Radiogenic Ca isotopes confirm post-formation K depletion of lower crust

Antonelli, M. A.; De Paolo, D. J.; Chacko, T.; Grew, E. S.; Rubatto, Daniela (2019). Radiogenic Ca isotopes confirm post-formation K depletion of lower crust. Geochemical perspectives letters, 9, pp. 43-48. European Association of Geochemistry 10.7185/geochemlet.1904

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Heat flow studies suggest that the lower crust has low concentrations of heat-producing elements. This could be due to either (i) greater fractions of basaltic rock at depth or (ii) metamorphic depletion of radioactive elements from rocks with more evolved (andesitic to granodioritic) compositions. However, seismic data suggest that lower crust is not predominantly basaltic, and previous studies (using Pb and Sr isotopes) have shown that lower crustal rocks have experienced significant losses of U and Rb. This loss, however, is poorly constrained for K, which is inferred to be the most important source of radioactive heat in the earliest crust. Our high precision Ca isotope measurements on a suite of granulite facies rocks and minerals from several localities show that significant losses of K (~60 % to >95 %) are associated with high temperature metamorphism. These results support models whereby reduction of heat production from the lower crust, and consequent stabilisation of continental cratons in the Precambrian, are largely due to high temperature metamorphic processes. Relative changes in whole rock K/Ca suggest that 20-30 % minimum (granitic) melt removal can explain the K depletions.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Isotope Geology

UniBE Contributor:

Rubatto, Daniela

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

2410-339X

Publisher:

European Association of Geochemistry

Language:

English

Submitter:

Daniela Rubatto

Date Deposited:

11 Jun 2019 16:10

Last Modified:

11 Jun 2019 16:10

Publisher DOI:

10.7185/geochemlet.1904

BORIS DOI:

10.7892/boris.130666

URI:

https://boris.unibe.ch/id/eprint/130666

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