Olivines in main-group pallasites: magma-ocean cumulates or partial melting residues?
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Keywords: Pallasite, early solar system, trace elements, rare earth elements, differentiation
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Abstract
Figures
 Figure 1 REE patterns of MGP olivines (Schmitt et al., 1963; Masuda, 1968; Saito et al., 1998; Barrat et al., 2016a; and this work) and phosphates (Davis and Olsen, 1991; Hsu, 2003) normalised to CI chondrite (Barrat et al., 2012). |  Figure 2 (Er/Lu)n vs. (La/Lu)n plot for MGP olivines. Data from Saito et al. (1998), Barrat et al. (2016a), and this work. |  Figure 3 (Er/Lu)n and (Ba/Lu)n vs. (La/Lu)n plot for MGP olivines. Data from Saito et al. (1998), Greenwood et al. (2015), Barrat et al. (2016a), and this work. Contamination trend uses the average upper continental crust composition from Rudnick and Gao (2014). |  Figure 4 Evolution of the (Er/Lu)n ratios in partial melting residues produced from a merrilite-bearing chondrite. Fractional melting and dynamic melting with Φ = 1 %, parameters and partition coefficients are taken from Barrat et al. (2016b). Olivine in equilibrium with a chondritic magma ocean, least contaminated pallasitic olivines (this work), and ureilites (Barrat et al., 2016b) are shown for comparison. |
| Figure 1 | Figure 2 | Figure 3 | Figure 4 |
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Introduction
Some meteorites (achondrites and irons) and samples collected during the Apollo and Luna missions represent materials from a few differentiated bodies, such as the Moon, Mars, and 4-Vesta, but also from other bodies accreted at 4.5 Ga, and now disrupted. Many of these samples are magmatic rocks. They are an invaluable source of information to reconstruct the differentiation history of their parent bodies, but also to understand the mechanisms of magma generation. Unfortunately, these rocks only allow an indirect approach to derive their parental mantle compositions and to understand the mantle processes involved in their formation. Among the achondrites, samples of mantles from fully differentiated rocky bodies are very uncommon. An additional source of mantle samples is provided by the pallasites, a relatively rare type of stony-iron meteorites of which about a hundred are known. Pallasites are essentially composed of metal and olivine (i.e. from isolated single crystals and/or polycrystalline aggregates to fragments of “dunitic pebbles”), with variable (low) amount of sulfide (troilite), minor schreibersite, phosphates, and in some cases chromite (e.g., Buseck and Holdsworth, 1977
Buseck, P.R., Holdsworth, E. (1977) Phosphate minerals in pallasites meteorites. Mineralogical Magazine 41, 91–102.
; Scott, 1977Scott, E.R.D. (1977) Formation of olivine-metal textures in pallasite meteorites. Geochimica et Cosmochimica Acta 41, 693–710.
). Although only a relatively small number of pallasites are known, their petrographic features and isotopic oxygen compositions show that they originate from at least six distinct parent bodies (e.g., Greenwood et al., 2017Greenwood, R.C., Burbine, T.H., Miller, M.F., Franchi, I.A. (2017) Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies. Chemie der Erde 77, 1–43.
). The most abundant ones, the MGPs, represent ≈85 % of all the pallasites. They display rather homogenous Δ17O values and are thought to derive from one (Greenwood et al., 2006Greenwood, R.C., Franchi, I.A., Jambon, A., Barrat, J.A., Burbine, T.H. (2006) Oxygen isotope variation in stony-iron meteorites. Science 313, 1763–1765.
, 2015Greenwood, R.C., Barrat, J.A., Scott, E.R.D., Haack, H., Buchanan, P.C., Franchi, I.A., Yamaguchi, A., Johnson, D., Bevan, A.W.R., Burbine, T.H. (2015) Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection. Geochimica et Cosmochimica Acta 169, 115–136.
) or perhaps two parent bodies (Ali et al., 2018Ali, A., Jabeen, I., Banerjee, N.R., Osinski, G.R., Nicklin I., Gregory, D., Herrmann, P. (2018) The oxygen isotope compositions of olivine in main group (MG) pallasites: New measurements by adopting an improved laser fluorination approach. Meteoritics & Planetary Science 53, 1223–1237.
).While pallasites are recognised to be mixtures of mantle and core materials, their formation is still debated (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.
; McKibbin et al., 2019McKibbin, S.J., Pittarello, L., Makarona, C., Hamann, C., Hecht, L., Chernonozhkin, S.M., Goderis, S., Clayes, P. (2019) Petrogenesis of main group pallasite meteorites based on relationships among texture, mineralogy, and geochemistry. Meteoritics & Planetary Science 54, 2814–2844.
; and references therein). Since the 1960s, they are usually seen as samples of the core-mantle transition zone of a differentiated body (e.g., Anders, 1964Anders, E. (1964) Origin, age and composition of meteorites. Space Science Reviews 3, 583–714.
; Wasson and Choi, 2003Wasson, J.T., Choi, B.-G. (2003) Main-group pallasites, chemical composition, relationship to IIIAB irons, and origin. Geochimica et Cosmochimica Acta 67, 3079–3096.
), but this hypothesis has been repeatedly challenged. Although there is no consensus on a precise formation model yet, the variety of textures shown by these meteorites, the diversity of cooling rates recorded by the metal among others, and also geophysical models suggest that the core-mantle mixing is related to catastrophic impacts (e.g., Scott, 1977Scott, E.R.D. (1977) Formation of olivine-metal textures in pallasite meteorites. Geochimica et Cosmochimica Acta 41, 693–710.
; 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.
; Bryson et al., 2015Bryson, J.F.J., Nichols, C.I.O., Herrero-Albillos, J., Fronast, F., Kasamas, T., Alimadadi, H., van der Laan, G., Nimmo, F., Harrison, R.J. (2015) Long-lived magnetism from solidification-driven convection on the pallasite parent body. Nature 517, 472–475.
; Nichols et al., 2016Nichols, C.I.O., Bryson, J.F.J., Herrero-Albillos, J., Kronast, F., Nimmo, F., Harrison, R.J. (2016) Pallasite paleomagnetism: quiescence of a core dynamo. Earth and Planetary Science Letters 441, 103–112.
; Walte et al., 2020Walte, N.P., Solferino, G.F.D., Golabek, G.J., Souza, D.S., Bouvier, A. (2020) Two-stage formation of pallasites and the evolution of their parent bodies revealed by deformation experiments. Earth and Planetary Science Letters 546, 116419.
), or is from an unknown type of volcanism (Johnson et al., 2019Johnson, B.C., Sori, M.M., Evans, A.J. (2019) Ferrovolcanism on metal worlds and the origin of pallasites. Nature Astronomy 4, 41–44.
). In any case, if the metal is well understood today (e.g., Wasson and Choi, 2003Wasson, J.T., Choi, B.-G. (2003) Main-group pallasites, chemical composition, relationship to IIIAB irons, and origin. Geochimica et Cosmochimica Acta 67, 3079–3096.
), it is not the case for the mantle fragments and olivines they contain. Some have suggested that mantle fragments could be melting residues after extraction of a large fraction of magma, or alternatively produced by the accumulation of olivines during the crystallisation of a global magma ocean (e.g., Buseck, 1977Buseck, P.R. (1977) Pallasite meteorites – mineralogy, petrology and geochemistry. Geochimica et Cosmochimica Acta 41, 711–740.
; Mittlefehldt, 1980Mittlefehldt, D.W. (1980) The composition of mesosiderite olivine clasts and implications for the origin of pallasites. Earth and Planetary Science Letters 51, 29–40.
; 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.
). The latter possibility would be consistent with the observed homogeneity of the Δ17O values for olivines which suggest a highly homogeneous or a well homogenised body (Δ17O = −0.187 ± 0.008 ‰ (1 σ, n = 103), Greenwood et al., 2015Greenwood, R.C., Barrat, J.A., Scott, E.R.D., Haack, H., Buchanan, P.C., Franchi, I.A., Yamaguchi, A., Johnson, D., Bevan, A.W.R., Burbine, T.H. (2015) Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection. Geochimica et Cosmochimica Acta 169, 115–136.
), however, this is not confirmed by trace element chemistry data (Mittlefehldt and Herrin, 2010Mittlefehldt, D.W., Herrin, J.S. (2010) Trace element compositions of pallasite olivine grains and pallasite origin. Meteoritical Society Meeting 73, #5386 (abstr.).
; McKibbin et al., 2013McKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013) 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.
). Therefore, the origin of mantle fragments remains open. Nevertheless, the behaviour of incompatible trace elements, and among them the REEs, should in principle allow their origin to be deciphered. The few pallasite olivines previously analysed showed totally unexpected light REE enrichments (Schmitt et al., 1963Schmitt, R.A., Smith, R.H., Lasch, J.E., Mosen, A.W., Olehy, D.A., Vasilevskis, J. (1963) Abundances of the fourteen rare-earth elements, scandium, and yttrium in meteoritic and terrestrial matter. Geochimica et Cosmochimica Acta 27, 577–622.
, 1964Schmitt, R.A., Smith, R.H., Olehy, D.A. (1964) Rare-earth, yttrium and scandium abundances in meteoritic and terrestrial matter-II. Geochimica et Cosmochimica Acta 28, 67–86.
; Masuda, 1968Masuda, A. (1968) Lanthanide concentrations in the olivine phase of the Brenham pallasite. Earth and Planetary Science Letters 5, 59–62.
; Saito et al., 1998Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
; Minowa and Ebihara, 2002Minowa, H., Ebihara, M. (2002) Rare Earth Elements in pallasite olivines. Lunar and Planetary Science XXXIII. Lunar and Planetary Institute, Houston, #1386 (abstr.).
; Greenwood et al., 2015Greenwood, R.C., Barrat, J.A., Scott, E.R.D., Haack, H., Buchanan, P.C., Franchi, I.A., Yamaguchi, A., Johnson, D., Bevan, A.W.R., Burbine, T.H. (2015) Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection. Geochimica et Cosmochimica Acta 169, 115–136.
).In this study, we analysed a series of olivine fractions and a dunite prepared from six main-group pallasites (samples and methods are described in Supplementary Information; Table S-1), with the aim of explaining the light REE enrichments described in previous studies, but also to constrain the formation processes of the mantle component contained in pallasites.
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Results and Discussion
Olivine separates were analysed after a very strong leaching step using hot HCl. Results are given in Table S-2. Abundances of Co (<115 μg/g) and P (<60 μg/g) are low and confirm that our leaching procedure has removed most traces of weathering (such as rust), and adhering traces of metal and/or phosphates. Calcium (from 27 to 281 μg/g) and Sc (from 0.31 to 2.16 μg/g) abundances are in the range of reported values for pallasite olivines (e.g., Mittlefehldt, 1980
Mittlefehldt, D.W. (1980) The composition of mesosiderite olivine clasts and implications for the origin of pallasites. Earth and Planetary Science Letters 51, 29–40.
; Hsu, 2003Hsu, W. (2003) Minor element zoning and trace element geochemistry of pallasites. Meteoritics & Planetary Science 38, 1217–1241.
; McKibbin et al., 2013McKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013) 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.
). The REE concentrations in pallasite olivines are low but highly variable, ranging from 6 × 10−5 to 0.6 times the CI abundances (including literature values; Fig. 1). All our samples display U-shaped REE patterns, due to light and heavy REE enrichments [(La/Lu)n = 0.04–6.9, (Er/Lu)n = 0.02–0.74]. Furthermore, the samples displaying the lowest light REE abundances also exhibit the most pronounced heavy REE enrichments (Fig. 1). An excellent relationship is obtained between (La/Lu)n and (Er/Lu)n ratios (Fig. 2). This is indicative of a mixing line between olivines (with low light REE abundances and heavy REE enrichments [i.e. extremely low Er/Lu ratios]), and another component (or other components) highly enriched in light REEs. This implies that the diversity of REE abundances displayed by the analysed olivines likely simply reflect the presence of impurities which need to be identified.Figure 1 REE patterns of MGP olivines (Schmitt et al., 1963
Schmitt, R.A., Smith, R.H., Lasch, J.E., Mosen, A.W., Olehy, D.A., Vasilevskis, J. (1963) Abundances of the fourteen rare-earth elements, scandium, and yttrium in meteoritic and terrestrial matter. Geochimica et Cosmochimica Acta 27, 577–622.
; Masuda, 1968Masuda, A. (1968) Lanthanide concentrations in the olivine phase of the Brenham pallasite. Earth and Planetary Science Letters 5, 59–62.
; Saito et al., 1998Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
; Barrat et al., 2016aBarrat, J.A., Dauphas, N., Gillet, P., Bollinger, C., Etoubleau, J., Bischoff, A., Yamaguchi, A. (2016a) Evidence from Tm anomalies for non-CI refractory lithophile element proportions in terrestrial planets and achondrites. Geochimica et Cosmochimica Acta 176, 1–17.
; and this work) and phosphates (Davis and Olsen, 1991Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasites 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 & Planetary Science 38, 1217–1241.
) normalised to CI chondrite (Barrat et al., 2012Barrat, J.A., Zanda, B., Moynier, F., Bollinger, C., Liorzou, C., Bayon, G. (2012) Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes. Geochimica et Cosmochimica Acta 83, 79–92.
).Figure 2 (Er/Lu)n vs. (La/Lu)n plot for MGP olivines. Data from Saito et al. (1998)
Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
, Barrat et al. (2016a)Barrat, J.A., Dauphas, N., Gillet, P., Bollinger, C., Etoubleau, J., Bischoff, A., Yamaguchi, A. (2016a) Evidence from Tm anomalies for non-CI refractory lithophile element proportions in terrestrial planets and achondrites. Geochimica et Cosmochimica Acta 176, 1–17.
, and this work.Among the different phases contained in pallasites, only phosphates and possibly melt inclusions would be rich enough in REEs to affect olivine concentrations. Phosphates in the Springwater pallasite are light REE enriched (Davis and Olsen, 1991
Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasites meteorites as probes of mantle processes in small planetary bodies. Nature 353, 637–640.
), and could explain the shape of the REE pattern displayed by the olivine fraction prepared from this meteorite (Fig. 1; Saito et al., 1998Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
). However, phosphates measured in other pallasites are generally light REE-depleted (Fig. 1), and, therefore, cannot account for the observed mixing line (Figs. 2, 3a). In addition, because we leached our samples before dissolution, and since their P contents are very low (Table S-2), this possibility is very unlikely.Figure 3 (Er/Lu)n and (Ba/Lu)n vs. (La/Lu)n plot for MGP olivines. Data from Saito et al. (1998)
Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
, Greenwood et al. (2015)Greenwood, R.C., Barrat, J.A., Scott, E.R.D., Haack, H., Buchanan, P.C., Franchi, I.A., Yamaguchi, A., Johnson, D., Bevan, A.W.R., Burbine, T.H. (2015) Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection. Geochimica et Cosmochimica Acta 169, 115–136.
, Barrat et al. (2016a)Barrat, J.A., Dauphas, N., Gillet, P., Bollinger, C., Etoubleau, J., Bischoff, A., Yamaguchi, A. (2016a) Evidence from Tm anomalies for non-CI refractory lithophile element proportions in terrestrial planets and achondrites. Geochimica et Cosmochimica Acta 176, 1–17.
, and this work. Contamination trend uses the average upper continental crust composition from Rudnick and Gao (2014)Rudnick, R.L., Gao, S. (2014) 4.1 – Composition of the Continental Crust. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry (Second Edition) Elsevier, Oxford, 1–51.
.It is known that the presence of a few melt inclusions can significantly change the shape of the REE pattern of an olivine (e.g., Barrat and Bachèlery, 2019
Barrat, J.A., Bachélery, P. (2019) La Réunion Island dunites as analogs of the Martian chassignites: tracking trapped melts with incompatible trace elements. Lithos 344–345, 452–463.
). However, unlike in magmatic rocks, the olivines in pallasites are very poor in glassy inclusions. Inclusions composed of tridymite and Al, K-rich glasses have been described in olivines from the Fukang pallasite (Dellagiustina et al., 2019Dellagiustina, D.N., Habib, N., Domanik, K.J., Hill, D.H., Lauretta, D.S., Goreva, Y.S., Killgore, M., Hexiong, Y., Downs, R.T. (2019) The Fukang pallasite: Characterization and implications for the history of the Main-group parent body. Meteoritics & Planetary Science 54, 1781–1807.
). Such inclusions are extremely rare, and, thus, it seems difficult to imagine that they could explain the light REE enrichments seen for all pallasite olivines analysed so far.Alternatively, the light REE enriched end members could be due to terrestrial contamination (Minowa and Ebihara, 2002
Minowa, H., Ebihara, M. (2002) Rare Earth Elements in pallasite olivines. Lunar and Planetary Science XXXIII. Lunar and Planetary Institute, Houston, #1386 (abstr.).
). Olivines from Esquel (Fig. 1c) show a positive anomaly in Ce (Ce/Ce* = 1.58), which is obviously related to some traces of rust not entirely removed by leaching (e.g., Braun et al., 1990Braun, J.J., Pagel, M., Muller, J.P., Bilong, P., Michard, A., Guillet, B. (1990) Cerium anomalies in lateritic profiles. Geochimica et Cosmochimica Acta 54, 781–795.
). The occurrence of trace amounts of dust or clays in the samples, not removed by leaching, should also be considered. Indeed, the REE patterns of the olivines with the highest REE abundances strongly resemble those of Earth’s continental crust rocks. The possibility that all the pallasite olivines analysed to date may be more or less contaminated, either during terrestrial residence time (i.e. weathering, soil contamination, etc.) or during sample preparation, should be seriously considered. We have tested this hypothesis using the mean composition of the terrestrial upper continental crust (Rudnick and Gao, 2014Rudnick, R.L., Gao, S. (2014) 4.1 – Composition of the Continental Crust. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry (Second Edition) Elsevier, Oxford, 1–51.
). On the (Ba/Lu)n or (Er/Lu)n vs. (La/Lu)n plots (Fig. 3), the pallasite olivines show a perfect match with the average terrestrial continental crust, strengthening the hypothesis of a contamination. We calculated that the fraction of olivines richest in REEs contain about 0.3 % of terrestrial material, whereas the poorest ones contain below 0.01 % of terrestrial material. Although this terrestrial contribution is extremely limited, it has a huge influence on the concentrations of incompatible elements in olivines.However, this contamination by a terrestrial component does not mask all the REE features of the olivines, in particular their extremely high heavy REE enrichments (Fig. 2): the least contaminated olivines, from Brenham, are characterised by (Er/Lu)n ≤ 0.03, ≤ 0.04 for Fukang, ≤ 0.05 for Admire, and ≤ 0.10 for Jepara and Finmarken meteorites. These values place very strong constraints on the formation of the olivines.
Our results allow us to evaluate some of the characteristics of the cumulates produced during the crystallisation of a magma ocean formed in the case of a planetesimal. Although the REE abundances of such a body are unknown, the relative proportions of those elements are certainly chondritic since they were not fractionated by core formation, and are also strongly lithophile. Partition coefficients for olivine are not constant but the ratios of heavy REE partition coefficients vary relatively little. For example, the data obtained by Evans et al. (2008)
Evans, T.M., O’Neill, H.St.C., Tuff, J. (2008) The influence of melt composition on the partitioning of REEs, Y, Sc, Zr and Al between forsterite and melt in the system CMAS. Geochimica et Cosmochimica 72, 5708–5721.
allow us to calculate that the DEr/DLu ratio of olivine is close to 0.3 (see Supplementary Information). The (Er/Lu)n ratios of olivines produced during the cooling of a magmatic ocean should be identical. Because the formation of a cumulate is invariably accompanied by magma trapping and possibly subsolidus redistribution, even minute amounts of trapped melt would tend to increase the Er/Lu ratio of olivines and of dunitic cumulates (Barrat and Bachelery, 2019Barrat, J.A., Bachélery, P. (2019) La Réunion Island dunites as analogs of the Martian chassignites: tracking trapped melts with incompatible trace elements. Lithos 344–345, 452–463.
). Therefore, the (Er/Lu)n ratios of dunites accumulated in a magma ocean cannot be less than ≈0.3.Nonetheless, the abundances of many trace elements, such as Ti, V, Cr and Ni were significantly modified in olivines during the mixing and cooling with metal (e.g., Hsu et al., 2003
Hsu, W. (2003) Minor element zoning and trace element geochemistry of pallasites. Meteoritics & Planetary Science 38, 1217–1241.
), and it is thus questionable whether REE distributions were also affected. We see at least two arguments suggesting that the heavy REE contents of the olivines were preserved. Firstly, REEs, as also Al and P, diffuse very slowly into olivine. The observed preservation of heterogeneities in Al and P, and the absence of diffusion profiles for these elements in the olivines (McKibbin et al., 2013McKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013) 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.
) support our hypothesis. Secondly, we have analysed olivines that have undergone different degrees of interaction with the metal (i.e. including a dunite sample, and both angular and rounded olivines), and the least contaminated of these samples are similarly enriched in heavy REEs. Therefore, it is very unlikely that interactions with core material fractionated heavy REEs in olivines.The (Er/Lu)n ratios of pallasite olivines, comprised between 0.03 and 0.1, are way too different from those estimated for olivines formed in a magma ocean (i.e. ≈0.3), and, thus, we can rule out this hypothesis. On the other hand, high heavy REE enrichments are an expected characteristic for partial melting residues, and, therefore, for dunites derived from a chondritic precursor (Fig. 4). However, it is impossible to estimate accurately the degree of partial melting of the pallasite mantle. The results are extremely model dependent (initial composition, phase proportions, fractional or dynamic melting, etc.), but in any case require very high melting degrees (probably above 40 %) to account for both the exhaustion of pyroxene and the low heavy REE abundances of the olivines (i.e. Lu concentration ranging from 3.10−3 to 4.10−2 × CI). The olivine contained in the main-group pallasites would have to be derived from a mantle even more residual or more refractory than the ureilites, for which melting degrees did not exceed 30 % (Barrat et al., 2016b
Barrat, J.A., Jambon, A., Yamaguchi, A., Bischoff, A., Rouget, M.L., Liorzou, C. (2016b) Partial melting of a C-rich asteroid: lithophile trace elements in ureilites. Geochimica et Cosmochimica Acta 194, 163–178.
).Figure 4 Evolution of the (Er/Lu)n ratios in partial melting residues produced from a merrilite-bearing chondrite. Fractional melting and dynamic melting with Φ = 1 %, parameters and partition coefficients are taken from Barrat et al. (2016b)
Barrat, J.A., Jambon, A., Yamaguchi, A., Bischoff, A., Rouget, M.L., Liorzou, C. (2016b) Partial melting of a C-rich asteroid: lithophile trace elements in ureilites. Geochimica et Cosmochimica Acta 194, 163–178.
. Olivine in equilibrium with a chondritic magma ocean, least contaminated pallasitic olivines (this work), and ureilites (Barrat et al., 2016bBarrat, J.A., Jambon, A., Yamaguchi, A., Bischoff, A., Rouget, M.L., Liorzou, C. (2016b) Partial melting of a C-rich asteroid: lithophile trace elements in ureilites. Geochimica et Cosmochimica Acta 194, 163–178.
) are shown for comparison.The very high homogeneity of the Δ17O values shown by MGP olivines is however puzzling. One can of course imagine that the parent body was very homogeneous in oxygen isotopes, but such a level of homogeneity, although conceivable, is not observed among any of the different groups of primitive meteorites (Greenwood et al., 2017
Greenwood, R.C., Burbine, T.H., Miller, M.F., Franchi, I.A. (2017) Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies. Chemie der Erde 77, 1–43.
). It is questionable to what extent the isotopic oxygen composition of the residues is preserved by partial melting. Greenwood et al. (2005)Greenwood, R.C., Franchi, I.A., Jambon, A., Buchanan, P. (2005) Widespread magma oceans on asteroidal bodies in the early solar system. Nature 435, 916–918.
estimated that at melting rates greater than 40 %, the Δ17O values are homogenised. In the case of fractional melting, or dynamic melting with low porosity, most of the melt is rapidly extracted from the restites, thus limiting the possibility of homogenisation, independent of the level of melting. Although the ureilites did not undergo such high fusion rates, their oxygen isotopic heterogeneities are still very well preserved, illustrating that the ability of partial melting to homogenise the Δ17O values was probably overestimated. Thus, if the dunites were derived from chondritic precursors, more variability in O isotopes would be expected. However, the O isotopic homogeneity of the main-group pallasites is very comparable to what is observed for the Earth, Moon, Mars, Vesta and the parent body of the angrites, for which magma oceans have been considered (Greenwood et al., 2005Greenwood, R.C., Franchi, I.A., Jambon, A., Buchanan, P. (2005) Widespread magma oceans on asteroidal bodies in the early solar system. Nature 435, 916–918.
, 2017Greenwood, R.C., Burbine, T.H., Miller, M.F., Franchi, I.A. (2017) Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies. Chemie der Erde 77, 1–43.
). Thus, re-melting of ultrabasic cumulates formed in the magma ocean can also lead to dunitic residues characterised by large heavy REE enrichments but also by homogeneous Δ17O.While there is no doubt that pallasite olivines derive from a residual mantle, we cannot specify whether this mantle simply formed from the melting of chondrites, or whether its formation was much more complex and involved the crystallisation of a magma ocean, then the melting of the produced cumulates. The homogeneity of the Δ17O values displayed by MGPs is, however, in favour of the second hypothesis.
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Acknowledgements
The Brenham olivines were kindly provided by Don Stimpson. We thank Helen Williams for the editorial handling, Claire Nichols and an anonymous reviewer for their constructive comments, and Richard Greenwood for helpful discussions. This work was supported by funds from the Programme National de Planétologie (CNRS-INSU) to the first author.
Editor: Helen Williams
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References
Ali, A., Jabeen, I., Banerjee, N.R., Osinski, G.R., Nicklin I., Gregory, D., Herrmann, P. (2018) The oxygen isotope compositions of olivine in main group (MG) pallasites: New measurements by adopting an improved laser fluorination approach. Meteoritics & Planetary Science 53, 1223–1237.
Show in context They display rather homogenous Δ17O values and are thought to derive from one (Greenwood et al., 2006, 2015) or perhaps two parent bodies (Ali et al., 2018).
View in article
Anders, E. (1964) Origin, age and composition of meteorites. Space Science Reviews 3, 583–714.
Show in context Since the 1960s, they are usually seen as samples of the core-mantle transition zone of a differentiated body (e.g., Anders, 1964; Wasson and Choi, 2003), but this hypothesis has been repeatedly challenged.
View in article
Barrat, J.A., Bachélery, P. (2019) La Réunion Island dunites as analogs of the Martian chassignites: tracking trapped melts with incompatible trace elements. Lithos 344–345, 452–463.
Show in context It is known that the presence of a few melt inclusions can significantly change the shape of the REE pattern of an olivine (e.g., Barrat and Bachèlery, 2019). However, unlike in magmatic rocks, the olivines in pallasites are very poor in glassy inclusions.
View in article
Because the formation of a cumulate is invariably accompanied by magma trapping and possibly subsolidus redistribution, even minute amounts of trapped melt would tend to increase the Er/Lu ratio of olivines and of dunitic cumulates (Barrat and Bachelery, 2019).
View in article
Barrat, J.A., Zanda, B., Moynier, F., Bollinger, C., Liorzou, C., Bayon, G. (2012) Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes. Geochimica et Cosmochimica Acta 83, 79–92.
Show in context REE patterns of MGP olivines (Schmitt et al., 1963; Masuda, 1968; Saito et al., 1998; Barrat et al., 2016a; and this work) and phosphates (Davis and Olsen, 1991; Hsu, 2003) normalised to CI chondrite (Barrat et al., 2012).
View in article
Barrat, J.A., Dauphas, N., Gillet, P., Bollinger, C., Etoubleau, J., Bischoff, A., Yamaguchi, A. (2016a) Evidence from Tm anomalies for non-CI refractory lithophile element proportions in terrestrial planets and achondrites. Geochimica et Cosmochimica Acta 176, 1–17.
Show in context REE patterns of MGP olivines (Schmitt et al., 1963; Masuda, 1968; Saito et al., 1998; Barrat et al., 2016a; and this work) and phosphates (Davis and Olsen, 1991; Hsu, 2003) normalised to CI chondrite (Barrat et al., 2012).
View in article
(Er/Lu)n vs. (La/Lu)n plot for MGP olivines. Data from Saito et al. (1998), Barrat et al. (2016a), and this work.
View in article
Data from Saito et al. (1998), Greenwood et al. (2015), Barrat et al. (2016a), and this work.
View in article
Barrat, J.A., Jambon, A., Yamaguchi, A., Bischoff, A., Rouget, M.L., Liorzou, C. (2016b) Partial melting of a C-rich asteroid: lithophile trace elements in ureilites. Geochimica et Cosmochimica Acta 194, 163–178.
Show in context The olivine contained in the main-group pallasites would have to be derived from a mantle even more residual or more refractory than the ureilites, for which melting degrees did not exceed 30 % (Barrat et al., 2016b).
View in article
Fractional melting and dynamic melting with Φ = 1 %, parameters and partition coefficients are taken from Barrat et al. (2016b).
View in article
Olivine in equilibrium with a chondritic magma ocean, least contaminated pallasitic olivines (this work), and ureilites (Barrat et al., 2016b) are shown for comparison.
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 While pallasites are recognised to be mixtures of mantle and core materials, their formation is still debated (Boesenberg et al., 2012; McKibbin et al., 2019; and references therein).
View in article
Some have suggested that mantle fragments could be melting residues after extraction of a large fraction of magma, or alternatively produced by the accumulation of olivines during the crystallisation of a global magma ocean (e.g., Buseck, 1977; Mittlefehldt, 1980; Boesenberg et al., 2012).
View in article
Braun, J.J., Pagel, M., Muller, J.P., Bilong, P., Michard, A., Guillet, B. (1990) Cerium anomalies in lateritic profiles. Geochimica et Cosmochimica Acta 54, 781–795.
Show in context Olivines from Esquel (Fig. 1c) show a positive anomaly in Ce (Ce/Ce* = 1.58), which is obviously related to some traces of rust not entirely removed by leaching (e.g., Braun et al., 1990).
View in article
Bryson, J.F.J., Nichols, C.I.O., Herrero-Albillos, J., Fronast, F., Kasamas, T., Alimadadi, H., van der Laan, G., Nimmo, F., Harrison, R.J. (2015) Long-lived magnetism from solidification-driven convection on the pallasite parent body. Nature 517, 472–475.
Show in context Although there is no consensus on a precise formation model yet, the variety of textures shown by these meteorites, the diversity of cooling rates recorded by the metal among others, and also geophysical models suggest that the core-mantle mixing is related to catastrophic impacts (e.g., Scott, 1977; Yang et al., 2010; Tarduno et al., 2012; Bryson et al., 2015; Nichols et al., 2016; Walte et al., 2020), or is from an unknown type of volcanism (Johnson et al., 2019).
View in article
Buseck, P.R. (1977) Pallasite meteorites – mineralogy, petrology and geochemistry. Geochimica et Cosmochimica Acta 41, 711–740.
Show in context Some have suggested that mantle fragments could be melting residues after extraction of a large fraction of magma, or alternatively produced by the accumulation of olivines during the crystallisation of a global magma ocean (e.g., Buseck, 1977; Mittlefehldt, 1980; Boesenberg et al., 2012).
View in article
Buseck, P.R., Holdsworth, E. (1977) Phosphate minerals in pallasites meteorites. Mineralogical Magazine 41, 91–102.
Show in context Pallasites are essentially composed of metal and olivine (i.e. from isolated single crystals and/or polycrystalline aggregates to fragments of “dunitic pebbles”), with variable (low) amount of sulfide (troilite), minor schreibersite, phosphates, and in some cases chromite (e.g., Buseck and Holdsworth, 1977; Scott, 1977).
View in article
Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasites meteorites as probes of mantle processes in small planetary bodies. Nature 353, 637–640.
Show in context REE patterns of MGP olivines (Schmitt et al., 1963; Masuda, 1968; Saito et al., 1998; Barrat et al., 2016a; and this work) and phosphates (Davis and Olsen, 1991; Hsu, 2003) normalised to CI chondrite (Barrat et al., 2012).
View in article
Phosphates in the Springwater pallasite are light REE enriched (Davis and Olsen, 1991), and could explain the shape of the REE pattern displayed by the olivine fraction prepared from this meteorite (Fig. 1; Saito et al., 1998).
View in article
Dellagiustina, D.N., Habib, N., Domanik, K.J., Hill, D.H., Lauretta, D.S., Goreva, Y.S., Killgore, M., Hexiong, Y., Downs, R.T. (2019) The Fukang pallasite: Characterization and implications for the history of the Main-group parent body. Meteoritics & Planetary Science 54, 1781–1807.
Show in context Inclusions composed of tridymite and Al, K-rich glasses have been described in olivines from the Fukang pallasite (Dellagiustina et al., 2019).
View in article
Evans, T.M., O’Neill, H.St.C., Tuff, J. (2008) The influence of melt composition on the partitioning of REEs, Y, Sc, Zr and Al between forsterite and melt in the system CMAS. Geochimica et Cosmochimica 72, 5708–5721.
Show in context For example, the data obtained by Evans et al. (2008) allow us to calculate that the DEr/DLu ratio of olivine is close to 0.3 (see Supplementary Information).
View in article
Greenwood, R.C., Franchi, I.A., Jambon, A., Buchanan, P. (2005) Widespread magma oceans on asteroidal bodies in the early solar system. Nature 435, 916–918.
Show in context Greenwood et al. (2005) estimated that at melting rates greater than 40 %, the Δ17O values are homogenised.
View in article
However, the O isotopic homogeneity of the main-group pallasites is very comparable to what is observed for the Earth, Moon, Mars, Vesta and the parent body of the angrites, for which magma oceans have been considered (Greenwood et al., 2005, 2017).
View in article
Greenwood, R.C., Franchi, I.A., Jambon, A., Barrat, J.A., Burbine, T.H. (2006) Oxygen isotope variation in stony-iron meteorites. Science 313, 1763–1765.
Show in context They display rather homogenous Δ17O values and are thought to derive from one (Greenwood et al., 2006, 2015) or perhaps two parent bodies (Ali et al., 2018).
View in article
Greenwood, R.C., Barrat, J.A., Scott, E.R.D., Haack, H., Buchanan, P.C., Franchi, I.A., Yamaguchi, A., Johnson, D., Bevan, A.W.R., Burbine, T.H. (2015) Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection. Geochimica et Cosmochimica Acta 169, 115–136.
Show in context They display rather homogenous Δ17O values and are thought to derive from one (Greenwood et al., 2006, 2015) or perhaps two parent bodies (Ali et al., 2018).
View in article
The latter possibility would be consistent with the observed homogeneity of the Δ17O values for olivines which suggest a highly homogeneous or a well homogenised body (Δ17O = −0.187 ± 0.008 ‰ (1 σ, n = 103), Greenwood et al., 2015), however, this is not confirmed by trace element chemistry data (Mittlefehldt and Herrin, 2010; McKibbin et al., 2013).
View in article
The few pallasite olivines previously analysed showed totally unexpected light REE enrichments (Schmitt et al., 1963, 1964; Masuda, 1968; Saito et al., 1998; Minowa and Ebihara, 2002; Greenwood et al., 2015).
View in article
Data from Saito et al. (1998), Greenwood et al. (2015), Barrat et al. (2016a), and this work.
View in article
Greenwood, R.C., Burbine, T.H., Miller, M.F., Franchi, I.A. (2017) Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies. Chemie der Erde 77, 1–43.
Show in context Although only a relatively small number of pallasites are known, their petrographic features and isotopic oxygen compositions show that they originate from at least six distinct parent bodies (e.g., Greenwood et al., 2017).
View in article
One can of course imagine that the parent body was very homogeneous in oxygen isotopes, but such a level of homogeneity, although conceivable, is not observed among any of the different groups of primitive meteorites (Greenwood et al., 2017).
View in article
However, the O isotopic homogeneity of the main-group pallasites is very comparable to what is observed for the Earth, Moon, Mars, Vesta and the parent body of the angrites, for which magma oceans have been considered (Greenwood et al., 2005, 2017).
View in article
Hsu, W. (2003) Minor element zoning and trace element geochemistry of pallasites. Meteoritics & Planetary Science 38, 1217–1241.
Show in context REE patterns of MGP olivines (Schmitt et al., 1963; Masuda, 1968; Saito et al., 1998; Barrat et al., 2016a; and this work) and phosphates (Davis and Olsen, 1991; Hsu, 2003) normalised to CI chondrite (Barrat et al., 2012).
View in article
Calcium (from 27 to 281 μg/g) and Sc (from 0.31 to 2.16 μg/g) abundances are in the range of reported values for pallasite olivines (e.g., Mittlefehldt, 1980; Hsu, 2003; McKibbin et al., 2013).
View in article
Nonetheless, the abundances of many trace elements, such as Ti, V, Cr and Ni were significantly modified in olivines during the mixing and cooling with metal (e.g., Hsu et al., 2003), and it is thus questionable whether REE distributions were also affected.
View in article
Johnson, B.C., Sori, M.M., Evans, A.J. (2019) Ferrovolcanism on metal worlds and the origin of pallasites. Nature Astronomy 4, 41–44.
Show in context Although there is no consensus on a precise formation model yet, the variety of textures shown by these meteorites, the diversity of cooling rates recorded by the metal among others, and also geophysical models suggest that the core-mantle mixing is related to catastrophic impacts (e.g., Scott, 1977; Yang et al., 2010; Tarduno et al., 2012; Bryson et al., 2015; Nichols et al., 2016; Walte et al., 2020), or is from an unknown type of volcanism (Johnson et al., 2019).
View in article
Masuda, A. (1968) Lanthanide concentrations in the olivine phase of the Brenham pallasite. Earth and Planetary Science Letters 5, 59–62.
Show in context The few pallasite olivines previously analysed showed totally unexpected light REE enrichments (Schmitt et al., 1963, 1964; Masuda, 1968; Saito et al., 1998; Minowa and Ebihara, 2002; Greenwood et al., 2015).
View in article
REE patterns of MGP olivines (Schmitt et al., 1963; Masuda, 1968; Saito et al., 1998; Barrat et al., 2016a; and this work) and phosphates (Davis and Olsen, 1991; Hsu, 2003) normalised to CI chondrite (Barrat et al., 2012).
View in article
McKibbin, S.J., O’Neill, H.St.C., Mallmann, G., Halfpenny, A. (2013) 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 The latter possibility would be consistent with the observed homogeneity of the Δ17O values for olivines which suggest a highly homogeneous or a well homogenised body (Δ17O = −0.187 ± 0.008 ‰ (1 σ, n = 103), Greenwood et al., 2015), however, this is not confirmed by trace element chemistry data (Mittlefehldt and Herrin, 2010; McKibbin et al., 2013).
View in article
Calcium (from 27 to 281 μg/g) and Sc (from 0.31 to 2.16 μg/g) abundances are in the range of reported values for pallasite olivines (e.g., Mittlefehldt, 1980; Hsu, 2003; McKibbin et al., 2013).
View in article
Firstly, REEs, as also Al and P, diffuse very slowly into olivine. The observed preservation of heterogeneities in Al and P, and the absence of diffusion profiles for these elements in the olivines (McKibbin et al., 2013) support our hypothesis.
View in article
McKibbin, S.J., Pittarello, L., Makarona, C., Hamann, C., Hecht, L., Chernonozhkin, S.M., Goderis, S., Clayes, P. (2019) Petrogenesis of main group pallasite meteorites based on relationships among texture, mineralogy, and geochemistry. Meteoritics & Planetary Science 54, 2814–2844.
Show in context While pallasites are recognised to be mixtures of mantle and core materials, their formation is still debated (Boesenberg et al., 2012; McKibbin et al., 2019; and references therein).
View in article
Minowa, H., Ebihara, M. (2002) Rare Earth Elements in pallasite olivines. Lunar and Planetary Science XXXIII. Lunar and Planetary Institute, Houston, #1386 (abstr.).
Show in context The few pallasite olivines previously analysed showed totally unexpected light REE enrichments (Schmitt et al., 1963, 1964; Masuda, 1968; Saito et al., 1998; Minowa and Ebihara, 2002; Greenwood et al., 2015).
View in article
Alternatively, the light REE enriched end members could be due to terrestrial contamination (Minowa and Ebihara, 2002).
View in article
Mittlefehldt, D.W. (1980) The composition of mesosiderite olivine clasts and implications for the origin of pallasites. Earth and Planetary Science Letters 51, 29–40.
Show in context Some have suggested that mantle fragments could be melting residues after extraction of a large fraction of magma, or alternatively produced by the accumulation of olivines during the crystallisation of a global magma ocean (e.g., Buseck, 1977; Mittlefehldt, 1980; Boesenberg et al., 2012).
View in article
Calcium (from 27 to 281 μg/g) and Sc (from 0.31 to 2.16 μg/g) abundances are in the range of reported values for pallasite olivines (e.g., Mittlefehldt, 1980; Hsu, 2003; McKibbin et al., 2013).
View in article
Mittlefehldt, D.W., Herrin, J.S. (2010) Trace element compositions of pallasite olivine grains and pallasite origin. Meteoritical Society Meeting 73, #5386 (abstr.).
Show in context The latter possibility would be consistent with the observed homogeneity of the Δ17O values for olivines which suggest a highly homogeneous or a well homogenised body (Δ17O = −0.187 ± 0.008 ‰ (1 σ, n = 103), Greenwood et al., 2015), however, this is not confirmed by trace element chemistry data (Mittlefehldt and Herrin, 2010; McKibbin et al., 2013).
View in article
Nichols, C.I.O., Bryson, J.F.J., Herrero-Albillos, J., Kronast, F., Nimmo, F., Harrison, R.J. (2016) Pallasite paleomagnetism: quiescence of a core dynamo. Earth and Planetary Science Letters 441, 103–112.
Show in context Although there is no consensus on a precise formation model yet, the variety of textures shown by these meteorites, the diversity of cooling rates recorded by the metal among others, and also geophysical models suggest that the core-mantle mixing is related to catastrophic impacts (e.g., Scott, 1977; Yang et al., 2010; Tarduno et al., 2012; Bryson et al., 2015; Nichols et al., 2016; Walte et al., 2020), or is from an unknown type of volcanism (Johnson et al., 2019).
View in article
Rudnick, R.L., Gao, S. (2014) 4.1 – Composition of the Continental Crust. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry (Second Edition) Elsevier, Oxford, 1–51.
Show in context Contamination trend uses the average upper continental crust composition from Rudnick and Gao (2014).
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We have tested this hypothesis using the mean composition of the terrestrial upper continental crust (Rudnick and Gao, 2014).
View in article
Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
Show in context The few pallasite olivines previously analysed showed totally unexpected light REE enrichments (Schmitt et al., 1963, 1964; Masuda, 1968; Saito et al., 1998; Minowa and Ebihara, 2002; Greenwood et al., 2015).
View in article
REE patterns of MGP olivines (Schmitt et al., 1963; Masuda, 1968; Saito et al., 1998; Barrat et al., 2016a; and this work) and phosphates (Davis and Olsen, 1991; Hsu, 2003) normalised to CI chondrite (Barrat et al., 2012).
View in article
(Er/Lu)n vs. (La/Lu)n plot for MGP olivines. Data from Saito et al. (1998), Barrat et al. (2016a), and this work.
View in article
Phosphates in the Springwater pallasite are light REE enriched (Davis and Olsen, 1991), and could explain the shape of the REE pattern displayed by the olivine fraction prepared from this meteorite (Fig. 1; Saito et al., 1998).
View in article
Data from Saito et al. (1998), Greenwood et al. (2015), Barrat et al. (2016a), and this work.
View in article
Scott, E.R.D. (1977) Formation of olivine-metal textures in pallasite meteorites. Geochimica et Cosmochimica Acta 41, 693–710.
Show in context Pallasites are essentially composed of metal and olivine (i.e. from isolated single crystals and/or polycrystalline aggregates to fragments of “dunitic pebbles”), with variable (low) amount of sulfide (troilite), minor schreibersite, phosphates, and in some cases chromite (e.g., Buseck and Holdsworth, 1977; Scott, 1977).
View in article
Although there is no consensus on a precise formation model yet, the variety of textures shown by these meteorites, the diversity of cooling rates recorded by the metal among others, and also geophysical models suggest that the core-mantle mixing is related to catastrophic impacts (e.g., Scott, 1977; Yang et al., 2010; Tarduno et al., 2012; Bryson et al., 2015; Nichols et al., 2016; Walte et al., 2020), or is from an unknown type of volcanism (Johnson et al., 2019).
View in article
Schmitt, R.A., Smith, R.H., Lasch, J.E., Mosen, A.W., Olehy, D.A., Vasilevskis, J. (1963) Abundances of the fourteen rare-earth elements, scandium, and yttrium in meteoritic and terrestrial matter. Geochimica et Cosmochimica Acta 27, 577–622.
Show in context The few pallasite olivines previously analysed showed totally unexpected light REE enrichments (Schmitt et al., 1963, 1964; Masuda, 1968; Saito et al., 1998; Minowa and Ebihara, 2002; Greenwood et al., 2015).
View in article
REE patterns of MGP olivines (Schmitt et al., 1963; Masuda, 1968; Saito et al., 1998; Barrat et al., 2016a; and this work) and phosphates (Davis and Olsen, 1991; Hsu, 2003) normalised to CI chondrite (Barrat et al., 2012).
View in article
Schmitt, R.A., Smith, R.H., Olehy, D.A. (1964) Rare-earth, yttrium and scandium abundances in meteoritic and terrestrial matter-II. Geochimica et Cosmochimica Acta 28, 67–86.
Show in context The few pallasite olivines previously analysed showed totally unexpected light REE enrichments (Schmitt et al., 1963, 1964; Masuda, 1968; Saito et al., 1998; Minowa and Ebihara, 2002; Greenwood et al., 2015).
View in article
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 Although there is no consensus on a precise formation model yet, the variety of textures shown by these meteorites, the diversity of cooling rates recorded by the metal among others, and also geophysical models suggest that the core-mantle mixing is related to catastrophic impacts (e.g., Scott, 1977; Yang et al., 2010; Tarduno et al., 2012; Bryson et al., 2015; Nichols et al., 2016; Walte et al., 2020), or is from an unknown type of volcanism (Johnson et al., 2019).
View in article
Walte, N.P., Solferino, G.F.D., Golabek, G.J., Souza, D.S., Bouvier, A. (2020) Two-stage formation of pallasites and the evolution of their parent bodies revealed by deformation experiments. Earth and Planetary Science Letters 546, 116419.
Show in context Although there is no consensus on a precise formation model yet, the variety of textures shown by these meteorites, the diversity of cooling rates recorded by the metal among others, and also geophysical models suggest that the core-mantle mixing is related to catastrophic impacts (e.g., Scott, 1977; Yang et al., 2010; Tarduno et al., 2012; Bryson et al., 2015; Nichols et al., 2016; Walte et al., 2020), or is from an unknown type of volcanism (Johnson et al., 2019).
View in article
Wasson, J.T., Choi, B.-G. (2003) Main-group pallasites, chemical composition, relationship to IIIAB irons, and origin. Geochimica et Cosmochimica Acta 67, 3079–3096.
Show in context Since the 1960s, they are usually seen as samples of the core-mantle transition zone of a differentiated body (e.g., Anders, 1964; Wasson and Choi, 2003), but this hypothesis has been repeatedly challenged.
View in article
In any case, if the metal is well understood today (e.g., Wasson and Choi, 2003), it is not the case for the mantle fragments and olivines they contain.
View in article
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 Although there is no consensus on a precise formation model yet, the variety of textures shown by these meteorites, the diversity of cooling rates recorded by the metal among others, and also geophysical models suggest that the core-mantle mixing is related to catastrophic impacts (e.g., Scott, 1977; Yang et al., 2010; Tarduno et al., 2012; Bryson et al., 2015; Nichols et al., 2016; Walte et al., 2020), or is from an unknown type of volcanism (Johnson et al., 2019).
View in article
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Supplementary Information
The Supplementary Information includes:
- Samples and Analytical Procedures
- Estimation of the DEr/DLu Ratio for Olivine
- Tables S-1 and S-2
- Figure S-1
- Supplementary Information References
Download the Supplementary Information (PDF).
Figures
Figure 1 REE patterns of MGP olivines (Schmitt et al., 1963
Schmitt, R.A., Smith, R.H., Lasch, J.E., Mosen, A.W., Olehy, D.A., Vasilevskis, J. (1963) Abundances of the fourteen rare-earth elements, scandium, and yttrium in meteoritic and terrestrial matter. Geochimica et Cosmochimica Acta 27, 577–622.
; Masuda, 1968Masuda, A. (1968) Lanthanide concentrations in the olivine phase of the Brenham pallasite. Earth and Planetary Science Letters 5, 59–62.
; Saito et al., 1998Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
; Barrat et al., 2016aBarrat, J.A., Dauphas, N., Gillet, P., Bollinger, C., Etoubleau, J., Bischoff, A., Yamaguchi, A. (2016a) Evidence from Tm anomalies for non-CI refractory lithophile element proportions in terrestrial planets and achondrites. Geochimica et Cosmochimica Acta 176, 1–17.
; and this work) and phosphates (Davis and Olsen, 1991Davis, A.M., Olsen, E.J. (1991) Phosphates in pallasites 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 & Planetary Science 38, 1217–1241.
) normalised to CI chondrite (Barrat et al., 2012Barrat, J.A., Zanda, B., Moynier, F., Bollinger, C., Liorzou, C., Bayon, G. (2012) Geochemistry of CI chondrites: Major and trace elements, and Cu and Zn isotopes. Geochimica et Cosmochimica Acta 83, 79–92.
).
Figure 2 (Er/Lu)n vs. (La/Lu)n plot for MGP olivines. Data from Saito et al. (1998)
Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
, Barrat et al. (2016a)Barrat, J.A., Dauphas, N., Gillet, P., Bollinger, C., Etoubleau, J., Bischoff, A., Yamaguchi, A. (2016a) Evidence from Tm anomalies for non-CI refractory lithophile element proportions in terrestrial planets and achondrites. Geochimica et Cosmochimica Acta 176, 1–17.
, and this work.
Figure 3 (Er/Lu)n and (Ba/Lu)n vs. (La/Lu)n plot for MGP olivines. Data from Saito et al. (1998)
Saito, T., Shimizu, H., Masuda, A. (1998) Experimental study of major and trace element partitioning among olivine, metallic phase and silicate melt using chondrite as starting material: implications for V-shaped REE patterns of the pallasites meteorites. Geochemical Journal 32, 159–182.
, Greenwood et al. (2015)Greenwood, R.C., Barrat, J.A., Scott, E.R.D., Haack, H., Buchanan, P.C., Franchi, I.A., Yamaguchi, A., Johnson, D., Bevan, A.W.R., Burbine, T.H. (2015) Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection. Geochimica et Cosmochimica Acta 169, 115–136.
, Barrat et al. (2016a)Barrat, J.A., Dauphas, N., Gillet, P., Bollinger, C., Etoubleau, J., Bischoff, A., Yamaguchi, A. (2016a) Evidence from Tm anomalies for non-CI refractory lithophile element proportions in terrestrial planets and achondrites. Geochimica et Cosmochimica Acta 176, 1–17.
, and this work. Contamination trend uses the average upper continental crust composition from Rudnick and Gao (2014)Rudnick, R.L., Gao, S. (2014) 4.1 – Composition of the Continental Crust. In: Holland, H.D., Turekian, K.K. (Eds.) Treatise on Geochemistry (Second Edition) Elsevier, Oxford, 1–51.
.
Figure 4 Evolution of the (Er/Lu)n ratios in partial melting residues produced from a merrilite-bearing chondrite. Fractional melting and dynamic melting with Φ = 1 %, parameters and partition coefficients are taken from Barrat et al. (2016b)
Barrat, J.A., Jambon, A., Yamaguchi, A., Bischoff, A., Rouget, M.L., Liorzou, C. (2016b) Partial melting of a C-rich asteroid: lithophile trace elements in ureilites. Geochimica et Cosmochimica Acta 194, 163–178.
. Olivine in equilibrium with a chondritic magma ocean, least contaminated pallasitic olivines (this work), and ureilites (Barrat et al., 2016bBarrat, J.A., Jambon, A., Yamaguchi, A., Bischoff, A., Rouget, M.L., Liorzou, C. (2016b) Partial melting of a C-rich asteroid: lithophile trace elements in ureilites. Geochimica et Cosmochimica Acta 194, 163–178.
) are shown for comparison.