Noble gas constraints on the layered structure of the mantle

Abstract

Several chemical models of mantle structure have been proposed^1–6 based on isotopic data such as Sr, Nd, Hf and Pb in volcanic rocks from both oceanic and continental areas. The most popular of these are two chemical models of the vertically-layered mantle: (1) that it is a depleted mantle from which mid-oceanic ridge basalt (MORB) is derived and (2) that it is a fertile mantle from which a mantle plume may arise^1–5. However, these models are almost always based on solid elements, such as Sr, Nd, Hf and Pb isotopes, and do not fully reflect the noble gas data. Because noble gases are liable to be lost from the solid earth when a mantle differentiation occurs on a global scale in the upper mantle, the data on noble gas isotopes could give constraints on the structure of the mantle that differ from those by solid element isotopes. Based on a consideration ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios of recent samples, I argue here that the observed data suggest that mixing occurs between the source materials of a mantle plume and of MORB. The data seem to favour the model which assumes that the source of a typical mantle plume is deeper than that of MORB.