The lithostratigraphy and alteration of volcanic basement from Holes 504B and 896A, located in different parts of a ridge flank circulation cell, are summarized and compared. The 290-m-thick volcanic section of Hole 896A is located on a basement high that coincides with high heat flow and upwelling fluids. The 571.5-m-volcanic section of Hole 504B is located ~1 km away in an area of ambient heat flow. Subtle differences in lithostratigraphy include slightly greater proportions of massive units and fewer pillow basalts in Hole 896A than in Hole 504B. The volcanic sections are geochemically similar, but there is no direct correlation of lithologic or geochemical units between the two sites. Veins are comparable in abundance in the two sections (~30 veins/m, mean vein width <1 mm), but carbonate veins and thick (>2 mm) saponite and carbonate veins are more abundant in Hole 896A than in Hole 504B. Permeabilty values of the upper basement sections in Holes 896A and 504B are similar (~10^–13 to 10^–14 m²), suggesting that the upper ~200 m of basement is sufficiently permeable on a regional scale to support circulation of seawater through basement.

Alteration effects in basement from Hole 896A are similar to those in the upper 320 m of volcanic rocks in Hole 504B. These include celadonitic phyllosilicates in fractures and alteration halos and reddish, Fe-oxyhydroxide-rich alteration halos along fractures. Dark gray rocks, characterized by the presence of saponite ± carbonate ± pyrite occur throughout Hole 896A and the entire Hole 504B volcanic section. Alteration reflects evolution from open circulation of cold, oxidizing seawater, to more restricted circulation of seawater caused by burial of the crust by sediments and sealing of fractures with saponite. Late-stage carbonates and minor zeolites formed in veins throughout both holes from reacted seawater fluids (decreased fluid Mg/Ca). Oxygen and strontium isotopic evidence indicate an early generation of carbonates in both holes that formed at relatively low temperatures (~25°-35°C) during open circulation, whereas later carbonates in Hole 896A formed at slightly higher temperatures (~50°-70°C) during more restricted circulation, possibly similar to the present ridge flank hydrothermal upflow conditions (~50°-80°C).

Chemical changes in altered upper crust include oxidation, increased alkalis, Mg, CO₂, and H₂O; local uptake of P; elevated delta¹⁸O, deltaD, delta¹¹B, and ⁸⁷Sr/⁸⁶Sr; and lower S contents and d34S. The greatest chemical changes occur in alteration halos and breccias, and the smallest chemical changes occur in the lower volcanic section of Hole 504B. Secondary minerals filling fractures and cementing breccias are sites of uptake of Mg, CO₂, and H₂O, in addition to changes occurring in altered rocks.

Date of initial receipt: 6 April 1995
Date of acceptance: 1 August 1995

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