SNF Professorships: Geochemistry / Laser ablation ICP-MS (completed in 2011)

Geochemistry: Fluids in subduction zones

This project, funded by the Swiss National Science Foundation to Thomas Pettke (SNF Förderungs­professur, 2005-2011) aims at constraining the chemical composition of the mobile phases in subduction zones. Constraints on properties of these fluids such as their chemical composition, how they move, how much mass flux they effect and over what scales in distance and time they operate have remained controversial to date.

Nature provides us direct samples of such fluids occurring at depths of up to some 200 km, preserved as inclusions of ~10 µm diameter in high-pressure minerals. Such inclusions occur globally, are not rare and testify to the presence of a free fluid phase in the rocks during at least some stages of the subduction cycle. Laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS) is the only suitable technique to date, allowing for the in-situ quantitative analysis of the total dissolved solid load in such tiny, heterogeneous inclusions. The direct analysis of individual fluid inclusions in high-P minerals will constrain the chemical characteristics of such fluids at great depth, for possibly more than 20 major to trace elements, from lithium to uranium - this forms the core of the project. The chemistry of minerals coexisting with this fluid will also be characterized in detail.

These data will constrain the fluid-mediated chemical cycling in subduction zones and should allow a better understanding of its effect on the global geochemical cycle of elements in general, and the formation of arc magmas in particular. The direct analysis of fluids coexisting with minerals at high pressure shall constrain the mobility of elements in the subducting oceanic slab. Focus is currently on the discontinuous devolatilisation reactions in serpentinites sampled in the field and examples of oceanic lithosphere in general, with an emphasis on fluid source tracing and migration by radiogenic isotopes. Together with the comprehensive data set on ocean floor serpentinites from various geotectonic settings, we aim at a coherent geochemical picture of the chemical effects of serpentinite subduction.