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 midcrustal 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

Fluid Distribution in the Central Andes Subduction Zone Imaged With Magnetotellurics

“…The electrical resistivity of rocks is a physical property that is highly sensitive to the presence of interconnected fluids or partial melts (Unsworth & Rondenay, ). Images of the electrical resistivity structure have been widely used to estimate the distribution of fluids in subduction zones (Worzewski et al, ).…”

The Lithospheric Structure of the Solonker Suture Zone and Adjacent Areas: Crustal Anisotropy Revealed by a High‐Resolution Magnetotelluric Study

“…Magnetotelluric (MT) data can produce resistivity models of the entire lithosphere and can constrain lithospheric composition and can complement seismic studies. Even small amounts of saline aqueous fluids and/or partial melt, or sulfides or graphite on the grain boundaries can lower the resistivity by 1 to 2 orders of magnitude compared to the normal lithospheric conductivity, while the relative change of seismic velocity caused by these changes in composition is smaller (Unsworth & Rondenay, ). Of relevance to this study is the ability of MT to image suture zones, which can sometimes be detected as a crustal low‐resistivity layer (Jones, ).…”

Decompression melting driving intraplate volcanism in Australia: Evidence from magnetotelluric sounding

“…In western Victoria, H 2 O‐CO 2 rich fluids are common products of metasomatism [e.g., O ' Reilly and Griffin , ]. Dissolution of carbon dioxide in water releases ions of H + and HCO 3 , which can act as charge carriers and increase the bulk conductivity [ Unsworth and Rondenay , ]. However, these minor phases can increase the bulk conductivity if they form well interconnected networks along grain boundaries [ Unsworth and Rondenay , ].…”

“…Dissolution of carbon dioxide in water releases ions of H + and HCO 3 , which can act as charge carriers and increase the bulk conductivity [ Unsworth and Rondenay , ]. However, these minor phases can increase the bulk conductivity if they form well interconnected networks along grain boundaries [ Unsworth and Rondenay , ]. Studies of volatile‐rich peridotite xenoliths show that, in contrast to silicate and carbonate melts, H 2 O‐CO 2 rich fluids tend to fill isolated pores along grain boundaries rather than forming interconnected films [ O ' Reilly and Griffin , ].…”