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 + CaCl 2), high-temperature (>340°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 hematite. A low-density, vapor-rich fluid bearing a small amount of CO 2 is closely associated with the briney fluids in one sample. All samples contain a population of high-temperature (150 to 400°C) aqueous liquid-dominated fluid inclusions, commonly of secondary origin, having seawaterlike 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 salinities, and extensive fluid-mafic rock interaction at low water:rock ratio to promote the calcium enrichment. Later introduction along microfractures of low-salinity, modified seawater at high water:rock ratio is recorded by the secondary fluid inclusions. The chemical complexity and association with CO 2 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 water:rock interaction, the host rock lithology, and the influence of a magmatic fluid component.