Abstract

The paper presents new data on the formation conditions of basalts from Menshiy Brat postcaldera volcano in the Medvezhia caldera, Iturup Island. The liquidus mineral assemblage consists of olivine (Fo 85.3–90.1 mol %) and chromium spinel (Cr# = 0.46–0.6), which crystallized at 1090–1170°C and oxygen fugacity at NNO + 0.6 (σ = 0.2) to NNO + 0.2 (σ = 0.14) in the course of the eruption. Data on melt inclusions in the liquidus olivine demonstrate that its parental melts were low-Al₂O₃ and low-K₂O, with up to 15.5 wt % MgO, and with an average H₂O content of 5.5 wt %. The newly obtained data on volatile contents in the olivine-hosted melt inclusions suggest that the mafic melts were derived by the partial melting of a peridotitic-rich source with a small admixture of an olivine-free component at 1225°C, under active influence of the slab-derived fluids. These fluids were separated from the subducting slab at 670–705°C and depths of 95–105 km beneath Iturup Island. Our results enhance our understanding of the evolution of basic magmas that serve as a heat and volatile sources during the formation of large calderas.

中文翻译：

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梅德韦日亚火山口的镁质玄武岩：主要岩浆及其来源，以千岛群岛伊图鲁普岛 Menshiy Brat 火山为例

摘要

该论文提供了有关伊图鲁普岛 Medvezhia 火山口 Menshiy Brat 火山口后火山玄武岩形成条件的新数据。液相线矿物组合由橄榄石 ( Fo 85.3–90.1 mol %) 和铬尖晶石 (Cr# = 0.46–0.6) 组成，结晶温度为 1090–1170°C，氧逸度为 NNO + 0.6 (σ = 0.2) 至 NNO + 0.2 (σ = 0.14) 在喷发过程中。液相线橄榄石中熔体包裹体的数据表明，其母熔体为低 Al ₂ O ₃和低 K ₂ O，MgO 含量高达 15.5 wt%，平均 H ₂O含量为5.5重量％。新获得的关于橄榄石熔体包裹体中挥发物含量的数据表明，镁铁质熔体是由富含橄榄岩的来源与少量无橄榄石成分的混合物在 1225°C 的积极影响下部分熔融而产生的。板片衍生的流体。这些流体在伊图鲁普岛下方 670-705°C、深度 95-105 公里处从俯冲板片中分离出来。我们的研究结果增强了我们对基性岩浆演化的理解，基性岩浆在大型破火山口的形成过程中充当热量和挥发物的来源。