Tracking and dating incipient melting of a new grouplet of primitive achondrites

Yun Jiang*, Ye Li, Jia Liu, Qing Wang, Li Ping Qin, Ke Zhu, Shiyong Liao, Zongjun Yin, Yongbo Peng, Piers Koefoed, Weibiao Hsu

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

2 Citations (Scopus)

Abstract

Primitive achondrites, representing ‘transitional’ samples between chondrites and differentiated achondrites, provide a great opportunity to decode the thermal histories of planetesimals in the early solar system. In this work, we report a comprehensive study of petrography, mineralogy, O-Cr isotopes and 53Mn-53Cr systematics of four ungrouped primitive achondrites, Northwest Africa (NWA) 12869, NWA 3250, NWA 11112, and Tafassasset. High resolution X-ray tomographic microscopy (HR-XTM) observations yield the 3D spatial distributions of metal, sulfide, and plagioclase, providing the direct petrologic evidence for incipient melting. In NWA 12869, NWA 3250, and NWA 11112, only FeNi-FeS eutectic melting occurs, with small fractions of metal and sulfide melt and amalgamate sporadically. As for Tafassasset, in addition to common metal and sulfide melt, plagioclase (±pyroxene) extensively melts and interconnects to form networks, with the generation of basaltic melts. The 53Mn-53Cr systematics in NWA 12869 [initial 53Mn/55Mn of (3.24 ± 0.32) × 10−6] and Tafassasset [initial 53Mn/55Mn of (3.48 ± 0.20) × 10−6] are determined with mineral separates and bulk samples. When anchored to the D'Orbigny angrite, NWA 12869 has a 53Mn-53Cr age of 4563.4 ± 0.6 Ma, while Tafassasset has an age of 4563.8 ± 0.4 Ma. The Mn-Cr ages of NWA 12869 and Tafassasset record the time of incipient melting within their parent bodies. Considering that NWA 12869, NWA 3250, and NWA 11112 exhibit the same olivine-dominated (47–69 vol%) mineral assemblages with recrystallized textures and a great similarity in mineral chemistry (olivine: Fa35–38; pyroxene: Fs27–30Wo3.0–3.5; plagioclase: An47–50; chromite: Cr#: 74–76; Mg#: 14.0–14.6), oxygen fugacity (IW-2 to IW-1), as well as 54Cr nucleosynthetic anomalies (ε54Cr: 1.01 ± 0.09 to 1.44 ± 0.18) and oxygen isotopes (Δ17O: −1.72 to −1.78 ± 0.05‰), we first classify them into a new grouplet of primitive achondrites, different from Tafassasset. This grouplet does not share a parent body with CR chondrites, but rather derives from incipient melting of a carbonaceous chondrite parent body which formed in a nebular reservoir physically close to that sampled by CR chondrites. More chondrite, differentiated achondrite, and/or iron samples associated with this new grouplet would provide further constraints on the rate and onset time of accretion, as well as the interior structure of the planetesimal in the outer solar system.

Original languageEnglish
Pages (from-to)1-15
Number of pages15
JournalGeochimica et Cosmochimica Acta
Volume345
DOIs
Publication statusPublished - 15 Mar 2023

Bibliographical note

Funding Information:
We are grateful to Bo Zhang for providing the sample NWA 12869 for this study. We also thank Drs. Z. Y. Fang and X. Q. He for their assistances with sample preparation for Mn-Cr analysis at USTC. Financial support for this work was provided by the National Key Research and Development Program of China (2021YFA0716100), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB 41000000), the pre-research Project on Civil Aerospace Technologies (No. D020202 and D020302), the National Natural Science Foundation of China (Grants Nos. 42173044, 42273038, 41873076, 41973060, 42073060), and the Minor Planet Foundation. JL thanks the financial support from the China Postdoctoral Science Foundation (2020M671880). KZ thanks Alexander von Humboldt foundation for a postdoc fellowship.

Publisher Copyright:
© 2023 Elsevier Ltd

Keywords

  • Incipient melt
  • Mn-Cr
  • Partial differentiation
  • Primitive achondrite
  • Tafassasset

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