Importance of Long-Term Shallow Degassing of Basaltic Magma on the Genesis of Massive Felsic Magma Reservoirs: a Case Study of Aso Caldera, Kyushu, Japan
Importance of Long-Term Shallow Degassing of Basaltic Magma on the Genesis of Massive Felsic Magma Reservoirs: a Case Study of Aso Caldera, Kyushu, Japan
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玄武质岩浆长期浅层脱气对大型长英质岩浆储层成因的重要性：以日本九州阿苏火山口为例
DOI：
10.1093/petrology/egad009
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发表时间：
2023
影响因子：
3.9
通讯作者：
Geshi Nobuo
中科院分区：
文献类型：
--
作者：
Miyagi Isoji;Hoshizumi Hideo;Suda Taichi;Saito Genji;Miyabuchi Yasuo;Geshi Nobuo
本文介绍了阿索火山口喷发产物中基质玻璃、斑晶熔融包裹体及其宿主矿物的化学组成（包括H2O和CO2）。我们发现一组熔融包裹体的钾含量明显低于之前报道的高K2O全岩成分（3-5 wt % K2O, 55-70 wt % SiO2） （50-70 wt % SiO2, 0.6-2 wt % K2O）。高K2O中长英质熔体包裹体多为蒸汽欠饱和，表现出H2O、CO2和K2O聚集的特征，而低K2O玄武岩熔体已经具有较高的H2O和CO2。我们用一个模型来调和这一差异：(1)富含挥发物的玄武岩岩浆在地表附近脱气，(2)迁移回地壳深处12公里，(3)长石和石英结晶产生高k2o的长英熔体，(4)未脱气、脱气和演化岩浆之间的岩浆混合。通过结晶，产生高k2o的长英质岩浆需要大约5倍的低k2o玄武岩岩浆。270 ka (Aso-1) ~ 90 ka （Aso-4）长英质岩浆喷发量要求玄武岩岩浆供给速率为18 ~ 31 km3/ka。这一岩浆供应速率与目前中takake Aso活跃中心的脱气岩浆生产速率（73 km3/ka）相当或更低。这些发现表明，在火山口火山休眠期间，火山气体通量监测有可能成为“玄武岩使用仪表”。
This paper presents the chemical composition (including H2O and CO2) of matrix glass, melt inclusions in phenocrysts, and their host minerals in eruptive products from Aso caldera. We found a group of melt inclusions with clearly lower potassium (0.6–2 wt % K2O at 50–70 wt % SiO2) than previously reported high-K2O whole-rock compositions (3–5 wt % K2O at 55–70 wt % SiO2). While most of the high-K2O intermediate to felsic melt inclusions are vapor undersaturated and show the features of H2O, CO2, and K2O accumulation, the low-K2O basaltic melt already has higher H2O and CO2. We reconcile this discrepancy with a model in which (1) the volatile-rich basalt magmas degas near the surface, (2) migrate back to depths of12 km in the crust, and (3) crystallize feldspar and quartz to produce high-K2O felsic melt, (4) magma mixing among the undegassed, degassed, and evolved magmas. By crystallization, about five times as much low-K2O basaltic magma is required to produce high-K2O felsic magma. The quantity of felsic magma ejected from 270 ka (Aso-1) to 90 ka (Aso-4) requires a basalt magma supply rate of 18 to 31 km3/ka. This magma supply rate is comparable to or less than the present-day production rate of degassed magma (73 km3/ka) at an active center of Aso, Nakadake. These findings suggest volcanic gas flux monitoring has the potential to be a ‘basalt usage meter’ during the dormant period of caldera volcanoes.