| Abstract |
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.
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