(Table 2) Geochemistry of K-feldspars and associated plagioclase phenocrysts and rims at DSDP Legs 69, 52, 51 and 19

Basalts from some holes of the Deep Sea Drilling Project contain secondary K-feldspar which forms pseudomorphs after calcic (>76% An) Plagioclase cores, whereas Plagioclase of rims and microlites (68-74% An) remains unaltered. In basalts of Hole 504B two such grains with relics of Plagioclase in the central parts of phenocysts were recovered. The composition of the Plagioclase rims and of non-replaced phenocrysts is An79-81; the composition of relics is An83. The An and Ab contents of the K-feldspar is higher than in K-feldspar from altered basalt in Hole 418A in the Atlantic Ocean near the Bermuda Rise. Replacement of plagioclases by K-feldspar evidently is caused by oxygen-rich nearbottom sea water penetrating into basalts. The temperature interval of K-feldspathization is probably in the range 30 to 80°C, more-calcic Plagioclase being replaced by K-feldspar at higher temperatures.

(Table 1) Sulfur isotopes, total sulfur, and degree of oxidation of basalt samples from DSDP Holes 69-504B and 70-504B

Whole-rock basalt samples from the upper half of Deep Sea Drilling Project Hole 504B have oxygen-isotope compositions typical of mid-ocean-ridge basalts which have experienced a moderate degree of low-temperature alteration by sea water. By contrast, d18O values in the lower half of the hole correspond to basalts which have experienced almost no detectable oxygen-isotope alteration. These observations suggest that the overall water/rock ratio was lower in the lower half of the drilled crust. A correlation between d18O values and 87Sr/86Sr ratios suggests that the water/rock ratio, rather than temperature variation, was the main factor determining basalt d18O values. Hydrogen-isotope data appear to be consistent with a low water/rock ratio in the lower part of the crust.