Electrical conductivity imaging of the Philippine Sea upper mantle using seafloor magnetotelluric data

摘要

We performed a three-year seafloor electromagnetic survey in the Philippine Sea, including the western edge of the Pacific Ocean, to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions (3-D). The project iterated one-year deployment of ocean bottom electromagnetometers (OBEMs) using a total of 37 instruments installed at 18 sites. The data obtained have been analyzed in the order of their recovery based on a magnetotelluric (MT) method. In this study, we attempt to obtain a one-dimensional (1-D) electrical conductivity model beneath the Philippine Sea and the Pacific region separately that can be used as a reference model in the first step toward the 3-D analysis. The resultant 1-D models show three main features: (1) The conductivity in the shallower 200km of the upper mantle depths of the two regions contrasts sharply, which is qualitatively consistent with the large difference in lithospheric age. (2) The conductivity at 200-300km depth in both regions is more or less the same at approximately 0.3Sm~(-1). (3) The conductivity just below 400km depth is higher for the Philippine Sea mantle than for the Pacific mantle. The conductivity structure can be interpreted in terms of the thermal structure, mantle hydration, and existence of partial melt using experimental results for the conductivity of mantle minerals. If the conductivity is interpreted simply as the effect of temperature, the mantle beneath the Philippine Sea could be hotter than the dry solidus of mantle peridotite and thus partially molten. However, beneath the Pacific region, the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration.