Jc. Alt, A SULFUR ISOTOPIC PROFILE THROUGH THE TROODOS OPHIOLITE, CYPRUS - PRIMARY COMPOSITION AND THE EFFECTS OF SEAWATER HYDROTHERMAL ALTERATION, Geochimica et cosmochimica acta, 58(7), 1994, pp. 1825-1840
The sulfide mineralogy, sulfur contents, and sulfur isotopic compositi
ons were determined for a section through the Troodos ophiolite, as re
presented by drillcore from Holes CY1A, CY2A, and CY4, plus selected o
utcrops, with the goals of understanding the geochemistry of S during
hydrothermal alteration of the crust and obtaining a mass balance for
sulfur in altered ophiolitic crust. Primary deltaS-34 values of the op
hiolite are 0-1parts per thousand, consistent with a MORB- or BAB-like
sulfur source. The volcanic rocks have low sulfur contents (mean = 40
ppm) and generally negative deltaS-34 values (to -26.1parts per thous
and). These are the results of sulfur loss through oxidation by seawat
er at low temperatures (<100-degrees-C) and isotopic fractionation dur
ing partial oxidation of igneous sulfides. The sheeted dike complex is
uniformly enriched in S-34 (deltaS-34 = 5.4parts per thousand) as the
result of mixing of reduced Cretaceous seawater sulfate (17parts per
thousand) with primary crustal sulfide. Sulfate was reduced through ox
idation of ferrous iron in the dikes and conversion of igneous pyrrhot
ite to secondary pyrite. Decreasing water/rock ratios with depth in th
e plutonic section led to a general decrease in deltaS-34 values downw
ard to igneous values in the lower gabbros. Seawater effects are prese
nt locally to the base of the section, however. Cooling of diffuse upw
elling hydrothermal fluids (T > 250-degrees-C) produced S and metal en
richments in the upper dikes (1.26 wt% S), whereas sulfur was lost fro
m the lower dikes and upper plutonic rocks (which contain 200 ppm and
10-1640 ppm S, respectively) through higher temperature (>350-degrees-
C) alteration reactions. Epidosites at the dikes-gabbro transition con
tain 30 ppm S with deltaS-34 = 5.2-6.5parts per thousand. These rocks
lost sulfur and metals during reaction with S-34-enriched deep hydroth
ermal fluids at high temperatures (350-400-degrees-C) and high water/r
ock ratios in basal hydrothermal upflow zones. The ultimate effect of
alteration of ophiolitic crust is redistribution of igneous sulfur wit
hin the crust and exchange of crustal sulfur for seawater sulfur, with
little net change in the sulfur content of the crust overall. The bul
k altered Troodos crust is enriched in S-34, having deltaS-34 = 3.6par
ts per thousand. Fluxes of sulfur between seawater and ophiolitic crus
t are an order of magnitude smaller than modern riverine input to and
sedimentary sulfide output from the oceans. The amount of seawater sul
fur that reacted with Troodos ophiolite crust is approximately twice t
hat for a composite section of oceanic crust, consistent with the gene
rally greater intensity of alteration of ophiolitic crust compared to
modern oceanic crust.