Hole 504B of the Ocean Drilling Program (ODP) provides the only available in situ samples of oceanic sheeted dikes. The rocks cored during Legs 137, 140, and 148 consist of altered diabase characterized by greenschist-facies secondary minerals. Fluid inclusions are abundant in plagioclase within highly altered vein halos, formed as apparently isolated inclusions or along healed planes of secondary inclusions. All fluid inclusions are two phase (liquid plus vapor) and liquid dominated (the volume of vapor is commonly 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×C. The corresponding fluid salinities range from 0.2 to 7.7 wt% NaCl equivalent, similar to the salinities of fluids exiting submarine hydrothermal vents at mid-ocean ridges (about 0.4–7.0 wt% NaCl equivalent). The mean salinities for individual samples 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 (less than 1.0 wt% NaCl equivalent).

The measured homogenization temperatures of fluid inclusions range from 124× to 211×C. The mean homogenization temperatures for individual samples increase systematically with depth, from 135×C at 1500 mbsf to 165×C at 2100 mbsf. This temperature range is much lower than that of hydrothermal alteration in the Hole 504B dike complex (250×–350×C), that of predicted greenschist-facies alteration (250×–450×C), and that of fluids in modern black smoker vents (250×–400×C). Assuming a gas-free fluid, hydrostatic-pressure-corrected "trapping temperatures" for the sheeted dike section from Legs 111, 137, 140, and 148 range from 150× to 201×C, which is consistent with the present-day borehole temperatures.

Lithostatic-pressure-corrected trapping temperatures are 15×–25×C higher than the present-day borehole temperatures. Crushing experiments show that inclusions contain on the order of 0.1 mol% compressible gas. If the presence of gas is considered, the lithostatic-pressurecorrected trapping temperatures correspond closely to the present borehole temperatures. The sheeted dike complex at Hole 504B was altered at the ridge axis 5.9 m.y. ago at high temperature (>350×C). The fluid inclusions, however, record temperatures appropriate for present-day conditions. Thus, fluid inclusions in hydrothermal plagioclase have been open systems, continually reequilibrating 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.

Date of initial receipt: 17 August 1994
Date of acceptance: 26 January 1995

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