Mg Isotope Variations In Microphases Of Unequilibrated Enstatite Chondrites

Magnesium, a major mineral-forming element of the inner solar system, is partitioned between the silicate and the unique sulfidic phases (niningerite, MgS) of enstatite chondrites (ECs) owing to the formation of EC under exceptionally reducing conditions. In this study, we have carried out mineralogical characterization of the distinct Mg- and Si-bearing phases of unequilibrated ECs (EH3). To evaluate the Mg isotope variations in such reduced planetary bodies, we have analyzed the Mg isotope composition of several enstatitic silicate phases, matrices (composed of mixed proportions of silicate and sulfidic phases), and bulk meteorite fractions micro milled from three EH3 chondrites. We found that the stable Mg isotope composition (expressed as delta Mg-25) of the microphase separates of EH3 chondrites ranged from -0.216 +/- 0.014 parts per thousand to -0.094 +/- 0.014 parts per thousand. Despite the dispersion in Mg isotope values, the average delta Mg-25 of the silicate fractions of the studied EH3 chondrites was similar to its matrix and bulk meteorite fractions. Mass-dependent Mg isotope fractionation was evinced among the phase separates of EH3 chondrites with the slope of the fractionation line on a delta Mg-25 versus delta Mg-26 plot being closer to kinetic fractionation trend. Experimental and theoretical considerations hint that Mg isotope exchange between the silicate and sulfide phases might have generated the observed Mg isotope variations among the microphase separates of EH3 chondrites. Based on our Mg isotope data, in combination with the Si isotope composition obtained in the same aliquot of silicate-matrix fractions of EH3 chondrites and its subsolar Al/Si ratio, we suggest that the high abundance of Si in EC (due to the partitioning of Si among diverse silicates, silica, and metallic phases) and the loss of Mg and refractory components from EC-forming regions could explain the lower Mg/Si ratio of EC compared to that of ordinary and carbonaceous chondrites.