Jc. Alt et Wc. Shanks, SULFUR IN SERPENTINIZED OCEANIC PERIDOTITES - SERPENTINIZATION PROCESSES AND MICROBIAL SULFATE REDUCTION, J GEO R-SOL, 103(B5), 1998, pp. 9917-9929
The mineralogy, contents, and isotopic compositions of sulfur in ocean
ic serpentinites reflect variations in temperatures and fluid fluxes.
Serpentinization of <1 Ma peridotites at Hess Deep occurred at high te
mperatures (200 degrees-400 degrees C) and low water/rock ratios. Oxid
ation of ferrous iron to magnetite maintained low fO(2) and produced a
reduced, low-sulfur assemblage including NiFe alloy. Small amounts of
sulfate reduction by thermophilic microbes occurred as the system coo
led, producing low-delta(34)S sulfide (1.5 parts per thousand to -23.7
parts per thousand). In contrast, serpentinization of Iberian Margin
peridotites occurred at low temperatures (similar to 20 degrees-200 de
grees C) and high water/rock ratios. Complete serpentinization and con
sumption of ferrous iron allowed evolution to higher fO(2). Microbial
reduction of seawater sulfate resulted in addition of low-delta(34)S s
ulfide (-15 to -43 parts per thousand) and formation of higher-sulfur
assemblages that include valleriite and pyrite. The high SO4/total S r
atio of Hess Deep serpentinites (0.89) results in an increase of total
sulfur and high delta(34)S Of total sulfur (mean approximate to 8 par
ts per thousand). In contrast, Iberian Margin serpentinites gained lar
ge amounts of S-34-poor sulfide (mean total S = 3800 ppm), and the hig
h sulfide/total S ratio (0.61) results in a net decrease in delta(34)S
of total sulfur (mean = -5 parts per thousand). Thus serpentinization
is a net sink for seawater sulfur, but the amount fixed and its isoto
pic composition vary significantly. Serpentinization may result in upt
ake of 0.4-14 x 10(12) g S yr(-1) from the oceans, comparable to isoto
pic exchange in mafic rocks of seafloor hydrothermal systems and appro
aching global fluxes of riverine sulfate input and sedimentary sulfide
output.