Recycling of Noble Gases in Ring-bearing Silicates

Brown University, Providence RI

Investigators

Abstract

Volatiles such as water (H2O) and carbon dioxide (CO2) are well-known to be key to the climate and atmosphere of the Earth, and the life that depends on them. Less well known is that these volatiles are also abundant in the Earth's interior. Indeed, there is probably as much water inside the Earth as on the surface. Thus the transfer of volatiles in and out of the Earth exerts a fundamental control on both the climate and the large-scale movement of the Earth's interior. However, it is difficult to geochemically trace the major volatiles (H2O, CO2, S, Cl and F) as they have few isotopes, which are key geochemical tracing tools. The noble gases (He, Ne, Ar, Kr and Xe) are also volatile. Though low in concentration, they are isotopically rich and so are ideal for tracing the movement of volatiles. However, there is little data on how the noble gases behave at high pressures inside the Earth, greatly restricting their use as tracers. Building on our previous high-pressure noble gas experiments, this study will measure the solubility of noble gases in a range of minerals. This will allow better estimates of volatile fluxes into and out of the Earth, as well as better tracing of the geochemical evolution of the Earth's interior. The fundamental issue that will be addressed by the project is how noble gases are incorporated into subducting lithosphere (slabs). Subduction is the main mechanism for bringing volatiles into the mantle. Most models have assumed that noble gases, having no charge, would not bind into minerals, and so slabs would not contain significant noble gas contents. In a previous study, my group demonstrated that He, Ne and Ar are incorporated into amphibole and that they are bound into Si-O ring structures in the crystal lattice. Such ring structures are present in a variety of minerals in subducting slabs. The proposed research will use high-pressure experiments combined with laser-ablation noble gas mass-spectrometry to determine noble gas solubilities in ring-structured minerals (e.g. serpentine, chlorite and mica). The data will be used to quantify both the amount of noble gases incorporated into slabs as well as how the noble gases are fractionated from each other. The data will be used to construct models aimed at matching the observed noble gas concentration and isotopic ratios in the mantle, which will constrain rates of cycling of noble gases into the mantle. As the noble gases should trace the movements of H2O and CO2, the noble gas flux model will constrain H2O and CO2 cycling as well. This work fits well with a number of recent geochemical observations that indicate recycling of noble gases is more important than previously thought. It will provide the first measurements of noble gas solubilities in a range of minerals and so will allow, for the first time, quantitative estimates of noble gas recycling rates.