al., 2012).
View in article
The inferred initial 53Mn/55Mn of 4.0 (±1.2) × 10-5 for Brenham and 8.4 (±4.5) × 10-6 for Brahin are very high compared to other early Solar System materials (Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998) and cannot represent fossil isochrons because they exceed or are close to the initial 53Mn/55Mn for the Solar System (9.1 (±1.7) × 10-6; Nyquist et al., 2009).
View in article
Mallmann, G., O’Neill, H.St.C. (2013) Calibration of an empirical thermometer and oxybarometer based on the partitioning of Sc, Y and V between olivine and silicate melt. Journal of Petrology 54, 933-949.
Show in context Mn and Cr are generally considered to be fairly neutrally partitioned between olivine and melt (e.g., Mallmann and O’Neill, 2013) although by comparison with other pallasites, Mn may have been more incompatible in olivine from the Brenham and Brahin environment (Boesenberg et al., 2012).
View in article
McKibbin, S.J., Ireland, T.R., Amelin, Y., O’Neill, H.St.C., Holden, P. (2013a) Mn-Cr relative sensitivity Factors for Secondary Ion Mass Spectrometry analysis of Mg-Fe-Ca olivine and implications for the Mn-Cr chronology of meteorites. Geochimica et Cosmochimica Acta 110, 216-228.
Show in context In this study, we re-investigate pallasite olivine trace elements and Mn-Cr isotopes using improved Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS; Spandler and O’Neill, 2010) and SIMS (McKibbin et al., 2013a) in two texturally different Main-Group pallasites: Brenham, with rounded olivines sharing equilibrated triple-junctions across a continuous network; and Brahin, with fragmental olivine of variable grain size (McKibbin et al., 2013b).
View in article
For SIMS Mn-Cr analysis, we modified previous Sensitive High-mass Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG) methodology (McKibbin et al., 2013a,c).
View in article
McKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013b) LA-ICP-MS mapping of olivine from the Brahin and Brenham meteorites: Complex elemental distributions in the pallasite olivine precursor. Geochimica et Cosmochimica Acta 119, 1-17.
Show in context In this study, we re-investigate pallasite olivine trace elements and Mn-Cr isotopes using improved Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS; Spandler and O’Neill, 2010) and SIMS (McKibbin et al., 2013a) in two texturally different Main-Group pallasites: Brenham, with rounded olivines sharing equilibrated triple-junctions across a continuous network; and Brahin, with fragmental olivine of variable grain size (McKibbin et al., 2013b).
View in article
Trace element variations and clear charge-balancing relationships between Cr and slow-diffusing Al in olivine (McKibbin et al., 2013b) indicates suppression of Cr diffusion.
View in article
McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P., Sugiura, N. (2013c) A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D’Orbigny using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 123, 181-194.
Show in context For SIMS Mn-Cr analysis, we modified previous Sensitive High-mass Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG) methodology (McKibbin et al., 2013a,c).
View in article
Data were reduced according to the ‘first ratio estimator’ method (Ogliore et al., 2011) which we earlier referred to as ‘ratio of total counts’ (McKibbin et al., 2013c).
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Figure 1 [...] Vertical dashed lines (blue and green for Brenham and Brahin respectively) represent limit of SIMS spatial resolution (~30 μm) when analysing near olivine margins. LA-ICP-MS tracks from this study and ablation pits from previous work (McKibbin et al., 2013c).
View in article
McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P. (2015) Mn-Cr dating of Fe- and Ca-rich olivine from ‘quenched’ and ‘plutonic’ angrite meteorites using Seconary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 157, 13-27.
Show in context Suitable areas of olivine ~30 μm in diameter were sputtered by a high current unfiltered primary ion beam (~16.9-56.6 nA mixed O- and O2-; McKibbin et al., 2015).
View in article
Isolation of late silico-phosphate melt before ~2.5 to 4 Myr after Solar System formation implies that it represents one of the earliest silicate differentiation products, potentially predating almost all basaltic eucrites and angrites (Wadhwa et al., 2009; Schiller et al., 2010; Brennecka and Wadhwa, 2012; McKibbin et al., 2015; Schiller et al., 2015).
View in article
Morse, S.A. (2011) The fractional latent heat of crystallizing magmas. American Mineralogist 96, 682-689.
Show in context This timescale, similar to or longer than the lifetime of 53Mn, accommodates a cooling history comprising 1) rapid accretion, heating and core-mantle separation (Kruijer et al., 2014); 2) rapid cooling to near-solidus temperatures followed by thermal buffering due to removal of crust, formation of regolith (Warren, 2011) and, potentially, crystallisation reactions (Morse, 2011; Namur et al., 2014); and 3) final quenching to preserve disequilibrium assemblages (Boesenberg et al., 2012; Fowler-Gerace and Tait, 2015).
View in article
Namur, O., Humphrey, M.C.S., Holness, M.B. (2014) Crystallisation of interstitial liquid and latent heat buffering in solidifying gabbros: Skaergaard Intrusion, Greenland. Journal of Petrology 55, 1389-1427.
Show in context This timescale, similar to or longer than the lifetime of 53Mn, accommodates a cooling history comprising 1) rapid accretion, heating and core-mantle separation (Kruijer et al., 2014); 2) rapid cooling to near-solidus temperatures followed by thermal buffering due to removal of crust, formation of regolith (Warren, 2011) and, potentially, crystallisation reactions (Morse, 2011; Namur et al., 2014); and 3) final quenching to preserve disequilibrium assemblages (Boesenberg et al., 2012; Fowler-Gerace and Tait, 2015).
View in article
Nyquist, L.E., Kleine, T., Shih, C.-Y., Reese, Y.D. (2009) The distribution of short-lived radioisotopes in the early solar system and the chronology of asteroid accretion, differentiation, and secondary mineralization. Geochimica et Cosmochimica Acta 73, 5115-5136.
Show in context Despite this diverse mineralogy, the short-lived isotope chronologies of pallasites remain poorly understood due to depletion in crustal components (plagioclase and pyroxene; Nyquist et al., 2009).
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The inferred initial 53Mn/55Mn of 4.0 (±1.2) × 10-5 for Brenham and 8.4 (±4.5) × 10-6 for Brahin are very high compared to other early Solar System materials (Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998) and cannot represent fossil isochrons because they exceed or are close to the initial 53Mn/55Mn for the Solar System (9.1 (±1.7) × 10-6; Nyquist et al., 2009).
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We assume a Solar System initial of 53Mn/55Mn of 9.1 × 10-6 (Nyquist et al., 2009) and that 53Mn decays completely, implying that the reservoir persists for ~10-20 Myr.
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Figure 3 [...] A Solar System initial 53Mn/55Mn of 9.1 × 10-6 (Nyquist et al., 2009), along with a conservatively estimated 53Cr isotope enrichment in pallasite olivine rims of 3.5 ± 1.4 per mille and a bulk 55Mn/52Cr for late interstitial melts of 40-60 (bounded by olivine and high Mn/Cr phosphates) constrains the isolation of evolved silico-phosphate melt to before ~2.5 to 4 Myr (yellow field) followed by decay of 53Mn to extinction (white field bound by olivine rims and phosphate minerals).
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We therefore favour a higher Solar System initial 53Mn/55Mn (Nyquist et al., 2009) and near complete decay of 53Mn (~10 Myr at eutectic temperatures) for 55Mn/52Cr in the range 40-60.
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Ogliore, R., Huss, G., Nakashima, K. (2011) Ratio estimation in SIMS analysis. Nuclear Instruments and Methods in Physics Research Section B 269, 1910-1918.
Show in context Data were reduced according to the ‘first ratio estimator’ method (Ogliore et al., 2011) which we earlier referred to as ‘ratio of total counts’ (McKibbin et al., 2013c).
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Pearce, N.J.G., Perkins, W.T., Westgate, J.A., Gorton, M.P., Jackson, S.E., Neal, C.R., Chenery, S.P. (1997) A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletter 21, 115–144.
Show in context Data were reduced according to established methods (Longerich et al., 1996; Spandler and O’Neill, 2010) with NIST 612 as the primary external standard (Pearce et al., 1997).
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Qin, L., Alexander, C.M.O’D., Carlson, R.W., Horan, M.F., Yokohama, T. (2010) Contributors to chromium isotope variations of meteorites. Geochimica et Cosmochimica Acta 74, 1122-1145.
Show in context Small variations (~0.1 per mille) in 53Cr/52Cr are likely present in Brenham pallasite olivine, with slightly higher (~2 per mille) excesses in coexisting metal (Hsu, 2005; Qin et al., 2010), due to cosmic ray exposure effects during transport to Earth.
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Anomalies related to pre-solar, nucleosynthetic isotope components are even smaller and unlikely to survive planetary differentiation (Trinquier et al., 2008; Qin et al., 2010).
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Schiller, M., Baker, J.A., Bizzarro, M. (2010) 26Al-26Mg dating of asteroidal magmatism in the young Solar System. Geochimica et Cosmochimica Acta 74, 4844-4864.
Show in context Isolation of late silico-phosphate melt before ~2.5 to 4 Myr after Solar System formation implies that it represents one of the earliest silicate differentiation products, potentially predating almost all basaltic eucrites and angrites (Wadhwa et al., 2009; Schiller et al., 2010; Brennecka and Wadhwa, 2012; McKibbin et al., 2015; Schiller et al., 2015).
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Schiller, M., Connelly, J.N., Glad, A.C., Mikouchi, T., Bizzarro, M. (2015) Early accretion of protoplanets inferred from a reduced inner solar system 26Al inventory. Earth and Planetary Science Letters 420, 45-54.
Show in context Isolation of late silico-phosphate melt before ~2.5 to 4 Myr after Solar System formation implies that it represents one of the earliest silicate differentiation products, potentially predating almost all basaltic eucrites and angrites (Wadhwa et al., 2009; Schiller et al., 2010; Brennecka and Wadhwa, 2012; McKibbin et al., 2015; Schiller et al., 2015).
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Spandler, C., O’Neill, H.St.C. (2010) Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications. Contributions to Mineralogy and Petrology 159, 791-818.
Show in contextIn this study, we re-investigate pallasite olivine trace elements and Mn-Cr isotopes using improved Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS; Spandler and O’Neill, 2010) and SIMS (McKibbin et al., 2013a) in two texturally different Main-Group pallasites: Brenham, with rounded olivines sharing equilibrated triple-junctions across a continuous network; and Brahin, with fragmental olivine of variable grain size (McKibbin et al., 2013b).
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Trace element analyses of olivine were carried out following a previously developed LA-ICP-MS profiling method (Spandler and O’Neill, 2010).
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Data were reduced according to established methods (Longerich et al., 1996; Spandler and O’Neill, 2010) with NIST 612 as the primary external standard (Pearce et al., 1997).
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A parallel trend is recorded by olivine geochemistry: olivine cores exhibit variable Cr and P (Fig. 1) while divalent Mn varies over a shorter distance of several tens of micrometres, consistent with an overgrowth or rapid diffusion of divalent elements (Zhukova et al., 2014a) and more slowly diffusing trivalent Cr (Ito and Ganguly, 2006) and pentavalent P (Spandler and O’Neill, 2010).
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Tarduno, J.A., Cottrell, R.D., Nimmo, F., Hopkins, J., Voronov, J., Erickson, A., Blackman, E., Scott, E.R.D., McKinley, R. (2012) Evidence for a dynamo in the main group pallasite parent body. Science 338, 939–942.
Show in context Pallasites may represent samples of quiescent core-mantle boundaries (Boesenberg et al., 2012), or violently formed mixtures of core and mantle materials (Yang et al., 2010; Tarduno et al., 2012) and are often discussed in terms of trapping of olivine in a liquid Fe-Ni metal groundmass progressively becoming saturated in chromite, troilite, schreibersite, and phosphate minerals during cooling (Boesenberg et al., 2012).
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Telus, M., Huss, G.R., Ogliore, R.C., Nagashima, K., Tachibana, S. (2012) Recalculation of data for short-lived radionuclide systems using less-biased ratio estimation. Meteoritics and Planetary Science 47, 2013-2030.
Show in context The 53Mn-53Cr decay scheme (t1/2 3.7 Myr) would seem ideal for such meteorites due to the fractionation of parent and daughter between olivine (generally with high Mn/Cr) and chromite (low Mn/Cr); however, pallasite chronologies have stalled since pioneering Thermal Ionisation Mass Spectrometry (TIMS) studies (Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998), with conflicting Secondary Ion Mass Spectrometry (SIMS) results having been later retracted (Huss et al., 2011; Telus et al., 2012).
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Trinquier, A., Birck, J.-L., Allègre, C.J., Ulfbeck, D. (2008) 53Mn-53Cr systematics of the early Solar System revisited. Geochimica et Cosmochimica Acta 72, 5146-5163.
Show in context Anomalies related to pre-solar, nucleosynthetic isotope components are even smaller and unlikely to survive planetary differentiation (Trinquier et al., 2008; Qin et al., 2010).
View in article
The difficulties of a lower Solar System initial, such as in Trinquier et al. (2008), or incomplete decay of 53Mn, are discussed below.
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Incomplete decay of 53Mn, or low values of initial 53Mn/55Mn for the Solar System (Trinquier et al., 2008), become difficult to accommodate.
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Wadhwa, M., Amelin, Y., Bogdanovski, O., Shukolyukov, A., Lugmair, G.W., Janney, P. (2009) Ancient relative and absolute ages for a basaltic meteorite: Implications for timescales of planetesimal accretion and differentiation. Geochimica et Cosmochimica Acta 73, 5189-5201.
Show in context Isolation of late silico-phosphate melt before ~2.5 to 4 Myr after Solar System formation implies that it represents one of the earliest silicate differentiation products, potentially predating almost all basaltic eucrites and angrites (Wadhwa et al., 2009; Schiller et al., 2010; Brennecka and Wadhwa, 2012; McKibbin et al., 2015; Schiller et al., 2015).
View in article
Warren, P.H. (2011) Ejecta–megaregolith accumulation on planetesimals and large asteroids. Meteoritics and Planetary Science 46, 53-78.
Show in context This timescale, similar to or longer than the lifetime of 53Mn, accommodates a cooling history comprising 1) rapid accretion, heating and core-mantle separation (Kruijer et al., 2014); 2) rapid cooling to near-solidus temperatures followed by thermal buffering due to removal of crust, formation of regolith (Warren, 2011) and, potentially, crystallisation reactions (Morse, 2011; Namur et al., 2014); and 3) final quenching to preserve disequilibrium assemblages (Boesenberg et al., 2012; Fowler-Gerace and Tait, 2015).
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Yang, J., Goldstein, J.I., Scott, E.R.D. (2010) Main-group pallasites: Thermal history, relationship to IIIAB irons, and origin. Geochimica et Cosmochimica Acta 74, 4471-4492.
Show in context Pallasites may represent samples of quiescent core-mantle boundaries (Boesenberg et al., 2012), or violently formed mixtures of core and mantle materials (Yang et al., 2010; Tarduno et al., 2012) and are often discussed in terms of trapping of olivine in a liquid Fe-Ni metal groundmass progressively becoming saturated in chromite, troilite, schreibersite, and phosphate minerals during cooling (Boesenberg et al., 2012).
View in article
Zhukova, I., O’Neill, H.St.C., Cambell, I.H., Kilburn, M.R. (2014a) The effect of silica activity on the diffusion of Ni and Co in olivine. Contributions to Mineralogy and Petrology 168, 1-15.
Show in context A parallel trend is recorded by olivine geochemistry: olivine cores exhibit variable Cr and P (Fig. 1) while divalent Mn varies over a shorter distance of several tens of micrometres, consistent with an overgrowth or rapid diffusion of divalent elements (Zhukova et al., 2014a) and more slowly diffusing trivalent Cr (Ito and Ganguly, 2006) and pentavalent P (Spandler and O’Neill, 2010).
View in article
That Cr might move slower than divalent cations (Ito and Ganguly, 2006), and that low silica activity can suppress divalent (Zhukova et al., 2014a) and trivalent cation diffusivity (Zhukova et al., 2014b) suggests that in pallasites, trace element variations might not be erased at 1000 °C even over millions of years.
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Zhukova, I., O’Neill H., Capbell, I. (2014b) The effect of mineral paragenesis on Al diffusion in olivine. EGU General Assembly Conference Abstracts 16, 573.
Show in context That Cr might move slower than divalent cations (Ito and Ganguly, 2006), and that low silica activity can suppress divalent (Zhukova et al., 2014a) and trivalent cation diffusivity (Zhukova et al., 2014b) suggests that in pallasites, trace element variations might not be erased at 1000 °C even over millions of years.
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Supplementary Information
Supplementary Methods and Results
The Brenham and Brahin stony-iron meteorites were selected because both have been assigned to the Main-Group of pallasites (Clayton and Mayeda, 1996
Clayton, R.N., Mayeda, T.K. (1996) Oxygen isotope studies of achondrites. Geochimica et Cosmochimica Acta 60, 1999-2017.
) and show similar distinctive olivine trace element characteristics, belonging to a low-Mn subgroup (Boesenberg et al., 2012Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
). They likely sampled a single geological unit on the same parent body but exhibit different textural characteristics. Samples were cast into epoxy along with terrestrial reference materials and polished for in situ measurement of trace elements by LA-ICP-MS and Mn-Cr isotopic ratios by SIMS, the latter using Sensitive High-mass Resolution Ion MicroProbe Reverse Geometry (SHRIMP-RG).Trace element analysis of olivine was conducted using the method developed by Spandler and O’Neill (2010)
Spandler, C., O’Neill, H.St.C. (2010) Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications. Contributions to Mineralogy and Petrology 159, 791-818.
. Sampling was undertaken with a 193 nm wavelength ArF Excimer laser using a second-generation custom-built ‘HelEx’ two-volume vortex ablation cell at RSES, ANU (Eggins et al., 2003Eggins, S., De Deckker, P., Marshall, J. (2003) Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration. Earth and Planetary Science Letters 212, 291-306.
). An aperture was used to shape the laser into a rectangle, which was aligned parallel to olivine rims for high spatial resolution sampling. The stage was driven at a rate of 1 μm s-1 to enable continuous sampling across the grains, from core to rim. Sampling of meteoritic olivine was bracketed by NIST 610 and 612 standard silicate glasses. The laser was tuned to achieve energies of 50 mJ and repetition rate of 5 Hz, with ablation occurring in a He atmosphere with gas flow augmented by Ar at the ablation site and H outside the sample chamber. Spatial resolution was limited to ~5 μm by the laser beam width and gas flow rates. The gas flow was directed to an Agilent 7500s ICP-MS. Various combinations of the isotopes 7Li, 27Al, 29Si, 31P, 43Ca, 45Sc, 47Ti, 49Ti, 51V, 52Cr, 53Cr, 55Mn, 59Co, 60Ni, 69Ga, 71Ga were investigated in Brenham and Brahin olivine. Data were reduced according to Longerich et al. (1996)Longerich, H.P., Jackson, S.E., Günther, D. (1996) Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. Journal of Analytical and Atomic Spectrometry 11, 899–904.
, with 29Si as the internal standard and NIST 612 as the primary external standard using the concentrations reported by Pearce et al. (1997)Pearce, N.J.G., Perkins, W.T., Westgate, J.A., Gorton, M.P., Jackson, S.E., Neal, C.R., Chenery, S.P. (1997) A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletter 21, 115–144.
. NIST 610 and BCR2g were analysed as secondary standards to check for accuracy. For Sc, we subtracted 0.2 ppm from nominal concentrations due to possible interference of 29Si16O+ on 45Sc+ (De Hoog et al., 2010De Hoog, J.C.M., Gall, L., Cornell, D.H. (2010) Trace-element geochemistry of mantle olivine and application to mantle petrogenesis and geothermobarometry. Chemical Geology 270, 196–215.
). Secondary standard concentrations were reproducible to ~5.2 % and ~8.5 % or better for NIST 610 and BCR2g respectively, with absolute concentrations within 16 % and 18 % or better for all elements in each standard respectively (Table S-1). Of the investigated elements, we selected a subset for further processing and present 27Al, 31P, 45Sc, 51V, 53Cr, 55Mn, 59Co, 60Ni, and 71Ga data for olivine from both meteorites (Tables S-2 and S-3 for Brenham and Brahin respectively). Each profile was smoothed using a 20-point moving average. LA-ICP-MS data for reference materials, Brenham and Brahin olivine are given in Table S-1, Table S-2, and Table S-3 respectively.Table S-1 Secondary standard reference materials for LA-ICP-MS.
| Standard | Al2O3 (wt. %) | P | Sc | V | Cr | Mn | Co | Ni | Ga |
| BCR2g | 14.23 | 1419 | 33.4 | 421.3 | 16.1 | 1523 | 37.6 | 14.0 | 21.3 |
| s.d. | 0.82 | 22 | 2.1 | 11.0 | 0.4 | 130 | 0.6 | 1.2 | 0.2 |
| n | 4 | ||||||||
| GeoREM | 13.4 | 1550 | 33.0 | 425.0 | 17.0 | 1550 | 38.0 | 13.0 | 23.0 |
| s.d. | 0.4 | 70 | 2.0 | 18.0 | 2.0 | 70 | 2.0 | 2.0 | 1.0 |
| NIST 610 | 2.05 | 405.9 | 494.7 | 486.5 | 467.0 | 470.6 | 435.8 | 472.0 | 449.5 |
| s.d. | 0.04 | 6.4 | 25.8 | 18.7 | 1.1 | 23.5 | 9.9 | 1.7 | 1.7 |
| n | 4 | ||||||||
| Pearce et al. (1997)Pearce, N.J.G., Perkins, W.T., Westgate, J.A., Gorton, M.P., Jackson, S.E., Neal, C.R., Chenery, S.P. (1997) A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletter 21, 115–144. | 1.95 | 413 | 455 | 450 | 408 | 444 | 410 | 458.7 | 433 |
Table S-2 LA-ICP-MS traverse data for Brenham olivine.
| Distance (um) | Traverse | Al27 | P31 | Sc45 | V51 | Cr53 | Mn55 | Co59 | Ni60 | Ga71 | Mn/Cr |
| 298.415 | Bren-11 | 23.27 | 44.78 | 0.94 | 7.49 | 128.13 | 1572.23 | 4.75 | 13.32 | 0.40 | 12.27 |
| 297.916 | Bren-11 | 23.84 | 51.62 | 0.92 | 6.78 | 136.28 | 1588.10 | 5.18 | 20.27 | 0.36 | 11.65 |
| 297.417 | Bren-11 | 27.53 | 68.11 | 1.06 | 6.79 | 138.20 | 1622.51 | 5.16 | 20.47 | 0.29 | 11.74 |
| 296.918 | Bren-11 | 26.97 | 71.07 | 1.20 | 6.98 | 131.74 | 1542.71 | 4.97 | 18.92 | 0.40 | 11.71 |
| 296.419 | Bren-11 | 27.66 | 74.95 | 1.05 | 7.15 | 131.43 | 1552.57 | 5.37 | 16.52 | 0.37 | 11.81 |
| 295.92 | Bren-11 | 29.42 | 69.03 | 1.22 | 7.23 | 131.78 | 1643.15 | 4.68 | 19.19 | 0.46 | 12.47 |
| 295.421 | Bren-11 | 26.73 | 63.47 | 1.44 | 6.75 | 128.10 | 1550.99 | 4.95 | 17.85 | 0.42 | 12.11 |
| 294.922 | Bren-11 | 26.59 | 65.83 | 0.98 | 6.32 | 130.15 | 1574.13 | 5.04 | 15.45 | 0.57 | 12.09 |
| 294.423 | Bren-11 | 25.49 | 45.89 | 0.99 | 6.69 | 128.56 | 1577.77 | 5.30 | 13.42 | 0.28 | 12.27 |
| 293.924 | Bren-11 | 25.28 | 50.78 | 1.07 | 6.85 | 124.17 | 1566.44 | 4.89 | 14.07 | 0.33 | 12.61 |
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Table S-3 LA-ICP-MS traverse data for Brahin olivine.
| Distance (um) | Traverse | Al27 | P31 | Sc45 | V51 | Cr53 | Mn55 | Co59 | Ni60 | Ga71 | Mn/Cr |
| 310.638 | Brah-01 | 9.70 | 57.19 | 1.16 | 5.65 | 56.32 | 1626.22 | 4.53 | 9.54 | 0.18 | 28.87 |
| 310.067 | Brah-01 | 10.03 | 58.46 | 1.02 | 5.28 | 58.99 | 1674.77 | 4.44 | 11.08 | 0.03 | 28.39 |
| 309.496 | Brah-01 | 9.32 | 49.86 | 1.34 | 5.59 | 63.14 | 1702.03 | 4.18 | 11.65 | 0.03 | 26.95 |
| 308.925 | Brah-01 | 10.10 | 51.03 | 1.11 | 5.32 | 60.87 | 1646.10 | 5.03 | 10.73 | 0.03 | 27.04 |
| 308.354 | Brah-01 | 9.19 | 52.05 | 1.11 | 5.63 | 66.60 | 1652.35 | 3.93 | 10.07 | 0.14 | 24.81 |
| 307.783 | Brah-01 | 9.12 | 42.97 | 1.30 | 5.31 | 57.66 | 1580.42 | 3.68 | 10.13 | 0.00 | 27.41 |
| 307.212 | Brah-01 | 8.51 | 50.76 | 1.25 | 5.50 | 61.61 | 1626.94 | 3.71 | 11.65 | 0.08 | 26.41 |
| 306.641 | Brah-01 | 10.26 | 45.27 | 1.12 | 5.46 | 67.95 | 1701.40 | 4.12 | 10.57 | 0.13 | 25.04 |
| 306.071 | Brah-01 | 9.06 | 50.47 | 1.32 | 5.34 | 59.40 | 1588.54 | 4.46 | 9.31 | 0.03 | 26.74 |
| 305.499 | Brah-01 | 9.62 | 53.26 | 1.20 | 5.57 | 62.15 | 1719.94 | 4.04 | 11.04 | -0.03 | 27.68 |
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For isotopic analysis of olivine using SHRIMP-RG, we have modified our previous methodology. Rather than using a filtered primary ion beam to sputter the surface (McKibbin et al., 2013a
McKibbin, S.J., Ireland, T.R., Amelin, Y., O’Neill, H.St.C., Holden, P. (2013a) Mn-Cr relative sensitivity Factors for Secondary Ion Mass Spectrometry analysis of Mg-Fe-Ca olivine and implications for the Mn-Cr chronology of meteorites. Geochimica et Cosmochimica Acta 110, 216-228.
) and due to small expected Cr-isotope anomalies arising from relatively high Cr concentrations in the range ~50-150 ppm (McKibbin et al., 2013bMcKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013b) LA-ICP-MS mapping of olivine from the Brahin and Brenham meteorites: Complex elemental distributions in the pallasite olivine precursor. Geochimica et Cosmochimica Acta 119, 1-17.
), we have used a high current unfiltered primary ion beam (~30 μm diameter; ~16.9-20.2 nA for Brenham and ~52.8-56.6 nA for Brahin; mixed O- and O2-). Secondary ions produced during sputtering were extracted by a potential of ~10 kV and passed through narrow source and collector slits of 350 and 500 μm respectively, giving mass resolution (m/Δm, 10 % peak height) of ~6000. This mass resolution allowed separation of molecular interferences such as oxides and hydrides. Ion beams of interest were directed sequentially to a single electron multiplier by cycling the magnet. Data were first collected for 52Cr+ and 55Mn+ in 10 replicates (the first 2-3 minutes of sputtering, 2 and 1 seconds per cycle respectively) because temporal changes in observed 55Mn+/52Cr+ associated with downhole effects as sputtering proceeds can be significant (McKibbin et al., 2013cMcKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P., Sugiura, N. (2013c) A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D’Orbigny using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 123, 181-194.
). Measurements of 52Cr+ and 53Cr+ followed for each spot, for which 80 replicates were collected (the next ~35 minutes) and to achieve a similar number of total counts for each of these masses, we used integration times of 1 second and 10 seconds respectively. The count rate for 55Mn+ was kept below ~1 Mcps to minimise wear on the electron multiplier. For SHRIMP-RG measurements in Brahin chromite, a ~4.0-5.5 nA filtered O2- primary ion beam was used to sputter the surface. Sequential collection of 52Cr+, 53Cr+, and 55Mn+ was undertaken in 10 replicates by cycling the magnet to direct the beams into a Faraday Cup. All measurements on meteoritic and terrestrial reference materials were conducted at the same conditions. Raw terrestrial reference material ion count ratios are given in Table S-4.Table S-4 Cr-isotope data for San Carlos olivine and terrestrial spinel standards.
| Analysis | PB nA | 52Cr+ kcps | 53Cr+/52Cr+ | 1-sigma |
| SC-3.08 (for Brenham olivine) | 16.9 | 54.2 | 0.11193 | 0.00006 |
| SC-3.10 (for Brenham olivine) | 17.2 | 56.1 | 0.11177 | 0.00006 |
| SC-3.13 (for Brenham olivine) | 18 | 54.5 | 0.11186 | 0.00007 |
| SC-3.15 (for Brenham olivine) | 18.3 | 58.2 | 0.11190 | 0.00006 |
| SC-3.16 (for Brenham olivine) | 19.5 | 58.6 | 0.11180 | 0.00006 |
| SC-3.17 (for Brenham olivine) | 20.1 | 58.5 | 0.11174 | 0.00006 |
| SC-3.18 (for Brenham olivine) | 20.2 | 55.8 | 0.11188 | 0.00007 |
| SC-10.04 (for Brahin olivine) | 52.9 | 79.6 | 0.11182 | 0.00007 |
| SC-6.06 (for Brahin olivine) | 55.2 | 87.8 | 0.11171 | 0.00005 |
| SC-6.07 (for Brahin olivine) | 52.9 | 70.0 | 0.11171 | 0.00006 |
| SC-6.08 (for Brahin olivine) | 52.8 | 83.5 | 0.11170 | 0.00006 |
| SC-10.05 (for Brahin olivine) | 54.4 | 97.1 | 0.11151 | 0.00007 |
| SC-7.06 (for Brahin olivine) | 53.2 | 125.7 | 0.11173 | 0.00006 |
| NewCaledonia-1.05 (for Brahin chromite) | 4.9 | - | 0.11240 | 0.00004 |
| NewCaledonia-1.06 (for Brahin chromite) | 4.9 | - | 0.11260 | 0.00004 |
| NewCaledonia-1.07 (for Brahin chromite) | 4.6 | - | 0.11226 | 0.00005 |
| NewCaledonia-1.12 (for Brahin chromite) | 4.9 | - | 0.11255 | 0.00002 |
| NewCaledonia-1.16 (for Brahin chromite) | 4.7 | - | 0.11250 | 0.00013 |
| NewCaledonia-1.20 (for Brahin chromite) | 5.2 | - | 0.11240 | 0.00004 |
| NewCaledonia-1.23 (for Brahin chromite) | 4.8 | - | 0.11239 | 0.00003 |
| NewCaledonia-1.25 (for Brahin chromite) | 5 | - | 0.11255 | 0.00003 |
| Tumut-1.08 (for Brahin chromite) | 5 | - | 0.11266 | 0.00002 |
| Tumut-1.10 (for Brahin chromite) | 5.3 | - | 0.11251 | 0.00002 |
| Tumut-1.14 (for Brahin chromite) | 4.9 | - | 0.11259 | 0.00021 |
| Tumut-1.18 (for Brahin chromite) | 4.8 | - | 0.11261 | 0.00006 |
| Tumut-1.22 (for Brahin chromite) | 5.1 | - | 0.11269 | 0.00002 |
| Tumut-1.24 (for Brahin chromite) | 4.5 | - | 0.11245 | 0.00008 |
| Tumut-1.26 (for Brahin chromite) | 4.3 | - | 0.11247 | 0.00004 |
Two previous studies on the Mn-Cr systematics of pallasites using small ion probes (Cameca IMS-3f) found elevated 53Cr/52Cr in phosphates and olivine (Hutcheon and Olsen, 1991
Hutcheon, I.D., Olsen, E. (1991) Cr isotopic composition of differentiated meteorites: a search for 53Mn. Lunar and Planetary Science Conference 22, 605-606.
; Hsu, 2005Hsu, W. (2005) Mn-Cr systematics of pallasites. Geochemical Journal 39, 311-316.
) and inferred initial 53Mn/55Mn values as high as 1-2 × 10-5. Huss et al. (2011)Huss, G.R., Ogliore, K., Nagashima, M., Telus, M., Jilly, C.E. (2011) Dangers of determining isotope ratios using means of individual ratios. Lunar and Planetary Science Conference 42, 2608.
revised the values of Hsu (2005)Hsu, W. (2005) Mn-Cr systematics of pallasites. Geochemical Journal 39, 311-316.
using improved data reduction techniques, and did not resolve any radiogenic anomaly, although some pallasites were not well constrained and could be consistent with initial 53Mn/55Mn of ~1 × 10-5. New measurements with improved ion probes (Cameca IMS-6f and 1280) also failed to resolve 53Cr* (Tomiyama et al., 2007Tomiyama, T., Huss, G., Nagashima, K., Krot, A.N. (2007) Ion microprobe analysis of 53Mn-53Cr systematics in pallasite olivines. Lunar and Planetary Science Conference 38, 2007.
). For our measurements, SIMS data were reduced according to the ‘First Ratio Estimator’ method (Ogliore et al., 2011Ogliore, R., Huss, G., Nakashima, K. (2011) Ratio estimation in SIMS analysis. Nuclear Instruments and Methods in Physics Research Section B 269, 1910-1918.
) which we have earlier referred to as ‘Ratio of Total Counts’ (McKibbin et al., 2013cMcKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P., Sugiura, N. (2013c) A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D’Orbigny using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 123, 181-194.
). This involves summing all counts for each isotope within an analysis (53Cr+, 55Mn+), and dividing by the sum of corresponding data for the normalising isotope for that analysis (52Cr+).Instrumental mass fractionation (IMF) on Cr isotopes was corrected by sample-standard bracketing with San Carlos olivine (for Brenham and Brahin olivine) or in-house reference spinels (for Brahin chromite) using a two-isotope correction procedure (53Cr/52Cr), assuming a ‘terrestrial’ value of 0.1134 for the reference materials. In a previous study, we used the median of absolute deviations (MAD) from polynomial curves fit to our time-interpolated count data (McKibbin et al., 2013c
McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P., Sugiura, N. (2013c) A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D’Orbigny using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 123, 181-194.
); this method somewhat underestimates the error and we now calculate internal spot-specific errors as the standard error on the mean of sub-ratios determined for each magnet cycle within a single analytical spot (Telus et al., 2012Telus, M., Huss, G.R., Ogliore, R.C., Nagashima, K., Tachibana, S. (2012) Recalculation of data for short-lived radionuclide systems using less-biased ratio estimation. Meteoritics and Planetary Science 47, 2013-2030.
; McKibbin et al., 2015McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P. (2015) Mn-Cr dating of Fe- and Ca-rich olivine from ‘quenched’ and ‘plutonic’ angrite meteorites using Seconary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 157, 13-27.
). The standard deviation of our terrestrial olivine or spinel 53Cr+/52Cr+ was propagated to meteoritic olivine and spinel 53Cr/52Cr ratios respectively. To convert the ion ratio 55Mn+/52Cr+ into the inter-element ratio 55Mn/52Cr, we use a Relative Sensitivity Factor (RSF) of 0.58 (±0.02) as a multiplier on olivine data (55Mn/52Cr = 55Mn+/52Cr+ × RSF). Olivine RSF is strongly dependent on composition; this value was determined using a range of ~Fo90 olivines (McKibbin et al., 2013aMcKibbin, S.J., Ireland, T.R., Amelin, Y., O’Neill, H.St.C., Holden, P. (2013a) Mn-Cr relative sensitivity Factors for Secondary Ion Mass Spectrometry analysis of Mg-Fe-Ca olivine and implications for the Mn-Cr chronology of meteorites. Geochimica et Cosmochimica Acta 110, 216-228.
), which is close to that found in Main-group pallasites. We have carefully investigated whether variations in primary beam current could have induced variable instrumental mass fractionation, but lack of correlation between this instrumental parameter and our observed 53Cr+/52Cr+ (Fig. S-1a,c) does not indicate this to be the case. The isotopic ratio is, as perhaps expected, negatively correlated with mean 52Cr+ count rate (Fig. S-1b,d).Figure S-1 Primary beam and mean 52Cr+ counts per second against raw 53Cr+/52Cr+ ion count ratios, uncorrected for instrumental mass fractionation, for olivine from Brenham (a and b) and Brahin (c and d), with San Carlos terrestrial olivine measurements in each of their respective analytical sessions.
We did not apply an RSF to Brahin chromite and in this case took measured 55Mn+/52Cr+ to be the same as 55Mn/52Cr (RSF = 1) because chromite 55Mn/52Cr is very low and a small degree of systematic bias in this variable will not change geological interpretations. For our chromite analyses, which were run in parallel with in-house terrestrial chromites, we found that the analytical scatter for terrestrial chromites was larger than for Brahin chromite. This may be due to matrix effects associated by variable Cr, Al, Mg and Fe. However, perhaps because these compositions approximately bracketed our meteoritic chromite, the 53Cr/52Cr for Brahin was indistinguishable from the terrestrial ratio. Therefore, we do not propagate the excess scatter from terrestrial chromites to the Brahin chromite. It should be noted that whether or not we propagate this extra uncertainty, our geological conclusions remain the same. Reduced and fractionation corrected Mn-Cr-isotope data for meteoritic materials are given in Table S-5.
Table S-5 Cr-isotope data for meteoritic olivine and chromite.
| Analysis | PB nA | 52Cr+ kcps | 55Mn/52Cr | 1-sigma | 53Cr/52Cr | 1-sigma |
| Brenham-1.31 | 17.3 | 29.5 | 15.47 | 0.53 | 0.11335 | 0.00011 |
| Brenham-1.33 | 18.2 | 16.8 | 26.63 | 0.92 | 0.11409 | 0.00014 |
| Brenham-1.34 | 18 | 26.5 | 16.42 | 0.57 | 0.11354 | 0.00011 |
| Brenham-1.35 | 18.4 | 49.9 | 8.86 | 0.31 | 0.11320 | 0.00009 |
| Brenham-1.37 | 18.4 | 28.3 | 17.01 | 0.59 | 0.11353 | 0.00011 |
| Brenham-1.41 | 19.3 | 19.0 | 24.43 | 0.84 | 0.11370 | 0.00014 |
| Brenham-1.42 | 20 | 18.8 | 26.59 | 0.92 | 0.11379 | 0.00012 |
| Brenham-1.43 | 20.1 | 18.4 | 32.09 | 1.11 | 0.11414 | 0.00014 |
| Brahin-3.01 | 60.5 | 27.7 | 30.10 | 1.07 | 0.11375 | 0.00014 |
| Brahin-4.01 | 61.7 | 21.0 | 34.87 | 1.22 | 0.11372 | 0.00014 |
| Brahin-4.03 | 60.8 | 25.6 | 24.80 | 0.93 | 0.11348 | 0.00014 |
| Brahin-4.06 | 64.3 | 24.6 | 39.12 | 1.36 | 0.11360 | 0.00014 |
| Brahin-4.07 | 64.6 | 18.4 | 39.48 | 1.37 | 0.11393 | 0.00016 |
| BrahinChromite-1.09 | 5.4 | - | 0.00578 | 0.00002 | 0.11338 | 0.00006 |
| BrahinChromite-1.11 | 4.8 | - | 0.00557 | 0.00002 | 0.11344 | 0.00004 |
| BrahinChromite-1.13 | 5.2 | - | 0.00576 | 0.00005 | 0.11336 | 0.00005 |
| BrahinChromite-1.15 | 5.2 | - | 0.00556 | 0.00002 | 0.11338 | 0.00003 |
| BrahinChromite-1.17 | 5.1 | - | 0.00585 | 0.00004 | 0.11343 | 0.00008 |
| BrahinChromite-1.19 | 4.4 | - | 0.00554 | 0.00002 | 0.11346 | 0.00005 |
| BrahinChromite-1.21 | 4.8 | - | 0.00584 | 0.00004 | 0.11334 | 0.00004 |
LA-ICP-MS trace element profiles (Fig. 1 and Tables S-2, S-3) exhibit decreasing Cr and increasing P and Mn near crystal edges. P is strongly enriched in the outermost ~20-30 μm. Grain-to-grain and intra-grain variations in P and Mn are more pronounced in rounded than in fragmental olivine, whereas Cr is very consistent in the former. Olivine cores for both meteorites are characterised by Mn concentrations of ~1400-1800 ppm, increasing to ~1700-2600 at the edges of Brenham olivine but only slightly increased to ~1500-1800 for Brahin. Cr in Brenham olivine cores is ~70-150 ppm; Brahin exhibit a wider range of ~60-190 ppm. For both meteorites, Cr in olivine rims is mostly in the range ~40-80 ppm. From these LA-ICP-MS data, Mn/Cr for olivine cores varies within and between grains, at 11-22 for Brenham and 9-30 for Brahin. Both increase near edges to 25-50 and 15-40 respectively (Fig. 1). SIMS 55Mn/52Cr and 53Cr/52Cr are well correlated in Brenham olivine over a range of ~9-32 and ~0.1132-0.1140 (~7 per mille variation) respectively, while for Brahin olivine they are more restricted at ~25-40 and ~0.1134-0.1139 (~4 per mille variation) respectively. Chromite was found to have low 55Mn/52Cr and normal 53Cr/52Cr (0.0055-0.0059 and 0.1134 ±0.0001 respectively; 2 standard deviations; Fig. 2 and Supplementary Information). 55Mn/52Cr and 53Cr/52Cr regression yields excessively high initial 53Mn/55Mn values for each meteorite, assuming in situ 53Mn decay and that the system was closed to disturbance. Brenham gives an initial 53Mn/55Mn of 4.0 (±1.2) × 10-5 with sub-terrestrial initial 53Cr/52Cr at 0.11285 (±0.00024) (2-sigma; MSWD 0.63). Brahin gives an initial 53Mn/55Mn of 8.4 (±4.5) × 10-6 with an initial 53Cr/52Cr of 0.11340 (±0.00009) (2-sigma, MSWD 0.49). SIMS Mn-Cr data are given in Table S-4. We do not regard these initial 53Mn/55Mn to be accurate because of their excessively high values and seemingly rotated isochron in the Brenham data; instead they reflect ingrowth in a different Mn-rich reservoir (a silico-phosphate melt) and subsequent crystallisation of 53Cr*-rich olivine rims.
Supplementary Information References
Boesenberg, J.S., Delaney, J.S., Hewins, R.H. (2012) A petrological and chemical re-examination of Main Group pallasite formation. Geochimica et Cosmochimica Acta 89, 134-158.
Show in context The Brenham and Brahin stony-iron meteorites were selected because both have been assigned to the Main-Group of pallasites (Clayton and Mayeda, 1996) and show similar distinctive olivine trace element characteristics, belonging to a low-Mn subgroup (Boesenberg et al., 2012).
View in Supplementary Information
Clayton, R.N., Mayeda, T.K. (1996) Oxygen isotope studies of achondrites. Geochimica et Cosmochimica Acta 60, 1999-2017.
Show in context The Brenham and Brahin stony-iron meteorites were selected because both have been assigned to the Main-Group of pallasites (Clayton and Mayeda, 1996) and show similar distinctive olivine trace element characteristics, belonging to a low-Mn subgroup (Boesenberg et al., 2012).
View in Supplementary Information
De Hoog, J.C.M., Gall, L., Cornell, D.H. (2010) Trace-element geochemistry of mantle olivine and application to mantle petrogenesis and geothermobarometry. Chemical Geology 270, 196–215.
Show in context For Sc, we subtracted 0.2 ppm from nominal concentrations due to possible interference of 29Si16O+ on 45/sup>Sc+ (De Hoog et al., 2010).
View in Supplementary Information
Eggins, S., De Deckker, P., Marshall, J. (2003) Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration. Earth and Planetary Science Letters 212, 291-306.
Show in context Sampling was undertaken with a 193 nm wavelength ArF Excimer laser using a second-generation custom-built ‘HelEx’ two-volume vortex ablation cell at RSES, ANU (Eggins et al., 2003).
View in Supplementary Information
Hsu, W. (2005) Mn-Cr systematics of pallasites. Geochemical Journal 39, 311-316.
Show in context Two previous studies on the Mn-Cr systematics of pallasites using small ion probes (Cameca IMS-3f) found elevated 53Cr/52Cr in phosphates and olivine (Hutcheon and Olsen, 1991; Hsu, 2005) and inferred initial 53Mn/55Mn values as high as 1-2 × 10-5.
View in Supplementary Information
Huss et al. (2011) revised the values of Hsu (2005) using improved data reduction techniques, and did not resolve any radiogenic anomaly, although some pallasites were not well constrained and could be consistent with initial 53Mn/55Mn of ~1 × 10-5.
View in Supplementary Information
Huss, G.R., Ogliore, K., Nagashima, M., Telus, M., Jilly, C.E. (2011) Dangers of determining isotope ratios using means of individual ratios. Lunar and Planetary Science Conference 42, 2608.
Show in context Huss et al. (2011) revised the values of Hsu (2005) using improved data reduction techniques, and did not resolve any radiogenic anomaly, although some pallasites were not well constrained and could be consistent with initial 53Mn/55Mn of ~1 × 10-5.
View in Supplementary Information
Hutcheon, I.D., Olsen, E. (1991) Cr isotopic composition of differentiated meteorites: a search for 53Mn. Lunar and Planetary Science Conference 22, 605-606.
Show in context Two previous studies on the Mn-Cr systematics of pallasites using small ion probes (Cameca IMS-3f) found elevated 53Cr/52Cr in phosphates and olivine (Hutcheon and Olsen, 1991; Hsu, 2005) and inferred initial 53Mn/55Mn values as high as 1-2 × 10-5.
View in Supplementary Information
Longerich, H.P., Jackson, S.E., Günther, D. (1996) Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation. Journal of Analytical and Atomic Spectrometry 11, 899–904.
Show in context Data were reduced according to Longerich et al. (1996), with 29Si as the internal standard and NIST 612 as the primary external standard using the concentrations reported by Pearce et al. (1997).
View in Supplementary Information
McKibbin, S.J., Ireland, T.R., Amelin, Y., O’Neill, H.St.C., Holden, P. (2013a) Mn-Cr relative sensitivity Factors for Secondary Ion Mass Spectrometry analysis of Mg-Fe-Ca olivine and implications for the Mn-Cr chronology of meteorites. Geochimica et Cosmochimica Acta 110, 216-228.
Show in context Rather than using a filtered primary ion beam to sputter the surface (McKibbin et al., 2013a) and due to small expected Cr-isotope anomalies arising from relatively high Cr concentrations in the range ~50-150 ppm (McKibbin et al., 2013b), we have used a high current unfiltered primary ion beam (~30 μm diameter; ~16.9-20.2 nA for Brenham and ~52.8-56.6 nA for Brahin; mixed O- and O2-).
View in Supplementary Information
Olivine RSF is strongly dependent on composition; this value was determined using a range of ~Fo90 olivines (McKibbin et al., 2013a), which is close to that found in Main-group pallasites.
View in Supplementary Information
McKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013b) LA-ICP-MS mapping of olivine from the Brahin and Brenham meteorites: Complex elemental distributions in the pallasite olivine precursor. Geochimica et Cosmochimica Acta 119, 1-17.
Show in context Rather than using a filtered primary ion beam to sputter the surface (McKibbin et al., 2013a) and due to small expected Cr-isotope anomalies arising from relatively high Cr concentrations in the range ~50-150 ppm (McKibbin et al., 2013b), we have used a high current unfiltered primary ion beam (~30 μm diameter; ~16.9-20.2 nA for Brenham and ~52.8-56.6 nA for Brahin; mixed O- and O2-).
View in Supplementary Information
McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P., Sugiura, N. (2013c) A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D’Orbigny using Secondary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 123, 181-194.
Show in context Data were first collected for 52Cr+ and 55Mn+ in 10 replicates (the first 2-3 minutes of sputtering, 2 and 1 seconds per cycle respectively) because temporal changes in observed 55Mn+/52Cr+ associated with downhole effects as sputtering proceeds can be significant (McKibbin et al., 2013c).
View in Supplementary Information
For our measurements, SIMS data were reduced according to the ‘First Ratio Estimator’ method (Ogliore et al., 2011) which we have earlier referred to as ‘Ratio of Total Counts’ (McKibbin et al., 2013c).
View in Supplementary Information
In a previous study, we used the median of absolute deviations (MAD) from polynomial curves fit to our time-interpolated count data (McKibbin et al., 2013c); this method somewhat underestimates the error and we now calculate internal spot-specific errors as the standard error on the mean of sub-ratios determined for each magnet cycle within a single analytical spot (Telus et al., 2012; McKibbin et al., 2015).
View in Supplementary Information
McKibbin, S.J., Ireland, T.R., Amelin, Y., Holden, P. (2015) Mn-Cr dating of Fe- and Ca-rich olivine from ‘quenched’ and ‘plutonic’ angrite meteorites using Seconary Ion Mass Spectrometry. Geochimica et Cosmochimica Acta 157, 13-27.
Show in context In a previous study, we used the median of absolute deviations (MAD) from polynomial curves fit to our time-interpolated count data (McKibbin et al., 2013c); this method somewhat underestimates the error and we now calculate internal spot-specific errors as the standard error on the mean of sub-ratios determined for each magnet cycle within a single analytical spot (Telus et al., 2012; McKibbin et al., 2015).
View in Supplementary Information
Ogliore, R., Huss, G., Nakashima, K. (2011) Ratio estimation in SIMS analysis. Nuclear Instruments and Methods in Physics Research Section B 269, 1910-1918.
Show in context For our measurements, SIMS data were reduced according to the ‘First Ratio Estimator’ method (Ogliore et al., 2011) which we have earlier referred to as ‘Ratio of Total Counts’ (McKibbin et al., 2013c).
View in Supplementary Information
Pearce, N.J.G., Perkins, W.T., Westgate, J.A., Gorton, M.P., Jackson, S.E., Neal, C.R., Chenery, S.P. (1997) A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletter 21, 115–144.
Show in context Data were reduced according to Longerich et al. (1996), with 29Si as the internal standard and NIST 612 as the primary external standard using the concentrations reported by Pearce et al. (1997).
View in Supplementary Information
Table S-1
View in Supplementary Information
Spandler, C., O’Neill, H.St.C. (2010) Diffusion and partition coefficients of minor and trace elements in San Carlos olivine at 1,300°C with some geochemical implications. Contributions to Mineralogy and Petrology 159, 791-818.
Show in context Trace element analysis of olivine was conducted using the method developed by Spandler and O’Neill (2010).
View in Supplementary Information
Telus, M., Huss, G.R., Ogliore, R.C., Nagashima, K., Tachibana, S. (2012) Recalculation of data for short-lived radionuclide systems using less-biased ratio estimation. Meteoritics and Planetary Science 47, 2013-2030.
Show in context In a previous study, we used the median of absolute deviations (MAD) from polynomial curves fit to our time-interpolated count data (McKibbin et al., 2013c); this method somewhat underestimates the error and we now calculate internal spot-specific errors as the standard error on the mean of sub-ratios determined for each magnet cycle within a single analytical spot (Telus et al., 2012; McKibbin et al., 2015).
View in Supplementary Information
Tomiyama, T., Huss, G., Nagashima, K., Krot, A.N. (2007) Ion microprobe analysis of 53Mn-53Cr systematics in pallasite olivines. Lunar and Planetary Science Conference 38, 2007.
Show in context New measurements with improved ion probes (Cameca IMS-6f and 1280) also failed to resolve 53Cr* (Tomiyama et al., 2007).
View in Supplementary Information
