The role of hydrous minerals in determining trace element and isotope geochemistry at subduction zones

Hydrous minerals play a significant role in determining subduction zone fluid compositions. The importance of hydrous minerals with respect to fluid compositions is due to (a) the compatibility of specific trace elements in these phases, and (b) the stability of hydrous phases over specific regions of pressure-temperature space. Understanding how elements are distributed between fluids, which rise to surface and contribute to crust formation, and residual slab minerals, which are recycled into the deep mantle, improves our understanding of the differentiation of Earth throughout its history. This work presents the results of a series of mineral-fluid partitioning experiments designed to determine the effects of hydrous phase stability on fluid and slab compositions in subduction zones.; Subduction zone magmatism is the result of aqueous fluid addition to the mantle. The fluids are generated by pressure-induced mineral breakdown reactions in the descending slab, and the composition of the fluids is determined by the residual mineralogy of the slab. New zoisite- and mica-fluid partition coefficients are combined with previously determined garnet-, clinopyroxene-, and rutile-fluid partition coefficients according to the relative abundances of these phases in the slab in order to determine bulk eclogite-fluid partition coefficients for warm and cold subduction zones. The mineralogical differences between warm and cold slabs, most importantly the absence of zoisite in the warm slab, result in significantly different eclogite-fluid trace element partition coefficients. Calculated trace element compositions in fluids derived from warm and cold slabs are likewise significantly different.; Fluids derived from the slab subsequently react with the overlying mantle wedge to form new hydrous minerals such as phlogopite. Mineral-fluid partition coefficients for phlogopite are determined at pressures and temperatures relevant to the mantle wedge. Many fluid-soluble trace elements, particularly Rb and Ba, are found to be compatible in phlogopite. Trace element partition coefficients for phlogopite are used to model the partitioning behavior of Ra, a radioactive uranium-series nuclide for which partition coefficients have not been determined directly. A model is developed to describe the development of steady-state 226Ra/230Th disequilibrium in hydrous minerals and fluids in the mantle due to diffusion of Ra between phases.