Experimental investigation of deep fluids of the lower crust and subduction zones

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Among the greatest geologic hazards we face as a society are the large earthquakes and volcanic eruptions associated with subduction zones, where two tectonic plates collide. Both hazard types are strongly influenced by the presence and movement of water-rich fluids at great depth. In the case of subduction-zone earthquakes, such fluids are liberated as rocks descend into the mantle, and they substantially modify the rates and style of the deformation that give rise to earthquakes. In the case of volcanic eruptions, the same fluids operate in the overriding plate to trigger melting, and ultimately to power massive, explosive eruptions at the surface. Yet the chemical properties of these fluids and the melts they produce remains very poorly understood, chiefly because the conditions at which they must be studied are quite challenging to generate in the laboratory. This project will exploit methods developed at UCLA for characterizing these deep fluids and melts, and how they dissolve and transport matter. The data and new models will provide unprecedented insights into the chemistry and properties of these deep fluids that figure so prominently in the hazards posed by subduction systems. Two graduate students will be mentored as part of this project, and the experimental laboratory at UCLA will host hands-on demonstrations during large UCLA science outreach events. All results of this work will be available to the rest of the geoscience community via incorporation in the Library for Experimental Phase Relations. The data and models produced will serve an international community of scientists investigating the role of fluids in high pressure geologic and geophysical processes. The work will involve three experimental investigations aimed at improving understanding of deep fluid-rock interaction. The first will be built around the system NaAlSi3O8-H2O, with and without NaCl. This chemical system is a useful model for silicate-rich fluids at depth. Experiments will focus on phase relations and thermodynamic data, allowing characterization of the stability of these fluids, their coexisting minerals, and what triggers their unmixing into separate fluids. The aim of the second project is to provide experimental constraints on the individual dissolved species found in deep fluids. We will use a new experimental design to investigate a wide compositional range of mineral saturation surfaces. The project will focus on dissolved Mg and Ca, which are most relevant to fluid-mantle interactions in subduction zones. The third project will combine experiments with new modeling to investigate aqueous silica in salty deep fluids. A modified model of quartz solubility based the densities of H2O-NaCl solutions will be extended to additional salt-H2O systems. New experiments on quartz solubility will be conducted along H2O-salt binaries, and in the H2O-NaCl-CO2 ternary. The data offers robust constraints on solution density, and can be used to constrain equations of state of salt-bearing deep fluids ranging from simple binaries with H2O, to more complex multicomponent systems expected in the deep crust and upper mantle. The end result of these investigations will be new data and models available to the broader geoscience community. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.