In situ element quantification in the hydrothermal diamond anvil cell using synchrotron X-ray fluorescence with applications toward subduction zone processes.

摘要

Yttrium is used in geochemical investigations of arc volcanics and metamorphic geothermometers. The ability to use Y as a geochemical tool is predicated on an understanding of the mobility of Y during fluid-saturated conditions attending metamorphic and igneous processes. The goal of this work was to use the hydrothermal diamond anvil cell (HDAC) and synchrotron radiation X-ray fluorescence to quantify, in situ, the concentration of Y in aqueous fluids at 2-5 GPa and 650--800°C; conditions likely at the oceanic lithosphere - mantle wedge interface in subduction zones. Previous studies have used modified diamond anvils which limits their maximum pressure to ∼2 GPa. I used unmodified diamond anvils to extend this range to greater pressure, hence simulating greater depth (about 80-140 km), where significant dehydration of the subducting oceanic lithosphere is hypothesized to occur. With the detectors oriented 10° relative to forward scattering geometry, Rayleigh scattering of the diamond anvils was sufficiently reduced to observe Y fluorescence within the HDAC. However, I observed a non-linear relationship between concentration and fluorescence. The new technique offers great promise, and future work to increase sensitivity, perhaps by increasing counting times, is warranted.