Glass-bearing inclusions in Nakhla (SNC meteorite) augite: heterogeneouslytrapped phases

Nakhla augite and olivine grains commonly contain glass-bearing inclusions. In contrast to olivines, augites host only one type of multiphase inclusio ns which consists of euhedral to subhedral augite, Ti-magnetite and pigeoni te plus silica-rich glass and a bubble. No fractures surround these inclusi ons, making it likely that they are of a pristine composition. Heating expe riments with a final temperature of 1150 degreesC were done for the first t ime with Nakhla augite inclusions. During heating the glass melted and crys tals inside the inclusions were dissolved in the melt whereby its chemical composition changed. The quenched glass is poorer in SiO2 and Al2O3 and ric her in CaO,FeO and MgO compared to unheated inclusion glass, Our in situ an alyses allowed us to estimate the initial composition of a liquid co-existi ng with Nakhla augite at 1150 degreesC and 1 atm pressure. Several features of Nakhla, such as the high Fe/Mg ratio of the augite, which is out of equ ilibrium with the glass, the highly variable alkali content and the Na/K ra tio of the glasses are incompatible with the standard model that states tha t SNC meteorites are all igneous rocks formed from basaltic magmas. Our res ults on re-melted glasses suggest a more complex and possibly non-magmatic genesis of Nakhla. Both types of glass-bearing inclusions (those hosted by augite or olivine) could represent heterogeneously trapped mineral + glass inclusions. Those hosted by augites mimic at least in part parental melt in clusions. However, the quenched glass is out of equilibrium with the host w ith respect to the Fe/Mg ratio and has too much compositional variation to be representative of a parental melt.