The structure and controls on fluid-rock interactions in ocean ridge hydrothermal systems: constraints from the Troodos ophiolite

Abstract

Abstract Rb-Sr isotopic compositions of rocks and minerals from the Troodos ophiolite have been analysed to constrain the rate limiting mechanisms that control fluid-solid exchange, the extent to which recharge fluids were channelled and the thermal evolution of oceanic hydrothermal systems. Systematic regional sampling has confirmed the Sr-isotopic alteration profile suggested by Bickle & Teagle (1992). This has been previously interpreted as a consequence of recharge fluids percolating down through the extrusive series with kinetically limited fluid-rock interaction but altering the underlying ≈1 km section of sheeted dykes with near equilibrium fluid-rock Sr-isotopic exchange. Detailed Sr-isotopic profiles reported here across structural heterogeneities such as pillows, dyke margins and faults have failed to show isotopic gradients related to channelling of fluid. Rather the fluid flow in the recharge was pervasive and the degree of alteration is largely controlled by the extent of mineral recrystallization, primarily by albitization of plagioclase, and to a lesser extent by recrystallization of pyroxene. Hydrothermal recharge should cool the sheeted dykes much faster than the progression of the Sr-isotope alteration through the crust and rocks at greenschist facies (∼250°C) or higher temperatures are predicted only to be found within a few hundred metres of the basal boundary layer of the system. The alteration pattern on Troodos with a zone a kilometre or more in thickness altered at greenschist facies temperatures and with significant Sr-isotope exchange is thus not easily explained.