Core-Mantle Co-Evolution E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Geophysical Monograph Series
• Sprache: Englisch

Seitenzahl: 888

Veröffentlichungsjahr: 2023

Beschreibung

New insights into interactions between the core and mantle. The Earth's deep interior is difficult to study directly but recent technological advances have enabled new observations, experiments, analysis, and simulations to better understand deep Earth processes. Core-Mantle Co-Evolution: An Interdisciplinary Approach seeks to address some of the major unsolved issues around the core-mantle interaction and co-evolution. It provides the latest insights into dynamics, structure, and evolution in the core-mantle boundary region. Volume highlights include: * Latest technological advances in high pressure experiments and their application to understanding the mineral physical properties and stability of phases in deep Earth * Recent progress in observational seismology, geochemical analysis, geoneutrino experiments, and numerical modeling for understanding the heterogeneity of the lower mantle * Theoretical investigations on thermal-chemical evolution of Earth's mantle and core * Exploring thermal-chemical-mechanical-electromagnetic interactions in the core-mantle boundary regions The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals.

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Core‐Mantle Co‐Evolution: An Interdisciplinary Approach

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Geophysical Monograph 276

Core‐Mantle Co‐Evolution

An Interdisciplinary Approach

Takashi NakagawaTaku TsuchiyaMadhusoodhan Satish‐KumarGeorge Helffrich

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LIST OF CONTRIBUTORS

Norikatsu Akizawa

Atmosphere and Ocean Research Institute

The University of Tokyo

Kashiwa, Japan

Thuany P. Costa de Lima

Research School of Earth Sciences

The Australian National University

Canberra, Australia

Christopher J. Davies

School of Earth and Environment

University of Leeds

Leeds, UK

David P. Dobson

Department of Earth Sciences

University College London

London, UK

Sanshiro Enomoto

Kavli Institute for the Physics and Mathematics of the Universe

The University of Tokyo

Kashiwa, Japan

and

Department of Physics

University of Washington

Seattle, Washington, USA

Sam Greenwood

School of Earth and Environment

University of Leeds

Leeds, UK

Satoru Haraguchi

Research Institute for Marine Geodynamics

Japan Agency for Marine‐Earth Science and Technology

Yokosuka, Japan

and

Earthquake Research Institute

The University of Tokyo

Tokyo, Japan

Simon A. Hunt

Department of Materials

University of Manchester

Manchester, UK

Tsuyoshi Iizuka

Department of Earth and Planetary Sciences

The University of Tokyo

Tokyo, Japan

Akira Ishikawa

Department of Earth and Planetary Sciences

Tokyo Institute of Technology

Tokyo, Japan

Shun‐ichiro Karato

Department of Earth and Planetary Sciences

Yale University

New Haven, Connecticut, USA

Kenji Kawai

Department of Earth and Planetary Sciences

The University of Tokyo

Tokyo, Japan

Tetsu Kogiso

Graduate School of Human and Environmental Studies

Kyoto University

Kyoto, Japan

Tomoaki Kubo

Department of Earth and Planetary Sciences

Kyushu University

Fukuoka, Japan

Yasuhiro Kuwayama

Department of Earth and Planetary Sciences

The University of Tokyo

Tokyo, Japan

Kyoko Matsukage

Department of Natural and Environmental Science

Teikyo University of Science

Uenohara, Japan

William F. McDonough

Research Center for Neutrino Science

Tohoku University

Sendai, Japan

and

Department of Earth Sciences

Tohoku University

Sendai, Japan

and

Department of Geology

University of Maryland

College Park, Maryland, USA

Takashi Nakagawa

Department of Planetology

Kobe University

Kobe, Japan

and

Department of Earth and Planetary System Science

Hiroshima University

Higashi‐Hiroshima, Japan

Yoichi Nakajima

Department of Physics

Kumamoto University

Kumamoto, Japan

Yu Nishihara

Geodynamics Research Center

Ehime University

Matsuyama, Japan

Kenji Ohta

Department of Earth and Planetary Sciences

Tokyo Institute of Technology

Tokyo, Japan

Toshiki Ohtaki

Geological Survey of Japan

National Institute of Advanced Industrial Science and Technology

Tsukuba, Japan

Yoshiyuki Okuda

Department of Earth and Planetary Sciences

Tokyo Institute of Technology

Tokyo, Japan

Thanh‐Son Phạm

Research School of Earth Sciences

The Australian National University

Canberra, Australia

Youhei Sasaki

Department of Information Media

Hokkaido Information University

Ebetsu, Japan

Gen Shimoda

Geological Survey of Japan

National Institute of Advanced Industrial Science and Technology

Tsukuba, Japan

Ryosuke Sinmyo

Department of Physics

Meiji University

Kawasaki, Japan

Katsuhiko Suzuki

Submarine Resources Research Center

Japan Agency for Marine‐Earth Science and Technology

Yokosuka, Japan

Shin‐ichi Takehiro

Research Institute for Mathematical Sciences

Kyoto University

Kyoto, Japan

Nozomu Takeuchi

Earthquake Research Institute

The University of Tokyo

Tokyo, Japan

Akiko Tanaka

Geological Survey of Japan

National Institute of Advanced Industrial Science and Technology

Tsukuba, Japan

Satoru Tanaka

Research Institute for Marine Geodynamics

Japan Agency for Marine‐Earth Science and Technology

Yokosuka, Japan

Andrew R. Thomson

Department of Earth Sciences

University College London

London, UK

Hrvoje Tkalčić

Research School of Earth Sciences

The Australian National University

Canberra, Australia

Yumiko Tsubokawa

Department of Earth and Planetary Sciences

Kyushu University

Fukuoka, Japan

Taku Tsuchiya

Geodynamics Research Center

Ehime University

Matsuyama, Japan

Noriyoshi Tsujino

Institute for Planetary Materials

Okayama University

Misasa, Japan

Kenta Ueki

Research Institute for Marine Geodynamics

Japan Agency for Marine‐Earth Science and Technology

Yokosuka, Japan

Hiroko Watanabe

Research Center for Neutrino Science

Tohoku University

Sendai, Japan

Daisuke Yamazaki

Institute for Planetary Materials

Okayama University

Misasa, Japan

Takashi Yoshino

Institute for Planetary Materials

Okayama University

Misasa, Japan

PREFACE

The Earth's deep interior, being physically inaccessible, is difficult to study directly. The necessarily indirect methods used in its study are best pursued collaboratively in order to bring all possible sources of knowledge to bear on the topic, hence the need for interdisciplinary research. Over recent decades, there have been advances in investigating the dynamics of the Earth's deep interior. In terms of experimental and observation work, there have been innovations in high‐temperature and high‐pressure experiments (employing diamond anvil cells and synchrotron radiation facilities), dramatically improved geochemical analyses aided by particle physics detectors, and seismic wave observations and theory. In terms of computational work, methodological innovations and increased computational power have facilitated theoretical calculations of mineral properties and fluid dynamical simulations at the micro and macro scale.

This monograph describes results of the research project “Core‐Mantle Co‐Evolution” that was selected by the Ministry of Education, Culture and Sports (MEXT) in Japan. It was a component of a national program of innovative research projects intended to apply technological innovations in an interdisciplinary framework to contemporary research questions, in this case the composition, dynamics and evolution of the Earth's deep interior.

Recent observational and experimental investigations have significantly advanced our understanding of the structure and constituent materials of the deep Earth. However, details of the chemical composition of the mantle, accounting for 85% of the volume of the entire Earth, and light elements expected to exist in the core, corresponding to the remaining 15%, have remained unclear even after 60 years of research in various fields of science related to deep Earth.

Seismological evidence suggests vigorous convection at the core‐mantle boundary region, whereas geochemical signatures suggest the presence of stable regions that hold the chemical fingerprints of early Earth's formation 4.6 billion years ago. In addition, the concentrations and disposition of various radioactive isotopes that act as the heat sources driving various dynamic behaviors of the deep Earth are also still largely unknown. With this backdrop the “Core‐Mantle Co‐Evolution” project attempted to tackle the unresolved mysteries of deep Earth science through comprehensive investigations of the interactions between the core and mantle by combining high‐pressure and high‐temperature experiments, geochemical analyses at different scales, high‐resolution geophysical observations, and large‐scale numerical simulations.

The 12 chapters comprise interdisciplinary contributions by internationally recognized researchers in Earth's deep interior. Part I summarizes recent research on the structure, composition, and dynamics of the Earth's deep mantle, which is the primary source of evidence for core‐mantle interaction. The first two chapters introduce the geoneutrino observations that are expected to reveal the distribution of heat sources in the Earth's deep mantle. Chapter 1 (McDonough and Watanabe) is a comprehensive review on the present status of geoneutrino observations. This is followed by Chapter 2 (Enomoto et al.) on delineating the heat source distribution in the deep mantle. The structure of seismic wave velocities at the deepest part of the mantle is described in Chapter 3 (Tanaka and Ohtaki), which evaluates the structure of the edge of a large‐scale low‐velocity region in the western Pacific region based on the deployment and observation of a seismic array in Thailand. Recent developments on experimental facilities for deformation at lower mantle conditions are elegantly presented in Chapter 4 on bridgmanite (Thomson et al.), which is the major mineral phase in the lower mantle. Next, in Chapter 5 (Suzuki et al.), a comprehensive geochemical and isotopic analysis of mantle‐derived rocks forms the basis of a discussion on core‐mantle co‐evolution where the authors suggest ways to decipher the isotope signature to fingerprint any core‐mantle interaction. Finally, in Chapter 6 (Tsuchiya et al.), the density structure of the deep mantle and its modeling of mantle dynamics are presented to provide a feasible interpretation of observational and experimental studies.

Part II summarizes the results of studies that invoke core‐mantle interaction from direct inferences. Chapter 7 (Karato) introduces the results of a high‐pressure mass diffusion experiment that found how iron originated from the core seeps into the mantle side, suggesting the possibility of an actual chemical interaction between the core and mantle. Chapter 8 (Okuda and Ohta) looks at heat flow across the core‐mantle boundary, the most important constraint for core‐mantle co‐evolution, which is measured under high‐pressure and high‐temperature conditions. Chapter 9 (Nakagawa et al.) shows that heat flow is also a crucial parameter in the fluid dynamics theory on the formation of stably stratified region below the core‐mantle boundary. A review of seismic velocity structure in the inner core is presented in Chapter 10 (Tkalčić et al.), while Chapter 11 (Sinmyo et al.) describes the continuing search for light‐element candidates for the Earth's core based on high‐temperature and high‐pressure experiments. Finally, Chapter 12 (Davies and Greenwood) presents a comprehensive discussion on the generation and maintenance mechanisms of the Earth's magnetic field based on chemical interactions between the core and mantle.

In assembling this monograph, our goal has been to show how an interdisciplinary approach can reveal what happens in Earth's deep interior. We hope that the next generation of brilliant researchers will build on the research results presented here and continue to produce innovative results that will further elucidate the dynamics and evolution of the Earth's deep interior.

We express sincere thanks to everyone who has contributed to this monograph as chapter authors and reviewers. We also thank Kate Lajtha, Editor in Chief AGU Books, and her team for their encouragement with this project, which had to pass through the tough period of pandemic‐related delays. We take this opportunity to thank the authors and publishers for their patience in completing this book.

Takashi Nakagawa

Kobe University and Hiroshima University, Japan

Taku Tsuchiya

Ehime University, Japan

Madhusoodhan Satish‐Kumar

Niigata University, Japan

George Helffrich

Tokyo Institute of Technology, Japan

Part IStructure and Dynamics of the Deep Mantle: Toward Core‐Mantle Co‐Evolution

1Neutrino Geoscience: Review, Survey, Future Prospects

William F. McDonough1,2,3 and Hiroko Watanabe1

1Research Center for Neutrino Science, Tohoku University, Sendai, Japan

2Department of Earth Sciences, Tohoku University, Sendai, Japan

3Department of Geology, University of Maryland, College Park, Maryland, USA

ABSTRACT

The Earth's surface heat flux is 46 ± 3 TW (terawatts, 1012 watts). Although many assume we know the Earth's abundance and distribution of radioactive HPEs (i.e., U, Th, and K), estimates for the mantle's heat production varying by an order of magnitude and recent particle physics findings challenge our dominant paradigm. Geologists predict the Earth's budget of radiogenic power at 20 ± 10 TW, whereas particle physics experiments predict TW (KamLAND, Japan) and TW (Borexino, Italy). We welcome this opportunity to highlight the fundamentally important resource offered by the physics community and call attention to the shortcomings associated with the characterization of the geology of the Earth. We review the findings from continent‐based, physics experiments, the predictions from geology, and assess the degree of misfit between the physics measurements and predicted models of the continental lithosphere and underlying mantle. Because our knowledge of the continents is somewhat uncertain ( TW), models for the radiogenic power in the mantle (3.5–32 TW) and the bulk silicate Earth (BSE; crust plus mantle) continue to be uncertain by a factor of ∼10 and ∼4, respectively. Detection of a geoneutrino signal in the ocean, far from the influence of continents, offers the potential to resolve this tension. Neutrino geoscience is a powerful new tool to interrogate the composition of the continental crust and mantle and its structures.

1.1 INTRODUCTION

Core‐mantle evolution involves understanding Earth's differentiation processes, which established the present‐day distribution of the HPEs, and its dynamic consequences (i.e., the radiogenic heat left in the mantle powering mantle convection, plate tectonics, and the geodynamo). The energy to drive the Earth's engine comes from two different sources: primordial and radiogenic. Primordial energy represents the kinetic energy inherited during accretion and core formation. Radiogenic energy is the heat of reaction from nuclear decay. We do not have a constraint on the proportion of these different energy sources driving the present‐day Earth's dynamics. In turn, this means that we do not have sufficient constraint on the thermal evolution of the planet, aside from first‐order generalities. You might ask, is this important? We ask the question – how much energy (and time) is left to keep the Earth habitable?

We understand that the Earth started out hot due to abundant accretion energy, the gravitational energy of sinking metal into the center, a giant impact event for the formation of the Earth's Moon, and energy from short‐lived (e.g., 26Al and 60Fe) and long‐lived (K, Th, and U) radionuclides. From this hot start the planet should quickly lose some of its initial energy, although the amount and rate are unknowns. There are many significant unknowns regarding the thermal evolution of the Earth: (1) the nature and presence (or absence) and lifetime of an early atmosphere, which has a thermal blanketing effect; (2) the compositional model for the Earth, particularly the absolute abundances of the HPEs (K, Th, and U); (3) the cooling rate of the mantle (present‐day estimates: 100 ± 50 K/Ga); and (4) the rate of crust formation and thus extraction of HPEs from the mantle.

The recent recognition (Krauss et al., 1984) and subsequent detection (Araki et al., 2005) of the planet's geoneutrino emission have opened up a new window into global scale geochemistry of the present‐day Earth. The measurement of this flux presents Earth scientists with a transformative opportunity for new insights into the composition of the Earth and its energy budget. For the most part, solid Earth geophysics measures and parameterizes the present‐day state of the planet. In contrast, solid Earth geochemistry measures and parameterizes its time‐integrated state, mostly on a hand sample scale and then extrapolates these insights to larger scales. The advent of measuring the Earth's geoneutrino flux allows us, for the first time, to get a global measure of its present‐day amount of Th and U.

This chapter is organized as follows: we provide the rationale for the field of neutrino geoscience and define some terms (section 1.2). We review the existing and developing detectors, the present‐day detection methods, and future technologies (section 1.3). We discuss the latest results from the physics experiments (section 1.4). We present the range of compositional models proposed for the Earth (section 1.5) followed by a discussion of the geological prediction of the geoneutrino fluxes at various detectors (section 1.6). We finish with a discussion on determining the radioactive power in the mantle (section 1.7) and future prospects (section 1.8).

1.2 NEUTRINO GEOSCIENCE

The field of neutrino geoscience focuses on constraining the Earth's abundances of Th and U and with these data we can determine: (1) the absolute concentration of refractory elements in the Earth and from that determine the BSE's composition (crust plus mantle), and (2) the amount of radiogenic power in the Earth driving the planet's major dynamic processes (e.g., mantle convection, plate tectonics, magmatism, and the geodynamo). These two constraints set limits on the permissible models for the composition of the Earth and its thermal evolutionary history.

First, the refractory elements are in constant relative abundances in all chondrites. There are 36 of these elements (e.g., Al, Ca, Sr, Zr, REE, Th, and U) and by establishing the absolute abundance of one defines all abundances, since refractory elements exist in constant ratios to each other (McDonough & Sun, 1995). Most of these elements are concentrated in the bulk silicate Earth, but not all (e.g., Mo, W, Ir, Os, Re, etc.) and these latter ones are mostly concentrated in the metallic core. Knowing the Earth's abundance of Ca and Al, two of the eight most abundant elements (i.e., O, Fe, Mg, Si, Ca, Al, Ni, and S) that make up terrestrial planets (i.e., 99%, mass and atomic proportions) define and restrict the range of accepted compositional models of the bulk Earth and BSE.

Second, the decay of 40K, 232Th, 238U, and 235U (i.e., HPE) provides the Earth's radiogenic power, accounts for 99.5% of its total radiogenic power, and is estimated to be TW (1 TW = 1012 watts). This estimate, however, assumes a specific BSE model composition (McDonough & Sun, 1995; Palme & O'Neill, 2014). It must be noted that there is no consensus on the composition of the BSE, and so predictions from competing compositional models span from about 10–38 TW (Agostini et al., 2020; Javoy et al., 2010). This uncertainty in our present state of knowledge means that the field of neutrino geoscience plays a crucial role in resolving fundamental questions in Earth sciences.

1.2.1 Background Terms

The field of neutrino geoscience spans the disciplines of particle physics and geoscience, including geochemistry and geophysics. The following list of terms are offered to support this interdisciplinary research field.

Alpha () decay: a radioactive decay process that reduces the original nuclide () by four atomic mass units by the emission of a He nucleus and reaction energy (). Commonly, the particle is emitted with between 4 and 9 MeV (1 MeV = 106 eV) of discrete kinetic energy. The basic form of decay is as follows:

(1.1)

Beta decay : a radioactive decay process that transforms the original nuclide () into an isobar (same mass ) with the next lower proton number () during either electron capture () or decays or, alternatively, the next higher proton number () during decay. During each decay, there is an exchange of two energetic leptons (i.e., beta particles) and reaction energy (). Basic forms are as follows:

(1.2)

Beta particles : first‐generation energetic leptons, either matter leptons (electrons and neutrinos: and ) or antimatter leptons (positrons and antineutrinos: and ).

Chondrite: an undifferentiated stony meteorite containing chondrules (flash‐melted spheres, sub‐mm to several mm across), matrix [fine grained (micron scale) aggregate of dust and crystals], and sometimes Ca‐Al‐inclusions and other refractory phases. They are typically mixtures of silicates and varying amounts of Fe‐Ni alloys and classified into groups based on their mineralogy, texture, and redox state. Three dominant groups are the carbonaceous, ordinary, and enstatite type chondrites, from most oxidized to reduced, respectively. Isotopic observations are also used to create a twofold classification of chondrites and related meteorites (i.e., the NC and CC groups). The NC (non carbonaceous) group includes enstatite and ordinary chondrites and is believed to have formed in the inner solar system inside of Jupiter. The CC (carbonaceous) group includes carbonaceous chondrites and is believed to have formed in the outer solar system from Jupiter and beyond. The CI carbonaceous chondrite type (the sole chondrite type lacking chondrules) is considered most primitive because its element abundances matches that of the solar photosphere 1:1 over 6 orders of magnitude, except for the noble and H‐C‐N‐O gases.

Geoneutrinos: naturally occurring electron antineutrinos (, with the over‐bar indicating it is an antimatter particle), mostly, and electron neutrinos (), much less so, produced during , and [ and ] decays, respectively. The interaction cross‐sections, which scale with their energy, for the detectable geoneutrinos (i.e., Th and U) is on the order of m2. Consequently, these particles rarely interact with matter in the Earth. The Earth's geoneutrino flux is 1025 s−1 (McDonough et al., 2020). Each neutrino leaving the Earth removes a portion of the Earth's radiogenic heat ().

Heat‐producing elements (HPEs): potassium, thorium, and uranium (i.e., K, Th, and U, or more specifically 40K, 232Th, 235U, and 238U) account for ∼99.5% of the Earth's radiogenic heating power.

Inverse Beta Decay (IBD): a nuclear reaction used to detect electron antineutrinos in large underground liquid scintillation detectors that are surrounded by thousands of photomultiplier tubes. The reaction [] involves a free proton (i.e., H atom) capturing a through going and results in two flashes of light close in space and time. The first flash of light involves annihilation (order a picosecond following interaction) and the second flash (∼0.2 ms later) comes from the capture by a free proton of the thermalizing neutron. This coincidence of a double light flash in space and time, with the second flash having 2.2 MeV light, reduces the background by a million‐fold. This reaction requires the to carry sufficient energy to overcome the reaction threshold energy of  MeV. Thus, the IBD reaction restricts us to detecting only antineutrinos from the decays in the 238U and 232Th decay chains.

Major component elements: a cosmochemical classification term for Fe, Ni, Mg, & Si, with half‐mass condensation temperatures ( between 1355 and 1250 K. These elements condense from a cooling nebular gas into silicates (first olivine, then pyroxene) and Fe, Ni alloys and together with oxygen represents 93% of terrestrial planet's mass (McDonough & Yoshizaki, 2021).

Earth and its parts: the Earth is chemically differentiated. It has a metallic core surrounded by the BSE (aka Primitive Mantle), which initially included the mantle, oceanic and continental crust, and the hydrosphere and atmosphere; the Primitive Mantle is the undegassed and undifferentiated Earth minus the core. The present‐day silicate Earth, less the hydrosphere and atmosphere, is made up of the mantle, including its bottom thermally conductive boundary layer (D”), and the top lithosphere; the latter composed of the crust an underlying lithospheric mantle. The lithosphere is the mechanically stiff (i.e., higher viscosity than the underlying asthenospheric mantle) thermally conductive boundary layer. In the continents, the zone above the Moho (a seismically defined boundary between the crust and mantle) is the continental crust and below the continental lithospheric mantle (CLM). Masses and thicknesses of these domains are listed in Table 1.1.

Table 1.1 Mass of the Earth and its parts

Domain/reservoir

Thickness km (±)

Mass kg (±)

Citation

a

Earth

6,371 ()

5.97218 (60) × 10

24

[1]

Bulk Silicate Earth (BSE)

2,895 (5)

4.036 (6) × 10

24

[2]

Modern mantle (DM

b

+ EM

c

domains)

2,867 (20)

d

4.002 (20) × 10

24

[2]

Oceanic crust

e

10.5 (4.3)

0.92 (11) × 10

22

[3]

Continental crust

e

40 (9)

2.22 (26) × 10

22

[3]

Continental lithospheric mantle (CLM)

115 (80)

6.3 (0.8) × 10

22

[3]

a cited source: 1 = Chambat et al. (2010), 2 = Dziewonski and Anderson (1981), 3 = Wipperfurth et al. (2020)

b DM = Depleted Mantle, the chemically depleted source of MORB (mid‐ocean ridge basalts), which is viewed as the chemical complement to the continental crust.

c EM = Enriched Mantle, a smaller (e.g., mass), deeper, and more chemically enriched source of OIB (ocean island basalts).

d From PREM, assuming a uniform surface crust of 24 km.

e Using a LITHO1.0 model, see Table 1 in Wipperfurth et al. (2020).

Moderately volatile elements: a cosmochemical classification term for elements with half‐mass condensation temperatures ( between 1250 and 600 K. These elements include the alkali metals (lithophile), some transition metals, all the other metals, less Al, and the pnictogens and chalcogens, less N and O.

Primordial energy: the energy in the Earth from accretion and core formation. Accretion kinetic energy is ∼1032 J, assuming an Earth mass (5.97 × 1024 kg) and 10 km/s as an average velocity of accreting particles. The gravitational energy of core formation, which translates to heating energy, is J, depending on the assumed settling velocity of a core‐forming metal in the growing Earth.

Radiogenic energy: energy of a nuclear reaction () resulting from radioactive decay, given in units of MeV (1 MeV = 106 eV) or pJ (1 pJ = 10−12 J), where 1MeV = 0.1602 pJ. For decays (MeV) = (massp – massd) × 931.494, with massp (mass of parent isotope), massd (mass of daughter isotope) in atomic mass units (1 amu = 1.660539 ×  kg = 931.494 MeV), and for decay, (MeV) = (massp – massd – mass) × 931.494, where , or 1 amu = 0.931494 GeV/c2.

Refractory elements: a cosmochemical classification term for elements with half‐mass condensation temperatures ( 1355 K; they condense at the earliest stage of the cooling of high‐temperature nebular gas. These elements are in equal relative proportion (±15%) in chondritic meteorites. In terrestrial planets, many of these elements are classified as lithophile (dominantly coupling with oxygen and hosted in the crust and mantle), or siderophile (dominantly metallically bonded and hosted in the core). The refractory elements include: Be, Al, Ca, Ti, Sc, V, Sr, Y, Zr, Nb, Mo, Rh, Ru, Ba, REE, Hf, Ta, W, Re, Os, Ir, Pt, Th, and U. The core contains 90% of the Earth's budget of Mo, Rh, Ru, W, Re, Os, Ir, and Pt, about half of its V, and potentially a minor fraction of its Nb.

Surface heat flux: the total surface heat flux from the Earth's interior is reported as 46 ± 3 TW (Jaupart et al., 2015) or 47 ± 2 TW (Davies, 2013). On average the surface heat flux is about 86 mW/m2, with that for the continents being 65 mW/m2 and for the oceans being 96 mW/m2 (Davies, 2013). Energy contributions to this surface flux come from the core (primordial, plus a minor [∼1% of the surface total] amount due to inner core crystallization), mantle (a combination of primordial and radiogenic), and crust (radiogenic). Other contributions include negligible additions from tidal heating and crust–mantle differentiation.

1.3 DETECTORS AND DETECTION TECHNOLOGY

Electron antineutrinos () come mostly from the radioactive decays of 40K, 232Th, 235U, and 238U (i.e., geoneutrinos; Krauss et al., 1984), plus contributions from local anthropogenic sources (i.e., nuclear reactor plants). The Earth emits some 1025 s−1 geoneutrinos (McDonough et al., 2020), with 65% coming from decays of 40K (Fig. 1.1).

Figure 1.1 Relative proportions of the Earth's present‐day flux of naturally occurring geoneutrinos (left) and the detectable geoneutrinos (right) from the 232Th decay chain (gray; 228Ac and 212Bi) and 238U decay chain (blue; 234Pa and 214Bi, not shown is the negligible contribution from 212Tl) (see also Table 1.2). 40K has two branches: to 40Ca (65%) and to 40Ar (8%).

The detection of an electron antineutrino uses the IBD reaction: , which has a reaction threshold energy of  MeV.

(1.3)

assuming the laboratory frame (i.e., stationary target) and where , , and are the masses of the proton (938.272 MeV), neutron (939.565 MeV), and electron (0.5110 MeV). The neutrino mass is unknown and contributes negligibly to this reaction. Although its upper limit is <0.8  (Aker et al., 2022), estimates of the neutrino's mass is of the order of 10s–100s of (de Salas et al., 2018). This energy threshold restricts the detectable antineutrinos to decays from the 238U and 232Th decay chains (Fig. 1.1).

1.3.1 Technical Details for Detecting Geoneutrinos

Here, we highlight some relevant aspects of the detection scheme.

Detection occurs when an antineutrino interacts with a free proton, transforming it to a neutron plus a positron, which then causes two flashes of light close in space and time. Each flash of light occurring in these large liquid scintillation spectrometers, which are sited 1–2 km underground to shield them from descending, atmospherically produced muons, are detected by the thousands of photomultiplier tubes covering the inner walls, each facing the detector's central volume.

Energy conservation requires that , with and being the kinetic energy of the positron and neutron. The prompt event involves the positron being annihilated in picoseconds by an electron, with the signal being the sum of the reaction releasing a 1.022 MeV energy flash (the sum of the masses of these two leptons) plus the characteristic kinetic energy inherited by the positron from the antineutrino (, or =  [MeV]). The accompanying emitted neutron undergoes a cascade of collisions (thermalizing events, loss of energy to its surroundings as it goes toward thermal equilibrium) over about 200 s and approximately 15 cm from the initial interaction point. This neutron is ultimately captured by a second free proton creating 2H, resulting in a 2.22 MeV (binding energy) flash. This rare event sequence is eminently detectable because of its characteristics: two flashes of light in space and time, with the second flash having a specific energy. This reaction chain eliminates most background and improves the signal to noise ratio by a million‐fold.

The organic liquid scintillator is mostly a long chain, aromatic ring hydrocarbon with approximately an H:C proportion of ∼2. A wavelength shifting fluor compound is added to the scintillator to set the fluorescence peak emission at ∼350–400 nm in order to reach the maximum quantum efficiency of the photomultiplier tubes. The photon yield for the liquid scintillator is typically a light yield of 200–400 photons/MeV.

The interaction cross‐section of antineutrinos (and neutrinos) scales with their energy. For each decay, there is a spectrum of emitted energies, which in turn means a spectrum of interaction cross‐sections. For IBD detection (i.e., starting at 1.806 MeV), the probability of a geoneutrino detection is low (order 1/1019), given that their cross‐sections are between and m2 (Vogel & Beacom, 1999). The overall emission above the energy threshold level are 0.40 U per decay and 0.156 Th per decay. There are four decay chains which produce detectable antineutrinos: two from the 232Th and two from the 238U decay chains (Table 1.2). The BSE has a Th/U mass ratio of 3.77 (or molar Th/U = 3.90) (Wipperfurth et al., 2018). However, despite Th being four times more abundant than U, attributes of the IBD detection method (i.e., , branching fraction, and ) make U much easier to detect.

Table 1.2 Detectable events

decay events

Max E

a

(MeV)

Branching fraction

Max IBD cross–section

b

( cm

2

)

% of total

c

signal

232

Th decay chain

228

Ac

228

Th

2.134

1.00

4.3

1

212

Bi

212

Po

2.252

0.64

4.8

20

238

U decay chain

234

Pa

234

U

2.197

1.00

4.6

31

214

Bi

214

Po

3.270

1.00

33

46

212

Tl

212

Po

4.391

0.0002

90

1%

aThere is a spectrum of energies for each antineutrino generated during a decay, where typically the takes about and the about of the reaction energy ().

bAs the energy of the antineutrino decreases so does its interaction cross‐section.

cNumbers from Tables 3 and 5 in Fiorentini et al. (2007).

1.3.2 Detectors: Existing, Being Built, Being Planned

Currently, there are two detectors (Fig. 1.2) measuring the Earth's geoneutrino flux: KamLAND (1 kt), in Kamioka, Japan (Araki et al., 2005; Watanabe, 2019), and SNO+ (1 kt) in Sudbury, Ontario, Canada (Andringa et al., 2016), experiments. The Borexino detector (0.3 kt), in Gran Sasso, Italy (Agostini et al., 2020), has finished. The JUNO (20 kt) experiment in Jiangmen, Guangdong province, China (An et al., 2016), is currently being built. The Jinping (∼4 kt) experiment in Jinping Mountains, Sichuan province, China (Beacom et al., 2017), is in development, with prototype detectors onsite testing future detector materials and technologies. Detectors in the proposal stage include Baksan in the Caucasus mountains in Russia (Domogatsky et al., 2005), Andes in Agua Negra tunnels linking the borders of Chile and Argentina (Dib et al., 2015), and a proposed ocean bottom detector (OBD). Significantly, the Andes detector is the only proposed detector to be sited in the Southern Hemisphere.

Figure 1.2 Present‐day global distribution of detectors counting geoneutrino (red), have counted (purple), and are in the development and/or planning stage (blue). JUNO (bold blue) is in the construction phase. Background figure is the calculated global geoneutrino flux (order 106 cm−2 s−1) (Usman et al., 2015); the relatively high flux density seen in the Himalayas is directly correlated with its greater crustal thickness.

There are ongoing developments for a ocean‐going detector. A team of particle physicists from the University of Hawaii put forth a proposal more than 10 years ago for an OBD called Hanohano (Learned et al., 2007). A team of Japanese particle physicists and engineers and Earth scientists from JAMSTEC (Japan Agency for Marine‐Earth Science and Technology) are currently moving forward with a project to deploy and test a mobile prototype detectors (“Ocean Bottom Detector” (OBD) scale is not yet set, but envisaged to up to 1 ton) off the coast of Japan. A mobile ocean‐going detector offers a complementary measurement to land‐based experiments. By sitting in the middle of the Pacific Ocean, 3,000 km from South America, 3,000 km from Australia, and ∼3,000 km from the core‐mantle boundary, such a detector gets a “mostly‐mantle” signal.

1.4 LATEST RESULTS FROM THE PHYSICS EXPERIMENTS

Results from the physics experiments follow counting statistics, with increasing exposure (time spent counting) uncertainties reduce. These experiments are attentive to systematic and statistical uncertainties and addressed these issues in great detail in their publications.

The measured geoneutrino flux is reported in cm−2 μs−1 for the KamLAND experiment and in TNU (Terrestrial Neutrino Unit) for the Borexino experiments. Mantovani et al. (2004) introduced TNU as a way to normalize the differences between detectors. A 1 TNU signal represents the detection of one event in a 1 kiloton liquid scintillation detector over a one‐year exposure with a 100% detection efficiency. A 1 kiloton liquid scintillation detector has free protons (the detection target). Each detector has its own efficiency relative to a 1 kiloton fiducial volume detector, which accounts for the differences in the size of detector, its photomultiplier coverage, its response efficiency of the scintillator, and other factors. The conversion factor between signal in TNU and flux in cm−2 μs−1 depends on the Th/U ratio and has a value of 0.11 cm−2 μs−1 TNU−1 for Earth's Th/U of 3.9.

The most recent results for the KamLAND and Borexino experiments are 32.1 ± 5.0 (Watanabe, 2019) and 47.0 (Agostini et al., 2020) TNU, respectively. [The SNO+ detector began counting in 2020 with a partial filled volume (with delays due to the covid‐19 pandemic) and is yet to report their data.] The conversion between TNU and TW depends on the geological model assumed for the distribution of Th and U. Using geological models developed for both experiments, the radiogenic heating of the Earth ranges from 14 to 25 TW for KamLAND and 19 to 40 TW for Borexino (McDonough et al., 2020; Wipperfurth et al., 2020). The combined KamLAND and Borexino results mildly favors an Earth model with 20 TW present‐day total power. Development of these and others geological models for each experiment are discussed later.

1.5 COMPOSITIONAL MODELS FOR THE EARTH

Earth scientists estimate the planet's radiogenic power within the bounds of 20 ± 10 TW (1012