Geoneutrino Fluxes From A Laterally Heterogeneous Lower Mantle: Constraints From Geophysics And Geochemistry

Knowledge of the abundances and spatial distribution of heat producing elements in the Earth's interior is crucial for understanding the heat budget of the Earth, its thermal evolution, the style and planform of mantle convection and the overall architecture of the mantle. However, mantle composition and heat budget remain poorly constrained. The present-day estimate of radiogenic heat production for the bulk silicate Earth (BSE) varies by a factor of about three between different models. Similar variation is found in compositional estimates of depleted MORB-source mantle. Mass balance of elements and isotopes in BSE generally requires the existence of a distinct chemical reservoir enriched in heat producing elements, usually assumed to be located in the lowermost mantle. Global seismic tomography studies and seismic waveform modeling suggest the existence of chemically distinct mantle structure in the lowermost mantle above the core-mantle boundary and below the Pacific and Africa (i.e., Pacific and African thermochemical piles or superplumes). Geodynamic modeling of mantle convection suggests that these two large thermochemical piles may be organized by the temporal and spatial patterns of plate subduction. Recent advances in experimental neutrino physics have led to the first measurements of anti-neutrino signal produced by radioactive decay in the Earth's mantle. Future experimental efforts at new detector sites and continued data accumulation will result in better statistics and more precise geoneutrino measurements that will provide better constraints on distribution of mantle radioactivity. Up to now, analyses of predicted mantle geoneutrino signal only consider compositionally uniform or, at most, layered spherically symmetric mantle, but ignore the Pacific and African thermochemical piles that may be significantly enriched in radioactive elements. Here, we construct mantle geoneutrino flux prediction for a seismic tomography-based geometry of the enriched reservoir. We also illustrate the geoneutrino flux spatial variation using simple conceptual models of enriched reservoir shape. Our calculations are performed for different proposed estimates of chemical abundances and the results are discussed in relation to existing geoneutrino flux measurements and possible future geoneutrino experiment sites. A representative model of mantle geoneutrino flux (url) shows spatial variability of up to 45% which needs to be taken into account when interpreting measurements at specific sites. For example, the flux at Kamioka, Japan (site of KamLAND experiment) and Sudbury, Canada (SNO+) are 10% and 13% below the average, while the flux at Gran Sasso, Italy (Borexino) is 3% above the average value and the flux at Hawaii (Hanohano; proposed) is 14% above average.