Petrography of the dike-gabbro transition at IODP Site 1256 (equatorial Pacific): The evolution of the granoblastic dikes

The Ocean Drilling Program (ODP)/Integrated Ocean Drilling Program (IODP) three-leg campaign at Site 1256 (Leg 206, Expeditions 309 and 312) provides the first continuous in situ sampling of fast spread ocean crust from the extrusive lavas, through the sheeted dikes, and down into the uppermost gabbros (Cocos plate; East Pacific Rise; eastern equatorial Pacific). The lowest similar to 60 m of the dikes above the gabbros were transformed to granoblastic dikes'' through a metamorphic overprint characterized by two-pyroxene domains formed under granulite-facies conditions. Equilibrium temperatures estimated by the two-pyroxene thermometer range between 930 degrees C and 1050 degrees C, implying that conditions within the granoblastic zone were appropriate for hydrous anatexis, with the potential to generate partial melts of trondhjemitic composition. The downhole evolution of the granoblastic overprint is expressed by systematic changes in texture, phase composition, and calculated equilibrium temperature, consistent with thermal metamorphism by a deeper heat source. Thermal modeling implies a long-lasting heat source located beneath the granoblastic dikes, providing thermal energy over several thousands of years. The most likely such source is a steady state, high-level axial magma chamber (AMC) located at the base of the sheeted dike section. We interpret the interval of granoblastic dikes as part of a dynamic conductive boundary overlying the AMC.