How Large is the Mantle Source of Ocean Island Basalts?

Abstract

The differentiation of the silicate Earth has resulted in formation of a continental crust enriched in elements that preferentially partition into silicate liquids such as Rb, Th, U, and the light rare earths, and a 'residual' mantle which is depleted in these elements. There have been many attempts to carry out a mass balance between the continents and the residual mantle using Nd and Sr isotope ratios and elemental abundances, in order to estimate the mean age of mantle depletion and the size of the depleted and undifferentiated mantle (e.g. Galer et al. 1989 and references therein). Most attempts have assumed that the source of mid-ocean ridge basalts (MORB) reasonably approximates the depleted mantle complement to the continental crust, and that ocean island basalt (OIB) sources are an insignificant portion of the depleted mantle. These studies have been used, for example, as geochemical arguments in favor of convective separation of the upper and lower mantle. However, the OIB source has been ignored mainly due to difficulties in constraining its size and mean isotopic composition. If OIB sources comprise a significant portion of the residual mantle, many conclusions based on balancing the continents with the MORB source alone would be invalid, as would corresponding inferences about the gross structure and composition of the mantle. In this abstract we present a path to constrain the sizes of the OIB, MORB, and primitive mantle reservoirs.