THE ROLE OF WATER IN THE GROWTH OF SPHERULITES IN RHYOLITIC LAVA FLOWS

Spherulites are common features in rhyolitic lava flows, and typically consist of radiating skeletal crystals of feldspar +/- quartz that nucleated on a crystal or a vapor bubble. Variations in water concentration in and around spherulites provide information about the behavior of incompatible compounds at the melt/crystal interface. Spherulites from the Hell's Gate lava flow (~24 Ma) of the Atascosa Mountains of southern Arizona consist of two or more generations of skeletal radiating crystals, with each successive generation overgrowing the previous generation. Maps of water concentration across radiating needle-shaped crystals indicate that water concentration increases along the length of the crystals, from approximately 0.6 wt.% to approximately 1.1 wt.%, and that each successive generation of overgrowing crystals has a higher range of water concentration than the previous generation. This variation in water concentration along crystals might record the rejection of water by the anhydrous minerals as they grew, and the partitioning of water into the boundary layer melt into which the crystals continued to grow.

In contrast to the spherulites from the Hell's Gate lava flow, spherulites from the Bartolo Mountain lava flow (~23 Ma), also from the Atascosa Mountains, in some instances host an interior concentric zone of glass. Skeletal crystals grew radially both prior to, and after the growth of the glass rind in these spherulites. Water concentration in the interior glass rinds is higher (to 2 wt.%) than it is in the preceding crystals (0.9 to 1.3 wt.%), indicating that water might have reached concentration levels in the boundary layer that drove feldspar from the liquidus and caused the melt to incorporate water into an amorphous solid rather than a crystalline one. The Bartolo Mountain lava flow is flow banded with white thicker bands hosting larger, more water-rich spherulites and glass, and the orange thinner bands hosting smaller, less water-rich spherulites and glass. In all cases the glass is more hydrous than the spherulites it hosts. These textures suggest that flow banding might reflect primary variations in water concentration in the melt, possibly associated with stretching of vesicles as the magma flowed.