Author: Administrator

Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite

Posted by

0

Noriyuki Kawasakia,b, Steven B. Simonc, Lawrence Grossmanc,d, Naoya Sakamotoe, Hisayoshi Yurimotob,a,e

Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.035]
aInstitute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
bDepartment of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
cDepartment of Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, Illinois 60637, USA
dThe Enrico Fermi Institute, The University of Chicago, 5640 S. Ellis Ave., Chicago, Illinois 60637, USA
eIsotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
Copyright Elsevier

TS34 is a Type B1 Ca-Al-rich inclusion (CAI) from the Allende CV3 chondrite, consisting of spinel, melilite, Ti-Al-rich clinopyroxene (fassaite) and minor anorthite in an igneous texture. Oxygen and magnesium isotopic compositions were measured by secondary ion mass spectrometry in spots of known chemical composition in all major minerals in TS34. Using the sequence of formation from dynamic crystallization experiments and from chemical compositions of melilite and fassaite, the oxygen isotopic evolution of the CAI melt was established. Oxygen isotopic compositions of the constituent minerals plot along the carbonaceous chondrite anhydrous mineral line. The spinel grains are uniformly 16O-rich (Δ17O = −22.7 ± 1.7 ‰, 2SD), while the melilite grains are uniformly 16O-poor (Δ17O = −2.8 ± 1.8 ‰) irrespective of their åkermanite content and thus their relative time of crystallization. The fassaite crystals exhibit growth zoning overprinting poorly-developed sector zoning; they generally grow from Ti-rich to Ti-poor compositions. The fassaite crystals also show continuous variations in Δ17O along the inferred directions of crystal growth, from 16O-poor (Δ17O ∼ −3 ‰) to 16O-rich (Δ17O ∼ −23 ‰), covering the full range of oxygen isotopic compositions observed in TS34. The early-crystallized 16O-poor fassaite and the melilite are in oxygen isotope equilibrium and formed simultaneously. The correlation of oxygen isotopic compositions with Ti content in the fassaite imply that the oxygen isotopic composition of the CAI melt evolved from 16O-poor to 16O-rich during fassaite crystallization, presumably due to oxygen isotope exchange with a surrounding 16O-rich nebular gas. Formation of spinel, the liquidus phase in melts of this composition, predates crystallization of all other phases, so its 16O-rich composition is a relic of an earlier stage. Anorthite exhibits oxygen isotopic compositions between Δ17O ∼ −2 ‰ and −9 ‰, within the range of those of fassaite, indicating co-crystallization of these two minerals during the earliest to intermediate stage of fassaite growth. The melilite and fassaite yield an 26Al−26Mg mineral isochron with an initial value of (26Al/27Al)0 = (5.003 ± 0.075) × 10−5, corresponding to a relative age of 0.05 ± 0.02 Myr from the canonical Al−Mg age of CAIs. These data demonstrate that both 16O-rich and 16O-poor reservoirs existed in the solar nebula at least ∼0.05 Myr after the birth of the Solar System.

New constraints on the Paleoarchean meteorite bombardment of the Earth – Geochemistry and Re-Os isotope signatures of spherule layers in the BARB5 ICDP drill core from the Barberton Greenstone Belt, South Africa

Posted by

0

Toni Schulza, Christian Koeberla,b, Ambre Luguetc, David Van Ackenc,d, Tanja Mohr-Westheidee,f, Seda Ozdemira, Wolf Uwe Reimolde,g,h

Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.040]
aDepartment of Lithospheric Research, University Vienna, Althanstrasse 14, 1090 Vienna, Austria
bNatural History Museum, Burgring 7, A-1010 Vienna, Austria
cSteinmann-Institut of Geology, Mineralogy and Palaeontology, University of Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany
dIrish Centre for Research in Applied Geosciences (iCRAG), University College Dublin, Belfield, Dublin 4, Ireland
eMuseum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstrasse 43, 10115 Berlin, Germany
fFreie Universität Berlin (FU Berlin), Institut für Geologische Wissenschaften, Malteserstrasse 74-100, 12249 Berlin, Germany
gHumboldt Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
hGeochronology Laboratory, University of Brasília, Brazil
Copyright Elsevier

Archean spherule layers, resulting from impacts by large extraterrestrial objects, to date represent the only remnants of the early meteorite, asteroid, and comet bombardment of the Earth. Only few Archean impact debris layers have been documented, all of them embedded in the 3.23 to 3.47 billion year old successions of the Barberton Greenstone Belt (BGB) in South Africa and the Pilbara Craton in Western Australia. Some of them might be correlated with each other. Given the scarcity of Archean spherule deposits, four spherule layer intersections from the recently recovered BARB5 drill core from the central Barberton Greenstone Belt, analyzed in this study, provide an opportunity to gain new insight into the early terrestrial impact bombardment.

Despite being hydrothermally overprinted, siderophile element abundance signatures of spherule-rich samples from the BARB5 drill core, at least in part, retained a meteoritic fingerprint. The impact hypothesis for the generation of the BARB5 spherule layers is supported by correlations between the abundances of moderately (Cr, Co, Ni) and highly siderophile (Re, Os, Ir, Pt, Ru and Pd) elements, whose peak concentrations and interelement ratios are within the range of those for chondrites. Rhenium-Osmium isotope evidence further support the impact hypothesis.

Collectively, this study provides evidence for extraterrestrial admixtures ranging between ∼40 and up to 100% to three of the four analyzed BARB5 spherule layers, and a scenario for their genesis involving (i) impact of a chondritic bolide into a sedimentary target, (ii) varying admixtures of meteoritic components to target materials, (iii) spherule formation via condensation in an impact vapor plume, (iv) transportation of the spherules and sedimentation under submarine conditions, followed by (v) moderate post-impact remobilization of transition metals and highly siderophile elements.

The effect of melt composition on metal-silicate partitioning of siderophile elements and constraints on core formation in the angrite parent body

Posted by

0

E.S. Steenstraa, A.B. Sitabia, Y.H. Lina, N. Raib,c, J.S. Knibbea, J. Berndtd, S. Matveeve, W. van Westrenena

Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.034]
aFaculty of Earth and Life Sciences, VU University Amsterdam, Amsterdam, The Netherlands
bCentre for Planetary Sciences, Birkbeck University of London, London, UK
cDepartment of Earth Sciences, Mineral and Planetary Sciences Division, Natural History Museum, London, UK
dDepartment of Mineralogy, University of Münster, Germany
eDepartment of Petrology, Utrecht University, The Netherlands
Copyright Elsevier

We present 275 new metal-silicate partition coefficients for P, S, V, Cr, Mn, Co, Ni, Ge, Mo, and W obtained at moderate P (1.5 GPa) and high T (1683-1883 K). We investigate the effect of silicate melt composition using four end member silicate melt compositions. We identify possible silicate melt dependencies of the metal-silicate partitioning of lower valence elements Ni, Ge and V, elements that are usually assumed to remain unaffected by changes in silicate melt composition. Results for the other elements are consistent with the dependence of their metal-silicate partition coefficients on the individual major oxide components of the silicate melt composition suggested by recently reported parameterizations and theoretical considerations. Using multiple linear regression, we parameterize compiled metal-silicate partitioning results including our new data and report revised expressions that predict their metal-silicate partitioning behavior as a function of P–T–X–fO2. We apply these results to constrain the conditions that prevailed during core formation in the angrite parent body (APB). Our results suggest the siderophile element depletions in angrite meteorites are consistent with a CV bulk composition and constrain APB core formation to have occurred at mildly reducing conditions of 1.4±0.5 log units below the iron-wüstite buffer (ΔIW), corresponding to a APB core mass of 18±11%. The core mass range is constrained to 21±8 mass% if light elements (S and/or C) are assumed to reside in the APB core. Incorporation of light elements in the APB core does not yield significantly different redox states for APB core-mantle differentiation. The inferred redox state is in excellent agreement with independent fO2 estimates recorded by pyroxene and olivine in angrites.

Water content in the Martian mantle: a Nakhla perspective

Posted by

0

Franz A. Weisa,b, Jeremy J. Belluccia, Henrik Skogbya, Roland Stalderc, Alexander A. Nemchina,d, Martin J. Whitehousea

Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.041]
aSwedish Museum of Natural History, Dept. of Geosciences, Box 50007, SE-104 05 Stockholm, Sweden
bUppsala University, Dept. of Earth Sciences, Center of Experimental Mineralogy, Petrology and Geochemistry (CEMPEG), SE-752 36 Uppsala, Sweden
cInnsbruck University, Inst. for Mineralogy and Petrography, A-6020 Innsbruck, Austria
dCurtin University, Dept. of Applied Geology, Perth, WA 6845, Australia
Copyright Elsevier

Water contents of the Martian mantle have previously been investigated using Martian meteorites, with several comprehensive studies estimating the water content in the parental melts and mantle source regions of the shergottites and Chassigny. However, no detailed studies have been performed on the Nakhla meteorite. One possible way to determine the water content of a crystallizing melt is to use the water content in nominally anhydrous minerals (NAMs) such as clinopyroxene and olivine. During or after eruption on the surface of a planetary body and during residence in a degassing magma, these minerals may dehydrate. By reversing this process experimentally, original (pre-dehydration) water concentrations can be quantified. In this study, hydrothermal rehydration experiments were performed at 2 kbar and 700 °C on potentially dehydrated Nakhla clinopyroxene crystals. Rehydrated clinopyroxene crystals exhibit water contents of 130 ± 26 (2σ) ppm and are thus similar to values observed in similar phenocrysts from terrestrial basalts. Utilizing clinopyroxene/melt partition coefficients, both the water content of the Nakhla parent melt and mantle source region were estimated. Despite previous assumptions of a relatively dry melt, the basaltic magma crystallizing Nakhla may have had up to 1.42 ±0.28 (2σ) wt.% H2O. Based on an assumed low degree of partial melting, this estimate can be used to calculate a minimum estimate of the water content for Nakhla’s mantle source region of 72 ±16 ppm. Combining this value with values determined for other SNC mantle sources, by alternative methods, gives an average mantle value of 102 ± 9 (2σ) ppm H2O for the Martian upper mantle throughout geologic time. This value is lower than the bulk water content of Earth’s upper mantle (∼250 ppm H2O) but similar to Earth’s MORB source (54-330 ppm, average ∼130 ppm H2O).

s-Processing from MHD-Induced Mixing and Isotopic Abundances in Presolar SiC Grains

Posted by

0

S. Palmerini, , O. Trippella, M. Busso, D. Vescovi, M. Petrelli, A. Zucchini, F. Frondini
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.030]
Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, Italia, INFN, Sezione di Perugia, Italia
Copyright Elsevier

In the past years the observational evidence that s-process elements from Sr to Pb are produced by stars ascending the so-called Asymptotic Giant Branch (or “ AGB”) could not be explained by self-consistent models, forcing researchers to extensive parameterizations. The crucial point is to understand how protons can be injected from the envelope into the He-rich layers, yielding the formation of presup13C and then the activation of the presup13C(α  ,n)presup16O reaction. Only recently, attempts to solve this problem started to consider quantitatively physically-based mixing mechanisms. Among them, MHD processes in the plasma were suggested to yield mass transport through magnetic buoyancy. In this framework, we compare results of nucleosynthesis models for Low Mass AGB Stars (M≲3M⊙), developed from the MHD scenario, with the record of isotopic abundance ratios of s-elements in presolar SiC grains, which were shown to offer precise constraints on the presup13C reservoir. We find that n-captures driven by magnetically-induced mixing can indeed account for the SiC data quite well and that this is due to the fact that our presup13C distribution fulfils the above constraints rather accurately. We suggest that similar tests should be now performed using different physical models for mixing. Such comparisons would indeed improve decisively our understanding of the formation of the neutron source.

Iron and nickel isotope compositions of presolar silicon carbide grains from supernovae

Posted by

0

János Kodolányia, Thomas Stephanb,c, Reto Trappitschb,c,d, Peter Hoppea, Marco Pignatarid,e, Andrew M. Davisb,c,f, Michael J. Pellinb,c,f,g
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.029]
aMax Planck Institute for Chemistry, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
bChicago Center for Cosmochemistry
cDepartment of the Geophysical Sciences, The University of Chicago, 5734 S Ellis Ave, Chicago, IL 60637, USA
dThe NuGrid Collaboration1
eE. A. Milne Centre for Astrophysics, University of Hull, Hull, HU6 7RX, UK
fEnrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
gMaterials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
Copyright Elsevier

We report the carbon, silicon, iron, and nickel isotope compositions of twenty-five presolar SiC grains of mostly supernova (SN) origin. The iron and nickel isotope compositions were measured with the new Chicago Instrument for Laser Ionization, CHILI, which allows the analysis of all iron and nickel isotopes without the isobaric interferences that plagued previous measurements with the NanoSIMS. Despite terrestrial iron and nickel contamination, significant isotopic anomalies in 54Fe/56Fe, 57Fe/56Fe, 60Ni/58Ni, 61Ni/58Ni, 62Ni/58Ni, and 64Ni/58Ni were detected in nine SN grains (of type X). Combined multi-isotope data of three grains with the largest nickel isotope anomalies (>100 ‰ or <−100 ‰ in at least one isotope ratio, when expressed as deviation from the solar value) are compared with the predictions of two SN models, one with and one without hydrogen ingestion in the He shell prior to SN explosion. One grain’s carbon-silicon-iron-nickel isotope composition is consistent with the prediction of the model without hydrogen ingestion, whereas the other two grains’ isotope anomalies could not be reproduced using either SN models. The discrepancies between the measured isotope compositions and model predictions may indicate element fractionation in the SN ejecta prior to or during grain condensation, and reiterate the need for three-dimensional SN models.

Rare Earth Element abundances in presolar SiC

Posted by

0

T.R. Irelanda,b, J.N. Ávilaa,b, M. Lugaroc, S. Cristallod, P. Holdena, P. Lanca, L. Nittlere, C.M.O’D. Alexandere, F. Gyngardf, S. Amarif
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.027]
aResearch School of Earth Sciences, The Australian National University, Canberra ACT 2601, Australia
bPlanetary Science Institute, The Australian National University, Canberra ACT 2601, Australia
cKonkoly Observatory, Hungarian Academy of Sciences, Budapest, Hungary
dINAF – Osservatorio Astronomico di Teramo, 64100, Italy
eDepartment of Terrestrial Magnetism, Carnegie Institution of Washington, Washington DC 20015, USA
fLaboratory for Space Sciences, Physics Dept., Washington University, St. Louis, MO 63130, USA
Copyright Elsevier

Individual isotope abundances of Ba, lanthanides of the rare earth element (REE) group, and Hf have been determined in bulk samples of fine-grained silicon carbide (SiC) from the Murchison CM2 chondrite. The analytical protocol involved secondary ion mass spectrometry with combined high mass resolution and energy filtering to exclude REE oxide isobars and Si-C-O clusters from the peaks of interest. Relative sensitivity factors were determined through analysis of NIST SRM reference glasses (610 and 612) as well as a trace-element enriched SiC ceramic. When normalised to chondrite abundances, the presolar SiC REE pattern shows significant deficits at Eu and Yb, which are the most volatile of the REE. The pattern is very similar to that observed for Group III refractory inclusions. The SiC abundances were also normalised to s-process model predictions for the envelope compositions of low-mass (1.5 to 3 fx1) AGB stars with close-to-solar metallicities (Z=0.014 and 0.02). The overall trace element abundances (excluding Eu and Yb) appear consistent with the predicted s-process patterns. The depletions of Eu and Yb suggest that these elements remained in the gas phase during the condensation of SiC. The lack of depletion in some other moderately refractory elements (like Ba), and the presence of volatile elements (e.g. Xe) indicates that these elements were incorporated in to SiC by other mechanisms, most likely ion implantation.

Platinum isotopes in iron meteorites: galactic cosmic ray effects and nucleosynthetic homogeneity in the p-process isotope 190Pt and the other Platinum isotopes

Posted by

0

Alison C. Hunt, Mattias Ek, Maria Schönbächler
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.026]
Institute of Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
Copyright Elsevier

Platinum isotopes are sensitive to the effects of galactic cosmic rays (GCR), which can alter isotope ratios and mask nucleosynthetic isotope variations. Platinum also features one p-process isotope, 190Pt, which is very low abundance and therefore challenging to analyse. Platinum-190 is relevant for early solar-system chronology because of its decay to 186Os. Here, we present new Pt isotope data for five iron meteorite groups (IAB, IIAB, IID, IIIAB and IVA), including high-precision measurements of 190Pt for the IAB, IIAB and IIIAB irons, determined by multi-collector ICPMS. New data are in good agreement with previous studies and display correlations between different Pt isotopes. The slopes of these correlations are well-reproduced by the available GCR models. We report Pt isotope ratios for the IID meteorite Carbo that are consistently higher than the predicted effects from the GCR model. This suggests that the model predictions do not fully account for all the GCR effects on Pt isotopes, but also that the pre-atmospheric radii and exposure times calculated for Carbo may be incorrect. Despite this, the good agreement of relative effects in Pt isotopes with the predicted GCR trends confirms that Pt isotopes are a useful in-situ neutron dosimeter. Once GCR effects are accounted for, our new dataset reveals s- and r-process homogeneity between the iron meteorite groups studied here and the Earth. New 190Pt data for the IAB, IIAB and IIIAB iron meteorites indicate the absence of GCR effects and homogeneity in the p-process isotope between these groups and the Earth. This corresponds well with results from other heavy p-process isotopes and suggests their homogenous distribution in the inner solar system, although it does not exclude that potential p-process isotope variations are too diluted to be currently detectable.

Simultaneous iron and nickel isotopic analyses of presolar silicon carbide grains

Posted by

0

Trappitscha et al. (>10)*

Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.031]
aDepartment of the Geophysical Sciences, The University of Chicago, 5734 S Ellis Ave, Chicago, IL 60637, USA
*Find the extensive, full author and affiliation list on the publishers website
Copyright Elsevier

Aside from recording stellar nucleosynthesis, a few elements in presolar grains can also provide insights into the galactic chemical evolution (GCE) of nuclides. We have studied the carbon, silicon, iron, and nickel isotopic compositions of presolar silicon carbide (SiC) grains from asymptotic giant branch (AGB) stars to better understand GCE. Since only the neutron-rich nuclides in these grains have been heavily influenced by the parent star, the neutron-poor nuclides serve as GCE proxies. Using CHILI, a new resonance ionization mass spectrometry (RIMS) instrument, we measured 74 presolar SiC grains for all iron and nickel isotopes. With the CHARISMA instrument, 13 presolar SiC grains were analyzed for iron isotopes. All grains were also measured by NanoSIMS for their carbon and silicon isotopic compositions. A comparison of the measured neutron-rich isotopes with models for AGB star nucleosynthesis shows that our measurements are consistent with AGB star predictions for low-mass stars between half-solar and solar metallicity. Furthermore, our measurements give an indication on the 22Ne(α,n)25Mg reaction rate. In terms of GCE, we find that the GCE-dominated iron and nickel isotope ratios, 54Fe/56Fe and 60Ni/58Ni, correlate with their GCE-dominated counterpart in silicon, 29Si/28Si. The measured GCE trends include the Solar System composition, showing that the Solar System is not a special case. However, as seen in silicon and titanium, many presolar SiC grains are more evolved for iron and nickel than the Solar System. This confirms prior findings and agrees with observations of large stellar samples that a simple age-metallicity relationship for GCE cannot explain the composition of the solar neighborhood.

Evaluating Crustal Contributions to Enriched Shergottites from the Petrology, Trace Elements, and Rb-Sr and Sm-Nd Isotope Systematics of Northwest Africa 856

Posted by

0

J. Ferdousa, A.D. Brandona, A.H. Peslierb, Z. Pirottec

Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.05.032]
aEarth and Atmospheric Sciences Department, University of Houston, Houston, TX 77204-5007, USA
bJacobs, NASA-Johnson Space Center, Mail Code X13, Houston TX 77058, USA
cDept. de Géologie, Université Libre de Bruxelles, 1050 Brussels, Belgium
Copyright Elsevier

The origin of the incompatible trace element (ITE) characteristics of enriched shergottites has been critical for examining two contradicting scenarios to explain how these Martian meteorites form. The first scenario is that it reflects ITE enrichment in an early-formed mantle reservoir whereas the second scenario attributes it to assimilation of ancient Martian crust (∼4-4.5 Ga) by ITE-depleted magmas. Strongly differentiated shergottite magmas may yield added constraints for determining which scenario can best explain this signature in enriched shergottites. The meteorite Northwest Africa (NWA) 856 is a basaltic shergottite that, unlike many enriched shergottites, lacks olivine and has undergone extensive differentiation from more primitive parent magma. In similarity to other basaltic shergottites, NWA 856 is comprised primarily of compositionally zoned clinopyroxenes (45% pigeonite and 23% augite), maskelynite (23%) and accessory minerals such as ulvöspinel, merrillite, Cl-apatite, ilmenite, pyrrhotite, baddeleyite and silica polymorph. The CI-chondrite normalized rare earth element (REE) abundance patterns for its maskelynite, phosphates, and its whole rock are flat with corresponding light-REE depletions in clinopyroxenes. The 87Rb-87Sr and 147Sm-143Nd internal isochron ages are 162±14 (all errors are ±2σ) Ma and 162.7±5.5 Ma, respectively, with an initial εNdI= -6.6±0.2. The Rb-Sr isotope systematics are affected by terrestrial alteration resulting in larger scatter and a less precise internal isochron age. The whole rock composition is used in MELTS simulations to model equilibrium and fractional crystallization sequences to compare with the crystallization sequence from textural observations and to the mineral compositions. These models constrain the depth of initial crystallization to a pressure range of 0.4-0.5 GPa (equivalent to 34-42 km) in anhydrous conditions at the Fayalite-Magnetite-Quartz buffer, and consistently reproduce the observed mineralogy throughout the sequence with progressive crystallization. The Ti/Al ratios in the clinopyroxenes are consistent with initial crystallization occurring at these depths followed by polybaric crystallization as the parent magma ascended to the surface. The REE abundances in the clinopyroxenes and maskelynite are consistent with progressive crystallization in a closed system.

The new results for NWA 856 are combined with other shergottite data and are compared to mixing and assimilation and fractional crystallization (AFC) models using depleted shergottite magmas and ancient Martian crust as end-members. The models indicate that the range of REE abundances and ratios, when taken in isolation, can be successfully explained for all shergottites by crustal contamination. However, no successful crustal contamination model can explain the restricted εNdI of -6.8±0.2 over the wide range of Mg# (0.65 to 0.25), and corresponding trace element variations from enriched shergottites to depleted shergottites. The findings indicate that the origin of the long-term ITE-enriched signature in enriched shergottites and the geochemical variability seen in shergottites is not a result of crustal contamination but instead reflects ancient mantle heterogeneity.