PROCESSES OF BRINE GENERATION AND CIRCULATION IN THE OCEANIC-CRUST - FLUID INCLUSION EVIDENCE FROM THE TROODOS OPHIOLITE, CYPRUS

Detailed temporal, thermal, and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the Troodos ophiolite, Cyprus provide the first documentation that generation of high-temperature brines may be common at depth in thc oceanic crust. Anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events. The earliest fracturing event, at temperatures >450-600-degrees-C resulted in entrapment of brine-rich aqueous fluids with salinities of 36-61 wt % NaCl equivalent. Homogenization of the brine inclusions by halite dissolution, the virtual absence of vapor-rich fluid inclusions throughout the upper level plutonic sequence, and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz-hosted, high-salinity inclusions. Cooling of the fluids during pulses of fluid migration associated with episodic fracturing events, resulted in entrapment of the brines in the deep-seated, high-temperature portion of the hydrothermal system. In localized areas, the high-temperature brines (NaCl+/-KCl+/-CaCl2) caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites. Arrays of low-temperature, low-salinity fluid inclusions, which in some samples crosscut fractures dominated by brine inclusions, indicate downward propagation of a cracking front subsequent to collapse of the high-temperature magmatic system, resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures >200-400-degrees-C. Hydration reactions under greenschist facies conditions, or limited mixing with brine-rich fluids, may have resulted in salinity variations from 70% below to 200% above seawater concentrations. Temperatures and compositions of the low-salinity 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.