Petrological and geochemical (major element, trace element, Sr–Nd isotope) data for recent (<5 kyr old) basalts that sporadically erupt on the western flank of Piton de la Fournaise (PdF), one of the most active volcanoes on Earth, allow the tracking of magma transfer and evolution from mantle to crustal depths. In the western peripheral area of PdF we document the broadly synchronous eruptions of (1) primitive olivine and olivine–clinopyroxene transitional basalts with tholeiitic affinity that are closely associated in space with (2) transitional olivine basalts with alkaline affinity, and (3) hybrid lavas, intermediate between the ‘alkaline’ and the ‘tholeiitic’ end-members. The composition of the latter overlaps with that of the lavas frequently erupted from the conduit system feeding the main summit cone. AlphaMELTS modelling, and fluid inclusion and clinopyroxene barometry, constrain the conditions of magma storage at 10–30 km, and the ascent of magma from the upper mantle to the shallow crustal plumbing system. Variable degrees of mantle melting, together with minor source heterogeneity and contamination with cumulate-derived partial melts, contribute to the diversity of PdF magmas. However, all these processes do not represent the dominant factors that produce the large variability we found in major element composition. Indeed, the composition of basalts erupted from PdF peripheral centers is strongly controlled by polybaric olivine–clinopyroxene fractionation at pressures higher than 3 kbar. Crystal textures and geochemical modelling suggest that fast magma ascent is critical to prevent clinopyroxene dissolution. Conversely, long-lasting magma stagnation promotes pyroxene resorption and magma differentiation. ‘Central’ eruptions occurring close to the PdF summit cone emit variably more evolved melts, which result from olivine–clinopyroxene–plagioclase differentiation at intermediate–shallow pressure (<3 kbar and in most cases <1 kbar). Deep and extensive magma mixing before injection into the crustal magma conduit system, located below the summit region, results in the apparent homogeneity of basalts erupted from the central area. As regards ‘peripheral’ eruptions, deep-seated stagnation of basaltic melts and differentiation at the mantle–crust transition zone (c. 4 kbar) produces a range of magma compositions. We demonstrate that rapid magma ascent from deep-seated reservoirs can bypass the central plumbing system. The eruptions of these magmas both in the central area and on the densely populated flanks have major consequences in terms of volcanic hazard at PdF.

中文翻译：

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Piton de la Fournaise（留尼汪岛，印度洋）西侧的喷发活动：对海洋火山岛岩浆转移、储存和演化的见解

最近（<5 kyr 老）玄武岩的岩石学和地球化学（主要元素、微量元素、Sr-Nd 同位素）数据，这些玄武岩在地球上最活跃的火山之一 Piton de la Fournaise (PdF) 的西侧偶尔喷发，允许跟踪从地幔到地壳深度的岩浆转移和演化。在 PdF 的西部外围区域，我们记录了 (1) 具有拉斑亲和性的原始橄榄石和橄榄石-斜辉石过渡玄武岩的广泛同步喷发，它们在空间中与 (2) 具有碱性亲和力的过渡橄榄石玄武岩密切相关，以及 (3) 混合熔岩，介于“碱性”和“拉斑”末端成员之间。后者的成分与经常从供给主峰顶锥体的管道系统喷出的熔岩的成分重叠。AlphaMELTS 建模，流体包裹体和斜辉石气压测量，限制了 10-30 公里岩浆储存的条件，以及岩浆从上地幔上升到浅地壳管道系统的条件。不同程度的地幔熔融，加上次要的来源非均质性和累积衍生的部分熔体的污染，促成了 PdF 岩浆的多样性。然而，所有这些过程并不代表产生我们在主要元素组成中发现的巨大可变性的主要因素。事实上，从 PdF 外围中心喷发的玄武岩的成分在高于 3 kbar 的压力下受到多元橄榄石-斜辉石分​​馏的强烈控制。晶体结构和地球化学模型表明，快速岩浆上升对于防止单斜辉石溶解至关重要。反过来，持久的岩浆停滞促进辉石吸收和岩浆分异。发生在 PdF 顶峰锥体附近的“中央”喷发释放出不同程度的演化熔体，这是由于橄榄石-斜辉石-斜长石在中-浅压力（<3 kbar，在大多数情况下 <1 kbar）下分化的结果。注入位于顶峰区域下方的地壳岩浆导管系统之前，深而广泛的岩浆混合导致中央区域喷发的玄武岩具有明显的均质性。至于“外围”喷发，玄武岩熔体的深层停滞和地幔-地壳过渡带（约 4 kbar）的分化产生了一系列岩浆成分。我们证明了从深部储层快速上升的岩浆可以绕过中央管道系统。