STRONTIUM ALTERATION IN THE TROODOS OPHIOLITE - IMPLICATIONS FOR FLUID FLUXES AND GEOCHEMICAL TRANSPORT IN MIDOCEAN RIDGE HYDROTHERMAL SYSTEMS

New and published strontium isotope analyses from the Troodos ophiolite constrain fluid-solid exchange processes, and the magnitude and circulation paths of the hydrothermal fluids. The Sr-87/Sr-86 profile reflects alteration in the recharge zone of an evolving hydrothermal system. Fluid-rock strontium isotope exchange in the upper approximately 1.5 km of extrusive lavas was kinetically limited and seawater-derived fluids emitted from the base of this zone were buffered to Sr-87/Sr-86 ratios between approximately 0.7047 and 0.7059. In contrast, over the next approximately 1 km depth interval of sheeted dykes and the uppermost plutonics, Sr-87/Sr-86 values cluster about 0.7054 +/- 7 (2sigma) and fluid flow is inferred to have been pervasive with near-equilibrium fluid-rock exchange. Quartz-chlorite and epidosite zones, the probable pathways of the concentrated, high-temperature upwelling fluids, have identical Sr-87/Sr-86 ratios to adjacent diabase dykes. On Troodos a time-integrated fluid flux in excess of 2.9 x 10(7) kg m-2 is required to transport the strontium isotope composition of approximately 0.7054, set in the kinetically controlled exchange zone, through the approximately 1 km of sheeted dykes and into the zones of concentrated upwelling. The uniformity of the Sr-87/Sr-86 ratios in the diabase sheeted dykes and high-temperature epidosite and quartz-chlorite rocks indicate that the strontium isotopic alteration took place during the high temperature phase of hydrothermal circulation. The inferred minimum time-integrated fluid flux of 2.9 x 10(7) kg m-2 substantially exceeds that of approximately 5 x 10(6) kg m-2 inferred from thermal models of high temperature circulation, but is comparable with estimates of the hydrothermal flux from oceanic budgets of He-3, Mg and Sr-87. The high flux estimate for Troodos is consistent with the ophiolite venting fluids, with Sr-87/Sr-86 elevated significantly above rock values, which contrasts with the near-MORB Sr-87/Sr-86 ratios of fluids from active high-temperature vents at mid-ocean ridges and the Sr-87/Sr-86 profile in DSDP hole 504B. This requires that either the modem systems are immature and there is a protracted phase of lower temperature circulation or that circulation in the Troodos ophiolite differed from that in "normal" mid-ocean ridges and oceanic strontium budgets are balanced by another source. The lack of very elevated Sr-87/Sr-86 values in the vent fluids precludes significant channelling in the recharge zones of the active systems.