Tracing the evolution of hydrothermal fluids in the upper oceanic crust: Sr-isotopic constraints from DSDP/ODP Holes 504B and 896A

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Abstract
Whole rock and secondary mineral 87Sr/86Sr determinations from ODP Holes 504B and 896A have been compiled into a profile for the upper 2 km of in situ ocean crust to constrain the composition and evolution of recharge and discharge fluids. There is a gradual trend from elevated 87Sr/86Sr whole rock ratios at the top of the crust towards primary MORB compositions at the base of the sheeted dykes. Rocks from the mineralized lava-dyke transition zone have 87Sr/86Sr ratios clustering in the range of black smoker fluids. Igneous and alteration phases are not in Sr-isotopic equilibrium, therefore whole rock strontium isotopic ratios reflect the proportion of secondary minerals and the extent of plagioclase recrystallization. The 87Sr/86Sr of anhydrite directly records the chemical evolution of seawater during recharge. There is a slight decrease in anhydrite 87Sr/86Sr with depth in the volcanic rocks but anhydrite from the lowermost dykes yields diabasic 87Sr/86Sr ratios. Epidotes in cross-cutting veins have higher 87Sr/86Sr than the background alteration recorded by amphibole in the lower dykes. This elevated signature reflects the 87Sr/86Sr of 504B black smoker-type fluids and requires less fluid evolution than the background alteration, suggesting channelling of recharge fluids locally.

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