Structure and Thermodynamics of Subduction Zone Fluids from Spectroscopic Studies

Abstract

Fluids1 in subduction zones have been a subject of growing interest for at least the last 20 years. Subduction zones are the major places on Earth for mass transfer, element recycling and chemical differentiation through magmatic processes in which fluids released from the slab play a central role. Hydrous subduction zone magmas are essential in the formation of numerous types of hydrothermal ore deposits and are the driving force behind magmatic-hydrothermal and geothermal systems in volcanic arcs. It is widely recognized that the characteristic geochemical signature of arc magmas results from the metasomatism of the magma source by a slab-derived flux (McCulloch and Gamble 1991; Elliott 2003). Large uncertainties remain however concerning the chemical composition and nature of the slab flux (aqueous fluid, hydrous melt or supercritical fluid?) and, ultimately, about the mechanisms of mass transfer from the slab to the mantle wedge. Correlating inputs and outputs in subduction zones to quantitatively estimate mass transfer and element recycling relies on thermodynamic modeling of fluid-rock interactions at pressure-temperature above 5 GPa and 1500 K (Manning 2004a). Key constraints on these models are the phase relations, mineral solubility, the behavior of trace elements (partition coefficients) and the physico-chemical and thermodynamic properties of high-pressure fluids at relevant conditions. Unfortunately, quantitative information on fluid properties at relevant P - T conditions are in most cases not available due to experimental difficulties associated with the non-quenchable character of fluid phases. New experimental designs, especially those using laser or synchrotron-based spectroscopic techniques combined with high temperature diamond anvil cells (Basset et al. 1993), are now opening the possibility for monitoring in situ the chemical composition of fluid phases and for the determination of structural parameters and thermodynamic quantities. In this chapter, I review recent progress in determining the thermodynamic properties and molecular-scale structure of …