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

迄今为止分析的洋岛玄武岩包括所有已知的地幔同位素组合的极端例子。o同位素比率，特别是在HIMU地区，范围非常大，具有很强的放射成因值，而且变化很大。有几种可能的解释。1)非均质幔源物质通过不同的岩浆生成机制被选择性地开采，从而排除了富Os岩浆中放射性成因的Os成分。然而，在缺乏Os的岩浆中，必须能够获得完整的Os同位素组成。2)岩石圈堆积橄榄石的聚集和可能的再吸收，形成了富Os岩浆，调节了这些岩浆的Os同位素组成。3)洋壳或洋岛火山堆中锰壳蚀变玄武岩物质的同化作用，足以提高贫氧岩浆的187Os/186Os比值，但不能提高富氧岩浆的187Os/186Os比值。研究岩浆中Os同位素比值变化的非地幔机制的相对重要性是必要的。为此，我们建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.