Reduced crustal magnetization beneath the active sulfide mound, TAG hydrothermal field, Mid-Atlantic Ridge at 26°N
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A detailed near-bottom magnetic field survey was carried out by the submersible Alvin over the actively venting mound located within the TAG (Trans-Atlantic Geotraverse) hydrothermal field on the Mid-Atlantic Ridge at 26-degrees 08’N, 44-degrees 49’W. Three-dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound. This magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure. In contrast, the sea surface magnetic anomaly is associated with a broad 2 x 8 km magnetization low elongated along the axis, that includes both the active and inactive mounds. The short-wavelength (250 m), near-bottom magnetic anomaly over the active mound is far too small to produce the long-wavelength (8 km) sea surface magnetic anomaly at TAG however, and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly. Other hypotheses, including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely. The existence of vigorous hydrothermal activity at TAG and the lack of microseismic activity in the TAG area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly. The thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long-wavelength sea surface anomaly. Pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long-wavelength magnetic signal could be preserved in the crust. The overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites. These studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late-stage intrusive bodies . While detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones, only drilling will ultimately test these hypotheses.