Petrographic studies of refractory inclusions from the Murchison meteorite

Abstract

Mineralogical and petrographic data are presented for ten refractory inclusions recovered by freeze‐thaw disaggregation and heavy liquid separation from the Murchison C2 chondrite. One hibonite‐, corundum‐rich and two hibonite‐spinel inclusions are completely unlike any previously described objects. The corundum‐bearing inclusion has a hibonite core that is partially replaced by corundum, which is itself partially replaced by hibonite. It formed either by condensation during varying physico‐chemical conditions or by partial incongruent melting of hibonite at ∼210O°K to form corundum and liquid, followed by distillation of calcium from the melt and reaction of the residual liquid with some of the corundum upon cooling. The melting temperature required in the latter model is far above astrophysical estimates of nebular temperatures, except perhaps at the very center of the nebula. The hibonite‐spinel inclusions are beautiful lacy aggregates of euhedral hibonite plates, some of which were pseudomorphically replaced by spinel. Textures provide compelling evidence of a vapor‐solid condensation origin and clearly show, that spinel formed by direct reaction of hibonite with the solar nebular gas, without the intervention of CaAl4O7 and prior to condensation of melilite, in conflict with equilibrium thermodynamic calculations. Four blue hibonitespinel spherules are described, three of which differ from all previously studied members of this class in being intensely altered to phyllosilicate and calcite. One shows evidence of two stages of alteration to phyllosilicates, possibly one in the solar nebula and the other in the parent body. Two of three pyroxene spinel‐, forsterite‐rich inclusions described herein differ from previously studied members of this group, one in containing a peculiar amoeba‐shaped spinel‐pyroxene core suggestive of a vapor‐solid condensation origin and the other in containing a spheroidal core suggestive of crystallization from a melt. The Murchison parent body sampled materials that formed under a wide range of nebular conditions and contains many objects composed of more refractory phase assemblages than are found in most Allende inclusions.