OSCILLATING WATER CONCENTRATIONS IN CLINOPYROXENE PHENOCRYSTS IN AN ANKARAMITE LAVA FROM THE DECCAN TRAPS: IMPLICATIONS FOR CRYSTAL GROWTH AND RESIDENCE TIME

Water concentrations were measured and mapped using FTIR spectroscopy in clinopyroxene phenocrysts in an ankaramite lava flow from Mumbai, in the western part of the Deccan flood basalt province. Chatterjee et al. (2014) documented an origin of the ankaramite involving: 1) intrusion of high-temperature primitive basaltic melt into a cumulate pile formed by earlier crystallization of a lower temperature evolved basaltic melt, 2) dissolution of orthopyroxene, and 3) crystallization of thick overgrowth rims on the existing large crystals of olivine and clinopyroxene in a large, middle to upper crustal (<23 km) magma chamber. Cpx crystals are euhedral and have concentric bands (100 to 200 microns thick) of fine (10-20 micron diameter) melt inclusions. Water concentrations in cpx range from 125 to 435 ppm (excluding water in melt inclusions). Cpx bands that host melt inclusions have higher concentrations of water than inclusion-free bands. Application of the partition coefficients determined by Aubaud et al. (2004) for H between nominally anhydrous minerals and basaltic melt indicates water concentrations for the parent hybrid magma between 0.5 to 1.89, based on water in cpx. Influx of water-richer magma may have produced bands of melt inclusion-rich and relatively water-rich cpx. Alternatively, oscillatory zoning of water-richer(melt inclusion-hosting) cpx and water-poorer (melt inclusion-free) cpx may be the product of a single magma batch in which water dissolved in the melt diffused to the growing crystal face rapidly, leaving a water-depleted melt zone behind. Water-rich melt may have enhanced the growth rate of cpx, resulting in capture of melt inclusions. With movement of the crystal/melt boundary zone out into water-depleted melt, the next zone of cpx growth would be water-poor. The single magma batch alternative is similar to the mechanism for development of oscillatory zoning in crystal composition described by Wang and Merino (1993). Their calculations of rates of diffusion of components to the boundary layer of a growing crystal are applicable to modeling the oscillation in water concentration and melt inclusion abundance in cpx in the Deccan ankaramite, and possibly imply a significant residence time for the magma in a middle- to shallow-crustal magma chamber prior to eruption.