ALBERT

Description: Hole 896A penetrates into the upper volcanic section of a ridge-flank hydrothermal upflow zone. Analyses of the secondary mineralogy and chemistry, whole-rock geochemistry, and oxygen, carbon, and strontium isotope ratios of whole rocks and secondary minerals were conducted to constrain the chemical and thermal evolution of hydrothermal alteration and its effects on the upper crust at Site 896. Celadonite +/- Fe-oxyhydroxides are the earliest secondary minerals and formed at low temperatures. The crust was open to free circulation of seawater, but solutions derived from deeper in the crust may have provided some of the Fe, Si, and alkalies required for celadonite formation. Whole-rock chemical changes involved increased alkalies, and slight increases in H2O, Fe3+/Fe(Total), delta18O and 87Sr/86Sr. Subsequently, Fe-oxyhydroxides formed reddish alteration halos in the rocks in relatively young crust, where open circulation of large volumes of seawater maintained oxidizing conditions and low temperatures. Whole-rock chemical changes are characterized mainly by oxidation, but include increased H2O, alkalies, U, P, deltal8O and 87Sr/86Sr; local losses of S and possibly Tl; and possible minor losses of Ca and Mg. The next alteration stage was characterized by the pervasive formation of saponite in slightly older crust, where circulation of seawater was more restricted, conditions were less oxidizing, and temperatures were probably higher though less than 100°-150°C. Whole-rock chemical changes include increased Mg, H2O, delta18O, and 87Sr/86Sr; slight alkali increases; and local gains of S and Tl. Significant uptake of Mg by the upper crust occurred through the formation of saponite in veins and breccias. Four saponites have 87Sr/86Sr = 0.70842 - 0.70875 indicating that fluids were partly evolved seawater, but one fibrous saponite has 87Sr/86Sr = 0.704363, requiring localized, rock-dominated fluid compositions. Calcium carbonates and zeolites were the last secondary phases to form. An early, lower temperature (26°-35°C) generation of carbonates, has low Mg, Fe, and Mn concentrations and high Sr contents. These carbonates formed from partly reacted seawater that had decreased Mg/Ca ratios and contained 2.5%-10% basaltic Sr (carbonate 87Sr/86Sr = 0.708775 +/- 0.000066 (2 sigma), N = 11). A second generation of carbonates formed at higher temperatures (47°-67°C), from seawater-derived fluids with lowered Mg/Ca and Sr/Ca ratios and elevated Fe, and Mn concentrations. Trace-element chemistry of the high-temperature carbonates in general, and the lower 87Sr/86Sr of rare high-temperature aragonites (0.7079 - 0.7084) suggest more restricted circulation of seawater and reducing conditions. The higher temperature carbonates formed at temperatures consistent with the present-day thermal regime at Site 896; a ridge-flank hydrothermal upflow zone with basement temperatures greater than 50°C. All rocks from Hole 896A have interacted with seawater at low temperatures, and samples commonly record the integrated legacy of superimposed alteration processes. The most intense chemical changes have occurred within hyaloclastite and fragmentation breccias that comprise at least 5% of the uppermost oceanic crust at Site 896. The sequence of alteration processes present in Hole 896A is broadly similar to that recorded in the upper crust (above 300 m sub-basement) of Hole 504B, which is located approximately 1 km to the northwest, in a zone of average regional heat flow. The main differences between the material from Holes 896A and 504B is the greater abundance of carbonates, and hyaloclastite and fragmentation breccias, and the common occurrence of thick (=l cm) saponite veins in the new hole.

Description: Rocks of the lower sheeted dike complex of Hole 504B sampled during Leg 140 were analyzed for major and trace element compositions to investigate the effects of igneous processes and hydrothermal alteration on the compositions of the rocks. The rocks are relatively uniform in composition and similar to the shallower dikes. They are moderately evolved mid-ocean-ridge basalts (MORB) with relatively high MgO (7.9-10 wt%) and Mg# (0.60-0.70), and have unusually low incompatible element contents (TiO2 = 0.42-1.1 wt%, Zr = 23-62 ppm). Discrete compositional intervals in the hole reflect varying degrees of differentiation, and olivine and plagioclase accumulation in the rocks, and may be related to injection of packets of dikes having similar compositions. Systematic depletions of total REE, Zr, Y, TiO2, and P2O5 in centimeter-size patches are most likely attributed to exclusion of highly differentiated, late-stage interstitial liquids from small portions of the rocks. The rocks exhibit increased H2O+ reflecting hydrothermal alteration. Replacement of primary plagioclase by albite and oligoclase led to local gains of Na2O, losses of CaO, and slightly positive Eu anomalies. Some mobility of P2O5 led to minor increases and decreases in P2O5 contents, and some local mobility of Ti may have occurred during alteration of titanomagnetite to titanite. Higher temperatures of alteration in the lower sheeted dikes led to breakdown of pyroxene and sulfide minerals and losses of Zn, Cu, and S to hydrothermal fluids. Later addition of anhydrite to the rocks in microfractures and replacing plagioclase caused local increases in sulfur contents. The lower sheeted dikes are a major source of metals to hydrothermal fluids for the formation of metal sulfide deposits on and within the seafloor.

Description: Drilling during Legs 137 and 140 of the Ocean Drilling Program deepened Hole 504B, the only hole to penetrate through the volcanic section and into the underlying hydrothermally altered sheeted dike complex, by 438.1 m to a total depth of 2000.4 meters below seafloor. This paper presents the secondary mineralogy, bulk-rock sulfur contents, and stable isotopic (O, S) compositions, plus oxygen isotopic compositions of secondary minerals from the lower sheeted dike complex drilled during Legs 137 and 140. Various evidence indicates higher temperatures of hydrothermal alteration in the lower dikes than in the upper dikes, including: the local presence of secondary clinopyroxene in the lower dikes; secondary anorthite and hornblende in the lower dikes vs. mainly actinolite and albite-oligoclase in the upper dikes; generally increasing Al and Ti contents of amphibole downward in the dike section; and greater 18O depletions of the lower dikes (d18O = 3.6-5.0 per mil) compared with the upper dikes. Early high-temperature alteration stages (T = 350°-500°C) resulted in 18O depletions and losses of metals (Cu, Zn) and sulfur from the rocks. Local incorporation of reduced seawater sulfate led to elevated d34S values of sulfide in the rocks (up to 2.5 per mil). Quartz + epidote formed in crosscutting veins at temperatures of 310°-320°C from more evolved fluids (d18O = 1 per mil). Late-stage lower-temperature (~250°C) reactions producing albite, prehnite, and zeolites in the rocks caused slight 18O enrichments, but these were insufficient to offset the 18O depletions caused by earlier higher-temperature reactions. Addition of anhydrite to the rocks during seawater recharge led to increased S contents of rocks that had previously lost S during axial hydrothermal alteration, and to further increases in d34S values of total S in the rocks (up to 12 per mil). Despite the evidence for seawater recharge to near the base of the sheeted dike complex, the paucity of late zeolites in the lower dikes suggests that late-stage, off-axis circulation was mainly restricted to the volcanics and shallowest dikes, or to localized high-permeability zones (faults) at depth.

Description: 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.