| Summary: | The structure and evolution of mid-ocean ridge hydrothermal systems and the magnitude of crustal alteration associated with different temperature fluid fluxes are poorly understood. This project has used strontium and oxygen isotopes together with metamorphic assemblages to investigate fluid flow geometry, fluid evolution and fluid-rock interaction during hydrothermal circulation in the Troodos Alteration Zone, the mesostasis has been altered to saponite but coarse clinopyroxene and plagioclase phenocrysts are unaltered. Homogeneous whole rock 90 Ma <SUP>87</SUP>Sr/<SUP>86</SUP>Sr profiles from the cores to rims of saponite-bearing pillows and dykes indicate the fluid flux passed through a matrix porosity rather than being channelled around the margins of the pillows or dykes. Fluid was channelled along cracks within glassy margins. A later celadonite-forming fluid was channelled along the margins of pillows and dykes. Within the Transition Zone, the mesostasis and clinopyroxene are altered to smectite/chlorite or chlorite, and plagioclase is variably altered to albite and alkali feldspar. Increasing whole rock 90 Ma <SUP>87</SUP>Sr/<SUP>86</SUP>Sr correlates with increasing albitisation of plagioclase indicating that magmatic strontium is retained in anorthite. There is no 90 Ma <SUP>87</SUP>Sr/<SUP>86</SUP>Sr gradient perpendicular to dyke margins suggesting that fluid flowed through the matrix porosity and was not channelled along dyke margins. Within the Basal Group and Sheeted Dyke Zone a greenschist facies assemblage is located structurally higher than an actinolite facies assemblage. Increasing 90 Ma <SUP>87</SUP>Sr/<SUP>86</SUP>Sr correlates with increasing albitisation of plagioclase indicating that magmatic strontium is retained in anorthite. Samples with albitised plagioclase had similar 90 Ma <SUP>87</SUP>Sr/<SUP>86</SUP>Sr values to the previously determined Sheeted Dyke Zone range of 0.7047-0.7059 (Bickle & Teagle, 1992).
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