87Sr/86Sr in Precambrian carbonates as an index of crustal evolution |
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| Authors: | Ján Veizer William Compston |
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| Institution: | Department of Geology, University of Ottawa, Ottawa, Ontario, Canada;Research School of Earth Sciences, Australian National University, Canberra, A.C.T., Australia |
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| Abstract: | The Sr isotopic composition of ‘seawater’, as measured on carbonate rocks, shows a composite pattern during geologic history. All known Archaean data are compatible with contemporaneous upper mantle 87Sr/86Sr values. This is followed by a strong increase in the radiogenic component during the 2.5–2.1 b.y. period, a less pronounced increase during the remaining portion of the Proterozoic and a decrease during the Phanerozoic. The trend closely resembles the K2O/Na2O secular variations in composition of igneous and sedimentary rocks (Engelet al, Bull. Geol. Soc. Amer. 85, 843–858, 1974) and probably reflects the fractionation state of the contemporary crust. The data are compatible with recent suggestions of three major tectonic regimes during geologic history: greenstone belts during the Archaean, mobile belts during the Proterozoic and plate tectonics during the Phanerozoic. They also indicate that continental crust during the Archaean contributed only subordinate Sr into the meteoric cycle. |
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