Noble gas constraints on the layered structure of the mantle
Noble gas constraints on the layered structure of the mantle
复制标题
稀有气体对地幔层状结构的约束
作者：
I. Kaneoka
基于大洋和大陆火山岩中Sr、Nd、Hf和Pb等同位素数据，提出了几种地幔结构的化学模型1 -6。其中最流行的是垂直分层地幔的两种化学模型：（1）它是一个贫化地幔，从中衍生出洋中脊玄武岩（MORB）;（2）它是一个肥沃的地幔，从中可能产生地幔柱1 -5。然而，这些模型几乎都是基于固体元素，如Sr，Nd，Hf和Pb同位素，并没有完全反映惰性气体数据。由于上地幔在全球范围内发生地幔分异时，惰性气体容易从固体地球中流失，因此惰性气体同位素数据可以给出不同于固体元素同位素的地幔结构约束。根据最近样品的~ 3 He/~ 4 He和~（40）Ar/~（36）Ar比值，本文认为，地幔柱和MORB的源区物质发生了混合。这些数据似乎有利于假设典型地幔柱的来源比MORB的来源更深的模型。
Several chemical models of mantle structure have been proposed1–6 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 arise1–5. 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.