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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Volatile element (B, Cl, F) behaviour in the roof of an axial magma chamber from the East Pacific Rise
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Volatile element (B, Cl, F) behaviour in the roof of an axial magma chamber from the East Pacific Rise

机译：东太平洋上升带轴向岩浆腔室顶板中的挥发性元素（B，Cl，F）行为

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摘要

Understanding the behaviour of volatile elements at mid-ocean ridges is important for reasons ranging from their influence on mantle viscosity through to their role as a food source for the deep biosphere. With the aim of constraining what processes control the distribution of volatiles in the ocean crust at fast-spreading ridges, we present a detailed study of the compositional variability in magmatic amphibole formed in the upper part of the plutonic sequence at the East Pacific Rise (EPR). These amphiboles are massively enriched in chlorine (by more than an order of magnitude), and moderately enriched in boron, with respect to magmatic amphiboles in cumulates from the Mid-Atlantic Ridge (MAR). Similar enrichments have been reported for basaltic glasses from the EPR and are interpreted as indicative of assimilation. The greater enrichments observed in the plutonic section suggest both that assimilation occurs at the roof of the axial magma chamber (AMC) and that lava compositions may record minimum amounts of exogenic contamination. Amphiboles with compositions indicative of crystallisation from a contaminated magma occur to depths of 800 m beneath the sheeted dyke complex. This is interpreted to indicate that at least this upper portion of the plutonic section forms via crystallisation within the AMC followed by subsidence of a crystal mush. Amphibole boron isotope compositions show that assimilation of altered sheeted dykes plus hydrothermal fluids drives AMC magmas to heavier δ~11B values (up to + 5.8‰). Subsequent degassing within a solidifying crystal mush leads to a negative trend in δ~(11)B-B with the most degassed magma having δ~(11)B as low as -21.2‰. This degassing was associated with hydrofracturing of the partially molten crystal mush and could have facilitated a temporal link with the overlying hydrothermal system.
机译：理解挥发性元素在大洋中脊的行为很重要，原因包括其对地幔粘度的影响到其作为深层生物圈食物的来源。为了限制哪些过程控制着快速扩散洋脊中洋壳中挥发物的分布，我们对东太平洋上升（EPR）岩体层序上部形成的岩浆闪石的成分变异性进行了详细研究。 ）。与来自中大西洋海岭（MAR）的岩浆闪石相比，这些闪石大量富集氯（超过一个数量级），并适度富集硼。据报道，EPR还对玄武玻璃进行了类似的富集，并被解释为同化作用的指示。在岩浆剖面中观察到的更大的富集表明，同化作用发生在轴向岩浆室（AMC）的顶部，并且熔岩成分可能记录了最少的外源污染。具有指示从受污染的岩浆中结晶的成分的闪石发生在成片的堤状复合体下方800 m的深度处。这被解释为表明，通过在AMC内结晶，然后沉降结晶糊状物，至少形成了深部剖面的该上部。闪石的硼同位素组成表明，改变后的薄片状堤坝和热液的同化作用使AMC岩浆达到较重的δ〜11B值（高达+ 5.8‰）。凝固晶体中随后的脱气会导致δ〜（11）B-B呈现负趋势，而脱气最多的岩浆的δ〜（11）B则低至-21.2‰。这种脱气与部分熔融的结晶糊状物的水力压裂有关，并且可能促进了与上覆的水热系统的暂时联系。

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