Ocean island volcanoes erupt a wide range of magmatic compositions via a diverse range of eruptive styles. Understanding where and how these melts evolve is thus an essential component in the anticipation of future volcanic activity. Here we examine the role of crustal structure and magmatic flux in controlling the location, evolution and ultimately composition of melts at Ascension Island. Ascension Island, in the South Atlantic, is an ocean island volcano which has produced a continuum of eruptive compositions from basalt to rhyolite in its 1Myr subaerial eruptive history. Volcanic rocks broadly follow a silica-undersaturated subalkaline evolutionary trend, and new data presented here show a continuous compositional trend from basalt through trachyte to rhyolite. Detailed petrographic observations are combined with in situ geochemical analyses of crystals and glass, and new whole-rock major and trace element data from mafic and felsic pyroclastic and effusive deposits that span the entire range in eruptive ages and compositions found on Ascension Island. These data show that extensive fractional crystallisation is the main driver for the production of felsic melts for Ascension Island; a volcano built on thin, young, oceanic crust. Strong spatial variations in the compositions of erupted magmas reveal the role of a heterogeneous lower crust; differing degrees of interaction with a zone of plutonic rocks are responsible for the range in mafic lava compositions, and for the formation of the central and eastern felsic complexes. A central core of nested, small-scale plutonic, or mush-like, bodies inhibits the ascent of mafic magmas, allowing sequential fractional crystallisation within the lower crust, and generating felsic magmas in the core of the island. There is no evidence for magma mixing preserved in any of the studied eruptions, suggesting that magma storage regions are transient, and material is not recycled between eruptions.

中文翻译：

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海洋岛火山（阿森松岛，南大西洋）小体积岩浆批次的下地壳非均质性和分晶化控制演化

大洋岛火山通过各种喷发方式喷发各种岩浆成分。因此，了解这些熔体在何处以及如何演化是预测未来火山活动的重要组成部分。在这里，我们研究了地壳结构和岩浆通量在控制阿森松岛熔体的位置、演化和最终组成方面的作用。阿森松岛位于南大西洋，是一座海洋岛屿火山，在其 1 米尔的地下喷发历史中产生了从玄武岩到流纹岩的连续喷发成分。火山岩大致遵循二氧化硅不饱和的亚碱性演化趋势，这里提供的新数据显示了从玄武岩到粗面岩再到流纹岩的连续成分趋势。详细的岩相学观察与晶体和玻璃的原位地球化学分析相结合，以及来自阿森松岛发现的跨越整个喷发年龄和成分范围的镁铁质和长英质火山碎屑和喷发沉积物的新全岩主要和微量元素数据。这些数据表明，广泛的分步结晶是阿森松岛生产长英质熔体的主要驱动力；建立在薄而年轻的海洋地壳上的火山。喷发岩浆成分的强烈空间变化揭示了非均质下地壳的作用；与深成岩区不同程度的相互作用是造成镁铁质熔岩成分范围不同的原因，也是形成中部和东部长英质复合体的原因。一个嵌套的、小规模的深成岩或糊状的中央核心，天体抑制基性岩浆的上升，允许在下地壳内连续分馏，并在岛的核心产生长英质岩浆。在任何研究的喷发中都没有保留岩浆混合的证据，这表明岩浆储存区域是短暂的，并且物质在喷发之间没有回收利用。