Sulfur cycling through subduction zones: identifying sources and constraining the output


DOEI Funded Project: 2008


The global sulfur cycle, closely linked to carbon and oxygen cycles, has important chemical implications for the geo-, atmo-, hydro- and biosphere.  Sulfur, along with other volatiles including water and CO2, is removed from the oceans via alteration of the oceanic crust and sedimentation, and is eventually recycled into the mantle at subduction zones.  The volatile rich subducting plate undergoes metamorphic dehydration reactions as it subducts to progressively higher temperatures and pressures.  These reactions release sulfur-bearing fluids to the overlying mantle wedge and ultimately lead to arc and back-arc volcanism.  The mechanism and extent of sulfur cycling through subduction systems remains largely elusive.  The reason for this has been largely due to analytical limitations, but also due to the lack of systematic studies targeting fractionation processes and identifying source values.

The aim of this project is to apply novel techniques to fingerprint sources and processes that influence sulfur during its subduction cycle.  In order to achieve this, we will measure sulfur abundance and isotope ratios (32S/34S) of volcanic gases, melt inclusions and submarine volcanic glasses from two contrasting subduction systems: the Central American arc and the Lau Basin.  In particular, we will collect new gas and lava samples from two volcanoes in Costa Rica, and we will analyze submarine glasses from the Lau Basin, samples which have been characterized for major and trace element geochemistry through Nicole Keller’s PhD thesis work.  Comparison between volcanic gasses and glass inclusions is a new approach, addressing the question of whether sulfur isotopes in near-surface geothermal systems are sensitive to degassing or not.  This work is essential for making progress towards understanding how sulfur behaves in volcanic systems.  By combining S analyses with other volatiles such as C and He, we can not only identify recycled slab vs.  mantle components, but also make absolute flux estimates for individual volcanoes (based on S/3He ratios and assumed He fluxes).

This proposal will provide analytical support for Nicole Keller, a newly appointed WHOI postdoctoral fellow and will help Alison Shaw develop analytical protocols for measuring the sulfur composition of volcanic gases using her newly built gas extraction system.  Keller will be involved in on-going developments for the analysis of sulfur isotopes in glasses and melt inclusions using WHOI’s 1280 ion probe through collaboration with Nobu Shimizu.  This analytical development opens a new field of study at WHOI; we are not aware of any other laboratories currently using SIMS techniques for S isotope measurements of glass samples.  The combination of melt inclusion data with gas analyses for the same volcano will allow us to evaluate processes which may modify intrinsic values during transport through the volcanic system.  These results will ultimately allow us to establish better quantitative mass balance models for sulfur through subduction systems.