Abstract
We have investigated the solubility and partitioning of Ar in natural anorthite, diopside, forsterite, spinel, and synthetic iron-free basaltic melts. The experiments used a new technique which obviates post-quenching phase separation. Minerals and melts known to be in equilibrium are held in separate crucibles in a one bar flowing noble gas atmosphere at 1300°C or 1332°C. After a specified time the samples are quenched and the gas concentrations measured by mass spectrometry. A reversal and a rate study for Ar in anorthite indicate a close approach to equilibrium solubilities in our experiments. The solubility of Ar in the minerals is surprisingly high. In addition, the solubility of Ar in different samples of a particular mineral run in the same experiment varies more than the solubility in the same sample run in different experiments. This result suggests that noble gases are held in lattice vacancy defects. Other evidence supports defect siting: 1. (i) gases are held in very retentive sites; 2. (ii) solubility trends do not favor interstitial siting. 3. (iii) TEM imaging revealed no anomalous microstructures or dislocation densities. 4. (iv) EXAFS studies of some samples show that Kr has no preferred site in the lattices. Argon solubilities in synthetic silicate melts are lower than those observed experimentally in natural basalts. This difference correlates with the greater molar volume and polymerization of the natural basalts compared to the synthetic melts. The solubility variations greatly affect the absolute values of Ar mineral/ melt partition coefficients. Average anorthite/melt (0.6 ± 0.5) and diopside/melt (0.6 ± 0.5) partition coefficient values suggest that Ar is moderately incompatible. However, given the evidence that Ar solubility in minerals depends on lattice vacancy defect concentrations, it may not be possible to specify the partition coefficient values in a manner analogous to ionic species.
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