Mapping the distribution of fluids in the crust and lithospheric mantle utilizing geophysical methods

Abstract

Geophysical imaging provides a unique perspective on metasomatism, because it allows the present day fluid distribution in the Earth’s crust and upper mantle to be mapped. This is in contrast to geological studies that investigate mid-crustal rocks have been exhumed and fluids associated with metasomatism are absent. The primary geophysical methods that can be used are (a) electromagnetic methods that image electrical resistivity and (b) seismic methods that can measure the seismic velocity and related quantities such as Poisson’s ratio and seismic anisotropy. For studies of depths in excess of a few kilometres, the most effective electromagnetic method is magnetotellurics (MT) which uses natural electromagnetic signals as an energy source. The electrical resistivity of crustal rocks is sensitive to the quantity, salinity and degree of interconnection of aqueous fluids. Partial melt and hydrogen diffusion can also cause low electrical resistivity. The effects of fluid and/or water on seismic observables are assessed by rock and mineral physics studies. These studies show that the presence of water generally reduces the seismic velocities of rocks and minerals. The water can be present as a fluid, in hydrous minerals, or as hydrogen point defects in nominally anhydrous minerals. Water can further modify seismic properties such as the Poisson’s ratio, the quality factor, and anisotropy. A variety of seismic analysis methods are employed to measure these effects in situ in the crust and lithospheric mantle and include seismic tomography, seismic reflection, passive-source converted and scattered wave imaging, and shear-wave splitting analysis. A combination of magnetotelluric and seismic data has proven an effective tool to study the fluid distribution in zones of active tectonics such as the Cascadia subduction zone. In this location fluids can be detected as they diffuse upwards from the subducting slab and hydrate the mantle wedge. In a continent-continent collision, such as the Tibetan Plateau, a pervasive zone of partial melting and aqueous fluids was detected at mid-crustal depths over a significant part of the Tibetan Plateau. These geophysical methods have also been used to study past metasomatism ancient plate boundaries preserved in Archean and Proterozoic aged lithosphere.