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The case for the angrite parent body as the archetypal first-generation planetesimal: Large, reduced and Mg-enriched

Tissot, François L. H. and Collinet, Max and Namur, Olivier and Grove, Timothy L. (2022) The case for the angrite parent body as the archetypal first-generation planetesimal: Large, reduced and Mg-enriched. Geochimica et Cosmochimica Acta, 338 . pp. 278-301. ISSN 0016-7037. doi:10.1016/j.gca.2022.09.031.

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Angrites are silica-undersaturated achondrites formed very early in the history of the Solar System, and the most volatile-depleted known meteorites. As such, the study of angrites can provide critical insights into the early stages of planetary formation, melting and differentiation. Yet, understanding the origins of angrites and the nature of their parent body has long been hindered by the initially small number of specimens available. Here, we leverage (i) the rapidly growing number of known angrites, and (ii) equilibrium crystallization experiments at various pressure, temperature and oxygen fugacity conditions (P-T-fO₂), to revisit the petrogenesis of angrites and constrain key features of the angrite parent body (APB), such as its composition and size. We observe that quenched (i.e., volcanic) angrites define two compositional groups, which we show are readily related by fractional crystallization. This crystallization trend converges on an olivine-clinopyroxene-plagioclase (Ol + Cpx + Plag) multiple saturation boundary, whose composition is sampled by D’Orbigny, Sahara 99555 and NWA 1296. Using the observation that some quenched specimens represent primitive angritic melts, we derive a self-consistent bulk composition for the APB. We find that this composition matches the proposed Mg/Si ratio of 1.3 derived from the angrite δ³⁰Si values, and yields a core size (18 ± 6 wt%) in agreement with the siderophile elements depletion in the APB mantle. Our results support a primary control of nebular fractionation (i.e., partial condensation) on the composition of the APB. To establish the liquid phase equilibria of angrites, a series of 1 atmosphere and high-pressure crystallization experiments (piston cylinder and internally heated pressure vessel) were performed on a synthetic powder of D’Orbigny. The results suggest that the APB was a large (possibly Moon-sized) body, formed from materials condensed at relatively high-temperature (∼1300–1400 K), and whose fO₂ changed from mildly reducing (∼IW-1.5) to relatively oxidizing (∼IW+1 ± 1) in the ∼ 3 Myr between its core formation and the crystallization of D’Orbigny-like (Group 2) angrites. Based on its timing of accretion and differentiation, its composition, redox, and size, we argue that the APB represents the archetype of the first-generation of refractory-enriched planetesimals and embryos formed in the innermost part of the inner Solar System (<1 AU), and which accreted in the telluric planets.

Item Type:Article
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URLURL TypeDescription
Tissot, François L. H.0000-0001-6622-2907
Collinet, Max0000-0002-8791-9751
Namur, Olivier0000-0001-9129-3264
Grove, Timothy L.0000-0003-0628-1969
Additional Information:Nilanjan Chatterjee’s maintenance of, and help with, the EMPA is gratefully acknowledged. This work was supported by a W.O. Crosby Postdoctoral Fellowship from MIT, NSF grants EAR-1824002 and MGG-2054892, a Packard Fellowship, a research award from the Heritage Medical Research Institute, and start-up funds (provided by Caltech) to FLHT. MC was funded by a NASA Emerging Worlds grant (NNX16AD29J) at MIT (to TLG) and DFG grant TRR-170 at DLR. This is TRR-170 contribution #177. We thank Paul C. Buchanan, Aaron S. Bell, and one anonymous reviewer for constructive reviews that helped improve the manuscript, and editor Audrey Bouvier for prompt and careful editorial handling.
Group:Division of Geological and Planetary Sciences, Heritage Medical Research Institute
Funding AgencyGrant Number
Massachusetts Institute of Technology (MIT)UNSPECIFIED
David and Lucile Packard FoundationUNSPECIFIED
Heritage Medical Research InstituteUNSPECIFIED
Deutsche Forschungsgemeinschaft (DFG)TRR-170
Record Number:CaltechAUTHORS:20221220-736506000.4
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:118572
Deposited By: Research Services Depository
Deposited On:25 Jan 2023 15:22
Last Modified:25 Jan 2023 18:39

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