Linking Sr systematics to the cooling of the lower oceanic crust - evidence from the geochemistry of oceanic gabbros

Hydrothermal systems at mid-ocean ridges are responsible for major heat and elemental fluxes that alter the chemistry of the oceans and oceanic crust, and contribute to the cooling of the lithosphere. Due to the distinct Sr isotopic signature of fresh MORB and seawater, whole rock 87Sr/86Sr can be used to quantify hydrothermal fluid fluxes in the axial region. The goal of this study is to constrain potential chemical and mineralogical controls on Sr isotopic exchange between fluid and rock, and to evaluate the effects these controls have on fluid flux calculations. To do this, gabbroic rocks that formed at the fast-spreading East Pacific Rise, and are now exposed along the northern scarp of the Hess Deep Rift Valley, were analyzed. This location provides a unique tectonic window with a nearly complete section of oceanic crust including a volcanic sequence, sheeted dike complex, and the upper few hundred meters of the plutonic section. The sample suite consists of gabbros, Fe-Ti-oxide gabbros, and gabbronorites that were variably altered (<2 to >50%) by pervasive fluid flow. Mineral assemblages and plagioclase-hornblende thermometry show that this alteration took place under upper greenshist to upper amphibolites facies conditions. Bulk rock Sr isotopic ratios range from 0.70244 to 0.70273, slightly elevated from the average magmatic value of 0.07025. There is no isotopic trend with depth within the gabbroic section but isotopic ratios are generally lower than in the overlying sheeted dike complex (average ~0.70283). Lack of correlation of isotopic shift with the percentage of hydrous alteration suggests that the precipitation of secondary hydrous minerals is not the major control on Sr exchange with hydrothermal fluids. Instead, we hypothesize that the Sr content and Sr isotopic signal of the altered samples are controlled by the replacement of primary plagioclase by secondary plagioclase. Sr contents in secondary plagioclase with anorthite compositions within the magmatic range (An40-An70) are similar to or higher than magmatic values, whereas more albitic secondary plagioclase can take up considerably higher amounts of Sr. Assuming interface-coupled dissolution-precipitation as the replacement mechanism of plagioclase, this suggests that the bulk rock isotopic shift is controlled by the composition and percentage of secondary plagioclase. This hypothesis is further supported by the slight positive correlation between bulk rock Na2O contents and 87Sr/86Sr. Therefore, secondary mineralogy and thus temperature conditions, as well as fluid and rock compositions, affect the transport of the isotopic signal. The development of a new approach to calculate fluid fluxes in mid-ocean ridge hydrothermal systems, considering the Sr budget in the different replacive phases, is in progress.