Carnegie Institution of WashingtonWA

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Carnegie Institution of WashingtonWA

United States
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Huang Y.,University of Maryland University College | Strati V.,University of Ferrara | Strati V.,National Institute of Nuclear Physics, Italy | Mantovani F.,University of Ferrara | And 3 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2014

The SNO+ detector that is currently under construction in Ontario, Canada, will be a new kiloton-scale liquid scintillation detector with the capability of recording geoneutrino events that can be used to constrain the strength of the Earth's radiogenic power, and in turn, to test compositional models of the bulk silicate Earth (BSE). We constructed a detailed 3-D model of the regional crust centered at SNO+ from compiled geological, geophysical, and geochemical information. Crustal cross sections obtained from refraction and reflection seismic surveys were used to characterize the crust and assign uncertainties to its structure. The average Moho depth in the study area is 42.3 ± 2.6 km. The upper crust was divided into seven dominant lithologic units on the basis of regional geology. The abundances of U and Th and their uncertainties in each upper crustal lithologic unit were determined from analyses of representative outcrop samples. The average chemical compositions of the middle and lower crust beneath the SNO+ region were determined by coupling local seismic velocity profiles with a global compilation of the chemical compositions of amphibolite and granulite facies rocks. Monte Carlo simulations were used to predict the geoneutrino signal originating from the regional crust at SNO+ and to track asymmetrical uncertainties of U and Th abundances. The total regional crust contribution of the geoneutrino signal at SNO+ is predicted to be 15.6-3.4+5.3 TNU (a Terrestrial Neutrino Unit is one geoneutrino event per 1032 target protons per year), with the Huronian Supergroup near SNO+ dominantly contributing 7.3-3.0+5.0 TNU to this total. Future systematically sampling of this regional unit and denser seismic surveys will better model its composition and structure, and thus reduce the uncertainty on geoneutrino signal at SNO+. The bulk crustal geoneutrino signal at SNO+ is estimated to be 30.7-4.2+6.0 TNU, which is lower than that predicted in a global-scale reference model that uses an average composition of the global upper continental crust, due to the fact that Archean to Proterozoic Canadian Shield has lower U and Th concentrations. Finally, without accounting for uncertainties on the signal from continental lithospheric mantle and convecting mantle, the total geoneutrino signal at SNO+ is predicted to be 40-4+6 TNU. © 2014. American Geophysical Union. All Rights Reserved.


Halldorsson S.A.,University of Iceland | Barnes J.D.,University of Texas at Austin | Stefansson A.,University of Iceland | Hilton D.R.,University of California at San Diego | And 2 more authors.
Geology | Year: 2016

The chlorine isotope composition of Earth's interior can place strong constraints on deep-Earth cycling of halogens and the origin of mantle chemical heterogeneity. However, all mantle-derived volcanic samples studied for Cl isotopes thus far originate from submarine volcanic systems, where the influence of seawater-derived Cl is pervasive. Here, we present Cl isotope data from subglacial volcanic glasses from Iceland, where the mid-ocean ridge system emerges above sea level and is free of seawater influence. The Iceland data display significant variability in d37δ37Cl values, from -1.8‰ to +1.4‰, and are devoid of regional controls. The absence of correlations between Cl and O isotope ratios and the lack of evidence for seawater-derived enrichments in Cl indicate that the variation in d37Cl values in Icelandic basalts can be solely attributed to mantle heterogeneity. Indeed, positive correlations are evident between δ37Cl values and incompatible trace element ratios (e.g., La/Y), and long-lived radiogenic Pb isotope ratios. The data are consistent with the incorporation of altered lithosphere, including the uppermost sedimentary package, subducted into the Iceland mantle plume source, resulting in notable halogen enrichments in Icelandic basalts relative to lavas from adjacent mid-ocean ridges. © 2016 Geological Society of America.

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