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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Hornblende, cummingtonite, and biotite hydrogen isotopes: Direct evidence of slab-derived fluid flux in silicic magmas of the Taupo Volcanic Zone, New Zealand
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Hornblende, cummingtonite, and biotite hydrogen isotopes: Direct evidence of slab-derived fluid flux in silicic magmas of the Taupo Volcanic Zone, New Zealand

机译：角闪石，方解石和黑云母氢同位素：新西兰陶波火山岩区硅质岩浆中板状流体通量的直接证据

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摘要

Hornblende, cummingtonite, and biotite 8D values from rhyolitic and mingled/mixed mafic rocks from the Central Taupo Volcanic Zone were determined in an effort to characterize the isotopic composition of the dissolved fluid of magmas in this predominantly rhyolitic volcanic zone. Hand samples were obtained from pyrodastic flow, airfall, and lava flow deposits as well as co-eruptive cognate plutonic blocks. Mineral 6D values range from - 88 to - 35‰> SMOW (Standard Mean Ocean Water). Cummingtonite phenocryst 8D overlaps with hornblende from the same rocks, while biotite values are consistently ~7‰ more negative than hornblende phenocrysts from the same hand sample. Calculated volcanic steam 6D values (ca. -46 to -25‰) are consistent with an island arc crustal source (ca. -40 to -15‰), with a negligible to absent mantle (ca. -100 to -60‰) component Earlier researchers suggested that the Taupo Volcanic Zone fluid flux included dilute concentrations of slab-derived phases (i.e. CO_2, Cl) similar to back-arc volcanic systems at other convergent margins. However, the island arc type fluid composition estimated from the minerals we studied is similar to hornblende and biotite compositions from pyrodastic deposits and volcanic fumaroles from the Kamchatka and Japanese arcs. The silicic rocks from the 2.0-1.0 Ma Mangakino caldera, the most inland caldera system we studied, have the lowest 8D hornblende and biotite, suggesting that silicic volcanism, distal to the subduction trench, was derived from a back-arc like (i.e. mixture of mantle and slab-fluid). However, we found that the currently active more proximal locus of silicic volcanism reflects typical arc-like fluid compositions. Our results further suggest that the composition of volcanic steam migrating through the upper crust was similar in composition to regional meteoric waters (ca. -45 to -23‰).
机译：确定了陶波火山中部流纹岩和混合/混合的镁铁质岩石中的角闪石，黑钙母岩和黑云母8D值，以表征该流纹岩性火山岩带中岩浆溶解流体的同位素组成。手工样品是从火山岩流，空降和熔岩流沉积物以及伴生的同源深成岩块中获得的。矿物质6D值的范围是-88至-35‰> SMOW（标准平均海水）。 Cummingtonite phenocryst 8D与来自同一块岩石的角闪石重叠，而黑云母值始终比同一手样品中的角闪石phenocrysts负〜7‰。计算出的火山蒸汽6D值（约-46至-25‰）与岛弧地壳源（约-40至-15‰）相一致，而缺少地幔则可忽略不计（约-100至-60‰）。组件早期的研究人员认为，陶波火山带的流体通量包括稀释浓度的板状相（即CO_2，Cl），类似于在其他会聚边缘的弧后火山系统。但是，从我们研究的矿物中估算出的岛弧型流体成分与来自堪察加半岛和日本弧的火山岩和火山岩的黑闪石和黑云母成分相似。来自我们研究的最内陆火山口系统2.0-1.0 Ma Mangakino火山口的硅质岩具有最低的8D角闪石和黑云母，这表明俯冲沟末端的硅质火山岩是由后弧状（即，混合物和板坯流体）。但是，我们发现硅酸火山岩目前较活跃的近端位点反映了典型的弧状流体成分。我们的结果进一步表明，通过上地壳迁移的火山蒸汽的成分与区域性流域水的成分相似（约-45至-23‰）。

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