Evidence for retrograde hydrothermal reactions in near surface sediments of Guaymas Basin, Gulf of California

Abstract

The Guaymas Basin, Gulf of California, is a sediment-covered portion of the East Pacific Rise, where hydrothermal fluids exit to the sea floor via both direct emanation from spires and chimneys, as well as by diffuse circulation through organic-matter-rich sediments. Thermal alteration of organic matter generates hydrothermal gas and petroleum which have wide ranges of compositions and maturities. Diffuse fluid discharge reaches the sediment/seafloor interface via channelized pathways that change with time. Mineralogical and geochemical analyses of sediments and pore fluids from an eight meter long piston core, PC 6, show superimposed geochemical signals indicating that presently active reactions are modifying previously formed authigenic materials. A slump deposit (140–250 cm) shows the presence of hydrothermal barite and a substantially changed chemical composition. Analysis of the extractable organic matter indicates that a significant amount of hydrothermally-generated oil has been transported laterally through the 140–250 cm depth interval, overwhelming the indigenous bituminous matter. Below 782 cm depth sandy layers have undergone substantial hydrothermal alteration as is evident from the presence of gypsum, dolomite, heulandite, well crystallized smectite, as well as significant changes in the bulk composition of the solid phases. The dolomite layers in the bottom part of the core act as a barrier towards upward fluid flow, allowing only diffusive communication with fluids of a hydrothermal origin. This is evident from the concentration profiles of dissolved chloride and potassium. Retrograde reactions (e.g., dissolution of gypsum), offsetting prograde hydrothermal reactions in PC 6 sediments, illustrate the ephemeral nature of fluid flow pathways in near surface Guaymas Basin sediments.