Epidosites (epidote- and quartz-rich rock with granoblastic texture) from the Solea graben are believed to form from upflowing hydrothermal fluid that deposited massive sulfide ores at the seawater-sea-floor interface of the Cretaceous age oceanic crust. Epidosite within the graben occurs as massive epidosite (epidosite concentration greater than 80%) and zones of incipient epidotization (between 5% and 80%). Regions of incipient epidotizations are characterized by the juxtaposition of greenschist facies alteration and epidosite; the fluids responsible for each of these alteration styles had similar oxygen isotope compositions and temperatures. Petrographic observations suggest that the epidosite protolith was a previously altered rock. In addition, evidence of secondary porosity is found in the areas of incipient epidotization, suggesting that the reactions creating epidosite lead to the creation of porosity.

Computer simulated fluid-rock interaction at 350 °C and 500 bar between a Ca-rich, seawater-derived hydrothermal fluid and a greenschist facies mineral assemblage (36% albite, 36% quartz, 18% chlorite, 6% magnetite, 3% clinopyroxene, and 1% epidote) produced an epidosite assemblage (epidote + quartz + chlorite + hematite/magnetite) at fluid/rock mass ratios of 1250:1 to 50:1. The results suggest that reaction of this hydrothermal solution with albite and chlorite, as opposed to fresh diabase, controls epidosite formation. The calculations also indicate initial reduction in rock volume, which correlates with the observed porosity in incipiently epidotized rocks of the Solea graben.

We suggest that hydrothermal circulation related to the localized intrusion of high-level gabbro in highly attenuated crust resulted in the transformation of green-schist facies rocks to epidosite. This is consistent with evidence from the Mid-Atlantic Ridge at the Kane Fracture Zone for reaction between upflowing hydrothermal solutions and previously altered basalt.