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

Thomas M Belgrano,Felix Marxer,Peter M Tollan,Larryn W Diamond

doi:10.1029/2021jb021966

Abstract

AbstractHydrostatic 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.

Highlights

Studies of the Samail and Troodos ophiolites have greatly influenced our interpretation of the formation, hydrothermal alteration, and ore mineralization of oceanic crust (e.g., Gillis & Banerjee, 2000; Hannington et al, 1998; Pearce et al, 1984)
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
For submarine volcanic terranes that accumulated below storm wave base, the three most promising lines of paleobathymetric evidence are the sedimentary record (e.g., pelagic carbonates indicating depths shallower than the carbonate compensation depth (CCD); Robertson, 2004), fluid inclusion evidence, and the concentrations of volatiles dissolved in volcanic glasses, which are partly dependent on pressure during quenching (Roberge et al, 2005)

Summary

Introduction

Studies of the Samail and Troodos ophiolites have greatly influenced our interpretation of the formation, hydrothermal alteration, and ore mineralization of oceanic crust (e.g., Gillis & Banerjee, 2000; Hannington et al, 1998; Pearce et al, 1984) Pressure fundamentally influences these processes and is a necessary parameter to know for many of the models and tools we use to describe and investigate them (e.g., Jupp & Schultz, 2004; Monecke et al, 2014). We determine the dissolved H2O concentrations and maximum permissible CO2 concentrations of volcanic glasses from the Samail ophiolite, and interpret these in terms of paleodepths of eruption together with a similar, recently published data set from the Troodos ophiolite (Woelki et al, 2020) We compare these predictions to sedimentary and fluid inclusion evidence from both ophiolites, finding

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