Element analyses of DSDP/ODP Hole 504B basalt and breccia (Table 2)

Geochemical well logs were used to measure the dry weight percent oxide abundances of Si, Al, Ca, Mg, Fe, Ti, and K and the elemental abundances of Gd, S, Th, and U at 0.15-m intervals throughout the basement section of Hole 504B. These geochemical data are used to estimate the integrated chemical exchange resulting from hydrothermal alteration of the oceanic crust that has occurred over the last 5.9 Ma. A large increase in Si in the transition zone between pillows and dikes (Layers 2B and 2C) indicates that mixing of hot, upwelling hydrothermal fluids with cold, downwelling seawater occurred in the past at a permeability discontinuity at this level in the crust, even though the low-to-high permeability boundary in Hole 504B is now 500 m shallower (at the Layer 2A/2B boundary). The observations of extensive Ca loss and Mg gain agree with chemical exchanges recorded in the laboratory in experiments on the reactions that occur between basalt and seawater at high temperatures. The K budget requires significant addition to Layer 2A from both high-temperature depletion in Layers 2B and 2C and low-temperature alteration by seawater. Integrated water/rock ratios are derived for the mass of seawater required to add enriched elements and for the mass of hydrothermal fluid required to remove depleted elements in the crust at Hole 504B.