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The δ 1 8 θ values of 108 basaltic samples from Holes 417A, 417D, and 418A range from 6.0 to 26.4%o. The analyses of separated igneous minerals and anhydrous basalts prove that initially the magmas had δ 1 8 θ values of 5.8°/oo, identical to all other unaltered mid-ocean ridge basalts. The highest δ 1 8 θ samples in all three holes are highly altered hyaloclastites, breccias, and flow margins. The δO-enriched nature of the basalts, in contrast to the normal δ 1 8 θ of igneous minerals, the high δ 1 8 θ values of 29 carbonates and one clay, prove that the wide variability of δ 1 8 θ values in the samples is due to alteration or weathering by cold sea water. A pronounced 1 O gradient with depth is found in Hole 417A, and lesser ones are found in the other two holes. The basalts from the lower parts of these holes have the same isotopic compositions as do basalts from young DSDP sites. Basalts from the upper parts of Holes 417D and 418A are about as enriched in 1 O as are Cretaceous and Jurassic samples from some previous DSDP legs. Those of Hole 417A are much richer in 1 O. Analyses of carbonates indicate a geothermal gradient of about 10°C/100 meters in Hole 417A but 5°C/100 meters in 418A. The general pattern of δ 1 8 θ of DSDP basalts with age is that of a rapid increase to about 7.5700 in the first 10 to 20 m.y., followed by a much slower rise to about 9%o. This pattern is strikingly similar to that observed for changes in seismic velocity in the upper crust, suggesting that low-temperature alteration or weathering causes the progressive change in geophysical properties associated with the aging of the sea floor. A simple model of the low-temperature alteration of the oceanic crust can be deduced from comparisons of the δ 1 8 θ data from Holes 417A, 417D, and 418A with analyses from other DSDP and dredged basalts of known age. The new sea floor is open to cold sea water circulation, which weathers the basalts more or less uniformly to a depth of at least 600 meters. The massive circulation ceases after about 10 m.y., but that is sufficient time to have had the δ 1 8 θ of the rocks raised to about 7.5%o. Some low-temperature alteration proceeds in the upper few hundred meters of the old oceanic crust but at a much slower rate and at a slightly warmer temperature, causing the O-gradients mentioned above. In addition, there are highly transmissive units throughout the crust through which sea water can circulate for much longer times (25 to 50 m.y.). The geochemical differences between Holes 417A and 417D or 418A can be attributed to the exposure of 417A to cold sea water for a much longer period.

The Alteration and Aging of the Basaltic Layer of Sea Floor: Oxygen Isotope Evidence from DSDP/IPOD Legs 51, 52, and 53