Siderophile element distribution in metal-sulfide nodules of EH3 Sahara 97072: A relict condensation signature overprinted by transient melting events

Evidence of a condensation signature has been detected in the textures and compositions of metal-sulfide nodules (MSN) from the unequilibrated enstatite chondrite Sahara 97072 (EH3). There are two end-member models, both of which can reproduce the siderophile element relations and distribution among the kamacite (αFeNi), schreibersite (FeNiP3), and perryite [(FeNi)x(SiP)y] observed in the MSN. These minerals may have condensed as a single metal alloy that subsequently decomposed in transient melting events or they could have condensed as separate phases. Most likely the minerals were produced from some combination of both processes. Kamacite, schreibersite, and perryite compositions in MSN can reasonably be recombined to produce a metallic phase with Co/Ni and P/Ni ratios similar to those in primitive CI meteorites. The recombined metal has siderophile element (Ir, Ru, Co, Pd, Au, Ga,) ratios closer to CI values than kamacite alone, suggesting it could have been a condensing metal that remained in equilibrium with a cooling reduced gas of near solar composition until Au and Ga condensed. Alternatively, nearly CI chondritic Pd/Ru ratios in schreibersite suggest it could be an early condensate. Perryite may have initially formed as a condensate, as a product of metal sulfurization, or both, but, if so, it appears to also have re-crystallized from a liquid rich in Ni and Si during partial melting of the MSN. Fe, Ni, and P distributions in kamacite near perryite and schreibersite suggest they are primitive minerals in disequilibrium with the metal, indicating they formed in transient melting events as opposed to slow metamorphic heating. The siderophile element distribution in Sahara 97072 MSN is consistent with early condensing material that was later reprocessed by transient melting events.