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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 |
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| Authors: | Alexander Borisov Andreas Pack Andreas Kropf Herbert Palme |
| Institution: | a Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences, Staromonetny 35, 109017 Moscow, Russia b Leibniz Universität Hannover, Institut für Mineralogie, Callinstraße 3, 30167 Hannover, Germany c Georg-August-Universität Göttingen, Geowissenschaftliches Zentrum, Goldschmidtstraße 1, 37077 Göttingen, Germany d Universität zu Köln, Institut für Geologie und Mineralogie, Zülpicher Straße 49b, 50674 Köln, Germany |
| Abstract: | We report experimentally determined 1 atm olivine/melt DNa partitioning data for low fO2, a variety of melt compositions and a temperature range of 1325-1522 °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 ∼10 μg/g. The mean olivine/melt DNa from 12 experimental runs is 0.0031 ± 0.0007 (1σ). 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. |
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