Model for the composition of the deep continental crust based on geochemical and geophysical data

The deep continental crust's chemical composition is central to the debate of crustal formation, evolution, strength, temperature, and bulk composition. The inaccessible nature of the deep crust (i.e., middle- and lower- crust, typically deeper than 15 km) forces reliance on analogue samples and modeling results to interpret its bulk composition and physical properties. However, major oxide compositions of small- to medium-scale samples collected from medium to high metamorphic grade terrains, as well as xenolith samples, can be related to large scale measurements of seismic velocities (Vp, Vs, Vp/Vs) via laboratory experiments. We use thermodynamic modeling software to relate observed seismic velocities to bulk compositions and mineralogy. We determine the best-fit geochemical model for bulk middle and lower crustal compositions. 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. Our model forecasts that more than three-quarters of the deepest continental crust has a concentration 45 to 55 wt.% SiO2. We also calculated trace element abundances as a function of depth from their relationships to major oxides, from which we can then calculate Moho heat flux. 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.