Metamorphic fluids involved in the chemical and petrological transformation of the igneous ocean crust to depths of at least 3 km below the seafloor occur in fluid inclusions from oceanic rocks and ophiolites, though the latter are subject to emplacement related metamorphism. Two fluid sources in oceanic rocks are seawater (SW) and magmatic volatiles (MV, primarily H2O and CO2). SW is the larger reservoir and of most importance in the upper crust, however MV are present in deeper, plutonic rocks.
Hydrothermal alteration within the top 2 km of ocean crust, comprising the extrusive volcanics and upper sheeted dikes,
is well understood from petrological, experimental, and theoretical studies. ODP Hole 504B in the eastern Pacific serves as
a model for such alteration (Alt, 1995). The fluid is heated modified SW, and the results of alteration include low-T oxidation, alkali and Mg fixation in the form of clay minerals and chlorite, and anhydrite precipitation. Below this zone of oxidative alteration is a zone of lower volcanics and upper sheeted dikes altered by modified SW that is largely Mg- and O2-depleted. Fluids here are responsible for a greeschist-grade assemblage of albite, Fe-chlorite and actinolite. Hole 504B was recently deepened to sample over 1 km of sheeted dikes, and initial alteration of the lower sheeted dikes is characterised by hornblende and highly-calcic plagiochase (bytownite and anorthite) with estimated fluid T of 400-500°C. This higher-grade assemblage most probably represents the petrology of the 'reaction zone' where black smoker-type fluids attain their final chemical characteristics before advecting upwards toward the seafloor.
As evidence of even deeper fluids within the ocean crust, nearly every plutonic rock suite sampled from the seafloor exhibits the effects of hydration reactions at amphibolite-grade conditions, with estimated temperatures as high as 700°C±50°C
and pressures up to 1 kb. Many, but not all such samples also exhibit a greenschist-grade overprint. Inclusions representing metamorphic fluids are ubiquitous, and commonly contain a population of high-salinity brines, in most cases suggesting the occurrence of supercritical phase separation.
Analyses of individual inclusions (by synchrotron XRF microprobe) show significant concentrations of Fe (10,000 to 90,000 ppm), Ca and Mn (several thousand to 30,000 ppm each) in addition to Na and Cl. Ratios between these metals are similar to those observed in modern black smoker fluids. Other constituents of deep-seated oceanic hydrothermal fluids include CO2 and CH4, which can have either a magmatic or a meteoric (i.e., SW) origin depending on the tectonic setting.
Alt, In Geophysical Monograph 91, (Humphris, et al., eds.) (American Geophysical Union, 1995).