Sulfur cycling through subduction zones: identifying
sources and constraining the output
|A view of Poas volcano from the crater's rim showing the fumaroles on the edge of the crater lake. These gases were sampled using various different techniques.
|Nicole and I on our hike down to the fumaroles with a view of Poas' resurgent dome in the background.
|Nicole is preparing to sample the gases
Dr. Nicole Keller (WHOI postdoctoral scholar), Dr. Shuhei Ono (MIT), Dr. Timothy Grove (MIT), Carlos Ramirez (UCR)
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).