Experiments on hydrothermal reactions between hemipelagic sediments and seawater

Laboratory experiments were conducted to examine the effects of carbonates, amorphous silica, organic matter, and basalt on the chemical composition of hydrothermal fluids and hydrothermally formed solids. Each of the reactive components was removed in turn from separate samples of unaltered homogenized Guaymas Basin sediment. In some experiments fresh East Pacific Rise basalt was added to bulk sediment. Reacting fluids included La Jolla seawater and simulated vent water similar in composition to EPR 21;Fluids reacted at low water/rock mass ratios were buffered to near neutral by reaction with the sediment, regardless of starting fluid pH or sediment type. Calculated saturation states of minerals in contact with fluid reacted in the presence of bulk sediment, and equilibria regulating dissolved H;Both smectite and calcite formation remove magnesium from the reacted fluid. Smectite, the most abundant secondary solid formed, influences the reacted fluid chemical composition by removing aluminum, calcium, potassium, magnesium, and silica, and may contribute to the fluid buffer capacity by removing OH;Dissolved metals were rapidly removed from solution by reaction of vent water with bulk sediment at low water/rock mass ratio, where neutral conditions exist. Even at the near neutral pH observed at low water/rock mass ratios, fluids reacted in the presence of sediment plus basalt retained elevated dissolved iron, 2.3 mM, and manganese, 0.5 mM, much higher than dissolved iron and manganese concentrations observed in natural Guaymas Basin hydrothermal fluids, 0.02 and 0.1 mM, respectively.