Geochemistry of Carbonate Nodules and Cements and Implications for Hydrothermal Circulation, Middle Valley, Juan de Fuca Ridge

Abstract

Carbonate cements and nodules from hydrothermal sediments of Middle Valley were analyzed for their mineralogical, chemical, and isotopic compositions. These diagenetic carbonates include calcite, high-magnesium calcite, and dolomite. The chemical sources for carbonate precipitation include alteration of basement rocks (calcium and strontium), diffusion from seawater (magnesium), recrystallization of biogenic calcite (calcium and carbon), oxidation of sedimentary organic matter (carbon), and oxidation of thermogenic methane (carbon). The shallowest diagenetic carbonates from Hole 857C that were analyzed are dolomites. These are succeeded downcore by high-magnesium calcites and, finally, low-magnesium calcites. The shallowest diagenetic carbonates from Hole 858D are highmagnesium calcites which are succeeded downcore by low-magnesium calcites. Dolomites also occur in a limited depth zone in Hole 858D. These dolomites have a seawater magnesium source, but their calcium must be solely derived from basement-derived hydrothermal sources. Temperature gradients calculated from oxygen isotopic compositions of the carbonates are 0.53°C/m in Hole 857C and 10.5°C/m in Hole 858D. The distribution of temperatures is consistent with conductive heat transfer over the analyzed sections. We hypothesize from chemical data that Hole 858D intersected a fault zone at 28 m below the seafloor. The fault zone is believed to be a major conduit for the hydrothermal fluids discharging at the nearby vent. The dissolved CO2 in this fluid has a calculated carbon isotopic composition of about -20‰ Peedee belemnite (PDB), implying its source from oxidation of sedimentary organic carbon. A small fraction of the hydrothermal fluid advects laterally at a sub-bottom depth of 3 to 7 m. This flow is not rapid enough to significantly affect the heat transport, but it does affect the mineralogical, chemical (iron and manganese contents), and isotopic compositions of the diagenetic carbonates.