Aqueous Aluminosilicate Polymers: Transport Agents in Crustal and Mantle Fluids?

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

The movement of fluids in the Earth's crust and mantle changes rock composition. Geologists see evidence for these changes on a variety of scales, ranging from mineral-filled microcracks to volcanoes such as Mount St. Helens. However, it is extremely difficult to assess how much flow is required to produce the observed changes, chiefly because we know little about how the rock-forming minerals dissolve in geofluids at high pressures and temperatures. This project will quantify the solubility of minerals in geofluids using novel experimental approaches. Key to the completion of the work is the recent recognition that aqueous silica is a dominant solute in geofluids, and that dissolved silica polymerizes significantly at high pressures and temperatures. Aluminum is readily incorporated in these structures, dramatically raising its solubility. Titanium exhibits similar behavior. Evidently, dissolution of these elements is aided by the availability of structural environments in the polymeric species that are more energetically favorable than those offered by pure H2O. These observations suggest the simple hypothesis that the aluminosilicate polymeric solutes may hold the key to understanding element mobility in deep metasomatic settings. The experiments to be carried out by the research team target three components of this hypothesis. The first will address the evolution of such structures, and their transport capacity, when they encounter new chemical environments by studying dissolution of mantle minerals (enstatite + forsterite) in Na2O-Al2O3-SiO2-H2O fluids. A second line of investigation will explore the solubility rare-earth elements in polymer-bearing fluid, using the minerals monazite and xenotime. Finally, the team will investigate the role played by polymeric solutes in Earth's volatile cycling by focusing on calcite. When complete, the experimental research will offer a test of a new mechanism for controlling element transfer in Earth's deep metasomatic environments.