EVIDENCE FOR RETROGRADE HYDROTHERMAL REACTIONS IN NEAR-SURFACE SEDIMENTS OF GUAYMAS BASIN, GULF-OF-CALIFORNIA

The Guaymas Basin, Gulf of California, is a sediment-covered portion o f the East Pacific Rise, where hydrothermal fluids exit to the sea flo or via both direct emanation from spires and chimneys, as well as by d iffuse circulation through organic-matter-rich sediments. Thermal alte ration of organic matter generates hydrothermal gas and petroleum whic h have wide ranges of compositions and maturities. Diffuse fluid disch arge reaches the sediment/seafloor interface via channelized pathways that change with time. Mineralogical and geochemical analyses of sedim ents and pore fluids from an eight meter long piston core, PC 6, show superimposed geochemical signals indicating that presently active reac tions are modifying previously formed authigenic materials. A slump de posit (140-250 cm) shows the presence of hydrothermal barite and a sub stantially changed chemical composition. Analysis of the extractable o rganic matter indicates that a significant amount of hydrothermally-ge nerated oil has been transported laterally through the 140-250 cm dept h interval, overwhelming the indigenous bituminous matter. Below 782 c m depth sandy layers have undergone substantial hydrothermal alteratio n as is evident from the presence of gypsum, dolomite, heulandite, wel l crystallized smectite, as well as significant changes in the bulk co mposition 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 pota ssium. Retrograde reactions (e.g., dissolution of gypsum). offsetting prograde hydrothermal reactions in PC 6 sediments, illustrate the ephe meral nature of fluid flow pathways in near surface Guaymas Basin sedi ments. Copyright (C) 1996 Elsevier Science Ltd