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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Hydrogen isotope investigation of amphibole and biotite phenocrysts in silicic magmas erupted at Lassen Volcanic Center, California
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Hydrogen isotope investigation of amphibole and biotite phenocrysts in silicic magmas erupted at Lassen Volcanic Center, California

机译：加利福尼亚拉森火山中心爆发的硅质岩浆中闪石和黑云母隐晶的氢同位素研究

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摘要

Hydrogen isotope ratio, water content and Fe~(3+)/Fe~(2+) in coexisting amphibole and biotite phenocrysts in volcanic rocks can provide insight into shallow pre- and syn-eruptive magmatic processes such as vesiculation, and lava drainback with mixing into less devolatilized magma that erupts later in a volcanic sequence. We studied four ～35 ka and younger eruption sequences (i.e. Kings Creek, Lassen Peak, Chaos Crags, and 1915) at the Lassen Volcanic Center (LVC), California, where intrusion of crystal-rich silicic magma mushes by mafic magmas is inferred from the varying abundances of mafic magmatic inclusions (MMIs) in the silicic volcanic rocks. Types and relative proportions of reacted and unreacted hydrous phenocryst populations are evaluated with accompanying chemical and H isotope changes. Biotite phenocrysts were more susceptible to rehydration in older vesicular glassy volcanic rocks than coexisting amphibole phenocrysts. Biotite and magnesiohornblende phenocrysts toward the core of the Lassen Peak dome are extensively dehydroxylated and reacted from prolonged exposure to high temperature, low pressure, and higher/_(02) conditions from post-emplacement cooling. In silicic volcanic rocks not affected by alteration, biotite phenocrysts are often relatively more dehydroxylated than are magnesiohornblende phenocrysts of similar size; this is likely due to the ca 10 times larger overall bulk H diffusion coefficient in biotite. A simplified model of dehydrogenation in hydrous phenocrysts above reaction closure temperature suggests that eruption and quench of magma ascended to the surface in a few hours is too short a time for substantial H loss from amphibole. In contrast, slowly ascended magma can have extremely dehydrogenated and possibly dehydrated biotite, relatively less dehydrogenated magnesiohornblende and reaction rims on both phases. Eruptive products containing the highest proportions of mottled dehydrogenated crystals could indicate that within a few days prior to eruption, degassed vesiculated magma or lava had drained back down the volcanic conduit and mixed with less devolatilized magma. The vesiculated magma contained hydrous phenocrysts with lattice damage, which locally raised the effective H diffusion coefficient by ca 10-100x and resulted in increased mineral dehydrogenation. Remobilization of dacite magma mush by relatively more reduced mafic magma appears to have generated further f_(02) variations in May 1915 as oxidized magma from shallow levels circulated to depths where dehydrogenation of hydrous phenocrysts began. The δD(Magmatic) tic H20 expressed in LVC acid hot springs is likely a mixture derived from devolatilized ascending mafic magmas and crystallizing silicic magma mush.
机译：火山岩中角闪石和黑云母共晶相共存的氢同位素比，水含量和Fe〜（3 +）/ Fe〜（2+）可以洞悉浅层的火山爆发前和爆发前的岩浆作用，例如溶泡作用以及熔岩流回混入挥发度较低的岩浆中，然后在火山喷发中喷发。我们在加利福尼亚的拉森火山中心（LVC）研究了四个〜35 ka和更年轻的喷发序列（即金斯克里克，拉森峰，混沌岩壁和1915年），从中推断出铁镁质岩浆对富含晶体的硅质岩浆侵入的影响。硅质火山岩中镁铁质岩浆包裹体（MMIs）的变化丰度。评估了反应态和未反应态水合晶族的类型和相对比例，以及伴随的化学和氢同位素变化。在较老的水泡状玻璃状火山岩中，黑云母比隐晶比共存的闪石变晶更易补液。朝向拉森峰穹顶核心的黑云母和镁晶角晶石被广泛脱羟基，并且由于放置后冷却，长时间暴露于高温，低压和较高/（02）条件下而发生反应。在不受蚀变影响的硅质火山岩中，黑云母隐晶通常比类似大小的镁角闪石隐晶具有更高的脱羟基作用。这可能是由于黑云母中整体H扩散系数大了大约10倍。在高于反应闭合温度的含水苯晶中脱氢的简化模型表明，在几个小时内上升到表面的岩浆的喷发和猝灭对于从闪石中大量释放H的时间太短了。相反，缓慢上升的岩浆可能具有高度脱氢的黑云母，并且可能具有脱水的黑云母，在两个相中脱氢的镁锰角闪石和反应边缘相对较少。含有最高比例的斑驳脱氢晶体的喷发产物可能表明，在喷发前几天内，脱气的囊状岩浆或熔岩已经从火山管道中流回，并与脱挥发分的岩浆混合。泡状岩浆含有含水晶格，晶格损伤，使有效氢扩散系数局部提高了约10-100x，并导致矿物脱氢增加。镁铁矿岩浆相对较弱的镁铁质岩浆的运移似乎在1915年5月产生了进一步的f_（02）变化，因为氧化岩浆从浅水位循环到开始水化变色层脱氢的深度。在LVC酸性温泉中表达的δD（岩浆）tic H20可能是脱挥发分的上升铁镁质岩浆和结晶硅质岩浆的混合物。

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