ISOTOPIC CHARACTERIZATION OF SULFUR CYCLING IN BASALT-HOSTED ALKALINE LAKES, SOUTH-CENTRAL OREGON
The isotopic composition of sulfur (d34S) in CFB reflects Earths mantle and generally is in the range of -0.5 to 1.0 per mil. Basalt samples from Warner Valley contain 200-1000 ppm sulfur. Sequential chemical extraction was used to isolate and purify sulfur species for determination of d34S. Disulfide minerals were volatilized with Cr reduction, yielding d34S values in the range of 0.7 to 1.8 per mil relative to V-CDT for fresh basalt. Sulfate with a d34S of 0.7 per mil was recovered from one fresh basalt sample, reflecting oxidation and retention of S with little isotope fractionation. In contrast, sulfate in weathered basalts ranges from 9.3 to 5.9 per mil.
Dissolved sulfate in cool springs has d34S values ranging from 7.5 to 10.2 per mil. Dissolved sulfate in hydrothermal springs has d34S values ranging from 8.6 to 9.4 per mil. Dissolved sulfate in lakes and evaporitic ponds has d34S values ranging from 9.4 and 14.8 per mil. Values of d34S for sulfate in cool springs and hydrothermal springs are indistinguishable from sulfate in weathered basalts. A marked positive shift in d34S values along the flow path for the Warner lakes probably reflects bacterial sulfate reduction and associated depletion of 32S in residual sulfate. Hydrothermal and biological processes pervasively overprint original igneous signatures in these basalt-hosted alkaline lakes.