Incompatible element-rich fluids released by antigorite breakdown in deeply subducted mantle

The Bengal Fan is covered afresh by systematic geological and geophysical investigations by National Institute of Oceanography (NIO), India and a detailed free-air gravity map of the fan is prepared. The map shows two strong gravity lows ¯ one corresponding to the continental shelf and the other to the 85°E Ridge. The Ninetyeast Ridge is brought out as a gravity high. The anomalies are inverted to determine the anomaly-producing interfaces, which suggest that the 85°E Ridge anomaly could not be explained by an isolated geophysical model invoking a negative density contrast for the ridge material. The 85°E Ridge anomaly and several other isolated gravity lows are attributed mostly to the depression-like structures in the Moho. Each depression of the Moho is associated with a basement high. The depression beneath the 85°E Ridge is about 6 km deep from the regional Moho boundary, which is at variance to the earlier results. It is suggested that the depressions may possibly have developed due to the surface volcanic loads emplaced on already evolved oceanic crust of the Bay of Bengal.