Strontium alteration in the Troodos ophiolite: implications for fluid fluxes and geochemical transport in mid-ocean ridge hydrothermal systems

Abstract

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 87Sr/ 86Sr profile reflects alteration in the recharge zone of an evolving hydrothermal system. Fluid-rock strontium isotope exchange in the upper ∼ 1.5 km of extrusive lavas was kinetically limited and seawater-derived fluids emitted from the base of this zone were buffered to 87Sr/ 86Sr ratios between ∼ 0.7047 and 0.7059. In contrast, over the next ∼ 1 km depth interval of sheeted dykes and the uppermost plutonics, 87Sr/ 86Sr values cluster about0.7054 ± 7 (2σ) 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 87Sr/ 86Sr ratios to adjacent diabase dykes. On Troodos a time-integrated fluid flux in excess of2.9 × 10 7 kg m −2 is required to transport the strontium isotope composition of ∼ 0.7054, set in the kinetically controlled exchange zone, through the ∼ 1 km of sheeted dykes and into the zones of concentrated upwelling. The uniformity of the 87Sr/ 86Sr 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 of2.9 × 10 7 kg m −2 substantially exceeds that of∼ 5 × 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 3He, Mg and 87Sr. The high flux estimate for Troodos is consistent with the ophiolite venting fluids, with 87Sr/ 86Sr elevated significantly above rock values, which contrasts with the near-MORB 87Sr/ 86Sr ratios of fluids from active high-temperature vents at mid-ocean ridges and the 87Sr/ 86Sr profile in DSDP hole 504B. This requires that either the modern 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 87Sr/ 86Sr values in the vent fluids precludes significant channelling in the recharge zones of the active systems.