On the Nature and Consequences of Hydrothermal Circulation in the Middle Valley Sedimented Rift: Inferences from Geophysical and Geochemical Observations, Leg 139

Abstract

the goal of characterizing the nature and consequences of hydrothermal circulation and venting in this sedimented rift setting. Four distinct parts of the hydrothermal system were examined, with downhole measurements and shipboard studies reported in the Leg 139 Initial Reports volume and shorebased studies reported in the papers in this volume. These results, particularly those of the geophysical studies, are synthesized herein. Site 855 is located along a rift-parallel normal fault scarp that bounds the valley. Pore-fluid compositions confirmed that seawater flows into the upper part of the igneous crust along the fault at this site, and heat-flow measurements provided constraints for an estimate of the rate of flow, roughly 0.5 m/yr. Site 857 is situated where the crust is covered by a 470-m-thick sequence of turbidite sediments. The concept of a permeable "hydrothermal basement" reservoir was confirmed at this site; measured and inferred permeabilities demonstrated that the sediment section is relatively impermeable (nominally 10~16 to 1CT17 m2) and serves to inhibit vertical advective heat transport, whereas the section below the sediment seal is sufficiently permeable (bulk permeability possibly as high as 10~12 m2) to support vigorous pore-fluid convection. This permeable hydrothermal basement reservoir comprises a highly altered sequence of interbedded basaltic sills and turbidite sediments. The nearly complete loss of potassium from the sediments, together with the current composition of pore fluids, suggests a minimum wateπrock mass ratio in the reservoir of about 50. At Site 858, a suite of holes was drilled in and immediately adjacent to a hydrothermal vent field, where fluids ascend through the locally attenuated section of sediment above a small basement edifice. Temperature measurements in the holes located adjacent to the field indicate that upflow is confined to the zone directly beneath the vent field. Rates of fluid flow through the sediment section less than 100 m from the vent field appear to be thermally insignificant as a result of the low permeabilities characteristic of the sediment surrounding the upflow zone. As within hydrothermal basement at Site 857, sediment alteration is accompanied by large physical changes. Seismic velocities increase and porosities decrease systematically with progressive degrees of alteration (well defined by observations at all sites), commonly reaching values of 3000-4000 m/s and 20%-30%, respectively, and over 5000 m/s and less than 10% in extreme cases. The strength gained by the sediment through extreme hydrothermal alteration allows fracture permeability to be supported, and hence fluid flow to be promoted. Efficient hydrologic interconnection between the vent field at Site 858 and hydrothermal basement at Site 857, roughly 2 km away, is suggested by observations and estimates of temperatures, water compositions, alteration assemblages, and formation pressures at both sites. The pressure gradient responsible for the fluid flow beneath and through the vent field is inferred to arise from the simple chimney effect of the hot, low-density fluids in the permeable basement edifice and overlying plug of altered sediment which are hydrologically isolated from the surrounding low-permeability sediments. Despite the probable long history of hydrothermal circulation and discharge at Sites 857 and 858, temperatures appear never to have exceeded significantly the present-day temperature of about 280°C. This is suggested by the alteration assemblages at both sites, by fluid inclusion temperatures determined for minerals precipitated in the upflow zone, and by the lack of significant accumulations of high-temperature (>350°C) hydrothermal mineral deposits in or beneath the vent field. It appears that spatial and temporal variations in heat supply have not resulted in large temperature variations, possibly because of the buffering effects of the large and efficiently mixed upper-crustal reservoir, and that on average the supply of heat has not been sufficient to raise temperatures in the reservoir to above about 300°C. This contrasts with the situation at Site 856, where holes were drilled in a transect crossing a large, relic massive sulfide deposit. A suspected genetic relationship between the deposit and an adjacent laccolithic intrusion was invalidated by stratigraphic and structural observations. Simple mass- and heat-balance calculations, based on estimates of the composition and size (>109 kg) of the deposit and of the composition and temperature of the fluid (c. 400°C), do not preclude formation of the deposit by fluids from a local reaction zone of hot rock or magma, however. A volume of less than 1 km of such hot rock or magma would be sufficient to supply the heat; a remote or regional source is not required. Although a reaction zone was not penetrated at this site, the composition of the massive sulfide provides information about the source of hydrothermal fluids. A mixture of fluid that reacted with purely igneous rock at high temperature (c. 400°C), and fluid that reacted with sediments at a lower temperature is suggested. This mixture may have resulted from the local high-temperature reaction zone having been fed by the regional hydrothermal basement reservoir characterized by Site 857.