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首页> 外文期刊>Bulletin of Volcanology >Heterogeneous initial Sr isotope compositions of highly evolved volcanic rocks from the Main Ethiopian Rift, Ethiopia
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Heterogeneous initial Sr isotope compositions of highly evolved volcanic rocks from the Main Ethiopian Rift, Ethiopia

机译：埃塞俄比亚主要埃塞俄比亚裂谷高度演化的火山岩的非均质初始Sr同位素组成

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Many individual mineral Sr isotope studies have revealed the complex evolution of highly evolved rocks in upper crustal magmatic systems, casting doubts on the meaning of whole-rock Sr isotopes in such samples. In this paper, whole-rock Sr isotope measurements were replicated (three to 13 times) on six highly evolved peralkaline rhyolites from the Main Ethiopian Rift (MER) to appraise their internal heterogeneity and, thus, the significance of such data. These rocks were all fresh samples of pumice, obsidian and lava. Their maximum Sr contents and ages were 15 μg/g and 1.7 Ma, respectively. Significant small-scale heterogeneities of both Sr isotopes and Rb and Sr contents were observed in most samples, although not necessarily associated with petrological characteristics suggesting possible inheritance processes. Only two, almost crystal-free, obsidian give fairly homogeneous Sr isotope ratios. These results outline the ambiguity of a single whole-rock Sr isotope determination on highly evolved peralkaline rocks, especially when no simultaneous accurate determination of the Rb/Sr is performed. They also suggest that the limitations of Sr whole-rock analyses are not restricted to phenocryst-rich samples as phenocryst-poor obsidian and almost aphyric pumices and lava are also concerned. These data further underscore that unreplicated whole-rock Sr isotope measurements should always be used with great caution in the petrogenetic modeling of highly evolved rocks. However, multiple determinations of whole rock 87Sr/86Sr in the same samples, combined with other geochemical and isotopic data, may provide constraints on the shallow level evolution of these magmas. It is suggested that selective upper crustal contamination and/or interactions with halogen-bearing hydrous fluids, typical of evolved peralkaline magmas, were probably involved in the late magmatic evolution of these MER rhyolites. The more pervasive character of fluid interaction processes would probably better account for the small-scale association of uncontaminated and contaminated signatures in a single sample. Thus, even fresh samples may have their Rb–Sr isotopic system significantly modified by fluid interactions, not as a secondary process but at the late magmatic stage.
机译：许多单独的矿物Sr同位素研究揭示了上地壳岩浆系统中高度演化的岩石的复杂演化，使人们对此类样品中全岩石Sr同位素的含义产生了怀疑。在本文中，在来自埃塞俄比亚大裂谷（MER）的六种高度演化的高碱性流纹岩上重复进行了全岩石Sr同位素测量（3到13次），以评估其内部异质性，从而评估了这些数据的重要性。这些岩石都是浮石，黑曜石和熔岩的新鲜样品。它们的最大Sr含量和年龄分别为15μg/ g和1.7 Ma。在大多数样品中都观察到了Sr同位素和Rb和Sr含量的显着小规模异质性，尽管不一定与岩石学特征相关，这表明可能存在遗传过程。只有两个几乎没有晶体的黑曜石具有相当均一的Sr同位素比率。这些结果概述了在高度演化的碱性碱性岩石上单个整岩Sr同位素测定的不确定性，尤其是在没有同时准确测定Rb / Sr的情况下。他们还表明，Sr全岩体分析的局限性不仅限于富含表晶的黑曜石，因为还缺乏表晶的黑曜石，而且几乎都含有磷灰石和熔岩。这些数据进一步强调，在高度演化的岩石的成岩模拟中，应始终谨慎使用不可重复的全岩Sr同位素测量。但是，在同一样品中对全岩石 87 Sr / 86 Sr的多次测定，结合其他地球化学和同位素数据，可能会限制这些岩浆的浅层演化。 。有人认为，选择性的上地壳污染和/或与碱性水成岩浆演化的典型的含卤素含水流体的相互作用，可能与这些MER流纹岩的后期岩浆演化有关。流体相互作用过程更普遍的特征可能会更好地解释单个样品中未受污染和受污染签名的小规模关联。因此，即使是新鲜样品，其Rb-Sr同位素系统也可能通过流体相互作用而显着改变，而不是作为第二过程，而是在岩浆后期。

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