Subsurface imaging of hydrothermal vent fields using high-resolution magnetic field mapping

Maurice A. Tivey

The volcanic lavas that erupt onto the seafloor and form the upper part of earth's ocean crust are typically highly magnetic due to their relatively high concentration of iron-rich magnetic minerals. These magnetic minerals record and preserve earth's magnetic field direction and intensity as the lavas cool after eruption. These magnetic properties can be used to discern the history and in some cases the age of the seafloor.

However, when we look in detail at the magnetic anomaly signal near the seafloor, we find that numerous processes can disrupt this magnetic recording. In particular, the hot and highly corrosive fluids found in hydrothermal vents can erase and destroy the magnetic minerals, replacing them with relative less magnetic minerals. Thus, the detailed maps of the magnetic field of ocean crust can be used to provide an estimate of the scale and geometry of the hydrothermal fluid flow through the crust at depth.

Such maps require a detailed survey that is only possible with remotely operated vehicles (ROVs), submersibles or autonomous underwater vehicles. Just such a survey was completed over the Endeavour segment of the northern Juan de Fuca Ridge in the northeast Pacific in 2000 and 2001 led by researchers from University of Washington and Woods Hole Oceanographic Institution. The figure shows the measured magnetic anomaly field over a well-known hydrothermal vent field: the Main Endeavour Field, a high-resolution bathymetric map, and the calculated magnetization map, which shows a string of magnetic "burn-holes."

The magnetic burn-holes are directly centered beneath the vent clusters and have diameters of ~100 m, which is indicative of near-vertical, narrow, pipe-like bodies of reduced magnetization located directly beneath the surface expression of the vent edifices. These reduced zones of magnetization are separated from each other by ~200 m, which further implies highly focused zones. These magnetic burn-holes are associated with both active and inactive/extinct vent areas, which indicates that alteration of the magnetic minerals in the crust rather than thermal demagnetization is the primary process responsible for the low magnetization.

The crustal magnetization patterns provide important constraints on the geometry of the subsurface plumbing beneath these hydrothermal vent systems. At Main Endeavour Field, the magnetic burn-holes appear to be semiregularly distributed along the trend of the rift valley, but individually distinct, suggesting that upward flow of hydrothermal fluid is highly partitioned at least in the near subsurface regime of a few hundred meters depth.

Figure 1: Left panel shows magnetic field anomalies observed along ROV tracklines shown in red. The red shaded areas are the published locations of Main Endeavour Field sulfide chimney edifices after Delaney et al. (1992, 1997). Vent chimney locations are only approximate; errors in location are ~10 m and have not been adjusted to the more detailed bathymetric map. Middle panel shows seafloor bathymetry (Johnson et al., 2002) measured by the ROV using a 200 kHz swath mapping sonar. Right panel shows the crustal magnetization calculated from the magnetic field data. Note the correlation of circular magnetization lows or magnetic burn-holes with both the active and inactive vent areas. (click for larger image)

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