The carbon geochemistry of serpentinized peridotites, and mafic rocks recovered during ICDP Expedition 5057, Oman Drilling Project Phase II (November 2017 to February 2018) on the Wadi Tayin Massif, Samail Ophiolite, was examined to characterize carbon sources, speciation, and the fate of organic and inorganic carbon during natural large-scale hydration and carbonization. The measurements were performed at the ETH Zurich, Geological Institute from 2017 to 2020 using a FlashEA 1112 Elemental Analyzer (EA) interfaced via a Conflo IV to a Delta V Plus Isotope Ratio Mass Spectrometer for total carbon (TC) and total non-carbonate carbon (NCC) content and isotope composition, and a GasBench II connected to a Delta V Mass Spectrometer for the total inorganic carbon (TIC) isotope composition. Results are reported as replicates. Analyzed samples are aliquots of samples used for microbiological studies and were collected under sterile conditions (denoted as MBio samples).