During Leg 148, Hole 896A was drilled 1 km southeast of Hole 504B. One of the scientific objectives of drilling Hole 896A was to compare the alteration features of the upper oceanic crust to those of the upper part of Hole 504 basalts, and to find evidence of the possible influence of their respective geothermal contexts on these features.
In this paper, basalts sampled along the 290 m drilled into the basement of Hole 896A were studied to document the mineralogy, petrology, and chemistry associated with low-temperature alteration and to compare these features with the upper part of Hole 504B, which was altered at low temperature. The methods used were optical microscopy (reflected and transmitted light), X-ray diffraction, electron microprobe, and bulk-rock chemical analyses.
Alteration effects in Hole 896A basalts are chiefly visible as veins, as breccia cements, and as variously colored halos that are 3 to 50 mm in thickness. No trend with depth downhole is apparent in either the alteration mineralogy and chemistry or in the overall extent to which alteration proceeded.
A strong lithological control on the extent and style of alteration is readily apparent in the section studied. Fresh basaltic glass is common, because of the difficult access to fluids in this amorphous material. The large number of highly oxidized basalts contained in the massive units, compared to the discrete oxidative alteration of the pillow basalts units, is explained by the fact that the diffusion of the fluids along grain boundaries is easier in the coarse-grained basalts composing the massive units.
Clay minerals are the most common and abundant secondary minerals in Hole 896A basalts. Most of them are saponites. Celadonite, celadonite-nontronite mixtures, celadonite-nontronite-saponite mixtures, and saponite-celadonite-iron oxide mixtures also occur commonly. These celadonite-bearing minerals are restricted to black halos, brown halos, and the internal part of the compound red halos. Calcite and aragonite are abundant in veins. Phillipsite, chabazite, and pyrite are the least abundant secondary minerals.
Seawater is the main fluid involved in the formation of oxidized (red and brown) alteration halos. Alteration started with an oxidative stage, with the replacement of olivine by iddingsite and Fe-hydroxides and the precipitation of these minerals in the primary voids. The chemical composition of the fluid evolved while moving into the basalt, leading to a reducing alteration stage with saponite precipitation in the adjacent gray basalt. Most of the late-stage secondary minerals (zeolites and Ca-carbonates) probably formed largely in response to an increase in pH, because of the consumption of H+ related to the breakdown of plagioclase.
The occurrence of celadonite-nontronite and the enrichment in K and Fet of Hole 896A black halos imply the contribution of Fe-rich, diluted hydrothermal fluids during their formation. We propose that the most internal zone of the compound red halos and the brown halos represent early black halos which would have been oxidized later by seawater.
The major mineralogical and chemical features of alteration evidenced in the 310 m of basalts forming the upper pillow alteration zone (UPAZ) of Hole 504B are similar to those observed in Hole 896A. However, the oxidizing, seawater dominated, alteration seems to be more widespread at Site 896, suggesting higher water/rock ratios, related to a higher primary permeability.
Date of initial receipt: 17 August 1994
Date of acceptance: 28 February 1995
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