1,2Alan E.Rubin,1Bidong Zhang,3Nancy L.Chabot
Geochimica et Cosmocchimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.05.020]
1Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
2Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, ME 04217, USA
3Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
Although IVA irons have O- and Cr-isotopic compositions resembling those of equilibrated LL chondrites, the bulk composition of refractory elements (e.g., Re, Ir, Pt) in the IVA core appears to be significantly lower than LL. These compositional discrepancies suggest known IVA irons may be missing early crystallized samples. We hypothesize the bulk composition of the IVA core is LL-like, but current collections do not include early fractional-crystallization IVA products. Our fractional-crystallization modeling of element vs. Au trends suggests that extant IVA irons are products of > 40% crystallization of the core, assuming an initial 2.9 wt.% S content. The model-derived bulk (Ni-normalized) composition of the IVA core is depleted relative to LL in most moderate volatiles: S (82% depletion), Ge (99.9% depletion), Ga (95% depletion), As (50% depletion); however, Au is enriched by 10%. Because moderate volatiles with depletions > 80% relative to LL have 50%-condensation temperatures < 1,020 K, it seems likely these depletions reflect post-accretion impact-induced volatilization of the IVA asteroid. The mean Ni-normalized compositions of analyzed IVA irons yield a lesser depletion of As (30%) and greater enrichment of Au (48%) relative to LL. The IVA asteroid may have experienced a complex parent-body thermal and collisional history: (1) differentiation, (2) impact-induced mantle stripping, devolatilization, and fractional condensation, (3) rapid crystallization of the core from the outside inwards, (4) shattering of the core after ∼75% crystallization, (5) quenching of thinly insulated samples (e.g., Fuzzy Creek), (6) formation of amorphous free silica in several IVA irons after impact-induced vaporization of portions of the overlying silicate mantle, followed by fractional condensation, (7) loss of portions of the core representing the first 40% of crystallization, (8) reaccretion of some core fragments, facilitating relatively slow cooling of a few IVA irons (e.g., Duchesne, Duel Hill (1854), Chinautla), and (9) collisional resetting of the Re-Os clock 4456 ± 25 Ma ago.