Chemistry of serpentinized peridotites from ODP Hole 109-670A

In this paper we describe textural relationships in hydrated upper mantle peridotites emplaced at a nonconstructive ridge segment. Development of serpentinites and partially serpentinized peridotites takes place in four main stages: (1) pervasive serpentinization forming mainly lizardite, (2) a tensional stage forming chrysotile + talc + chlorite, (3) a deformational stage forming antigorite + tremolite, and (4) a late local tensional stage forming another generation of chrysotile veinlets. Mineral chemistry of serpentine pseudomorphs reflects in part primary mineral compositions. Olivine pseudomorphs are typically nickeliferous and depleted in aluminum and chromium. Orthopyroxene pseudomorphs have lower nickel contents and relatively high iron, aluminum, and chromium contents. Clinopyroxene pseudomorphs have very low nickel contents and relatively high aluminum and chromium contents. These chemical patterns in the serpentinites can be used to help discriminate between harzburgitic and lherzolitic protoliths. Oxygen isotopes and mineral parageneses suggest serpentine is derived from circulation of hydrothermal (200?C) fluids through the peridotite body. Crystallization of tremolite, talc, and chlorite may have occurred at temperatures up to 525?C if C02/H20 ratios were less than 0.25. Open fissures developing in aging upper mantle provide paths for important seawater circulation through a thin basaltic carapace down to shallow mantle rocks.

Isotopic composition and chemistry of pore waters from ODP Site 195-1201 sediments, West Philippine Basin

This report summarizes chemical and isotopic data from Ocean Drilling Program Leg 195 Site 1201. Pore water is divided into three intervals based on the rate of chemical change with depth. The shallowest interval is the red clay unit between 1.26 and 56.40 meters below seafloor (mbsf). In this section, there are overall decreases in the concentrations of alkalinity, boron, lithium, magnesium, potassium, sodium, and sulfate, whereas concentrations of calcium and chloride increase. Values of d18O and dD plot near standard mean ocean water to the right of the global meteoric water line (GMWL). Five samples from 72.60 and 83.33 mbsf yielded pore water for analyses. These samples help define a trend in the second interval, which is between 56.4 and 238.98 mbsf. Here, concentrations of magnesium, potassium, sodium, and sulfate decease, whereas concentrations of boron, calcium, and chloride increase. Concentrations of alkalinity and lithium remain roughly constant. The deepest interval, between 238.04 and 504.8 mbsf, has comparatively slower decreases of sodium and sulfate, increases of calcium and chloride, slow increases of alkalinity and lithium, and roughly constant concentrations of magnesium, potassium, and boron. Values of d18O and dD in pore water between 146.98 and 504.80 mbsf plot in a linear trend to the right of the GMWL.