Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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Siderophile and chalcophile element abundances in oceanic basalts, Pb isotope evolution and growth of the Earth's core

Authors:	HE Newsom WM White KP Jochum AW Hofmann

Institution:	¹Max-Planck-Institut für Chemie, Saarstr. 23, 6500 Mainz F.R.G. ²Institute of Meteoritics and Department of Geology, University of New Mexico, Albuquerque, NM 87131U.S.A. ³College of Oceanography, Oregon State University, Corvallis, OR 97331U.S.A.

Abstract:	We have investigated the hypothesis that mantle Pb isotope ratios reflect continued extraction of Pb into the Earth's core over geologic time. The Pb, Sr and Nd isotopic compositions, and the abundance of siderophile and chalcophile elements (W, Mo and Pb) and incompatible lithophile elements have been determined for a suite of ocean island and mid-ocean ridge basalt samples. Over the observed range in Pb isotopic compositions for oceanic rocks, we found no systematic variation of siderophile or chalcophile element abundances relative to abundances of similarly incompatible, but lithophile, elements. The high sensitivity of theMo/Pr ratio to segregation of Fe-metal or S-rich metallic liquid (sulfide) and the observed constantMo/Pr ratio rules out the core formation model as an explanation for the Pb paradox. The mantle and crust have the sameMo/Pr and the sameW/Ba ratios, suggesting that these ratios reflect the ratio in the Earth's primitive mantle. Our data also indicate that thePb/Ce ratio of the mantle is essentially constant, but the presentPb/Ce ratio in the mantle ( 0.036) is too low to represent the primitive value ( 0.1) derived from Pb isotope systematics. HigherPb/Ce ratios in the crust balance the lowPb/Ce of the mantle, and crust and mantle appear to sum to a reasonable terrestrialPb/Ce ratio. The constancy of thePb/Ce ratio in a wide variety of oceanic magma types from diverse mantle reservoirs indicates this ratio is not fractionated by magmatic processes. This suggests crust formation must have involved non-magmatic as well as magmatic processes. Hydrothermal activity at mid-ocean ridges may result in significant non-magmatic transport of Pb from mantle to crust and of U from crust to mantle, producing a higherU/Pb ratio in the mantle than in the total crust. We suggest that the lower crust is highly depleted in U and has unradiogenic Pb isotope ratios which balance the radiogenic Pb of upper crust and upper mantle. The differences between thePb/Ce ratio in sediments, this ratio in primitive mantle, and the observed ratio in oceanic basalts preclude both sediment recycling and mixing of primitive and depleted reservoirs from being important sources of chemical heterogeneities in the mantle.

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