Hole 504B of the ocean Drilling Program (ODP) provides the only available in situ samples of oceanic sheeted dikes. The rocks cored during ODP Legs 137, 140, and 148 consist of altered diabase characterized by greenschist-like alteration. Fluid inclusions are abundant in plagioclase within highly altered vein halos, present as apparently isolated inclusions or along healed planes of secondary inclusions. All fluid inclusions are two-phase (liquid plus vapor) and liquid-dominated (the volumes of vapor are mostly less than 10%). Microthermometric results from plagioclase-hosted inclusions in the lower sheeted dikes reveal fluids with ice-melting temperatures ranging from -0.1 to -4.9 degrees C. The corresponding salinities of these fluid inclusions range from 0.2 to 7.7 wt% NaCl equivalent, similar to those of fluids exiting submarine hydrothermal vents at mid-ocean ridges (about 0.4-7.0 wt% NaCl equivalent). The mean salinities are between 1.8 and 3.8 wt% NaCl equivalent, close to that of seawater-3.2 wt% NaCl equivalent. The fluid inclusions in some secondary planes display much lower salinities (mostly less than 1.0 wt% NaCl equivalent). This may reflect at least two episodes of fluid penetration during the alteration of the ocean crust, one with the composition of seawater and the next with lower salinity. The measured homogenization temperatures of fluid inclusions range from 124 to 211 degrees C. The mean homogenization temperatures increase systematically with the depth, from 135 degrees C at 1500 mbsf to 165 degrees C at 2100 mbsf. This temperature range is much lower than that of the hydrothermal metamorphism of the Hole 504B dike complex (250-350 degrees C), that of predicted greenschist facies alteration (250-450 degrees C), and that of fluids in modern hydrothermal vents ( approximately 350 degrees C). Assuming a gas-free condition, hydrostatic-pressure-corrected "trapping temperatures" for sheeted dike section from legs 111, 137, 140, and 148 range from 150 to 201 degrees C, consistent with the present-day borehole temperatures; lithostatic-pressure-corrected "trapping temperatures" are 15-25 degrees C higher than the present-day borehole temperatures. However, crushing experiments show that inclusions contain a small amount of compressible gas at room temperature with a partial pressure of about 10 atm. Therefore, if considering the presence of gas, the lithostat-corrected "trapping temperatures" are likely to correspond to the present borehole temperature. The sheeted dike complex at Hole 504B was altered at the ridge axis 5.9 m.y. ago at high temperature (>350 degrees C). The fluid inclusions, however, record the temperatures appropriate for the present-day conditions. Thus, fluid inclusions in hydrothermal plagioclase have been more of less open systems that have continually reequilibrated to the ambient conditions. Inclusions must be interpreted in terms of the history of temperature and pressure changes undergone by the young ocean crust, as it formed at the mid-ocean ridge axis and was then transported from the rift environment.