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Science 26 June 1981:
Vol. 212. no. 4502, pp. 1502 - 1506
DOI: 10.1126/science.212.4502.1502
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Articles

Eastern Indian 3800-Million-Year-Old Crust and Early Mantle Differentiation

A. R. BASU 1, S. L. RAY 2, A. K. SAHA 2, and S. N. SARKAR 3

1 Department of Geological Sciences, University of Rochester, Rochester, New York 14627, and Branch of Isotope Geology, U.S. Geological Survey, Denver, Colorado 80225
2 Department of Geology, Presidency College, Calcutta 700073, India
3 Indian School of Mines, Dhanbad, India

Samarium-neodymium data for nine granitic and tonalite gneisses occurring as remnants within the Singhbhum granite batholith in eastern India define an isochron of age 3775 ± 89 x 106 years with an initial 143Nd/144Nd ratio of 0.50798 ± 0.00007. This age contrasts with the rubidium-strontium age of 3200 x 106 years for the same suite of rocks. On the basis of the new samarium-neodynium data, field data, and petrologic data, a scheme of evolution is proposed for the Archean crust in eastern India. The isotopic data provide evidence that parts of the earth's mantle were already differentiated with respect to the chondritic samarium-neodymium ratio 3800 x l06 years ago.

Submitted on January 16, 1981
Revised on March 17, 1981


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