Mafic enclaves preserve a record of deep differentiation of primitive magmas in arc settings. We analyze the petrology and geochemistry of mafic enclaves from Shiveluch volcano in the Kamchatka peninsula to determine the differentiation histories of primitive magmas and to estimate their pressures, temperatures, and water contents. Amphibole inclusions in high forsterite olivine suggest that the primitive melt was superhydrous (i.e., > 8 wt% H2O) and was fractionating amphibole and olivine early on its liquid line of descent. We find that the hydrous primitive melt had liquidus temperatures of 1062 ± 48 °C and crystallized high Mg# amphibole at depths of 23.6–28.8 km and water contents of 10–14 wt% H2O. The major and trace element whole-rock chemistry of enclaves and of published analyses of andesites suggest that they are related through fractionation of amphibole-bearing assemblages. Quantitative models fractionating olivine, clinopyroxene, and amphibole reproduce geochemical trends defined by enclaves and andesites in variation diagrams. These models estimate 0.2–12.2% amphibole fractionated from the melt to reproduce the full range of enclave compositions, which overlaps with estimates of the amount of amphibole fractionated from parental melts based on whole-rock dysprosium contents. This contribution extends the published model of shallow processes at Shiveluch to greater depths. It provides evidence that primitive magmas feeding arc volcanoes may be more hydrous than estimated from other methods, and that amphibole is an important early fractionating phase on the liquid line of descent of superhydrous, primitive mantle-derived melts.

中文翻译：

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堪察加半岛 Shiveluch 火山基性飞地超水原始弧岩浆的证据

镁铁质飞地保留了弧形环境中原始岩浆深度分化的记录。我们分析了来自堪察加半岛 Shiveluch 火山的基性飞地的岩石学和地球化学，以确定原始岩浆的分化历史，并估计它们的压力、温度和含水量。高镁橄榄石中的角闪石包裹体表明原始熔体是超水合的（即 > 8 wt% H2O）并且在其液态下降线早期分馏角闪石和橄榄石。我们发现含水原始熔体的液相线温度为 1062 ± 48 °C，在 23.6-28.8 公里的深度结晶出高 Mg# 角闪石，水含量为 10-14 wt% H2O。飞地的主要和微量元素全岩化学以及已发表的安山岩分析表明它们是通过含闪石组合的分馏相关联的。橄榄石、斜辉石和角闪石的定量模型在变化图中再现了由飞地和安山岩定义的地球化学趋势。这些模型估计从熔体中分离出 0.2-12.2% 的闪石以重现整个范围的飞地组成，这与基于全岩镝含量从母体熔体中分离出的闪石数量的估计重叠。这一贡献将 Shiveluch 浅层过程的已发表模型扩展到了更深的层次。它提供的证据表明，原始岩浆喂养弧形火山可能比其他方法估计的更含水，