A light, chondritic xenolith in the Murchison (CM) chondrite - Formation by fluid-assisted percolation during metasomatism?

The main mineralogical characteristics of a large light-colored clast within the Murchison CM breccia are discussed in detail including data on the mineralogy, bulk chemistry, organics, and oxygen isotopes. Petrographic study shows that the white clast consists of two areas with different granoblastic textures: (1) a coarse-grained (average grain size: similar to 200 mu m) and (2) a fine-grained lithology (average grain-size: similar to 20 mu m). The Fa-content of olivine in the clast is the same as Fa within olivine from Rumuruti (R) chondrites (Fa: similar to 38 mol%); however, the concentrations of the elements Ni and Ca in olivine are significantly different. The fragment also contains Ca-rich pyroxene, similar to An(30-38)-plagioclase/maskelynite, Cr-rich spinel, several sulfide phases, a nepheline-normative glass, and traces of merrillite and metal. The occurrence of maskelynite and nepheline-normative amorphous phase in restricted areas of the well-recrystallized rock may indicate remarkable P-T-excursions during shock metamorphism. The O-isotope composition of the clast falls below the terrestrial fractionation line (TFL), lying in the field of CM chondrites and is significantly different from data for bulk R chondrites. The study of the soluble organic matter revealed a highly-oxidized carbon chemistry and organomagnesium compounds reflecting high temperature and pressure processes