Metal and Sulfides in Eucrites and Diogenites

Introduction: Fe-Ni metal and sulfides are common accessory minerals in eucrites and diogenites. We have invesigated the occurrence, mineralogy and composition of these phases in 4 eucrites (EET-92023, Petersburg, Stannern, Camel Donga, hereafter CD) and in two diogenites (Johnstown and Tatahouine). In addition to conventional techniques (SEM, EMP), LA-ICP-MS was used to specifically measure the highly siderophile elements (HSE) abundances of sulfides and Fe-Ni metal [1,2]. Results: In eucrites, except for CD, Fe-Ni Metal (kamacite-tetra-taenite ± taenite) is associated with troilite. In diogenites, troilite is associated with pentlandite and Cu-rich sulfide as previously described by [3]. HSE abundances are extremely variable from sample to sample and between phases. For instance Fe-Ni metal has high Os and Ir abundances ((Os) N ≈ 15; N, Chondrite CI1 normalised) in EET-92023. The normalized HSE pattern shows a slight negative slope toward Pd and Re ((Re) N ≈10), while Au and Ag are much more depleted relative to Ir ((Au/Ir) N <0.5, (Ag/Ir) N <0.01). The coexisting troilite shows an inverse HSE pattern, with low Os and Ir content (e.g. (Os) N <0.1), positive slope toward Pd ((Pd) N >0.1) and extreme Au and Ag enrichment ((Au/Ir) N >10). These two types of complementary patterns are found in all eucrites and diogenites (except Stan-nern), including CD which contains only Ni-poor metal. This suggests that part of the metal is due to desulfidation of preexist-ing troilite in CD, as suggested by [4]. Overall the high abundance of HSE in metal (±sulfide) suggest that metal was not formed by reduction of silicate. Similarly, petrographic observations and metal/sulfide compositions (major elements and HSE) indicate that metal was not formed by S-loss of sulfide due to thermal event(s)-except for Ni-free metal grains in CD (see above)-and inversely troilite was not formed by sul-furization of metal [3]. Our observations rather suggest the evolution (common to Diogenites and Eucrites) of a HSE-rich Fe-Ni-S magmatic liquid, with sequential crystallisation of Fe-Ni-metal, followed by the crystallisation of an imiscible sulfide melt.