3. The geochemistry of sulfur during serpentinization of
oceanic ultramafic rocks Slow-spreading mid-ocean ridges undergo
tectonic extension, resulting in large exposures of ultramafic rocks.
My past work has examined hydrothermal exchange of sulfur between ocean
crust and seawater, and this project fills a missing gap. Results on three
sample suites (Hess Deep, Iberian Margin, and MARK) reveal serpentinization
is a function of temperatures and fluid fluxes (Alt and Shanks, 1998a,b).
Where temperatures and fluid fluxes are high, inorganic sulfate reduction
leads to sulfide and 34S enrichment (Alt and Shanks, 1998b). At low
temperatures microbial sulfate reduction can be significant, utilizing
hydrogen and methane produced during serpentinization, and resulting in
addition of 34S-depleted sulfide to the rocks (Alt and Shanks, 1998a).
Serpentinization results in an estimated sink for seawater sulfur of 0.2-6.0
x 1012 g S y-1 (Alt and Shanks, 1998b). This is comparable to sulfur
exchange in hydrothermal systems in mafic ocean crust at mid-ocean ridges
and on ridge flanks (2.1-2.5 x 1012 g S y-1; Alt, 1995), making serpentinization
a significant part of the sulfur budget for ocean crust.