Santa Elena samples:
Sample preparation and mineralogy: Ultramafic basement rocks were sampled throughout the Santa Elena Ophiolite. Carbonate deposits associated with Mg-HCO₃ and Ca-OH springs in the eastern part of the ophiolite were sampled at various locations within riverbeds. All analytical measurements and sample preparation of the Santa Elena peridotites and carbonates were performed at the Department of Geosciences at Virginia Tech. The carbonate samples were prepared by drilling out individual depositional layers of carbonates to reveal heterogeneities within different layers or crushed with an agate mortar for bulk rock analysis. Carbonate precipitates from the rivers were first dried at 40 degrees C in the oven. Before sample analyses all samples were homogenized by hand with the agate mortar. The mineralogy of the carbonates was then determined by X-ray diffraction on a Rigaku MiniFlex XRD using the powder diffraction analysis package PDXL.

Bulk rock powders were prepared for all the ultramafic rocks. To remove contamination from weathering, the outermost 1-2 cm of the rock samples were cut away. The samples were then cleaned in an ultrasonic bath prior to powdering them with a shatter box using an alumina dish. These bulk rock powders were subsequently analyzed for their carbon and sulfur geochemistry.

Sulfur geochemistry:
We determined the isotopic composition and contents of the acid volatile sulfide (AVS or also referred to elsewhere as the monosulfide fraction), chromium reducible sulfide (CRS or also called disulfide fraction) and the sulfate. Sulfur extractions were performed following a modified version of the methods of Canfield et al. (1986). Acid volatile sulfide was first extracted by reacting 20-25g of bulk rock powder with 6N HCl in an inert N₂-atmosphere. The residual sample was then reacted with an acidified CrCl₂ solution to extract the chromium-reducible sulfide. In both cases the liberated H₂S was precipitated as ZnS in a zinc acetate solution and subsequently converted to Ag₂S through reaction with a 0.1M AgNO₃ solution. The sulfate fraction was recovered by reacting the solution from the AVS extraction with BaCl₂ to form BaSO₄. The amounts of AVS, CRS and sulfate were determined gravimetrically and were subsequently corrected based on the sulfur content of the precipitate as determined on the EA, since co-precipitation of other phases during the wet chemical extraction could not be completely prevented.

The isotopic composition of the AVS, CRS and sulfate were determined on a Vario ISOTOPE EA attached to an Isoprime 100 IRMS. To ensure complete combustion during EA-analyzes, vanadium pentoxide (V₂O₅) was added to the samples. Sulfur isotope values are reported in standard delta-notation relative to the Vienna-Canyon Diablo Troilite (V-CDT) standard. During measurements the international sulfide (Ag₂S) standards IAEA-S-1 (d³⁴S = -0.3‰), IAEA-S-2 (d³⁴S = +22.7‰) and IAEA-S-3 (d³⁴S = -32.3‰) and the sulfate (BaSO₄) standards IAEA-SO-5 (d³⁴S = +0.5‰), IAEA-SO-6 (d³⁴S = -34.1‰), and NBS127 (d³⁴S =+20.3‰) were used to place our samples on the V-CDT scale. Reproducibility is better than 0.2‰ for all sulfur analyses (samples and standards), and the relative precision of sulfur contents is within 3%.