Bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure
Bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure
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DOI：
10.1016/j.chemgeo.2006.08.006
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发表时间：
2007-01
影响因子：
4.1
通讯作者：
James E. Gardner
中科院分区：
文献类型：
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作者：
James E. Gardner
H2O气泡在岩浆中的成核作用被认为是火山爆发的触发因素。为了确定气泡如何在硅酸盐熔体中不均匀地成核，进行了实验，其中高二氧化硅流纹岩熔体在800 °C和50或125 MPa下水合，然后在550和700 °C之间的温度下减压20-91 MPa，并在淬火之前保持在较低压力下10-720 s。气泡的成核数密度（NB）介于3× 107和2× 108 cm −3之间。块状磁铁矿或针形赤铁矿的末端作为成核的网站，但只有当超过最低过饱和度，这随着熔体粘度的增加而增加。气泡没有成核沿着长度的赤铁矿针，也没有斜长石。随着熔体粘度的增加，成核事件的开始和结束时间都增加。使用成核理论的预测，无论是缓慢的成核速率，也不改变激活的成核网站可以充分解释的温度，水扩散系数，或粘度的差异。相反，成核动力学的变化最好解释为熔体和蒸汽之间的表面张力的变化，导致在较低的温度和水含量的熔体的聚合增加。由于在流纹岩熔体中，只有约108个cm− 3的气泡在磁铁矿上成核，几乎不考虑实验条件，这项研究的结果可能无法直接与不同成分的火山浮石中的气泡数量进行比较。
The nucleation of H2O bubbles in magmas has been proposed as a trigger for volcanic eruptions. To determine how bubbles nucleate heterogeneously in silicate melts, experiments were carried out in which high-silica rhyolitic melts were hydrated at 800 °C and either 50 or 125 MPa, and then decompressed by 20–91 MPa at temperatures that ranged between 550 and 700 °C, and held at the lower pressures for 10–720 s before being quenched. Bubbles nucleated in number densities (NB) that vary between 3×107and 2×108cm−3. Blocky shaped magnetite or the ends of needle-shaped hematite acted as sites for nucleation, but only if a minimum super-saturation was exceeded, which increases with increasing melt viscosity. Bubbles did not nucleate along the lengths of hematite needles nor on plagioclase. Both the beginning and ending times of the nucleation event increases with increasing melt viscosity. Using nucleation theory predictions, neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature, water diffusivity, or viscosity. Instead, the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor, resulting from the increasing polymerization of the melt at lower temperatures and water contents. Because only ∼108bubbles cm−3nucleate on magnetite in the rhyolite melt used, almost regardless of experimental conditions, results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions.