Diabase and basalt were reacted with Na-Ca-K-Cl fluids of seawater chlorinity at 375--425 ¿C, 375--400 bars, and fluid/rock mass ratios of 0.5--1.0 to assess the role of basalt chemistry and texture on Sr and Ca mobility during high-temperature hydrothermal alteration. An additional experiment, utilizing an 84Sr spiked fluid, was performed to help quantify reaction rates of processes affecting Sr mobility.

The experimental results help to constrain reaction processes responsible for the chemistry of hot spring fluids at midocean ridges. Alteration of basalt and diabase is characterized by formation of tremolite-actinolite-smectite-chlorite and clinozoisitic epidote-smectite-chlorite, respectively. Diabase alteration produced dissolved Sr/Ca ratios similar to those observed for ridge crest hot spring fluids, whereas alteration of a mostly crystalline basalt produced significantly lower ratios. This observation supports the premise that the Sr/Ca ratios observed in vent fluids may be produced during deep-seated reaction of the hydrothermal fluids with diabase dikes and/or gabbro at relatively low fluid/rock ratios and suggests that hydrothermal alteration of primary igneous minerals at relatively high temperatures (400 ¿C) leads formation of a mineral with low DSr/Ca, possibly hydrothermal plagioclase.

Results of the 84Sr spiked experiment indicated that only 4% of the Sr in basalt was mobilized after 800 hours of reaction despite the fact that B and Li were nearly quantitatively leached. It is thus suggested that B and Li are good indicators of the amount of fresh rock encountered by fluids, while Sr concentration and isotopic data can be used to estimate the degree of alteration that the Sr-bearing primary minerals have undergone. The degree of alteration of subsurface reservoirs may be estimated by dividing the fluid/rock ratio obtained for highly mobile elements by the fluid/rock ratio obtained from path-integratred Sr isotope and concentration data. Based on this method, for example, it can be estimated that only 5% of the primary Sr-bearing minerals encountered by fluids in the subsurface at EPR 21 ¿N are actually converted to secondary phases. ¿ American Geophysical Union 1988