On the Estimation of the Isotope Ratios in the Mantle

Abstract

Up to the present time, the question of the chemical composition and the isotopic ratios in the interior of the earth has been much debated. It is supposed that the average composition of the interior layers of the earth resembles that of certain types of rocks or meteorites. For example, the mantle may be represented by dunite, eclogite and peridotite. Or it can be represented by chondritic composition. For any theories concerning the model composition of the interior, one might conclude that elements such as Li, Na, Ca, Al and halogens are concentrated toward the earth's surface, while S, Mg, Fe and Ni are enriched in the deeper layers.In the similar way, the isotope ratios of the mantle could be assumed. At first, the geochemical aspects of the isotope ratios are discussed. One general feature of the geochemical cycles of boron and lithium isotopes is discussed by comparing them to the sulfur isotope ratio which has been reported by many individual data. In order to estimate the isotope ratios in the mantle, the following cases are considered. (1) The isotope ratios in the mantle have values similar to the meteoritic values. (2) The isotope ratios in the mantle have values similar to the terrestrial values. And (3) the isotope ratios in the mantle are different from those of either the meteoritic composition or the terrestrial composition. The mean values of isotopic composition for the mantle is important for the estimation of isotope ratios in the nuclear synthesis theory or in cosmochemistry also.