Direct measurements of oceanic crustal density at the northern Juan de Fuca Ridge

Density and porosity of oceanic crust are critically important to understanding the circulation of hydrothermal fluid at mid-ocean ridges. To determine these parameters directly, we completed a fixed, on-bottom gravity survey on the Endeavour Segment of the Juan de Fuca Ridge, which is a well studied spreading center that is both hydrothermally and tectonically active. A Bell-Aerospace gravity meter mounted within the submersible ALVIN was used to make gravity measurements from the floor of the axial valley to the summit of the west axial ridge. Analysis of these data gave a density of 2240+/-50 kg/m(3) for the upper 130 m of crust. Comparison of the outcrop density with that of recovered rock samples resulted in a calculated porosity of 38+/-4%. Forward modeling of the seafloor gravity data indicated that both the valley floor and bounding walls have unusually low density crust that is flanked by higher density crust.