Compositional Attributes of the Deep Continental Crust Inferred From Geochemical and Geophysical Data

• Published in: Journal of geophysical research. Solid earth Vol. 127; no. 8
• Main Authors: Sammon, Laura G., McDonough, William F., Mooney, Walter D.
• Format: Journal Article
• Language: English
• Published: 01.08.2022
• Subjects: crust composition; geochemical; geophysical; lower crust; middle crust; seismology
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB024041

This study provides a global assessment of the abundance of the major oxides in the deep continental crust. The combination of geochemistry and seismology better constrains the composition of the middle and lower continental crust better than either discipline can achieve alone. The inaccessible nature of the deep crust (typically >15 km) forces reliance on analog samples and modeling results to interpret its bulk composition, evolution, and physical properties. A common practice relates major oxide compositions of small‐ to medium‐scale samples (e.g., medium to high metamorphic grade terrains and xenoliths) to large scale measurements of seismic velocities (Vp, Vs, Vp/Vs) to determine the composition of the deep crust. We provide a framework for building crustal models with multidisciplinary constraints on composition. 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. Our 3D compositional model of the deep crust uses the USGS Global Seismic Structure Catalog (Mooney, 2015) and a compilation of geochemical analyses on amphibolite and granulite facies lithologies (Sammon & McDonough, 2021, https://doi.org/10.1029/2021JB022791). We find a SiO2 gradient from 61.2 ± 7.3 to 53.3 ± 4.8 wt.% from the middle to the base of the crust, with the equivalent lithological gradient ranging from quartz monzonite to gabbronorite. In addition, we calculate trace element abundances as a function of depth from their correlations with major oxides. From here, other lithospheric properties, such as Moho heat flux (21.6−5.6+16.0 ${21.6}_{-5.6}^{+16.0}$ mW/m2), are derived. Plain Language Summary Using many different geophysical and geochemical techniques together helps us understand the composition of the bottom two‐thirds of the continental crust. We cannot sample much of the continental crust directly because of how deep it is. Instead, we rely on rocks that have been brought to the surface and measurements of the speed of seismic waves traveling through the crust in order to determine what the deepest parts of the crust are made of. Accounting for various factors, such as crust temperature and tectonic setting, allows us to create a large‐scale model for the composition of the deep crust. Key Points We present a global model for the composition of the deep continental crust constrained by geochemical and geophysical data Crustal SiO2 content decreases with increasing depth, and compositions correlate to relative depth rather than absolute depth Moho heat flux is predicted at 21.6−5.6+16.0 ${21.6}_{-5.6}^{+16.0}$ mW/m2