CALCIUM-RICH BRINES AND OTHER HYDROTHERMAL FLUIDS IN FLUID INCLUSIONS FROM PLUTONIC ROCKS, OCEANOGRAPHER TRANSFORM, MID-ATLANTIC RIDGE

Fluid inclusions in metagabbro, plagiogranite, and epidosite from the Oceanographer Transform, Mid-Atlantic Ridge, were investigated using microthermometric measurements and laser Raman spectroscopy. Inclusions in quartz reveal the presence of a high-salinity (average 40 wt% NaCl + CaCl2), high-temperature (> 340-degrees-C), calcium-rich (Na:Ca about 1, by weight) fluid in metagabbro and plagiogranite. At room temperature these fluids bear a halite daughter crystal, and may also contain small crystals of calcite, titanite, and hermatite. A low-density, vapor-rich fluid bearing a small amount of CO2 is closely associated with the briney fluids in one sample. All samples contain a population of high-temperature (150 to 400-degrees-C) aqueous liquid-dominated fluid inclusions, commonly of secondary origin, having seawater-like salinity (3 to 8 wt% NaCl equivalent). We conclude that the calcium-rich brines are produced by a combination of two processes: fluid phase separation and segregation of brines from vapor prior to trapping to generate the high salmitics, and extensive fluid-mafic rock interaction at low waterrock ratio to promote the calcium enrichment. Later introduction along microfractures of low-salinity, modified seawater at high waterrock ratio is recorded by the secondary fluid inclusions. The chemical complexity and association with CO2 may indicate that there is a magmatic component to the brines in addition to a seawater source. Differences between the Oceanographer Transform brines and those reported in previous studies of oceanic and ophiolite samples are attributed to variations in the degree of waterrock interaction, the host rock lithology, and the infleunce of a magmatic fluid component.