Parent body processing and constraints on an IAB iron connection from Re-Os isotopes and highly siderophile element abundances in winonaites

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Xinmu J. Zhang, James M.D. Day, Robert W. Nicklas, Yash Srivastava

Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 7 November 2025

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“Winonaites are a group of primitive achondrites with broadly chondritic bulk compositions and relict chondritic textures that reflect variable and limited extents of partial melting due to incomplete parent-body metal-silicate segregation. Winonaites therefore provide a snapshot of the role of incipient heating and melting in chondritic parent bodies. Among different winonaite samples, a high degree of inherent textural and chemical heterogeneities on small scales (<1 cm) is observed, likely resulting from variations in heating, metamorphism, impact brecciation, and reassembly. To investigate parent-body heterogeneities and early metal-silicate differentiation, we present whole-rock highly siderophile element (HSE; including Re, Os, Ir, Ru, Pt, Pd) abundances and 187Re-187Os data for seven winonaites. We also report HSE abundances (Re, Os, Ir, Ru, Rh, Pt, Pd, Au) for winonaite metal and sulfides measured in situ. Bulk-rock winonaites all have broadly chondritic measured 187Os/188Os (0.1267 ± 0.0010, 2SE). Among the bulk winonaites analyzed, Winona and Tierra Blanca show the greatest degrees of inter-element HSE fractionation. By contrast, Graves Nunataks 12510, Yamato-8005, and Pontlyfni have near-chondritic bulk major- and trace-elements abundances, and relative HSE abundance patterns. Unlike other primitive achondrite groups, supra-chondritic Pd/Pt and Pd/Os ratios are characteristic of winonaite metals, possibly suggesting a nebular or parent-body process (e.g., fractional fusion) origin. The HSE abundances of winonaite metals suggest compositional similarities with the IAB sLL iron meteorite subgroup and argue against a genetic connection with main group IAB irons, demonstrating the need for future nucleosynthetic and chronological studies to reevaluate the previously proposed winonaite-IAB connection.”

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