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The composition of the deep continental crust inferred from geochemical and geophysical data
  • Laura G Sammon,
  • William F mcdonough,
  • Walter D. Mooney
Laura G Sammon
University of Maryland

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William F mcdonough
University of Maryland
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Walter D. Mooney
USGS Earthquake Science Center
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Combing geochemical and seismological results constrains the composition of the middle and lower continental crust better than either field can achieve alone. The inaccessible nature of the deep crust (typically >15 km) forces reliance on analogue 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 deep crust global compositional model uses the USGS global seismic database (Mooney, 2015) and a compilation of geochemical analyses on amphibolite and granulite facies lithologies (Sammon McDonough, 2021). We find 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. In addition, we calculate trace element abundances as a function of depth from their relationships to major oxides. From here, other lithospheric properties, such as Moho heat flux, are derived (18.8 ± 8.8 mW/m2). This study provides a global assessment of major element composition in the deep continental crust.