Fluid evolution in an Oceanic Core Complex: A fluid inclusion study from IODP hole U1309 D—Atlantis Massif, 30°N, Mid‐Atlantic Ridge

In the detachment mode of slow seafloor spreading, convex‐upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor. Large, long‐lived hydrothermal systems such as TAG are situated off axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro, and trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30°N, Mid‐Atlantic Ridge), reveals evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and previously phase‐separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 to 35 wt % NaCl, although the most common inclusions have salinities close to seawater (3.4 wt % NaCl). Homogenization temperatures range from 160 to >400°C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low‐salinity inclusions trapped in quartz veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped.