A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge

We report here results from a deep tow magnetic survey over Middle Valley, Juan de Fuca Ridge. A series of track lines are combined to generate a high -resolution map of the magnetic field anomaly within a 10 x 12 km region su rrounding the Bent Hill massive sulfide (BHMS) deposit. A uniformly magneti zed body (5 A/m) with a cross section approximating the body inferred from Ocean Drilling Program (ODP) drilling can account for the observed near-bot tom magnetic anomaly amplitude. Assuming this magnetization is entirely ind uced, the average susceptibility (0.11 SI) corresponds to similar to3.5% ma gnetite + pyrrhotite by volume, consistent with the abundance of these phas es observed in drill core samples. However, this uniform magnetization mode l significantly underestimates the magnetic anomaly measured a few meters a bove the seafloor by submersible, indicating that the upper portion of the sulfide mound must have a significantly higher magnetization (similar to 10 % magnetite + pyrrhotite) than at deeper levels. On a larger scale, the nea r-bottom magnetic anomaly data show that basement magnetizations are not un iformly near zero, as had been inferred from analysis of the sea surface an omaly pattern. We interpret this heterogeneity as reflecting primarily diff erences in the degree of hydrothermal alteration. Our results highlight the potential of magnetic anomaly data for characterizing hydrothermal deposit s where extensive drill core sampling is not available.