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Abstract 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 SiO2gradient 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 ( mW/m2), are derived.
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A Geochemical Review of Amphibolite, Granulite, and Eclogite Facies Lithologies: Perspectives on the Deep Continental Crust
Abstract Debate abounds regarding the composition of the deep (middle + lower) continental crust. Exhumed medium‐ and high‐grade metamorphic rocks, which range in composition from mafic to felsic, provide information about the bulk composition of the deep crust. This study presents a global compilation of geochemical data on amphibolite (n = 6,500), granulite (n = 4,000), and eclogite (n = 200) facies lithologies and quantifies trends, uncertainties, and sources of bias in the deep crust sampling. The continental crust's Daly Gap is well documented in amphibolite and most granulite facies lithologies. Igneous differentiation processes likely control the compositional layering in the crust. Al2O3, Lu, and Yb vary little from top to bottom of the crust. In contrast, SiO2, light rare earth elements, Th, and U show a wider range of abundances throughout. Because of oversampling of mafic lithologies, our predictions are a lower bound on middle crustal composition. Additionally, the distinction between granulite facies terrains (intermediate SiO2, high heat production, high incompatibles) or granulite facies xenoliths (low SiO2, low heat production, low incompatibles) as being the best analogs of the deep crust remains disputable. We have incorporated both rock types, along with amphibolite facies lithologies, to define a deep crustal composition that approaches 57.6 wt.% SiO2. This number, however, represents a compositional middle ground; the shallower parts of the deep crust (middle crust) resemble quartz monzonite while the deepest portions (lower crust) more resemble a Ca‐rich monzonite. Future studies should analyze more closely the depth dependent trends in deep crustal composition to develop composition models that are not limited to a three‐layer crust.
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- Award ID(s):
- 2050374
- PAR ID:
- 10367052
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 126
- Issue:
- 12
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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