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Published May 2017 | Published + Supplemental Material
Journal Article Open

In search of the Earth-forming reservoir: Mineralogical, chemical, and isotopic characterizations of the ungrouped achondrite NWA 5363/NWA 5400 and selected chondrites


High-precision isotope data of meteorites show that the long-standing notion of a "chondritic uniform reservoir" is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this "isotopic crisis" and to better understand the genetic relations of meteorites and the Earth-forming reservoir, we performed a comprehensive petrographic, elemental, and multi-isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent-body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium-tungsten chronology yields a model age of 2.2 ± 0.8 Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent-body is not the "missing link" that could explain the composition of the Earth by the mixing of known meteorites. Until this "missing link" or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth.

Additional Information

© 2017 The Meteoritical Society. Received 08 July 2016; revision accepted 26 December 2016; First published: 20 March 2017. This work was supported by a SNF postdoc fellowship (SNF PBE2PZ-145946) (C.B.) and NASA grants (NNX14AK09G, OJ-30381-0036A, NNX15AJ25G) (N.D.). We thank G. Budde for running the spiked samples in Münster, and T. Burbine and Associate Editor M. Caffee for reviews.

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Published - cb8ce84e2bbca2ea00c6eb9f293bddf0dc202b6a4f29e985a393205cd86d36f1.pdf

Supplemental Material - maps12834-sup-0001-TableS1.xls

Supplemental Material - maps12834-sup-0002-Supinfo.xls


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