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

Primitive achondrites, representing 'transitional' samples between chondrites and differentiated achon-drites, provide a great opportunity to decode the thermal histories of planetesimals in the early solar sys-tem. 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) observa-tions 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 melt-ing 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 intercon-nects to form networks, with the generation of basaltic melts. The 53Mn-53Cr systematics in NWA 12869 [initial 53Mn/55Mn of (3.24 +/- 0.32) x 10-6] and Tafassasset [initial 53Mn/55Mn of (3.48 +/- 0.20) x 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 bod-ies.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 (e54Cr: 1.01 +/- 0.09 to 1.44 +/- 0.18) and oxygen isotopes (D17O:-1.72 to-1.78 +/- 0.05 parts per thousand), we first classify them into a new grou-plet 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, differ-entiated achondrite, and/or iron samples associated with this new grouplet would provide further con-straints on the rate and onset time of accretion, as well as the interior structure of the planetesimal in the outer solar system.(c) 2023 Elsevier Ltd. All rights reserved.