Collaborative Research: Relating Bulk Composition to Seismic Properties in Crustal Rocks

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The Earth's continental crust is a reservoir for minerals and other natural resources. Moreover, the chemical composition of the crust controls its material properties (e.g., strength), and thus influences the way in which the crust deforms and evolves over geologic time. However, while crustal composition at the Earth's surface can be determined by direct sampling, at depth composition must be inferred from indirect methods such as geophysical imaging. One of the most common geophysical imaging methods employs seismic waves, which travel at different speeds through different rock types. By measuring seismic wave speeds the 3-D internal structure of the crust can be determined using an approach similar to x-ray computed tomography (CT scan) in medical studies. In this project, we will take seismic velocity models for the Earth?s continental crust and invert them to infer crustal composition. These compositional models will then be used to determine variations in crustal strength, as well as regions with different natural resource potential.In the past decades a wealth of seismic data characterizing the continental crust have become available through NSF programs such as Earthscope, GeoPRISMS, and the Cascadia Initiative including co-located P-wave data from controlled-source experiments, VP/VS constraints from receiver functions, and S-wave data from ambient noise tomography. These datasets provide the foundation for future research on the nature and composition of the continental crust. However, despite the abundance of information provided by the seismic data, a quantitative relationship between rock composition and seismic velocity is missing. In this project we will develop a new suite of tools to relate crustal composition to seismic velocity. Our approach is divided into four steps: (1) derive a representative database for crustal composition space, (2) perform thermodynamic calculations of the equilibrating mineral assemblages for each composition at various equilibrium P,T, H2O and fO2 conditions, (3) calculate VP and VS for each phase assemblage for different in situ P,T conditions, and (4) use the resulting dataset to establish forward relationships between crustal composition and seismic velocity. These data will be compiled into a publicly available online database, which can be used by others to derive relationships between major element chemistry and seismic velocity under a specified range of crustal conditions. The project will support MIT/WHOI Joint Program student William Shinevar. To maximize the utility of the compositional inversion to the community we will develop a web-based Java applet to allow users to directly invert major element compositions from their own seismic datasets.