The effects of Antarctic alteration and sample heterogeneity on Sm-Nd and Lu-Hf systematics in H chondrites

Ryoga Maeda, Steven Goderis, Vinciane Debaille, Hamed Pourkhorsandi, Geneviève Hublet, Philippe Claeys

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Abstract

Long-lived radioactive isotope systematics, such as Sm-Nd and Lu-Hf, are useful tools as important chronometers and tracers for chemical differentiation processes. Even though Antarctic meteorites include rare meteorites such as ungrouped meteorites, the effects of Antarctic alteration on the Sm-Nd and Lu-Hf systems in chondrites have not yet been evaluated in detail. Moreover, the heterogeneity of Sm-Nd and Lu-Hf data in bulk chondrites prevents the determination of precise average Sm-Nd and Lu-Hf values (e.g., for individual chondrite groups). To examine the effects of Antarctic alteration and sample heterogeneity on the Sm-Nd and Lu-Hf isotope systematics, ten Antarctic H chondrites (HCs) and three HCs from hot deserts were characterized for their modal abundances, elemental abundances, and Sm-Nd and Lu-Hf isotopic compositions. Regardless of the classical weathering index for Antarctic meteorites and the normalized Rb abundance used as a chemical alteration indicator in this study, the modal and elemental abundances in Antarctic HCs appear to be in good agreement with those in non-Antarctic HCs. The Sm-Nd and Lu-Hf isotopic compositions of the characterized H chondrites fall within the range measured for both HC falls and for falls of other chondrite groups, except in the case of the most heavily altered sample. Consequently, the effects of Antarctic alteration processes on the Sm-Nd and Lu-Hf systematics in HCs appear to be limited, except in the case of Asuka 09516. The latter meteorite exhibits severe mineralogical and chemical alteration, with considerable losses of even the rare earth elements (REEs), which are considered relatively immobile. The 147Sm/144Nd, 143Nd/144Nd, 176Lu/177Hf, and 176Hf/177Hf of bulk HCs correlate with their P/Mg and Y/Mg. Furthermore, the Lu-Hf ratios correlate strongly with their P/Ca and Y/Ca as well as their P/Mg and Y/Mg. Thus, the distribution of the elements between constituent minerals in ordinary chondrites (OCs) may control the heterogeneity observed for the bulk Sm-Nd and Lu-Hf data. In this context, the weight ratio of Ca-phosphates to Ca-pyroxene, or at least that of Ca-phosphates to silicates, may be a key factor leading to the observed elemental and isotopic variations. This observation indicates that the nugget effect of Ca-phosphates in OCs as the result of insufficient homogenization or terrestrial alteration leads to the heterogeneities displayed by the Sm-Nd and Lu-Hf data. Moreover, it also indicates that the use of equilibrated OCs for the determination of Sm-Nd and Lu-Hf data is affected more by sample heterogeneity, especially with respect to Ca-phosphates, than is the case for unequilibrated OCs, based on the re-distribution of REEs during thermal metamorphism on their parent bodies. This study demonstrates that Antarctic meteorites commonly preserve their original Sm-Nd and Lu-Hf isotopic compositions as much as chondrite falls, although exceptions are possible in the case of severe alteration. Similar to previous studies, we recommend the use of unequilibrated chondrites, for which the re-distribution of REEs is less extensive, for the determination of well-constrained average Sm-Nd and Lu-Hf isotopic compositions for individual chondrite groups as well as their robust Chondritic Uniform Reservoir values.
Original languageEnglish
Pages (from-to)106-129
Number of pages24
JournalGeochimica et Cosmochimica Acta
Volume305
DOIs
Publication statusPublished - 15 Jul 2021

Bibliographical note

Funding Information:
We would like to thank S. Cauchies and W. Debouge for assistance in the lab, J. De Jong for helping with the isotope analysis, and N. J. de Winter for guidance with the μXRF. We also thank the Royal Belgian Institute of Natural Sciences, Belgium, and the National Institute of Polar Research, Japan, for the loan of the Antarctic meteorites and meteorites collected from hot deserts used in this study. RM, SG, VD, and PhC acknowledge support from the Excellence of Science (EoS) project “ET-HoME”. SG, VD, and GH thank the BRAIN-Be Belgian Science Policy (BELSPO) project “BAMM”. VD thanks the ERC StG ISoSyC and FRS-FNRS for support. HP acknowledges receiving funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 801505. PhC acknowledges support from the VUB Strategic Research Program and the Research Foundation Flanders (FWO Hercules grant) for the purchase of the µXRF instrument. Finally, we are grateful to Audrey Bouvier for her editorial work and insightful comments during the review process. We also thank Erik Scherer, Minako Righter, and an anonymous reviewer for their constructive and detailed reviews.

Funding Information:
We would like to thank S. Cauchies and W. Debouge for assistance in the lab, J. De Jong for helping with the isotope analysis, and N. J. de Winter for guidance with the μXRF. We also thank the Royal Belgian Institute of Natural Sciences, Belgium, and the National Institute of Polar Research, Japan, for the loan of the Antarctic meteorites and meteorites collected from hot deserts used in this study. RM, SG, VD, and PhC acknowledge support from the Excellence of Science (EoS) project “ET-HoME”. SG, VD, and GH thank the BRAIN-Be Belgian Science Policy (BELSPO) project “BAMM”. VD thanks the ERC StG ISoSyC and FRS-FNRS for support. HP acknowledges receiving funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 801505. PhC acknowledges support from the VUB Strategic Research Program and the Research Foundation Flanders (FWO Hercules grant) for the purchase of the µXRF instrument. Finally, we are grateful to Audrey Bouvier for her editorial work and insightful comments during the review process. We also thank Erik Scherer, Minako Righter, and an anonymous reviewer for their constructive and detailed reviews.

Publisher Copyright:
© 2021 Elsevier Ltd

Copyright:
Copyright 2021 Elsevier B.V., All rights reserved.

Keywords

  • Sm-Nd
  • Lu-Hf
  • Chondrite
  • Antarctic alteration
  • Sample heterogeneity
  • Rare earth element

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