Sulfur cycling through subduction zones: identifying sources and constraining the output
DOEI Funded Project: 2008
Abstract
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.