top

Abstract

Figures and Tables

Supplementary Figures and Tables

top

Introduction

Early Solar System chronology is largely based on short-lived, currently extinct radioisotopes that only provide relative ages. Anchoring these ages to the absolute timescale requires long-lived chronometers that are accurate and precise. With the exception of Pb-Pb, such chronometers are based on the measured proportion of a radioactive parent isotope (P) to its decay product (daughter, D). Thus in addition to high-temperature (diffusional) isotopic re-equilibration, these systems may also be disturbed by recent changes in P/D, which can occur even during low-temperature processes such as alteration and weathering.

The long-lived ¹⁷⁶Lu-¹⁷⁶Hf chronometer benefits from a large range in P/D among different minerals and a high closure temperature in silicates (e.g., Scherer et al., 2000

Scherer, E.E., Cameron, K.L., Blichert-Toft, J. (2000) Lu-Hf garnet geochronology: Closure temperature relative to the Sm-Nd system and the effects of trace mineral inclusions. Geochimica et Cosmochimica Acta 64, 3413–3432.

) and apatite (Barfod et al., 2003

Barfod, G.H., Otero, O., Albarède, F. (2003) Phosphate Lu-Hf geochronology. Chemical Geology 200, 241–253.

); therefore, it is potentially precise and robust against post-crystallisation heating and shock. Unsupported ¹⁷⁶Hf has been observed in many meteorites however, resulting in Lu-Hf dates that are up to 300 Myr older than the Pb-Pb age of the Solar System (e.g., Blichert-Toft et al., 2002

Blichert-Toft, J., Boyet, M., Télouk, P., Albarède, F. (2002) ¹⁴⁷Sm–¹⁴³Nd and ¹⁷⁶Lu–¹⁷⁶Hf in eucrites and the differentiation of the HED parent body. Earth and Planetary Science Letters 204, 167-181.

; Bizzarro et al., 2012

Bizzarro, M., Connelly, J.N., Thrane, K., Borg L.E. (2012) Excess hafnium-176 in meteorites and the early Earth zircon record. Geochemistry Geophysics Geosystems 13, doi: 10.1029/2011GC004003.

). The origin of this component is vigorously debated, with hypotheses including high-energy irradiation (Albarède et al., 2006

Albarède, F., Scherer, E. E., Blichert-Toft, J., Rosing, M. T., Simionovici, A., Bizzarro, M. (2006) γ-ray irradiation in the early Solar System and the conundrum of the ¹⁷⁶Lu decay constant. Geochimica et Cosmochimica Acta 70, 1261–1270.

; Thrane et al., 2010

Thrane, K., Connelly, J.N., Bizzarro, M., Meyer, B.S., The, L.-S. (2010) Origin of excess ¹⁷⁶Hf in meteorites. Astrophysical Journal Letters 717, 861–867.

) and diffusive re-equilibration on the meteorite parent body (Debaille et al., 2011

Debaille, V., Yin, Q. Z., Amelin, Y. (2011) The Role of Phosphates for the Lu-Hf Chronology of Meteorites. LPI Contributions 1639, 9066.

, 2013

Debaille, V., Yin, Q.-Z., Amelin, Y. (2013) Can diffusion cause discrepant Lu-Hf isochrons in meteorites? Mineralogical Magazine 77, 957.

, 2014

Debaille, V., Van Orman, J., Yin, Q. Z., Amelin, Y. (2014) The Role of Diffusion During Metamorphism for the Lu-Hf Systematics of Chondrites. Meteoritics and Planetary Science 49, A5238.

; Bloch et al., 2016

Bloch, E., Watkins, J., Ganguly, J. (2016) Diffusion kinetics of Lu in clinopyroxene and applications to Lu-Hf ages of eucrites. Abstract submitted to the Goldschmidt conference in Japan.

). However, our investigation of a sample of the recent Almahata Sitta meteorite fall precludes these mechanisms. Instead, we propose that the observed discrepancies may in general arise from terrestrial contamination, terrestrial weathering, or both.

top

Samples and Methods

Almahata Sitta fell onto the Nubian Desert in Sudan on October 7^th, 2008 (Jenniskens et al., 2009

Jenniskens, P., Shaddad, M.H., Numan, D., Elsir, S., Kudoda, A.M., Zolensky, M.E., Le, L., Robinson, G.A., Friedrich, J.M., Rumble, D., Steele, A., Chesley, S.R., Fitzsimmons, A., Duddy, S., Hsieh, H.H., Ramsay, G., Brown, P.G., Edwards, W.N., Tagliaferri, E., Boslough, M.B., Spalding, R.E., Dantowitz, R., Kozubal, M., Pravec, P., Borovicka, J., Charvat, Z., Vaubaillon, J., Kuiper, J., Albers J., Bishop, J.L., Mancinelli, R.L., Sandford, S.A., Milam, S.N., Nuevo, M., Worden, S.P. (2009) The impact and recovery of asteroid 2008 TC3. Nature 458, 485–488.

). Among polymict ureilitic and chrondritic fragments (Bischoff et al., 2010

Bischoff, A., Horstmann, M., Pack, A., Laubenstein, M., Haberer, S. (2010) Asteroid 2008 TC3-Almahata Sitta: A spectacular breccia containing many different ureilitic and chondritic lithologies. Meteoritics and Planetary Science 45, 1638-1656.

; Horstmann and Bischoff, 2014

Horstmann, M., Bischoff, A. (2014) The Almahata Sitta polymict breccia and the late accretion of Asteroid 2008 TC₃ - Invited Review. Chemie der Erde - Geochemistry 74, 149-184.

), the trachyandesitic sample ALM-A was found as a fresh 24.2 g piece on October 5^th, 2009. It consists mostly of feldspar (anorthoclase and plagioclase), low-Ca pyroxene, and Cr-bearing Ca pyroxene with numerous inclusions of alkali-rich melt glass, feldspar, Ti,Fe-oxides, troilite, and metal. Accessory phases include apatite, merrillite, ilmenite, Ti,Cr,Fe-spinel, troilite, and Fe-metal. All minerals appear unaltered in thin section.

ALM-A is a unique sample of the differentiated crust of the ureilite parent body (Bischoff et al., 2014

Bischoff, A., Horstmann, M., Barrat, J.A., Chaussidon, M., Pack, A., Herwartz, D., Ward, D., Vollmer, C., Decker, S. (2014) Trachyandesitic volcanism in the early Solar System. Proceedings of the National Academy of Sciences 111, 12689-12692.

). Its Pb-Pb age of 4562.0 ± 3.4 Ma (Amelin et al., 2015

Amelin, Y., Koefoed, P., Bischoff, A., Budde, G., Brennecka, G., Kleine, T. (2015) Pb Isotopic Age of ALM-A - A Feldspar-Rich Volcanic Rock from the Crust of the Ureilite Parent Body. LPI Contributions 1856, 5344.

) is consistent with its Al-Mg model age of 6.5 +0.5/-0.3 Myr after Ca-Al-rich inclusions (Bischoff et al., 2014

Bischoff, A., Horstmann, M., Barrat, J.A., Chaussidon, M., Pack, A., Herwartz, D., Ward, D., Vollmer, C., Decker, S. (2014) Trachyandesitic volcanism in the early Solar System. Proceedings of the National Academy of Sciences 111, 12689-12692.

), suggesting that ALM-A has not been disturbed by heating or shock after ~4.56 Ga. It is therefore ideal for investigating the cause of spurious Lu-Hf isochrons in meteorites.

A 2 g piece of ALM-A devoid of fusion crust was crushed in an agate mortar and sieved to <63, 63-125, and 125-250 μm fractions. Mineral concentrates were prepared using standard magnetic separation and heavy liquid techniques (see Supplementary Information for more details). Pure, mono-mineralic grains were handpicked under a binocular microscope, but impure separates dominated by one of the major minerals were also analysed (Fig. 1). When enough material was available, fractions were split, washing one aliquot with 2 M HNO₃ for 30 minutes, while leaving the other aliquot unwashed. The wash solutions were carefully pipetted off and analysed separately. The analytical procedure follows that of Bast et al. (2015)

Bast, R., Scherer, E.E., Sprung, P., Fischer-Gödde, M., Stracke, A., Mezger, K. (2015) A rapid and efficient ion-exchange chromatography for Lu–Hf, Sm–Nd, and Rb–Sr geochronology and the routine isotope analysis of sub-ng amounts of Hf by MC-ICP-MS. Journal of Analytical Atomic Spectrometry 30, 2323–2333.

and is detailed in the Supplementary Information. Isochron regressions (Table 1) are calculated using Isoplot/Ex v3.76 (Ludwig, 2003

Ludwig, K.R. (2003) Isoplot/Ex 3, A geochronological toolkit for Microsoft Excel. Berkeley Geochronology Center Special Publication No. 5.

) and the ¹⁷⁶Lu decay constant λ = 1.867 ×10^­-11 yr^-1 (Scherer et al., 2001

Scherer, E.E., Münker, C., Mezger, K. (2001) Calibration of the Lutetium-Hafnium Clock. Science 293, 683-687.

, 2003

Scherer, E.E., Mezger, K., Münker, C. (2003) The ¹⁷⁶Lu decay constant discrepancy: terrestrial samples vs. meteorites. Meteoritics and Planetary Science 38, A136.

; Söderlund et al., 2004

Söderlund, U., Patchett, P.J., Vervoort, J.D., Isachsen, C.E. (2004) The ¹⁷⁶Lu decay constant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafic intrusions. Earth and Planetary Science Letters 219, 311–324.

).

Full size image | Download in Powerpoint

Fractions	n	Date (Ma)	176Hf/177Hfi	MSWD	Fig.
All bulk & mineral fractions	20	4606 ± 84	0.279801 (39)	45	2
Washed residues, excl. fine	10	4578 ± 66	0.279807 (29)	15	2
Unwashed grains, excl. WR & fine	7	4659 ± 23	0.279765 (11)	2.1	2
All washes & purest mineral grains	13	4569 ± 24	0.279796 (11)	1.3	3
Purest mineral grains only	3	4571 ± 29	0.279796 (14)	0.012	3

The numbers in parentheses after ¹⁷⁶Hf/¹⁷⁷Hf_i indicate the uncertainties in the least significant digits.

Download in Excel

top

Results

The Lu-Hf data for all bulk and mineral fractions are given in Table S-1 and shown in Figure 2 together with a reference isochron that is based on the ¹⁷⁶Lu-¹⁷⁶Hf parameters of the chondritic uniform reservoir (CHUR, Bouvier et al., 2008

Bouvier, A., Verwoort, J.D., Patchett, P.J. (2008) The Lu–Hf and Sm–Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth and Planetary Science Letters 273, 48-57.

) and the maximum age of the Solar System (4568 Ma, e.g., Bouvier et al., 2011

Bouvier, A., Brennecka, G.A., Wadhwa, M. (2011) Absolute chronology of the first solids in the Solar System. LPI Contribution 1639, 9054.

). About 2/3 of the data plot above this reference, with the WR and fine fractions deviating the most. Regressing all 20 points yields an errorchron with an MSWD of 45 (Table 1) indicating excessive scatter (Wendt and Carl, 1991

Wendt, I., Carl C. (1991) The statistical distribution of the mean squared weighted deviations. Chemical Geology 86, 275–285.

). The 10 washed mineral fractions (residues, filled symbols in Fig. 2) also yield an errorchron (4578 ± 66 Ma, MSWD = 15; Table 1). However, the unwashed, impure mineral separates (open circles in Fig. 2) define a low-scatter trend (MSWD = 2.1, n = 7; Table 1) with a slope of 0.09088, which corresponds to a date of 4659 ± 23 Ma and a ¹⁷⁶Hf/¹⁷⁷Hf_i of 0.279765 ± 0.000011.

Full size image | Download in Powerpoint

Washed residues generally have lower ¹⁷⁶Lu/¹⁷⁷Hf than their unwashed counterparts (Fig. 2), and the washes have complementary high ¹⁷⁶Lu/¹⁷⁷Hf (0.31–0.67), and radiogenic ¹⁷⁶Hf/¹⁷⁷Hf (Fig. 3). Owing to the low Lu- (0.6-1.9 ng) and Hf contents (0.1-0.9 ng) of the washes, the isochron points have relatively large uncertainties (see Supplementary Information), but they are not systematically offset from the Solar System reference. A regression of the purest, handpicked mineral grains and all washes yields a 13-point isochron (MSWD = 1.3) with an age of 4569 ± 24 Ma and ¹⁷⁶Hf/¹⁷⁷Hf_i of 0.279796 ± 0.000011 (Fig. 3).

Full size image | Download in Powerpoint

top

Discussion

A reasonable Lu-Hf age that is concordant with the Pb-Pb age of the sample is obtained for the purest major mineral fractions and the 2 M HNO₃ washes, which are interpreted to represent selectively digested phosphate minerals. Thus, the ¹⁷⁶Lu-¹⁷⁶Hf systematics of ALM-A have not been disturbed after initial closure with respect to feldspars, pyroxenes, and phosphate minerals. Because irradiation, resetting during parent body brecciation, or terrestrial alteration would have disturbed those minerals, such processes can be ruled out for ALM-A. Nevertheless, most of the bulk and impure mineral fractions scatter above the Solar System reference (Fig. 2) – a feature that has previously been observed in other achondrite samples (e.g., Blichert-Toft et al., 2002

Blichert-Toft, J., Boyet, M., Télouk, P., Albarède, F. (2002) ¹⁴⁷Sm–¹⁴³Nd and ¹⁷⁶Lu–¹⁷⁶Hf in eucrites and the differentiation of the HED parent body. Earth and Planetary Science Letters 204, 167-181.

; Bouvier et al., 2015

Bouvier, A., Blichert-Toft, J., Boyet, M., Albarède, F. (2015) ¹⁴⁷Sm−¹⁴³Nd and ¹⁷⁶Lu−¹⁷⁶Hf systematics of eucrite and angrite meteorites. Meteoritics and Planetary Science 50, 1896–1911.

; Sanborn et al., 2015

Sanborn, M.E., Carlson, R.W., Wadhwa, M. (2015) ^147,146Sm–^143,142Nd, ¹⁷⁶Lu–¹⁷⁶Hf, and ⁸⁷Rb–⁸⁷Sr systematics in the angrites: Implications for chronology and processes on the angrite parent body. Geochimica et Cosmochimica Acta 171, 80–99.

).

On the basis of our ALM-A Lu-Hf data, we infer that terrestrial contamination is the source of the excess radiogenic Hf that affects the most impure separates, especially the fine fraction. (See Supplementary Information for more details on the terrestrial contaminant.) This terrestrial component is not effectively removed by washing in 2 M HNO₃ (Table S-1), as indicated by the scatter among the washed residues of the impure fractions (i.e. pyroxene and feldspar concentrates, impure picking dregs, both composites, and the fine fraction, Table 1). This is consistent with the isotope compositions of the washes, which reflect meteoritic phosphate minerals that were selectively dissolved from all fractions. These observations suggest that the terrestrial contaminant comprises fine-grained silicate material that, while insoluble in 2 M HNO₃, does dissolve during the HF–HNO₃ digestion. The contaminant was not identified optically. We assume that only small amounts of terrestrial material are present in cracks in the meteorite or adhering to grains. To cause the observed deviations from the Solar System reference, the contaminant must be isotopically distinct (i.e. more radiogenic at lower ¹⁷⁶Lu/¹⁷⁷Hf) from the meteorite minerals. Thus it is more likely that the contamination is terrestrial than introduced during parent body brecciation. We assume that the terrestrial contaminant is similar to average loess (i.e. 6.6 ppm Hf, ¹⁷⁶Lu/¹⁷⁷Hf = 0.0095 ± 0.0049, ¹⁷⁶Hf/¹⁷⁷Hf = 0.282428 ± 0.000030; Chauvel et al., 2014

Chauvel, C., Garçon, M., Bureau, S., Besnault, A., Jahn, B., Ding, Z. (2014) Constraints from loess on the Hf–Nd isotopic composition of the upper continental crust. Earth and Planetary Science Letters 388, 48-58.

). The deviations of, e.g., the whole rock and fine fractions from the Solar System isochron can be explained by ~0.3 and 1.1 wt. % of this terrestrial contaminant, respectively (Table S-1).

Apparently, low-scatter trends that would not be immediately identified as errorchrons (e.g., unwashed, impure fractions; MSWD of 2.1; Table 1) can yield spurious dates and low ¹⁷⁶Hf/¹⁷⁷Hf_i values. A similarly good isochron fit along a steep slope was previously observed for the quenched angrite Sahara 99555, and this was taken as evidence for accelerated ¹⁷⁶Lu decay caused by irradiation in the early Solar System (Bizzarro et al., 2012

Bizzarro, M., Connelly, J.N., Thrane, K., Borg L.E. (2012) Excess hafnium-176 in meteorites and the early Earth zircon record. Geochemistry Geophysics Geosystems 13, doi: 10.1029/2011GC004003.

). However, the requisite ¹⁷⁶Lu depletions have never been observed in meteorites (Scherer et al., 2005

Scherer, E.E., Münker, C., Kleine, T., Mezger, K. (2005) The isotopic composition of Lu in meteorites and lunar rocks: Implications for the decay constant of ¹⁷⁶Lu. Geophysical Research Abstracts 7, 10486.

; Wimpenny et al., 2015

Wimpenny, J., Amelin, Y., Yin, Q.-Z. (2015) The Lu isotopic composition of achondrites: Closing the case for accelerated decay of ¹⁷⁶Lu. Astrophysical Journal Letters 812, L3–5.

). On the basis of our ALM-A Lu-Hf data, we argue instead that terrestrial contamination can also produce an apparently steep isochron if the high-Lu/Hf points included in the regression (e.g., our impure pyroxene-rich fractions) are offset.

Evidently, terrestrial contamination can readily affect the ¹⁷⁶Lu-¹⁷⁶Hf systematics of meteorites, even during short terrestrial residence times. However, we infer from the accurate low-scatter isochron of the purest fractions (i.e. feldspar, low-Ca pyroxene, and Cr-pyroxene, 4571 ± 29 Ma, MSWD = 0.012, Table 1) that the terrestrial component is progressively removed during the mineral separation procedure. Sieving removes the fine-grained dust, which is most affected by contamination, and further sample handling during successive magnetic and density separations and the handpicking may help eliminate grain surface contamination. Washing minerals in 2 M HNO₃, in contrast, only increases the spread along isochrons toward lower ¹⁷⁶Lu/¹⁷⁷Hf values via phosphate removal without removing silicate-hosted contamination. The comparison of handpicked, impure, and bulk fractions reveals the importance of a thorough mineral purification, and we suggest the use of the most coarse-grained, mono-mineralic fractions available when applying the Lu-Hf chronometer to meteorites.

top

Conclusion

Despite its short terrestrial residence and lack of visible alteration, ALM-A bears evidence − in the form of unsupported ¹⁷⁶Hf − of terrestrial contamination. Meteorites having longer residence times (i.e. finds and some falls) may be affected in a similar manner, but with the added complication of aqueous alteration. The latter could potentially redistribute parent and daughter isotopes among meteoritic and terrestrial minerals, not only disturbing isochrons but also rendering the contamination difficult to remove. Contaminated mineral and bulk fractions can define overly steep trends, potentially without obvious geologic scatter if some data are excluded from the regression. The possibility of such effects should be carefully evaluated before invoking such exotic mechanisms as early Solar System irradiation to explain spuriously old Lu-Hf dates. For ALM-A, the contamination was effectively removed by our elaborate mineral separation procedure based on grain size, magnetic properties, density, and, importantly, handpicking to optically identify and exclude impurities. The purest mineral fractions and all washes provide a crystallisation age for ALM-A of 4569 ± 24 Ma. The ¹⁷⁶Hf/¹⁷⁷Hf_i of the ALM-A isochron, 0.279796 ± 0.000011, is identical to 1) the value of 0.279794 ± 0.000011 derived from the average composition of unequilibrated chondrites (Bouvier et al., 2008

Bouvier, A., Verwoort, J.D., Patchett, P.J. (2008) The Lu–Hf and Sm–Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth and Planetary Science Letters 273, 48-57.

) calculated back to the start of the Solar System using λ¹⁷⁶Lu = 1.867 ×10^­-11 yr^-1 and 2) the value of 0.279781 ± 0.000018 measured in eucrite zircon by Iizuka et al. (2015)

Iizuka, T., Yamaguchi, T., Hibiya, Y., Amelin, Y. (2015) Meteorite zircon constraints on the bulk Lu-Hf isotope composition and early differentiation of the Earth. Proceedings of the National Academy of Sciences 112, 5331–5336.

. These estimates are all clearly higher than that of the Sahara 99555 regression (0.279685 ± 0.000019; Bizzarro et al., 2012

Bizzarro, M., Connelly, J.N., Thrane, K., Borg L.E. (2012) Excess hafnium-176 in meteorites and the early Earth zircon record. Geochemistry Geophysics Geosystems 13, doi: 10.1029/2011GC004003.

). Although some eucrite whole rock regressions yield ¹⁷⁶Hf/¹⁷⁷Hf_i similar to our ALM-A value (e.g., 0.279751 ± 0.000030 to 0.27977 ± 0.00008; Bouvier et al., 2015

Bouvier, A., Blichert-Toft, J., Boyet, M., Albarède, F. (2015) ¹⁴⁷Sm−¹⁴³Nd and ¹⁷⁶Lu−¹⁷⁶Hf systematics of eucrite and angrite meteorites. Meteoritics and Planetary Science 50, 1896–1911.

), they generally exhibit elevated slopes and less precise ¹⁷⁶Hf/¹⁷⁷Hf_i values whose meaning remains unclear because of unexplained excess scatter (MSWD = 4.5–11; e.g., Blichert-Toft et al., 2002

Blichert-Toft, J., Boyet, M., Télouk, P., Albarède, F. (2002) ¹⁴⁷Sm–¹⁴³Nd and ¹⁷⁶Lu–¹⁷⁶Hf in eucrites and the differentiation of the HED parent body. Earth and Planetary Science Letters 204, 167-181.

; Bouvier et al., 2015

Bouvier, A., Blichert-Toft, J., Boyet, M., Albarède, F. (2015) ¹⁴⁷Sm−¹⁴³Nd and ¹⁷⁶Lu−¹⁷⁶Hf systematics of eucrite and angrite meteorites. Meteoritics and Planetary Science 50, 1896–1911.

). We therefore agree with the assessment of Bouvier et al. (2015)

Bouvier, A., Blichert-Toft, J., Boyet, M., Albarède, F. (2015) ¹⁴⁷Sm−¹⁴³Nd and ¹⁷⁶Lu−¹⁷⁶Hf systematics of eucrite and angrite meteorites. Meteoritics and Planetary Science 50, 1896–1911.

that existing eucrite isochron data cannot be used to precisely constrain the Lu-Hf parameters of the Solar System or Earth. Nevertheless, the consistency among three kinds of independent ¹⁷⁶Hf/¹⁷⁷Hf_i estimates (i.e. our ALM-A isochron, average bulk chondrite compositions, and low-P/D mineral compositions) for samples from different parent bodies provides evidence for the isotopic homogeneity of Hf at the beginning of the Solar System and suggests that the chondritic ¹⁷⁶Hf/¹⁷⁷Hf_i also applies to Earth. This, in turn, constitutes a vital reference for Hf isotope studies of Earth’s early crust-mantle evolution.

top

Acknowledgements

We gratefully acknowledge funding by the Special Priority Program 1385 − "The first 10 Million Years of the Solar System − A Planetary Materials Approach" of the Deutsche Forschungsgemeinschaft (grant SCHE 1579/1-1/2/3). We thank editor Helen Williams and two anonymous reviewers for their constructive comments.

Editor: Helen Williams

top

References

top

Supplementary Information

Analytical Procedure

Two grams of ALM-A were crushed in an agate mortar and sieved to <63, 63-125, and 125-250 μm fractions. A 100 mg chip was reserved for the whole rock powder. The chip was treated in an ultrasound ethanol bath, rinsed with ethanol, dried, and then powdered in agate. Minerals were separated by magnetic properties using a Frantz^® magnetic barrier separator and by density using methylene iodide (MI, 3.3 g/cm³) and an MI-acetone mixture (3.0 g/cm³). Density fractions were rinsed several times in acetone to remove all heavy liquid residues. Feldspar, low-Ca pyroxene, and Cr-bearing Ca pyroxene were further purified by dry hand-picking mono-mineralic grains from the respective mineral concentrates (125-250 μm; Fig. 1). Care was taken to avoid grains with any adhering material or coatings. The bulk and mineral fractions were weighed into Savillex Teflon^® vials, in which some aliquots (Table S-1) were washed in 2 M HNO₃ and rinsed multiple times with milli-Q water. The HNO₃ plus water rinses were all collected in another Savillex vial and constitute the “washes”. The residual samples after the HNO₃ washing procedure are referred to as “washed samples” or “residues”. All samples and washes were spiked with a mixed ¹⁷⁶Lu-¹⁸⁰Hf tracer. The wash solutions were dried down with a drop of HClO₄. The mineral fractions were digested in 2:1 HF:HNO₃ on a hotplate at 120 °C for 2 days, whereas the whole-rock powder was autoclave-digested at 180 °C for 5 days. After drying the samples, fluorides were evaporated three times with 1 ml of concentrated HNO₃, and complete dissolution was achieved when converting the samples to chlorides using 10 M HCl.

Fraction	Sample weight (mg)	Washed?	Lu (ng)	Hf (ng)	Lu (ppm)	Hf (ppm)	176Lu/177Hf	est. % 2 s.d.	176Hf/177Hf	2 s.e.	est. % 2 s.d.	wt. % terr. comp.
WR	48.57	no	10.8	62.5	0.222	1.29	0.02451	0.25	0.282005	(3)	0.0015	0.3
fine	24.05	no	5.34	27.3	0.222	1.14	0.02776	0.27	0.282344	(4)	0.0028	0.8
fine-r	24.95	yes‡	3.57	26.3	0.143	1.05	0.01925	0.25	0.281629	(4)	0.0029	1.1
fine-w			1.4	0.6			0.3421	2.8	0.310141	(30)	0.26
px conc	50.81	no	20.5	94.1	0.403	1.85	0.03091	0.27	0.282567	(3)	0.0018	0.3
px conc-r1	48.68	yes‡	19.2	91.1	0.394	1.87	0.02985	0.25	0.282455	(3)	0.0019	0.1
px conc-r2	49.60	yes	19.3	93.7	0.389	1.89	0.02922	0.26	0.282421	(3)	0.0021	0.4
px conc-w			1.4	0.5			0.4160	1.1	0.316665	(120)	0.25
*px-r	14.22	yes‡	6.76	57.1	0.476	4.01	0.01681	0.27	0.281293	(4)	0.0023	0.1
px-w			0.4	0.1			0.4500	3.9	0.320195	(72)	0.45
*Cr-px-r	24.42	yes‡	13.4	50.3	0.547	2.06	0.03773	0.27	0.283156	(3)	0.0023	0.1
Cr-px-w			0.6	0.1			0.6698	4.4	0.339548	(76)	0.74
px imp-1	5.08	no	1.92	10.2	0.379	2.00	0.02688	0.25	0.282222	(7)	0.0035	0.6
px imp-2	50.48	no	21.2	100	0.419	1.98	0.03000	0.25	0.282489	(3)	0.0019	0.4
px imp-r	53.14	yes‡	21.6	109	0.407	2.06	0.02804	0.26	0.282340	(3)	0.0022	0.9
px imp-w			1.7	0.5			0.4504	1.0	0.320497	(34)	0.13
comp1	33.14	no	14.5	71.3	0.438	2.15	0.02886	0.28	0.282391	(3)	0.0017	0.5
comp1-r	11.46	yes	4.53	25.3	0.396	2.21	0.02540	0.26	0.282077	(5)	0.0031	0.4
comp1-w			1.0	0.3			0.4559	3.6	0.320205	(47)	0.43
comp2	49.78	no	7.39	49.2	0.148	0.988	0.02131	0.25	0.281706	(4)	0.0057	0.1
comp2-r	51.75	yes	6.66	51.4	0.129	0.994	0.01837	0.25	0.281464	(3)	0.0025	0.3
comp2-w			1.3	0.4			0.4176	2.6	0.315563	(53)	0.28
fsp conc	105.2	no	3.72	35.5	0.0353	0.337	0.01486	0.25	0.281114	(3)	0.0020	0.0
fsp conc-r	100.8	yes‡	1.90	37.1	0.0189	0.368	0.007290	0.25	0.280456	(3)	0.0022	0.0
fsp conc-w			1.9	0.9			0.3098	1.9	0.307352	(25)	0.15
*fsp-r	95.44	yes	1.73	13.8	0.0181	0.144	0.01781	0.25	0.281383	(4)	0.0026	0.0
fsp-w			1.3	0.4			0.4944	3.8	0.323039	(31)	0.48
fsp imp	99.99	no	5.89	48.5	0.0589	0.485	0.01724	0.25	0.281332	(3)	0.0017	0.0
fsp imp-r	104.9	yes	4.41	51.1	0.0421	0.487	0.01226	0.25	0.280906	(4)	0.0022	0.1
fsp imp-w			1.3	0.5			0.4023	2.5	0.314287	(26)	0.26

* Pure, handpicked mineral separate. For each mineral fraction, one aliquot (-r, residue) was washed in 2 M HNO₃ for 30 min at room temperature or ‡ at 65 °C, and the wash solutions (-w) were analysed separately. Abbreviations as in Figure 1 in the main text, est. % 2 s.d.: estimated external reproducibility in %, 2 s.e.: absolute internal measurement uncertainty on ¹⁷⁶Hf/¹⁷⁷Hf in the 6th digit, wt. % terr. comp.: weight % of terrestrial component needed to explain the positive deviation from the reference isochron, calculated assuming average Lu-Hf parameters (see main text) and 6.6 ppm Hf in the loess (Chauvel et al., 2014).

Download in Excel

Hafnium was separated from the rare earth elements (REE) on an initial 2 ml cation column (AG 50W-X8, 200-400 mesh) and further purified using a dedicated 2 ml Ln-Spec column (Bast et al., 2015). Both Hf and Lu cuts were analysed on a Neptune Plus MC-ICP-MS following the procedure described in Bast et al. (2015). For most bulk and mineral fractions, the Hf isotope composition was measured at concentrations around 40 ppb in sufficiently clean solutions (Zr/Hf ≤ 1, ¹⁷⁵Lu/¹⁷⁶Σ monitors ≤ 0.0005). For the washes, Hf was analysed at the 1 ppb-level with ≤0.35 V on ¹⁷⁷Hf, and the ¹⁷⁵Lu/¹⁷⁶Σ monitors were somewhat elevated at 0.0001 to 0.006. These Lu interferences were corrected using 1) natural Lu, and 2) the ¹⁷⁶Lu/¹⁷⁵Lu of the spiked sample. An average ¹⁷⁶Hf/¹⁷⁷Hf is reported, and the reported uncertainties enclose both end-member values. Procedural blanks were continuously monitored and ranged from 1 to 20 pg Hf and 1 to 5 pg Lu. To ensure a robust blank correction for samples with low sample-to-blank ratios (i.e. ~20-140 for the washes), the reported error ellipses include additional uncertainties for subtracting minimum and maximum blanks. Otherwise, the external reproducibility is estimated as described in Bast et al. (2015). The results reported in Table S-1 are normalised to ¹⁷⁶Hf/¹⁷⁷Hf = 0.282160 for the Ames Hf-standard, which is isotopically equivalent to JMC-475.

Additional Details on the Terrestrial Contamination in ALM-A

In the main text, the accuracy and lack of excess scatter in the isochrons of 1) handpicked minerals alone, and 2) handpicked minerals plus selectively dissolved phosphate minerals (washes) were used to eliminate several explanations (irradiation, diffusive resetting, and terrestrial alteration) for the excess ¹⁷⁶Hf found in most of the bulk and fine-grained samples. Given that the coarse, hand-picked grains were devoid of excess ¹⁷⁶Hf, the simplest remaining way to explain its presence in bulk and fine fractions is the presence of very fine grained (<63 µm) terrestrial material that infiltrated (dry or suspended in water) the meteorite along cracks or grain boundaries. Considering the sample preparation method used, we would thus expect the whole rock (powdered directly from a chip) and the <63 µm fractions to be affected the most by contamination. Owing to geochemical cycling over Earth’s history, low-Lu/Hf terrestrial samples are generally more radiogenic than low-Lu/Hf meteoritic minerals that have been isolated since ~4.56 Ga. Thus mixtures of meteoritic material with terrestrial contamination will be displaced above (and to the left of) the true isochron (Fig. 2, main text). That the contamination is effectively removed by sieving for coarse grains and hand-picking suggests that it is hosted by a separate, particulate material that has not chemically reacted, or combined, with the meteorite minerals e.g., by hydrous alteration. This may explain why the meteorite can be contaminated yet still appear fresh and unaltered. We speculate that the contamination was derived from ambient desert material over the 1 year residence time between the Almahata Sitta fall and the collection of ALM-A. (We consider it unlikely that contamination was introduced by sample curation, but cannot rule out this possibility.) If the terrestrial contamination is indeed the result of even short residence times on Earth, then potentially all meteorite finds (and many falls) may subject to similar effects. In cases such as ALM-A, where the contamination is not accompanied by chemical alteration or weathering, simple physical means, such as sieving, may be effective at removing the contamination. For whole rock analyses, it might be better to first isolate a coarse grained bulk fraction by sieving (e.g., Amelin et al., 2015) rather than simply powdering a rock chip as we have done here. However, the result may not be representative of the bulk meteorite if fine-grained meteoritic components (e.g., accessory minerals) are removed along with the contaminant. If the contamination occurs in connection with the chemical alteration that commonly affects meteorite finds (e.g., Crozaz and Wadhwa, 2001; Crozaz et al., 2003) and perhaps recently found pieces of earlier falls, it might prove more difficult to remove.

Supplementary Information References