Evidence for superhydrous primitive arc magmas from mafic enclaves at Shiveluch volcano, Kamchatka
Evidence for superhydrous primitive arc magmas from mafic enclaves at Shiveluch volcano, Kamchatka
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堪察加半岛希韦鲁奇火山镁铁质飞地中存在超水原始弧岩浆的证据
DOI：
10.1007/s00410-020-01746-5
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发表时间：
2020
影响因子：
3.5
通讯作者：
Ruprecht, Philipp
中科院分区：
文献类型：
--
作者：
Goltz, Andrea E.;Krawczynski, Michael J.;Gavrilenko, Maxim;Gorbach, Natalia V.;Ruprecht, Philipp
镁铁质包体保留了弧环境中原始岩浆深度分异的记录。我们分析了堪察加半岛 Shiveluch 火山镁铁质包体的岩石学和地球化学，以确定原始岩浆的分异历史并估计其压力、温度和含水量。高镁橄榄石中的角闪石包裹体表明，原始熔体是超水的（即> 8 wt% H2O），并且在其液体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 km 深度处结晶出高 Mg# 角闪石，水含量为 10-14 wt% H2O。飞地的主量元素和微量元素全岩石化学以及已发表的安山岩分析表明，它们通过含角闪石组合的分馏而相关。分馏橄榄石、单斜辉石和角闪石的定量模型再现了变化图中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馏出 0.2-12.2% 的角闪石，以再现全范围的飞地成分，这与基于全岩镝含量从母熔体中分馏出的角闪石数量的估计重叠。这一贡献将已发表的 Shiveluch 浅层过程模型扩展到了更深的深度。它提供的证据表明，供给弧火山的原始岩浆可能比其他方法估计的含水量更高，并且角闪石是超含水的原始地幔熔体下降液体线上的重要早期分馏阶段。
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.
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