THE COMPOSITION OF THE DEEP CONTINENTAL CRUST FROM GEOCHEMICAL AND SEISMOLOGICAL DATA

The modeling of combined geochemical, petrological, and seismological data places strong constraints on the composition of the deep continental crust. Here we focus on the bulk composition, evolution, and physical properties of the inaccessible middle- and lower-crust (typically deeper than 15 km). Major oxide compositions of small- to medium-scale samples (e.g., rocks from medium to high metamorphic grade terrains and xenoliths) can be related to large scale measurements of seismic velocities (Vp, Vs, Vp/Vs) via known physical properties. We present a global deep crustal model that documents compositional changes with depth and accounts for uncertainties in Moho depth, temperature, and physical and chemical properties. We found a compositional gradient from 61.2 ± 7.3 to 53.8 ± 3.0 wt.% SiO₂ from the middle to the base of the crust, with the equivalent lithological gradient ranging from quartz monzonite to gabbronorite. We also calculated the concentrations of eight other major oxides. In addition, we calculated trace element abundances as a function of depth from their relationships to major oxides. From here, we can calculate other lithospheric properties, such as Moho heat flux. This study provides a global assessment of major element composition in the deep continental crust and a framework for building future multiply constrained crustal composition models.

*Corresponding author: lsammon@umd.edu