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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Magma systems of the Kutcharo and Mashu volcanoes (NE Hokkaido, Japan):Petrogenesis of the medium-K trend and the excess volatile problem
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Magma systems of the Kutcharo and Mashu volcanoes (NE Hokkaido, Japan):Petrogenesis of the medium-K trend and the excess volatile problem

机译：Kutcharo和Mashu火山的岩浆系统（日本北海道）：中K趋势的成岩作用和过度挥发的问题

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摘要

Despite the existence of a growing database of the amount of H_2O in melt inclusions, it is difficult to calculate the bulk H_2O content for vapor-saturated magma because the observable H-2O concentration in melt is limited by its solubility. To overcome this problem, we focus on K_2O, which shows similar geochemical behavior to H_2O but is largely retained within melt in the case of volatile saturation and magma degassing. As an initial trial of this approach, we document the evolution of K_2O content for the Kutcharo and Mashu volcanoes in northeastern Hokkaido, Japan, which are located close to each other and have similar timing of eruptive ac-tivity, but are chemically distinct, with Kutcharo being medium-K in composition and Mashu being low-K. Despite the contrasting bulk-rock compositions of the two volcanoes, the melt inclusions show a smooth compositional trend that is continuous from low-K basalt to andesite, rhyolite, medium-K rhyolite, and finally high-K rhyolitic melt. The observed medium-K series probably is made by mixing of low-K basalt and medium-K rhyolite which derive from solidified low-K basalt. Thermodynamic computations using MELTS show that the observed chemical variation in melt inclusions can be explained by fractional crystallization and subsequent re-melting of low-K basalt. In addition, the similar Sr, Nd, and Pb isotopic compositions of the basalt and rhyolite, combined with the progressive enrichment of trace elements from basalt to rhyolite, support the interpretation that the two rock types are genetically related. The observed evolution of K_2O contents suggests that the total volume of basalt supplied intermittently beneath the volcanoes was approx-imately 10 times the volume of erupted rhyolite magma. The solidified basalt, which contains H_2O≤5 wt.%, S≤0.3 wt.%, and Cl = -0.01 wt.%, supplied a much larger amount of magmatic volatiles to the overlying felsic magma than could be dissolved.
机译：尽管存在关于熔体夹杂物中H_2O含量不断增长的数据库，但是由于熔体中可观察到的H-2O浓度受到其溶解度的限制，因此很难计算出蒸汽饱和岩浆的总H_2O含量。为了克服这个问题，我们集中研究了K_2O，它表现出与H_2O类似的地球化学行为，但是在挥发性饱和和岩浆脱气的情况下，大部分保留在熔体中。作为这种方法的初步试验，我们记录了日本北海道东北部的喀彻鲁火山和玛舒火山的K_2O含量的演变，这些火山彼此靠近且具有相似的爆发活动时间，但化学性质不同，具有Kutcharo的成分为中K，Mashu为低K。尽管两个火山的块岩成分形成对比，但熔体包裹体仍显示出平滑的成分趋势，从低钾玄武岩到安山岩，流纹岩，中钾流纹岩，最后是高钾流纹岩熔体是连续的。所观察到的中钾系列可能是由低钾玄武岩和衍生自凝固低钾玄武岩的中钾流纹岩混合而成的。使用MELTS进行的热力学计算表明，熔体夹杂物中观察到的化学变化可以通过分步结晶和随后的低K玄武岩重熔来解释。另外，玄武岩和流纹岩的相似的Sr，Nd和Pb同位素组成，再加上从玄武岩到流纹岩的微量元素的逐步富集，支持了两种岩石类型与遗传有关的解释。观察到的K_2O含量的演变表明，间歇性地向火山下方供应的玄武岩的总体积大约是流纹岩浆岩浆体积的10倍。包含H_2O≤5wt。％，S≤0.3wt。％和Cl = -0.01 wt。％的凝固玄武岩为上覆的长岩浆提供了大量的岩浆挥发物，而无法溶解。

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  • 来源
    《Journal of Volcanology and Geothermal Research》 |2012年第15期|p.50-60|共11页
  • 作者

    Isoji Miyagi; Junichi Itoh; Nguyen Hoang; Yuichi Morishita;

  • 作者单位

    Geological Survey of Japan, Tsukuba Central 7, 1-1-7 Higashi, Tsukuba, Ibaraki 305-8567, Japan;

    Geological Survey of Japan, Tsukuba Central 7, 1-1-7 Higashi, Tsukuba, Ibaraki 305-8567, Japan;

    Geological Survey of Japan, Tsukuba Central 7, 1-1-7 Higashi, Tsukuba, Ibaraki 305-8567, Japan;

    Geological Survey of Japan, Tsukuba Central 7, 1-1-7 Higashi, Tsukuba, Ibaraki 305-8567, Japan;

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  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

    caldera; magmatic volatiles; medium-K; low-K;

    机译：破火山口;岩浆挥发物;中等K;低介电常数;

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