Paleobathymetry of Submarine Lavas in the Samail and Troodos Ophiolites: Insights From Volatiles in Glasses and Implications for Hydrothermal Systems

Hydrostatic pressure exerted by the ocean water column fundamentally influences magmatic and hydrothermal processes in submarine volcanic settings and is therefore an important parameter to know when investigating such processes. Currently, there are few reliable methods for reconstructing past ocean depths for ancient volcanic terranes. Here, we develop and test an empirically calibrated statistical approach for determining paleodepths of eruption from the concentrations of H2O and CO2 dissolved in volcanic glasses, utilizing the well‐defined pressure‐dependent solubility of these volatiles in silicate melts. By comparing newly determined and published glass compositions from the Samail and Troodos ophiolites with sedimentary and fluid inclusion evidence, we propose that the Samail lavas erupted at ocean depths of ∼3.4 km, and the Troodos lavas at ∼4.1 km. These depths are 1–2 km deeper than those assumed in most previous studies of hydrothermal activity in the two ophiolites. These high depths imply high hydrostatic pressures within the underlying oceanic crust. Such pressures may have allowed convecting hydrothermal fluids to attain significantly higher temperatures (e.g., >450°C) than in typical modern ocean ridge hydrothermal systems during metal leaching in the crust and metal precipitation in seafloor sulfide deposits. Plain Language Summary The pressure of seawater exerts a fundamental influence on volcanism and related processes in Earth's oceanic crust. This article presents an improved method for estimating the depth of seawater above ancient oceanic volcanic rocks as they were forming. Determining these depths allows us to better define the pressure under which this ancient crust formed and hence more accurately model it and compare it to modern examples. Key Points The H2O–CO2 contents of volcanic glasses in the ophiolites are used to estimate ocean depths during lava extrusion on the seafloor Ocean depth during volcanism is constrained to between 3.2 and 4.0 km for the Samail ophiolite and 3.6 and 4.9 km for the Troodos ophiolite High subseafloor fluid pressures may have allowed convecting hydrothermal fluids to reach temperatures above 450°