Special Collection: Glasses, Melts, and Fluids, as Tools for Understanding Volcanic Processes and Hazards. Melt inclusion CO2 contents, pressures of olivine crystallization, and the problem of shrinkage bubbles
Special Collection: Glasses, Melts, and Fluids, as Tools for Understanding Volcanic Processes and Hazards. Melt inclusion CO2 contents, pressures of olivine crystallization, and the problem of shrinkage bubbles
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特别收藏：玻璃、熔体和流体，作为了解火山过程和危害的工具。
DOI：
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发表时间：
2015
期刊：
影响因子：
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通讯作者：
P. Cervantes
中科院分区：
文献类型：
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作者：
P. Wallace;Vadim Kamenetsky;P. Cervantes
摘要熔体包裹体中H2O和CO2的含量可以用来推断结晶压力和包裹体截留压力，因为H2O-CO2混合蒸汽的溶解度已经在很宽的熔体组成范围内通过实验确定。然而，熔融夹杂物通常在包裹后冷却和结晶过程中产生收缩气泡，因为这些过程导致夹杂物中的压力下降。这种压降导致蒸汽泡成核，导致低溶解度CO2从被捕获的熔体中出溶。为了研究CO2进入这种气泡的损失，我们实验性地加热了来自莫纳罗亚山苦橄岩的橄榄石（Fo含量为88.1 ± 0.2）中的大的天然玻璃质熔体包裹体，以使包裹体重新均匀化。使用高温加热阶段快速加热至1420 °C将收缩气泡溶解到熔体中。对于加热的内含物，通过FTIR光谱法测量并重新计算与橄榄石主体平衡的熔体的CO2含量为224-505 ppm（n = 11），远高于来自相同样品的自然淬火内含物的CO2含量（38-158 ppm; n = 8）。包裹体截留的压力计算从水和CO2数据的加热包裹体的范围从0.5到1.1千巴，表明富镁橄榄石结晶在很浅的深度下的莫纳罗亚山表面。我们的研究结果表明，40-90%（平均75%）的原始CO2溶解在熔体中的夹杂物截留时，可以失去的收缩气泡在后截留冷却。我们表明，Riker（2005）的计算方法，预测喷发前收缩气泡大小作为捕获温度和喷发前温度之间的差异的函数，成功地再现了我们的实验结果。我们的研究结果表明，必须考虑收缩气泡中所含的CO2的质量，以准确地推断熔体包裹体的初始结晶压力。然而，效果预计将更小的富H2O的熔融包裹体比这里研究的，因为在这样的包裹体中的蒸汽泡将有较低的摩尔分数的CO2比低H2O包裹体在我们的研究。
Abstract The H2O and CO2 contents of melt inclusions can potentially be used to infer pressures of crystallization and inclusion entrapment because the solubility of mixed H2O-CO2 vapor has been determined experimentally for a wide range of melt compositions. However, melt inclusions commonly develop a shrinkage bubble during post-entrapment cooling and crystallization because these processes cause a pressure drop in the inclusion. This pressure drop causes a vapor bubble to nucleate, leading to exsolution of low-solubility CO2 from the trapped melt. To investigate the loss of CO2 into such bubbles, we experimentally heated large, naturally glassy melt inclusions in olivine (Fo contents of 88.1 ± 0.2) from a Mauna Loa picrite to rehomogenize the inclusions. Rapid heating to 1420 °C using a hightemperature heating stage dissolved the shrinkage bubbles into the melt. CO2 contents measured by FTIR spectroscopy and recalculated for melt in equilibrium with the olivine host are 224-505 ppm (n = 11) for heated inclusions, much higher than the CO2 contents of naturally quenched inclusions from the same sample (38-158 ppm; n = 8). Pressures of inclusion entrapment calculated from the H2O and CO2 data for the heated inclusions range from 0.5 to 1.1 kbar, indicating that Mg-rich olivine crystallized at very shallow depths beneath the surface of Mauna Loa. Our results indicate that 40-90% (average 75%) of the original CO2 dissolved in the melt at the time of inclusion entrapment can be lost to the shrinkage bubble during post-entrapment cooling. We show that the computational method of Riker (2005), which predicts the pre-eruption shrinkage bubble size as a function of the difference between trapping temperature and pre-eruption temperature, successfully reproduces our experimental results. Our results demonstrate that the mass of CO2 contained in shrinkage bubbles must be considered to accurately infer original pressures of crystallization for melt inclusions. However, the effect is expected to be smaller for more H2O-rich melt inclusions than those studied here because the vapor bubble in such inclusions will have lower mole fractions of CO2 than the low-H2O inclusions in our study.