Fluid circulation in the oceanic crust: Contrast between volcanic and plutonic regimes

Vein carbonates from a variety of ocean crust environments have been analyzed for Sr contents and Sr and oxygen isotopic compositions. Veins from upper crustal lithologies typically have low formation temperatures (<100°C) and form within 10–15 m.y. after crust formation, from solutions containing a basaltic component of very high Ca/Sr (>30 times that of hot smoker vent fluids). Veins from the gabbro lithologies at site 735B show both high (>150°C) and very low (<10°C) formation temperatures. The high‐temperature carbonate veins have a large basaltic component (low 87Sr/86Sr) with low Ca/Sr and probably formed before unroofing of this deep crustal block. The low‐temperature veins have only a minor basaltic component, and formed after unroofing, but within 0–10 m.y. of crust formation. Hot smoker vent fluids represent the low Ca/Sr limit of fluids recorded by carbonate veins and thus cannot describe the totality of the seawater‐ocean crust Ca/Sr exchange budget.