Chemistry of secondary minerals from the deep sheeted dike complex of ODP Holes 137-504B and 140-504B

Dolerites sampled from the lower sheeted dikes from Hole 504B during Ocean Drilling Program Legs 137 and 140, between 1562.4 and 2000.4 mbsf, were examined to document the mineralogy, petrography, and mineral parageneses associated with secondary alteration, to constrain the thermal history and composition of hydrothermal fluids. The main methods used were mineral chemical analyses by electron microprobe, X-ray diffraction, and cathodoluminescence microscopy. Temperatures of alteration were estimated on the basis of single and/or coexisting mineral chemistry. Permeability is important in controlling the type and extent of alteration in the studied dike section. At the meter-scale, intervals of weakly altered dolerites containing fresh olivine are interpreted as having experienced restricted exposure to hydrothermal fluids. At the centimeter- or millimeter-scale, alteration patches and extensively altered halos adjacent to veins reflect the permeability related to intergranular primary porosity and cracks. Most of the sheeted dike alteration in this case resulted from non-focused, pervasive fluid-rock interaction. This study confirms and extends the previous model for hydrothermal alteration at Hole 504B: hydrothermal alteration at the ridge axis followed by seawater recharge and off-axis alteration. The major new discoveries, all related to higher temperatures of alteration, are: (1) the presence of hydrothermal plagioclase (An80-95), (2) the presence of deuteric and/or hydrothermal diopside, and (3) the general increasing proportion of amphiboles, and particularly magnesio-hornblende with depth. We propose that the dolerites at Hole 504B were altered in five stages. Stage 1 occurred at high temperatures (less than 500° to 700°C) and involved late-magmatic formation of Na- and Ti-rich diopside, the hydrothermal formation of Na, Ti-poor diopside and the hydrothermal formation of an assemblage of An-rich plagioclase + hornblende. Stage 2 occurred at lower temperatures (250°-320°C) and is characterized by the appearance of actinolite, chlorite, chlorite-smectite, and/or talc (in low permeability zones) and albite. During Stage 3, quartz and epidote precipitated from evolved hydrothermal fluids at temperatures between 310° and 320°C. Anhydrite appeared during Stage 4 and likely precipitated directly from heated seawater. Stage 5 occurred off-axis at low temperatures (250°C) with laumontite and prehnite from evolved fluids.

Geochemical composition of olivine, spinel and amphibole in ODP Hole 209-1271B

Dunite and gabbroic materials recovered from Hole 1271B, Ocean Drilling Program (ODP) Leg 209, were examined for mineral chemistry to understand melt flow and melt-mantle reactions in the shallowest upper mantle of the Mid-Atlantic Ridge near the 15°20' Fracture Zone. Hole 1271B was drilled to 103.8 meters below seafloor on the inner corner high along the south wall of the 15°20' Fracture Zone. The total length of core collected was 15.9 m (recovery = ~15%). The dominant rock type in Hole 1271B is dunite, followed by brown amphibole gabbro, olivine gabbro, and troctolite, along with minor amounts of harzburgite and olivine gabbronorite. A large proportion of the dunite is associated with gabbroic rocks in Hole 1271B, similar to those observed in the Mohorovicic (Moho) transition zone of the Oman ophiolite, indicating significant magmatic activity in this region near the 15°20' Fracture Zone. Olivine Fo content varies from 89.2 to 91.2 in impregnated dunite and from 85.6 to 88.6 in troctolite, olivine gabbro, and olivine gabbronorite. Spinel Cr# (= 100 x Cr/[Cr + Al] molar ratio) ranges from 38.9 to 62.7 in dunite and from 46.3 to 57.6 in troctolites, olivine gabbro, and olivine gabbronorite. Compositional trends for spinel from dunite through troctolite toward olivine gabbro/gabbronorite are characterized by increases in TiO2, Cr#, and Fe3+#, very similar to those reported from Hess Deep Site 895. Olivine gabbro, olivine gabbronorite, and troctolite in Hole 1271B are considered to have formed as hybrid rocks between dunite and an evolved melt in the walls of a melt channel in the shallowest upper mantle that is tens of meters wide. The melt trapped in the wall rock crystallized plagioclase and clinopyroxene. On the other hand, dunite in the center of the melt channel became more refractory by melt-mantle reactions, increasing spinel Cr# to 62.5.

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.

Geochemistry and stable isotope record of carbonate oozes and chalks of ODP Hole 130-807B

Light greenish gray and pale purple color bands are common in the ooze and chalk of the Ontong Java Plateau. Analyses of Pleistocene and Pliocene ooze samples that contain abundant bands indicate that the purple bands are colored by finely disseminated iron sulfide, whereas the green bands are colored by finely disseminated Fe- and Al-bearing silicates (probably clays). No local contrasts in the total organic carbon contents, carbon and oxygen isotopic compositions, and grain sizes were found. Band abundances, counted from core photographs of all Leg 130 holes, can be correlated from hole to hole on the basis of age rather than depth. The temporal distribution of these color bands is also comparable with that of the green bands described from the Lord Howe Rise, which were previously interpreted as products of altered volcanic glass. This may indicate that the green and purple bands on the Ontong Java Plateau originate from the early alteration of volcanic ash. The crosscutting relationships between the green and purple bands and original structures in the host sediment indicate that the bands have been locally altered by redox conditions in the sediments after the bands were formed.