The deepest parts of Earth's crust are widely inaccessible to traditional geochemical sampling and so their composition is poorly understood. Only in areas where eruptions have brought xenoliths to the surface or where tectonic activity has exhumed medium and high grade metamorphic terrains are we able to partially determine the composition of the deep (middle and lower) continental crust. Even so, these ex situ, aged, weathered, and transported rocks may not adequately represent the overall, current composition of the deep crust. Such inaccessibility has challenged geochemists for decades (Dumond et al., 2018), leading to competing models for continental crust and bulk silicate Earth (BSE) compositions, formation, and evolution (e.g., Javoy et al. (2010);McDonough et al. (2020); Turcotte and Schubert (2014)). Dissonance in the geochemical community stems from known and unknown unknowns; that is, we are mostly certain of the uncertainties in our geochemical and petrological measurements, but we are uncertain if our samples are truly representative of large swathes of the deep crust or if they are merely point samples. Xenoliths and terrains are the sum of the processes that form them, which may cause them to differ from what is presently at 15-45 km and deeper.Seismological techniques, however, provide another piece of the cipher by directly measuring the physical state of large sections of the deep crust. Physical properties (e.g., density, Poisson's ratio, Vp, and Vs) determined from these in situ geophysical experiments can be compared to laboratory experiments on rocks of known compositions, particularly medium to high grade metamorphic lithologies (amphibolite and granulite facies rocks) to

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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The composition of the lower continental crust is well studied but poorly understood because of the difficulty of sampling large portions of it. Petrological and geochemical analyses of this deepest portion of the continental crust are limited to the study of high‐grade metamorphic lithologies, such as granulite. In situ lower crustal studies require geophysical experiments to determine regional‐scale phenomena. Since geophysical properties, such as shear wave velocity (Vs), are nonunique among different compositions and temperatures, the most informative lower crustal models combine both geochemical and geophysical knowledge. We explored a combined modeling technique by analyzing the Basin and Range and Colorado Plateau of the United States, a region for which plentiful geochemical and geophysical data are available. By comparing seismic velocity predictions based on composition and thermodynamic principles to ambient noise inversions, we identified three compositional trends in the southwestern United States that reflect three different geologic settings. The Colorado Plateau (thick crust), Northern Basin and Range (medium crust), and Southern Basin and Range (thin crust) have intermediate, intermediate‐mafic, and mafic deep crustal compositions. Identifying the composition of the lower crust depends heavily on its temperature because of the effect it has on rock mineralogy and physical properties. In this region, we see evidence for a lower crust that overall is intermediate‐mafic in composition (53.7 ± 7.2 wt.% SiO2) and notably displays a gradient of decreasing SiO2 with depth.

<p>The deep continental crust's chemical makeup is central to the debate of crustal formation, evolution, strength, and bulk composition. The impenetrable depths and pressures of the deep (roughly > 10 km) crust force geoscientists to rely on indirect sampling methods, studying medium- to high-grade metamorphic terrains and xenoliths to ascertain the composition of the middle and lower continental crust. Analyzing the deep crust in situ requires geophysical data, such as seismic velocities: Vp, Vs, and the Vp/Vs ratio. Each method provides a different perspective on deep crustal composition, but alone, neither is definitive. </p><p>To address the nonuniqueness in crust composition modeling, we use thermodynamic modeling software (i.e. Perple_X) to relate observed seismic velocities to bulk compositions and mineralogies. We present a multidisciplinary model for the composition of Earth's deep crust, using geochemical and geophysical data. Through a Monte Carlo modeling approach, we determine the best-fit geochemical model for bulk middle and lower crustal compositions. For 12 different tectonic regimes, we quantify uncertainties in crustal composition, temperature, and seismic velocity while recognizing our own scientific biases. We present a global model of deep crustal composition conclude that regional scale geological variations benefit from a higher resolution model. Overall, our model forecasts 77% of the deepest continental crust has 45 to 55 wt.% SiO<sub>2</sub>; 15% 55 to 65 wt.% SiO<sub>2</sub>; 8% may have > 65 wt.% SiO<sub>2</sub>. Of perhaps equal or greater importance, however, we present a scalable, modular program that can be altered to incorporate additional petrological and geophysical constraints, allowing geoscientists to more easily compare different scenarios for the deep crust.</p>