MIXTURE OF SEVERAL TYPES OF Fe,Ni METAL GRAINS IN THE ISHEYEVO METEORITE

Introduction: Isheyevo is a metal-rich carbonaceous chon-drite with metal-rich and metal-poor lithologies that are most similar to the CBb and CH chondrites, respectively [1]. All consist of similar components – Fe,Ni metal, chondrules, refractory inclusions, amoeboid olivine aggregates and heavily hydrated lithic clasts. We continued our investigation of these components of Isheyevo and found that Fe,Ni metal grains are more variable in compositions than we had described earlier. Here we report results on chemical compositions of different types of Fe,Ni metal grains and discuss how they accreted in the Isheyevo parent asteroid. Result and discussion: Isheyevo contains several types of Fe,Ni metal grains: 1) chemically homogeneous grains (un-zoned), containing 4.2-14.3 wt% Ni, 0.2-0.5 wt% Co, and 0.03-0.6 wt %; Ni and Co are positively correlated with solar Co/Ni ratio; sometimes these grains contain blebs of forsterite; 2) homogeneous grains enriched in Ni (>30 wt%) and Co (1.9 wt%); 3) unzoned Ni-poor or slightly decomposed grains containing inclusions of Cr-rich (2.5-13 wt%) troilite; 4) about 20-30% chemically zoned Fe,Ni metal grains with Ni and Co decreasing and Cr increasing towards the edges of the grains with solar Co/Ni ratio. Most zoned grains show evidence for decomposition into very fine-grained Ni-rich and Ni-poor phases; 5) < 5%, un-zoned Fe,Ni grains enriched in Si (up to 8 wt%), Cr (up to 1.29 wt%), and P (up to 0.8 wt%) with Co/Ni ration 0.07, higher than solar. Among grains of type 5, we discovered one round Fe,Ni grain, like a chondrule, with different type of zoning-Si (6.9-11.6 wt%) and P (0.73-0.94 wt%) decrease, and Cr (0.09-0.46) increases towards the edges of the grains with a negative correlation between Si and Cr, and a positive correlation between Si and P. Ni (6.4 wt%) and Co (0.44 wt%) are not various, and Co/Ni ratio is 0.07. This grain contains inclusions of Cr-rich (6.8 wt%) troilite. This set of Fe,Ni metal grains is more typical for CH chon-drites than for CBb chondrites [2,3]. Zoned grains enriched in Si and P were described at first. High-Cr and high-Si particles have been observed in ALH 85085 [3]. Si-bearing metal may have condensed along with other metal but may have resulted from local fluctuations of gas pressure [3] or redox conditions during condensation. On the other hand Ni-rich and Si-rich grains could form in different environments in the nebula, as discussed in [4]. Conclusions. Isheyevo contains several types of Fe,Ni …