Diverse sources of volatile release across volcanic arcs: Insights from Kozelsky and Khangar volcanoes, Kamchatka

堪察加半岛是地球上火山活动最活跃的地区之一，也是世界上研究俯冲的旧（冷）海洋地壳命运的最佳地点之一。独特的是，这个弧上的火山在几个，而不是一个，广泛的火山线条中喷发，可能在弧前，在主火山锋下，也可能从弧后区域取样岩浆流体和熔融源。最近的地球物理和地球化学结果表明，堪察加火山确实从低至50公里深至350公里深的板块深度取样流体。虽然大多数弧火山（这里和其他弧上的其他地方）是由枯竭弧下地幔楔源的通量熔融造成的，但弧火山岩的地球化学（包括10Be， U-系列，B和Li同位素，B/Be， B/Nb和Sr/Y比值等）也表明，有时有来自俯冲板块的重要元素和同位素输入。这些可能包括俯冲沉积物、蚀变玄武岩地壳和/或板块蛇纹化部分或弧前蛇纹化（次弧）地幔楔脱水产生的流体/熔体。尽管枯竭的地幔源主导了喷发产物的主要和次要元素，但从板块释放的成分是地球化学可追溯的，并且已被证明为弧岩浆提供了最重要的H2O， CO2，卤素和流体流动元素（B, Cs, Li, As, Sb） （Portnyagin et al., 2007）。然而，所涉及的个别板块岩性及其与深度的相对影响，特别是源于深俯冲蛇纹岩脱水反应的板块岩性，尚未被量化。该项目旨在通过研究沿东南西北向纵深样带喷发的火山产物的地球化学和岩石学来解决这一问题，该纵深样带横跨东部火山弧锋，深入后弧（Sredinny Ridge）区域。在之前的实地考察活动中，我们对阿瓦钦斯基火山和巴肯宁火山进行了广泛的采样，这两个火山理想地位于交叉弧样带的中间，分别代表了120至200公里之间的板块顶部的深度。在这里，我们的目标是在堪察加半岛（2020年和/或2021年）进行实地调查，并在同一（弧前正交）样带的另一个浅源（Kozelsky小于100公里）和一个深源（Khangar小于300公里）火山中取样和研究挥发性和流体流动元素系统。含橄榄石和辉石的基性矿渣可用于提取熔融包裹体（MI）。该MI将补充现有的（在达勒姆和利兹）堪察加弧MI系列。新的MI将首次增加对活火山弧上的石化变化的高分辨率观察。此外，我们将有独特的机会来研究随着离板深度的增加，板衍生通量的组成变化。拟议的样带将与北部的交叉弧样带相补充（Churikova等人，2007），尽管与新提议的样带相比，那里的板块深度/几何形状和覆盖的地形边界不太清楚，在新提议的样带中，贝尼奥夫带被深部区域地震活动和层析成像清楚地描绘出来。该项目将涉及培训和使用各种原位测量技术，如SEM、EPMA、LA-ICP-MS和SIMS（用于MI）。我们将提供基于大块岩石和MI数据集的纹理/岩石学观察、样品选择和制备以及地球化学建模方面的培训（主要是内部培训）。参考文献：Churikova等人（2007,Contrib Mineral Petrol, 154, 217-239）。Portnyagin等人（2007）。地球的星球。科学。《社会科学》，255,53-69。

The Kamchatka peninsula is one of the most volcanically active regions on Earth and is among the best places in the world to study the fate of subducted old (cold) oceanic crust. Uniquely the volcanoes of this arc erupt in several, rather than one, broad volcanic lineaments, possibly sampling magmatic fluid and melt sources in the forearc, beneath the main volcanic front and also from the behind-the-arc region. Recent geophysical and geochemical results show that the Kamchatka volcanoes indeed sample fluids derived from slab depths from as little as 50km to as deep as 350 km. While the majority of arc volcanoes (here and elsewhere in other arcs) result from flux melting of depleted subarc mantle wedge source, the geochemistry of the arc volcanic rocks (incl. 10Be, U- series, B & Li isotopes, B/Be, B/Nb and Sr/Y ratios, among others) also shows, sometimes significant elemental and isotopic inputs from subducted slabs. These may include subducted sediments, altered basaltic crust and/or fluids/melts resulting from the dehydration of serpentinized sections of the slabs or forearc-serpentinized (subarc) mantle wedge. Although the depleted mantle sources dominate the major and minor element budgets of the erupted products, the components released from the slab are geochemically traceable and have been shown to deliver the most important H2O, CO2, halogen and fluid mobile element (B, Cs, Li, As, Sb) contributions to arc magmas (Portnyagin et al. , 2007). However, the individual slab lithologies involved and their relative influence with depth, especially the one originating from dehydration reactions in the deeply subducted serpentinites, has not been quantified yet. This project aims to address this issue by examining the geochemistry and petrology of volcanic products erupted along a SE-NW oriented depth transect across the Eastern Volcanic Arc Front and deep into the rear arc (Sredinny Ridge) region. In previous fieldwork campaigns we have extensively sampled Avachinsky and Bakening volcanoes that are ideally situated in the middle of a cross-arc transect that represents depth to the top of the slab between 120 and 200 km, respectively. Here we will aim to conduct fieldwork in Kamchatka (2020 and/or 2021) and sample and study the volatile and fluid mobile element systematics in one additional shallow-sourced (<100 km, Kozelsky) and one very deeply sourced (>300 km Khangar) volcano from the same (arc front orthogonal) transect. Olivine- and pyroxene- bearing mafic scoria will be used to extract melt inclusions (MI). The MI will be complementary to the already available (in Durham and Leeds) Kamchatka arc MI collection. The new MI will be adding, for the first time, an exceptional high resolution look at the petrochemical variations across active volcanic arc. Moreover, we will uniquely have a chance to examine compositional changes of slab derived fluxes with increasing depth-to-slab. The proposed transect will be complementary to a cross arc transect to the north (Churikova et al., 2007), although the slab depths/geometry and the overlaying terrain boundaries there are less clear when compared with the newly proposed transect, where the Benioff zone is clearly delineated by deep regional seismicity and tomography. The project will involve training and use of various in-situ measurement techniques such as SEM, EPMA, LA-ICP-MS and SIMS (for the MI). We will provide training (mostly in-house) in textural/petrological observations, sample selection+ preparation and geochemical modelling based on bulk rock and MI datasets.References used:Churikova et al. (2007, Contrib Mineral Petrol 154, 217-239.Portnyagin et al. (2007). Earth Planet. Sci. Lett. 255, 53-69.