Os Isotopes in Ocean Island Basalts: An Investigation Aimed at Identifying Primary Mantle-Source Variations

迄今为止分析的大洋岛玄武岩包括所有已知的地幔同位素组合的极端例子。OS同位素比率，特别是在HIMU地区，从放射性成因值到非常大的辐射值，变化很大。有几种可能的解释。1)不同的岩浆生成机制选择性地开发了非均质地幔物质，从而排除了富Os岩浆中的放射性成因Os成分。然而，必须在贫Os的岩浆中获得全系列的Os同位素组成。2)岩石圈堆积橄榄石的堆积和可能的再吸收作用，形成富Os岩浆，调节了这些岩浆的Os同位素组成。3)洋壳或洋岛火山堆积的含锰蚀变玄武岩物质的同化作用足以提高贫Os岩浆的187Os/186Os比值，但不能提高富Os岩浆的187Os/186Os比值。研究非地幔机制对改变岩浆中Os同位素比值的相对重要性势在必行。为此，我们提议在加那利群岛的拉帕尔马岛上进行一系列实验。

Ocean Island basalts analyzed to date include extreme examples of all known mantle isotopic associations. Os isotope ratios, particularly at HIMU localities, range to very radiogenic values and are quite variable. There are several possible explanations. 1) Heterogeneous mantle source materials are tapped selectively by differing magma generation mechanisms in such a way as to preclude radiogenic Os compositions in Os-rich magmas. The full range of Os isotopic compositions must be attainable, however in Os-poor magmas. 2) Accumulation and possible resorption of lithospheric cumulate olivine, resulting in Os-rich magmas, modulates the Os isotopic compositions of these magmas. 3) Assimilation of manganese-encrusted altered basaltic material, in the ocean crust or oceanic- island volcanic pile, is sufficient to elevate the 187Os/186Os ratios of Os-poor magmas, but not Os-rich magmas. Investigation of the relative importance of non- mantle mechanisms for the modification of Os isotope ratios in magmas is imperative. To this end, we propose a series of experiments to be performed on the island of La Palma in the Canary Islands.