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  1. Abstract

    Debate continues on the amount and distribution of radioactive heat producing elements (i.e., U, Th, and K) in the Earth, with estimates for mantle heat production varying by an order of magnitude. Constraints on the bulk‐silicate Earth's (BSE) radiogenic power also places constraints on overall BSE composition. Geoneutrino detection is a direct measure of the Earth's decay rate of Th and U. The geoneutrino signal has contributions from the local (40%) and global (35%) continental lithosphere and the underlying inaccessible mantle (25%). Geophysical models are combined with geochemical data sets to predict the geoneutrino signal at current and future geoneutrino detectors. We propagated uncertainties, both chemical and physical, through Monte Carlo methods. Estimated total signal uncertainties are on the order of20%, proportionally with geophysical and geochemical inputs contributing30% and70%, respectively. We find that estimated signals, calculated using CRUST2.0, CRUST1.0, and LITHO1.0, are within physical uncertainty of each other, suggesting that the choice of underlying geophysical model will not change results significantly, but will shift the central value by up to15%. Similarly, we see no significant difference between calculated layer abundances and bulk crustal heat production when using these geophysical models. The bulk crustal heat production is calculated as 7  2 TW, which includes an increase of 1 TW in uncertainty relative to previous studies. Combination of our predicted lithospheric signal with measured signals yield an estimated BSE heat production of 21.5  10.4 TW. Future improvements, including uncertainty attribution and near‐field modeling, are discussed.

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  2. Abstract

    Terrestrial planets (Mercury, Venus, Earth, and Mars) are differentiated into three layers: a metallic core, a silicate shell (mantle and crust), and a volatile envelope of gases, ices, and, for the Earth, liquid water. Each layer has different dominant elements (e.g., increasing iron content with depth and increasing oxygen content to the surface). Chondrites, the building blocks of the terrestrial planets, have mass and atomic proportions of oxygen, iron, magnesium, and silicon totaling ≥ 90% and variable Mg/Si (∼ 25%), Fe/Si (factor of ≥2), and Fe/O (factor of ≥ 3). What remains an unknown is to what degree did physical processes during nebular disk accretion versus those during post-nebular disk accretion (e.g., impact erosion) influence these planets final bulk compositions. Here we predict terrestrial planet compositions and show that their core mass fractions and uncompressed densities correlate with their heliocentric distance, and follow a simple model of the magnetic field strength in the protoplanetary disk. Our model assesses the distribution of iron in terms of increasing oxidation state, aerodynamics, and a decreasing magnetic field strength outward from the Sun, leading to decreasing core size of the terrestrial planets with radial distance. This distribution enhances habitability in our solar system and may be equally applicable to exoplanetary systems.

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  3. Abstract

    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 (53.77.2 wt.% SiO) and notably displays a gradient of decreasing SiOwith depth.

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  4. Abstract

    Potassium (K) informs on the radiogenic heat production, atmospheric composition, and volatile element depletion of the Earth and other planetary systems. Constraints on the abundance of K in the Earth, Moon, and other rocky bodies have historically hinged on K/U values measured in planetary materials, particularly comparisons of the continental crust and mid‐ocean ridge basalts (MORBs), for developing compositional models of the bulk silicate Earth (BSE). However, a consensus on the most representative K/U value for global MORB remains elusive despite numerous studies. Here, we statistically analyze a critical compilation of MORB data to determine the K/U value of the MORB source. Covariations in the log‐normal abundances of K and U establish that K is 3–7 times less incompatible than U during melting and/or crystallization processes, enabling inverse modeling to infer the K/U of the MORB source region. These comprehensive data have a mean K/U for global MORB = 13,900 ± 200 (2σm;n = 4,646), and define a MORB source region with a K/U between 14,000 and 15,500, depending on the modeled melting regime. However, this range represents strictly a lower limit due to the undefined role of fractional crystallization in these samples and challenges preserving the signatures of depleted components in the MORB mantle source. This MORB source model, when combined with recent metadata analyses of ocean island basalt (OIB) and continental crust, suggests that the BSE has a K/U value >12,100 and contains >260 × 10−6 kg/kg K, resulting in a global production of3.5 TW of radiogenic heat today and 1.5 × 1017 kg of40Ar over the lifetime of the planet.

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  5. Abstract

    We report the Earth's rate of radiogenic heat production and (anti)neutrino luminosity from geologically relevant short‐lived radionuclides (SLR) and long‐lived radionuclides (LLR) using decay constants from the geological community, updated nuclear physics parameters, and calculations of theβspectra. We track the time evolution of the radiogenic power and luminosity of the Earth over the last 4.57 billion years, assuming an absolute abundance for the refractory elements in the silicate Earth and key volatile/refractory element ratios (e.g., Fe/Al, K/U, and Rb/Sr) to set the abundance levels for the moderately volatile elements. The relevant decays for the present‐day heat production in the Earth (19.9 ± 3.0 TW) are from40K,87Rb,147Sm,232Th,235U, and238U. Given element concentrations in kg‐element/kg‐rock and densityρin kg/m3, a simplified equation to calculate the present‐day heat production in a rock isurn:x-wiley:ggge:media:ggge22244:ggge22244-math-0001

    The radiogenic heating rate of Earth‐like material at solar system formation was some 103to 104times greater than present‐day values, largely due to decay of26Al in the silicate fraction, which was the dominant radiogenic heat source for the first10 Ma. Assuming instantaneous Earth formation, the upper bound on radiogenic energy supplied by the most powerful short‐lived radionuclide26Al (t1/2= 0.7 Ma) is 5.5×1031 J, which is comparable (within a factor of a few) to the planet's gravitational binding energy.

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    Composition of terrestrial planets records planetary accretion, core–mantle and crust–mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 × CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile element depletion might correlate with the abundances of chondrules in the accreted materials, planetary size, and their accretion timescale, which provides insights into composition and origin of Mercury, Venus, the Moon-forming giant impactor, and the proto-Earth. During its formation before and after the nebular disk’s lifetime, the Earth likely accreted more chondrules and less matrix-like materials than Mars and chondritic asteroids, establishing its marked volatile depletion. A giant impact of an oxidized, differentiated Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced proto-Earth produced a Moon forming debris ring with mostly a proto-Earth’s mantle composition. Chalcophile and some siderophile elements in the silicate Earth added by the Mars-like impactor were extracted into the core by a sulfide melt (~0.5% of the mass of the Earth’s mantle). In contrast, the composition of Mars indicates its rapid accretion of lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars’ rapid cooling and early loss of its dynamo likely led to the absence of plate tectonics and surface water, and the present-day low surface heat flux. These similarities and differences between the Earth and Mars made the former habitable and the other inhospitable to uninhabitable. 
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  8. Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth’s composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulkMars and its core. The Martian volatility trend is consistent with <7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core. 
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