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Detailed temporal, thermal, and compositional data on aqueous fluid inclusions from a suite of plutonic rocks from the Troodos ophiolite, Cyprus provide the first documentation that generation of high-temperature brines may be common at depth in the oceanic crust. Anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence are believed to record episodic fracturing events. The earliest event at temperatures >400-500oC resulted in pervasive entrapment of brine-rich aqueous fluids with salinities greater than 15 times that of seawater. The high-temperature brines are most reasonably interpreted in terms of two-phase separation of either pore or magmatic fluids at, or near, solidus temperatures. Migration and segregation of brine and vapor phases along fractures near the margins of crystallizing plagiogranite and gabbroic bodies resulted in preferential entrapment of the brines in the deep-seated, high-temperature brines (NaCl+KCl+CaCl2) caused extreme alteration of plagiogranite bodies and formation of podiform epidosites. Lack of brine-enrichment in fluid inclusions hosted in rocks of the sheeted dike complex suggests development of multi-tiered circulation cells in the upper plutonic sequence and sheeted dikes. Arrays of low-temperature, low-salinity fluid inclusions, which crosscut fractures dominated by brine inclusions, indicate penetration of seawater during subsequent fracturing events at temperatures >200-400oC. Hydration reactions under greenschist facies conditions, or limited mixing with phase-separated fluids, resulted in salinity variations in these fluids from 70% below to 200% above seawater concentrations. Temperatures and compositions of the inclusions are similar to those found in stockwork systems beneath Troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid-ocean ridge spreading centers. The low-temperature fracture networks may represent an extensive deep-seated feeder system which coalesced to form zones of concentrated hydrothermal upflow.

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Supervisor: Paul Ribinson

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