Partitioning of Na between olivine and melt: An experimental study with application to the formation of meteoritic Na2O-rich chondrule glass and refractory forsterite grains

Abstract

We report experimentally determined 1 atm olivine/melt D-Na partitioning data for low fO(2), a variety of melt compositions and a temperature range of 1325-1522 degrees C. We demonstrated that high-current electron microprobe analyses (EPMA, I = 500 nA, 600 s on the peak) allow quantitative determination of Na2O in olivine down to similar to 10 mu g/g. The mean olivine/melt D-Na from 12 experimental runs is 0.0031 +/- 0.0007 (1 sigma). This is the recommended value for low pressures and a wide range of natural compositions. This result is applied to the problem of the origin of alkalis in chondrules and the formation of chondritic refractory forsterite grains. The data on Semarkona (LL3.0) chondrules show that Na2O is primordial and was present during olivine crystallization. For refractory forsterite grains from Murchison (CM2), we demonstrate that high CaO contents are not a result of equilibration with Na2O-rich melts, but require high activities of CaO during their formation. (C) 2008 Elsevier Ltd. All rights reserved.