Abundances of uranium and thorium elements in Earth estimated by geoneutrino spectroscopy

TL;DR

This study uses 18 years of geoneutrino data from KamLAND to estimate Earth's uranium and thorium abundances, constraining their heat contributions and challenging certain Earth models.

Contribution

First spectroscopic measurement of uranium and thorium abundances in Earth using long-term geoneutrino observations, providing new constraints on Earth's heat sources.

Findings

KamLAND data constrains uranium and thorium heat contributions.

High-Q Earth model is disfavored at 99.76% confidence level.

Fully radiogenic model is excluded at 5.2 sigma.

Abstract

The decay of the primordial isotopes $^{238}\mathrm{U}$, $^{235}\mathrm{U}$, $^{232}\mathrm{Th}$, and $^{40}\mathrm{K}$ have contributed to the terrestrial heat budget throughout the Earth's history. Hence the individual abundance of those isotopes are key parameters in reconstructing contemporary Earth model. The geoneutrinos produced by the radioactive decays of uranium and thorium have been observed with the Kamioka Liquid-Scintillator Antineutrino Detector (KamLAND). Those measurements have been improved with more than 18-year observation time, and improvements in detector background levels mainly by an 8-year almost rector-free period now permit spectroscopy with geoneutrinos. Our results yield the first constraint on both uranium and thorium heat contributions. Herein the KamLAND result is consistent with geochemical estimations based on elemental abundances of chondritic meteorites and mantle peridotites. The High-Q model is disfavored at 99.76% C.L. and a fully radiogenic model is excluded at 5.2$\sigma$ assuming a homogeneous heat producing element distribution in the mantle.