The fidelity of melt inclusions as records of melt composition |
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Authors: | Don R Baker |
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Institution: | (1) Department of Earth and Planetary Sciences, GEOTOP-UQAM-McGill Research Centre, McGill University, 3450 rue University, Montréal, QC, H3A 2A7, Canada;(2) Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy |
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Abstract: | A series of experiments created melt inclusions in plagioclase and pyroxene crystals grown from a basaltic melt at 1,150°C,
1.0 GPa to investigate diffusive fractionation during melt inclusion formation; additionally, P diffusion in a basaltic melt
was measured at 1.0 GPa. Melt inclusions and melts within a few 100 microns of plagioclase–melt interfaces were analyzed for
comparison with melt compositions far from the crystals. Melt inclusions and melt compositions in the boundary layer close
to the crystal–melt interface were similar, but both differ significantly in incompatible element concentrations from melt
found greater than approximately 200 microns away from the crystals. The compositional profiles of S, Cl, P, Fe, and Al in
the boundary layers were successfully reproduced by a two-step model of rapid crystal growth followed by diffusive relaxation
toward equilibrium after termination of crystal growth. Applying this model to investigate possible incompatible element enrichment
in natural melt inclusions demonstrated that at growth rates high enough to create the conditions for melt inclusion formation,
∼10−9–10−8 m s−1, the concentration of water in the boundary layer near the crystal was similar to that of the bulk melt because of its high
diffusion coefficient, but sulfur, with a diffusivity similar to major elements and CO2, was somewhat enriched in the boundary layer melt, and phosphorus, with its low diffusion coefficient similar to other high-field
strength elements and rare earth elements, was significantly enriched. Thus, the concentrations of sulfur and phosphorus in
melt inclusions may over-estimate their values in the bulk melt, and other elements with similar diffusion coefficients may
also be enriched in melt inclusions relative to the bulk melt.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | Melt inclusions Phosphorus diffusion Crystal growth Diffusive Fractionation Volatiles in melts |
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