TAG Hydrothermal Vent Site : Mid Atlantic Ridge 26°N
The TAG hydrothermal area is located on the MAR at 26°8'N and 44°49'W (Figure 1) which has been the focus of several studies over the last two decades [Rona, 1973; Scott et al., 1974]. TAG stands for Trans Atlantic Geotraverse which was one of the earliest geologically-oriented surveys of the Mid-Atlantic Ridge of the modern era.
An anomalous magnetic anomaly low was discovered over the rift valley in the TAG region of the MAR after the first detailed survey of the area by Rona & Scott . The dredging of hydrothermally altered rocks prompted the interpretation that the magnetic anomaly low was related to hydrothermal alteration of the underlying crust [Mcgregor & Rona, 1975, Mcgregor et al., 1977].
It was not until much later that evidence for active hydrothermal activity was observed with geochemical water column surveys [Klinkhammer et al., 1985]. Subsequent deeptow camera surveys and submersible surveys discovered low temperature hydrothermal deposits on the east wall of the rift valley [Rona, 1981; Temple et al., 1979] and a high temperature black smoker mound at the base of the east wall [Rona, 1985; Rona et al., 1986; Thompson et al., 1988].
Magnetic data at TAG
Early analysis of sea surface magnetic data over the TAG area was based on two-dimensional sea surface profiles [McGregor et al., 1977]. Inversion of these profiles assuming 2D lineated sources produced a zone of low magnetization in the rift valley. McGregor et al. suggested that hydrothermal activity was responsible for this low. A recent 3D analysis of this same 1972 dataset by Wooldridge et al.,  shows the three-dimensional nature of the anomaly more clearly. The inversion of the 1972 data results in a magnetization low over the location of the TAG black smoker mound area. The "jerky" nature of the 1972 data set is most probably due to errors in positioning since navigation was based on transit satellite and dead reckoning. This study presents new data collected using GPS navigation and shiptrack error correction using Sea Beam bathymetry. The magnetic data from this survey appear much smoother although the main features of the field are similar to the 1972 dataset.
The 3D representation of the magnetic field over the TAG area clearly shows that the anomaly low often associated with the TAG hydrothermal field is in fact part of a magnetic high magnetic low anomaly pair (Figure 3). This high/low anomaly pair has the shape of a ying/yang symbol with a magnetic high to the northwest and a low to the southeast. The ying/yang anomaly has a peak to trough amplitude of approx. 200 nT and a horizontal distance of about 8 km (Figure 3). The maximum gradient between the high and low appears to be located directly over the black smoker vent site. A two-dimensional profile across this area would clearly give misleading information concerning the nature of this type of anomaly. This type of anomaly is characteristic of a negative point source type of feature [Tivey et al., 1989a]. Figure 4 shows the magnetic field of a negative point source with parameters similar to those at TAG. In reality the source of this anomaly is likely to be a discrete region/volume of oceanic crust rather than a point source. The point source character of the anomaly is more likely to be a result of the resolution of the data which is in this case about 4 km due to the water depth and line spacing of the data set [Miller, 1977].
The magnetic field has been inverted for crustal magnetization using the Parker and Huestis  Fourier inversion technique discussed in the previous section. The inversion (Figure 5a) shows a magnetization low at center of the Brunhes. The low is located at base of east wall of rift valley and encompasses the TAG black smoker mound and surrounding area. The low is clearly separated from the neovolcanic zone (i.e. spreading axis) by about 4 km. The low is also slightly elongated in the northeast direction extending about 8 km northeast of the black smoker mound (see figure ???) and averages about 2 km wide. The black smoker mound is at the southern end of the magnetization low suggesting the possibility of further hydrothermal activity or deposits to the northeast of the mound (Figure 5b). Deeptow camera surveys and submersible traverses in this area have found inactive hydrothermal deposits undergoing tectonic disturbance to the northeast of the mound [Rona et al., 1990] providing additional evidence to support the idea that the hydrothermally affected region extends over a scale of a few kilometers. There has also been some suggestion of active venting to the northeast of the black smoker mound but as yet no active vents have been found [Klinkhammer et al., 1986].
An estimate of the volume of demagnetized crust from the magnetization inversion gives approximately 2x8x0.5 or 8 km3 of affected crust. Wooldridge et al.  calculate a volume of xx km of based on forward modelling a uniformly magnetized sphere to produce the observed anomaly amplitude and peak-torugh distance.
Published in Earth and Planetary Science Letters, vol. 115, 1993.
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