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首页> 外文期刊>Journal of Geophysical Research. Biogeosciences >Gradients in H_2O, CO_2, and exsolved gas in a large-volume silicic magma system: Interpreting the record preserved in melt inclusions from the Bishop Tuff
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Gradients in H_2O, CO_2, and exsolved gas in a large-volume silicic magma system: Interpreting the record preserved in melt inclusions from the Bishop Tuff

机译：大体积硅质岩浆系统中H_2O，CO_2和溶解气体中的梯度：解释Bishop Tuff熔体包裹体中保存的记录

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Infrared spectroscopic analyses of approx 140 melt inclusions in quartz phenocrysts from the zoned Bishop rhyolitic tuff demonstrate that systematic gradients in dissolved magmatic H_2O and CO_2 concentrations were present during preeruptive crystallization of the magma body. Melt inclusions from the earliest erupted samples contain lower H_2O (5.3 +- 0.4 wt %) and CO_2 (62 +- 37 ppm) than inclusions from the middle of the eruption (5.7 +- 0.2 wt % H_2O; 120 +- 60 ppm CO_2). Melt inclusions from late erupted samples have much lower H_2O (4.1 +- 0.3 wt %) and higher and variable CO_2 (150-1085 ppm). Trace element analyses of melt inclusions by ion microprobe show that inclusions within single pumice clasts from the early and middle Bishop Tuff have an inverse correlation between CO_2 and incompatible elements. This pattern indicates that the magma was gas-saturated during crystallization, with CO_2 partitioning into a coexisting gas phase. Quantitative modeling using H_2O-CO_2 solubility relations reveals a preeruptive gradient in exsolved gas, with gas contents varying from approx 1 wt % in the deeper regions of the magma body to nearly 6 wt % near the top. Dissolved Cl, B, Li, and Be in melt inclusions correlate negatively with CO_2. Mass balance modeling of Cl loss to exsolving H_2O-rich gas during crystallization provides strong corroborating evidence for the mass fractions of exsolved gas estimated from H_2O, CO_2, and trace element data. Pressures of quartz crystallization and melt inclusion entrapment calculated from inclusion H_2O-CO_2 data are consistent with progressive downward tapping of a zoned magma body during the eruption. Melt inclusion gas saturation pressures, magma volume estimates, and time-stratigraphic-compositional relations suggest that early erupted magma was stored at the top of a downward widening magma body. Melt inclusion data and the inferred gradients in dissolved H_2O, CO_2 and exsolved gas in the Bishop magma body suggest that gas saturation plays an important role in the formation and subsequent preservation of compositional gradients in silicic magma reservoirs.
机译：区域Bishop流纹岩凝灰岩中石英细晶中约140个熔体包裹体的红外光谱分析表明，在岩浆体的隆升结晶过程中，存在溶解的岩浆H_2O和CO_2浓度的系统梯度。最早喷出的样品中的熔体夹杂物比喷发中期的夹杂物（5.7 +-0.2 wt％的H_2O; 120 +-60 ppm的CO_2）的H_2O（5.3 +-0.4 wt％）和CO_2（62 +-37 ppm）更低）。来自后期喷发样品的熔体夹杂物具有低得多的H_2O（4.1±0.3 wt％）和更高且可变的CO_2（150-1085 ppm）。离子微探针对熔体夹杂物的痕量元素分析表明，Bishop Tuff早期和中期的单个浮石碎屑中的夹杂物与CO_2和不相容元素之间呈负相关。该模式表明岩浆在结晶过程中是饱和的，CO_2分配为共存的气相。使用H_2O-CO_2溶解度关系进行的定量建模揭示了溶解气体中的喷发梯度，气体含量从岩浆体深处的约1 wt％到顶部附近的近6 wt％不等。熔融夹杂物中溶解的Cl，B，Li和Be与CO_2负相关。结晶过程中溶解富H_2O气体时Cl损失的质量平衡模型为根据H_2O，CO_2和痕量元素数据估算的溶解气体的质量分数提供了有力的佐证。由夹杂物H_2O-CO_2数据计算出的石英结晶压力和熔体夹杂物夹带压力与喷发过程中分区的岩浆体的逐步向下拍击一致。熔体夹杂物气体的饱和压力，岩浆体积估计以及时间地层-成分关系表明，早期喷出的岩浆被储存在向下扩展的岩浆体的顶部。熔岩夹杂物数据和Bishop岩浆体中溶解的H_2O，CO_2和溶解气体中的推断梯度表明，气体饱和度在硅质岩浆储层的成分梯度的形成和随后的保存中起着重要作用。

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