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Register a new userGeo-neutrinos and Silicate Earth Enrichment of U and Th
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Steve Dye
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The terrestrial distribution of U, Th, and K abundances governs the thermal evolution, traces the differentiation, and reflects the bulk composition of the earth. Comparing the bulk earth composition to chondritic meteorites estimates the net amounts of these radiogenic heat-producing elements available for partitioning to the crust, mantle, and core. Core formation enriches the abundances of refractory lithophile elements, including U and Th, in the silicate earth by ~1.5. Global removal of volatile elements potentially increases this enrichment to ~2.8. The K content of the silicate earth follows from the ratio of K to U. Variable enrichment produces a range of possible heat-producing element abundances in the silicate earth. A model assesses the essentially fixed amounts of U, Th, and K in the approximately closed crust reservoir. Subtracting these sequestered crustal amounts from the variable amounts in the silicate earth results in a range of possible mantle allocations, leaving global dynamics and thermal evolution poorly constrained. Terrestrial antineutrinos from {beta}-emitting daughter nuclei in the U and Th decay series traverse the earth with negligible attenuation. The rate at which large subsurface instruments observe these geo-neutrinos depends on the distribution of U and Th relative to the detector. Geo-neutrino observations with sensitivity to U and Th in the mantle are able to estimate silicate earth enrichment, leading to a more complete understanding of the origin, accretion, differentiation, and thermal history of the planet.
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We present the current status of geo-neutrino measurements and their implications for radiogenic heating in the mantle. Earth models predict different levels of radiogenic heating and, therefore, different geo-neutrino fluxes from the mantle. Seismic tomography reveals features in the deep mantle possibly correlated with radiogenic heating and causing spatial variations in the mantle geo-neutrino flux at the Earth surface. An ocean-based observatory offers the greatest sensitivity to the mantle flux and potential for resolving Earth models and mantle features. Refinements to estimates of the geo-neutrino flux from continental crust reduce uncertainty in measurements of the mantle flux, especially measurements from land-based observatories. These refinements enable the resolution of Earth models using the combined measurements from multiple continental observatories.
The deepest hole that has ever been dug is about 12 km deep. Geochemists analyze samples from the Earths crust and from the top of the mantle. Seismology can reconstruct the density profile throughout all Earth, but not its composition. In this respect, our planet is mainly unexplored. Geo-neutrinos, the antineutrinos from the progenies of U, Th and K40 decays in the Earth, bring to the surface information from the whole planet, concerning its content of natural radioactive elements. Their detection can shed light on the sources of the terrestrial heat flow, on the present composition, and on the origins of the Earth. Geo-neutrinos represent a new probe of our planet, which can be exploited as a consequence of two fundamental advances that occurred in the last few years: the development of extremely low background neutrino detectors and the progress on understanding neutrino propagation. We review the status and the prospects of the field.
We review a new interdisciplinary field between Geology and Physics: the study of the Earths geo-neutrino flux. We describe competing models for the composition of the Earth, present geological insights into the make up of the continental and oceanic crust, those parts of the Earth that concentrate Th and U, the heat producing elements, and provide details of the regional settings in the continents and oceans where operating and planned detectors are sited. Details are presented for the only two operating detectors that are capable of measuring the Earths geo-neutrinos flux: Borexino and KamLAND; results achieved to date are presented, along with their impacts on geophysical and geochemical models of the Earth. Finally, future planned experiments are highlighted.
Radiogenic heating is a key component of the energy balance and thermal evolution of the Earth. It contributes to mantle convection, plate tectonics, volcanoes, and mountain building. Geo-neutrino observations estimate the present radiogenic power of our planet. This estimate depends on the quantity and distribution of heat-producing elements in various Earth reservoirs. Of particular geological importance is radiogenic heating in the mantle. This quantity informs the origin and thermal evolution of our planet. Here we present: currently reported geo-neutrino observations; estimates of the mantle geo-neutrino signal, mantle radiogenic heating, and mantle cooling; a comparison of chemical Earth model predictions of the mantle geo-neutrino signal and mantle radiogenic heating; a brief discussion of radiogenic heating in the core, including calculations of geo-neutrino signals per pW/kg; and finally a discussion of observational strategy.
Geo-neutrino observations probe the quantities and distributions of terrestrial heat-producing elements uranium and thorium. The quantities of these elements gauge global radiogenic power, offering insights into the origin and thermal history of the Earth. The distributions reveal the initial partitioning and subsequent transport of these trace elements between metallic core, silicate mantle, and crust types. Ongoing observations at underground sites in Japan and Italy record the energies but not the directions of geo-neutrinos from uranium and thorium. Without directions pointing back to source regions, disentangling the signals from various reservoirs requires resolution of differing rates or energy spectra at separate sites. Due to limited statistics and site contrast, however, the observations at Japan and Italy do not yet measure distinct rates or energy spectra. Further analyses of the observations that derive fluxes, determine a signal from the mantle, and assess the global radiogenic power of uranium and thorium, depend on geochemical assumptions and model predictions. This letter discusses opportunities for eliminating or minimizing these dependencies through observations at dissimilar sites, producing robust geo-neutrino results.
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