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Register a new userTesting a proposed second continent beneath eastern China using geoneutrino measurements
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Bed\\v{r}ich Roskovec
Publication date
2018
fields
Physics
and research's language is
English
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Models that envisage successful subduction channel transport of upper crustal materials below 300 km depth, past a critical phase transition in buoyant crustal lithologies, are capable of accumulating and assembling these materials into so-called second continents that are gravitationally stabilized at the base of the Transition Zone, at some 600 to 700 km depth. Global scale, Pacific-type subduction (ocean-ocean and ocean-continent convergence), which lead to super continent assembly, were hypothesized to produce second continents that scale to about the size of Australia, with continental upper crustal concentration levels of radiogenic power. Seismological techniques are incapable of imaging these second continents because of their negligible difference in seismic wave velocities with the surrounding mantle. We can image the geoneutrino flux linked to the radioactive decays in these second continents with land and/or ocean-based detectors. We present predictions of the geoneutrino flux of second continents, assuming different scaled models and we discuss the potential of current and future neutrino experiments to discover or constrain second continents. The power emissions from second continents were proposed to be drivers of super continental cycles. Thus, testing models for the existence of second continents will place constraints on mantle and plate dynamics when using land and ocean-based geoneutrino detectors deployed at strategic locations.
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Constraints on the Earths composition and on its radiogenic energy budget come from the detection of geoneutrinos. The KamLAND and Borexino experiments recently reported the geoneutrino flux, which reflects the amount and distribution of U and Th inside the Earth. The KamLAND and Borexino experiments recently reported the geoneutrino flux, which reflects the amount and distribution of U and Th inside the Earth. The JUNO neutrino experiment, designed as a 20 kton liquid scintillator detector, will be built in an underground laboratory in South China about 53 km from the Yangjiang and Taishan nuclear power plants. Given the large detector mass and the intense reactor antineutrino flux, JUNO aims to collect high statistics antineutrino signals from reactors but also to address the challenge of discriminating the geoneutrino signal from the reactor background.The predicted geoneutrino signal at JUNO is 39.7 $^{+6.5}_{-5.2}$ TNU, based on the existing reference Earth model, with the dominant source of uncertainty coming from the modeling of the compositional variability in the local upper crust that surrounds (out to $sim$ 500 km) the detector. A special focus is dedicated to the 6{deg} x 4{deg} Local Crust surrounding the detector which is estimated to contribute for the 44% of the signal. On the base of a worldwide reference model for reactor antineutrinos, the ratio between reactor antineutrino and geoneutrino signals in the geoneutrino energy window is estimated to be 0.7 considering reactors operating in year 2013 and reaches a value of 8.9 by adding the contribution of the future nuclear power plants. In order to extract useful information about the mantles composition, a refinement of the abundance and distribution of U and Th in the Local Crust is required, with particular attention to the geochemical characterization of the accessible upper crust.
Uranium and thorium are the main heat producing elements in the earth. Their quantities and distributions, which specify the flux of detectable antineutrinos generated by the beta decay of their daughter isotopes, remain unmeasured. Geological models of the continental crust and the mantle predict different quantities and distributions of uranium and thorium. Many of these differences are resolvable with precision measurements of the terrestrial antineutrino flux. This precision depends on both statistical and systematic uncertainties. An unavoidable background of antineutrinos from nuclear reactors typically dominates the systematic uncertainty. This report explores in detail the capability of various operating and proposed geo-neutrino detectors for testing geological models.
Gravimetric methods are expected to play a decisive role in geophysical modeling of the regional crustal structure applied to geoneutrino studies. GIGJ (GOCE Inversion for Geoneutrinos at JUNO) is a 3D numerical model constituted by ~46 x 10$^{3}$ voxels of 50 x 50 x 0.1 km, built by inverting gravimetric data over the 6{deg} x 4{deg} area centered at the Jiangmen Underground Neutrino Observatory (JUNO) experiment, currently under construction in the Guangdong Province (China). The a-priori modeling is based on the adoption of deep seismic sounding profiles, receiver functions, teleseismic P-wave velocity models and Moho depth maps, according to their own accuracy and spatial resolution. The inversion method allowed for integrating GOCE data with the a-priori information and regularization conditions through a Bayesian approach and a stochastic optimization. GIGJ fits the homogeneously distributed GOCE gravity data, characterized by high accuracy, with a ~1 mGal standard deviation of the residuals, compatible with the observation accuracy. Conversely to existing global models, GIGJ provides a site-specific subdivision of the crustal layers masses which uncertainties include estimation errors, associated to the gravimetric solution, and systematic uncertainties, related to the adoption of a fixed sedimentary layer. A consequence of this local rearrangement of the crustal layer thicknesses is a ~21% reduction and a ~24% increase of the middle and lower crust expected geoneutrino signal, respectively. Finally, the geophysical uncertainties of geoneutrino signals at JUNO produced by unitary uranium and thorium abundances distributed in the upper, middle and lower crust are reduced by 77%, 55% and 78%, respectively. The numerical model is available at http://www.fe.infn.it/u/radioactivity/GIGJ
Deep-Earth volatile cycles couple the mantle with near-surface reservoirs. Volatiles are emitted by volcanism and, in particular, from mid-ocean ridges, which are the most prolific source of basaltic volcanism. Estimates of volatile extraction from the asthenosphere beneath ridges typically rely on measurements of undegassed lavas combined with simple petrogenetic models of the mean degree of melting. Estimated volatile fluxes have large uncertainties; this is partly due to a poor understanding of how volatiles are transported by magma in the asthenosphere. Here, we assess the fate of mantle volatiles through numerical simulations of melting and melt transport at mid-ocean ridges. Our simulations are based on two-phase, magma/mantle dynamics theory coupled to idealised thermodynamic model of mantle melting in the presence of water and carbon dioxide. We combine simulation results with catalogued observations of all ridge segments to estimate a range of likely volatile output from the global mid-ocean ridge system. We thus predict global MOR crust production of 66-73 Gt/yr (22-24 km3/yr) and global volatile output of 52-110 Mt/yr, corresponding to mantle volatile contents of 100--200~ppm. We find that volatile extraction is limited: up to half of deep, volatile-rich melt is not focused to the axis but is rather deposited along the LAB. As these distal melts crystallise and fractionate, they metasomatise the base of the lithosphere, creating rheological heterogeneity that could contribute to the seismic signature of the LAB.
We present evidence for an uninterrupted continuation of Indian continental lithospheric mantle into the adjoining Bay of Bengal to a distance of 400-500 km away from the passive margin. The inference is based on the shear wave velocity image of the uppermost mantle beneath the Bay of Bengal, Bangladesh, and the adjoining Indian craton, computed using ambient noise and earthquake waveform data. The Indian lithospheric mantle is characterized by a shear wave velocity of ~ 4.1-4.3 km at the Moho depth of 35-40 km, progressively increasing to ~4.5-4.7 km/s at least up to a depth of 140 km. This velocity structure continues uninterrupted to about 86{deg} E in the Bay of Bengal. Further east, the thickness of the lithospheric lid decreases to ~90 km and is underlain by reduced shear wave velocity (~4.1-4.3 km/s) in the uppermost mantle. We postulate that the Indian craton is embedded in the western Bay of Bengal and the continent-ocean boundary lay around 86{deg} E. The craton possibly submerged soon after the India-Australia-Antractica rifting at around 136 Ma. The significantly reduced shear wave velocity beneath the eastern Bay of Bengal may be due to reheating of the mantle as a consequence of its interaction with the Kergulean hotspot around 90 Ma.
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