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.

Carbon geochemistry:
Carbon in these systems is typically detected as inorganic (e.g. carbonate carbon) or organic carbon. Organic carbon has also been referred to as reduced carbon or non-carbonate carbon. Here we refer to it as total organic carbon (TOC) even though the presence of traces of graphite and other reduced carbon species cannot entirely be excluded. Total carbon (TC) and total inorganic carbon (TIC) contents, d¹³C of TC, d¹³C of TOC and d¹³C and d¹⁸O of TIC were analyzed on the basement rocks, while only d¹³C and d¹⁸O of TIC were determined for the carbonate deposits and precipitates.

TC contents, d¹³C_TC and d¹³C_TOC were determined on a Vario ISOTOPE elemental analyzer (EA) coupled to an Isoprime 100 isotope ratio mass spectrometer (IRMS). For analyses of d¹³C_TOC, bulk rock samples were reacted for three days with 3N HCl to remove all acid soluble carbon. They were then rinsed with H₂O, dried at 40 degrees C in the oven and homogenized in the agate mortar. TC and TOC isotope values are reported in the standard delta notation relative to the Vienna-Pee Dee Belemnite (V-PDB) standard and calibrated to this scale using international [IAEA-CH-6 (sucrose; d¹³C = -10.449‰) and IAEA-CH-7 (polyethylene; d¹³C = -32.151‰)] and commercial standards [Elemental Microanalysis wheat flour; d¹³C = -27.21‰]. TOC was calculated from the difference between TC and TIC and errors are within 10% of the TOC contents. %TIC was calculated as TIC/TC*100. Reproducibility of d¹³C_TC and d¹³C_TOC is better than 0.1‰.

The d¹³C_TIC and d¹⁸O_TIC and TIC contents were analyzed on a MultiFlowGeo headspace sampler attached to an Isoprime 100 IRMS. Samples were prepared in septum vials, flushed with helium and acidified with phosphoric acid. Samples were then reacted for at least 3 hours at room temperature if no magnesite was present (as determined by XRD analysis). Magnesite-bearing samples were reacted for 72 hours at 90 degrees C. Carbon and oxygen isotope values are reported in the standard delta-notation relative to the Vienna-Pee Dee Belemnite (V-PDB) standard and calibrated to this scale using the international standards IAEA-CO-1 (marble; d¹³C = +2.492‰, d¹⁸O = -2.4‰), IAEA-CO-9 (BaCO₃; d¹³C = -47.321‰, d¹⁸O = -15.6‰) and NBS18 (calcite, d¹³C = -5.014‰, d¹⁸O = -23.2‰). Reproducibility for the analysis of the samples was better than +/- 0.07‰ for d¹³C and better than +/- 0.3‰ for d¹⁸O. Reproducibility of TIC contents is typically better than 5%.