Relationship between the Sr and O isotope compositions of hydrothermal fluids and the spreading and magma-supply rates at oceanic spreading centers

We find a first-order correlation between the Sr and O isotope compositions of hydrothermal fluids and spreading rate at oceanic spreading centers. Th e isotope signatures of hydrothermal fluids at slow-spreading ridges are mo re rock dominated than those at fast-spreading ridges, implying that they h ave undergone a greater extent of Sr and O exchange with the crust, The loc al spatial and temporal variability in Sr and O isotope signatures within i ndividual hydrothermal sites can be attributed to short-term variations in the hydrothermal system in response to magmatic or tectonic events, phase s eparation at depth, and mixing with seawater in the upwelling zone. However , these processes cannot explain the systematic global relationship of an i ncreasing rock signature in hydrothermal fluids with decreasing spreading r ate. A greater extent of Sr and O exchange between rocks and the circulatin g fluids can be achieved by (1) a longer fluid-flow pathway through the oce anic lithosphere, (2) more fractured rock that exposes a greater reaction a rea, (3) an increased residence time of the fluid due to slower hydrotherma l circulation, or (4) a combination of all three. Our preferred interpretat ion is that fluid-flow paths are longer, and depths of penetration are like ly greater, at slow-spreading ridges.