In January 2014, January 2015, and February 2016, fluids were obtained from deep wells (up to ~475 m deep) previously drilled in peridotite and gabbro for the Oman Ministry of Regional Municipalities and Water Resources. Over the course of these three field seasons, 20 water samples, including particulates and dissolved gases, were collected from a total of 12 wells. Geographic coordinates, elevations, and depths of boreholes are reported in Table 1 ; all wells were drilled vertically (90 degrees). A map of sampling sites is provided in Figure 1 . This series of wells spans the crust/mantle transition of the Samail Ophiolite in the Tayin block (Figure 1 ). These wells access the water table, and they commonly intersect anoxic fluids that have extensively reacted with mafic to ultramafic rocks. We classified wells as belonging to one of three lithologies (peridotite, gabbro, or contact) based on field observations of surrounding geology (described in Table 1 ). Wells that were classified as “contact” sit in gabbro or peridotite, but are in close proximity (<1 km) to the surface boundary between crustal and mantle rocks. Large changes in rock permeability may occur at these boundaries, giving rise to the potential injection of hyperalkaline fluids into adjacent higher permeability gabbros (Dewandel et al., 2005 (link)). Faulting at the crust-mantle boundary has also been documented (Boudier and Coleman, 1981 (link); Nicolas et al., 2000 (link)), which may facilitate the mixing of gabbro- and peridotite-reacted fluids.
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