Low-temperature exsolution in refractory siderophile element-rich opaque assemblages from the Leoville carbonaceous chondrite

A model for the formation of opaque assemblages in refractory inclusions in C3V chondrites was proposed by Blum et al. 1,2], in which the assemblages formed not by direct condensation but by crystallization from siderophile element-rich droplets within molten silicate inclusions, followed by low-temperature exsolution and oxidation and/or sulfidation. Although the data presented do not address the high-temperature histories of these objects, we describle herein two opaque assemblages in a Leoville refractory inclusion which provide the strongest textural and chemical evidence yet found in natural samples for the relatively low-temperature equilibration called for in this model. The assemblages have fine ( < 1 μm across) lamellae of εRuFe in γNiFe, with sharp contacts between phases. The larger (35 × 35 μm) of the two assemblages, OA1, has sets of regularly spaced, parallel lamellae, demonstrating crystallographic control. In the other, OA2, Ir and Pt partitioning between lamellae and host is consistent with equilibration at 873 K, as are the Fe, Ni and Ru contents of coexisting εRuFe and γNiFe and the narrow widths of the lamellae in both assemblages. The bulk compositions of the two assemblages bracket a previously inferred phase boundary in the FeNiRu system at 873 K. In these assemblages, the observed phases and their compositions are as predicted by the inferred phase relations at this temperature, suggesting that the position of the inferred phase boundary is correct. This is the same equilibration temperature estimated by Blum et al. 2] for another opaque assemblage from Leoville, also on the basis of alloy compositions and inferred phase equilibria in the FeNiRu system. The similarity of results for different Leoville samples is consistent with the suggestion 2] that final equilibration of the assemblages was post-accretionary.