Density and porosity of the upper oceanic crust from seafloor gravity measurements

The exposure of 1300 meters of upper oceanic crust at the Blanco Fracture Z one allows near-bottom gravity measurements to determine the in situ densit y of the seafloor as a function of depth. Gravity measurements along the no rth wall of the Blanco Depression indicate an outcrop density of 2530 +/- K g/m(3) for the upper 800 meters of crust and a calculated porosity of 23%. The lower 500 meters of crust (800 to 1300 meters below the sea floor) has a measured density of 2710 +/- 130 Kg/m(3) and a porosity of 14%. These dat a indicate that most of the extrusive volcanic oceanic crust is highly poro us and can act as an aquifer and large-scale reservoir for hydrothermal flu ids. These direct crustal density measurements also support previous interp retations that low seismic velocities observed in Layer 2 are due to the hi gh porosity of the upper extrusive section.