CO 2 bubble nucleation upon pressure release in potassium-rich silicate magmas
CO 2 bubble nucleation upon pressure release in potassium-rich silicate magmas
复制标题
富钾硅酸盐岩浆压力释放时二氧化碳气泡成核
DOI：
10.1016/j.chemgeo.2016.12.033
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发表时间：
2017
期刊：
影响因子：
3.9
通讯作者：
Schmidt B.C.
中科院分区：
文献类型：
--
作者：
Fanara S;Sottili G;Silleni A;Palladino D.M;Schmidt B.C.
首次报道了压力释放对硅酸盐岩浆中CO2气泡成核和生长的影响。起始材料为标称无水粗面岩和粗面玄武岩玻璃，其在氧化条件下在1200 °C和300 MPa下CO2饱和至少48 h。这些起始玻璃掺杂有不同的晶体相关的研究熔体组合物：橄榄石，铬尖晶石，单斜辉石，钾长石和白榴石。在1128-1156 °C和300 MPa下初始平衡15 - 45 h后进行减压实验。随后以4 MPa/min的减压速率将压力降至最终值30 MPa。使用扫描电子显微镜收集淬火产物的背散射电子显微照片。在所有实验中，我们观察到均匀和非均匀气泡成核的事件，通过在晶体边缘以及残余熔体中形成直径达30 μm的囊泡来证明。在含钾长石和白榴石的样品中，在这些晶体附近存在另一个直径达130 μm的气泡，这表明早期的异相成核事件已经发生。其可用作晶体作为成核位置的效率的代表。计算表明，在所研究的岩浆中，气泡成核活化能降低了一个系数φ，最高可达约1.5%。0.64实验结果表明，CO2气泡非均质成核具有以下特点：（1）CO2泡囊在不同晶体上无差别地形成，其分布范围在40° ~ 70°之间;（2）CO2气泡不优先在囊壁上成核;（3）晶体形状似乎不影响气泡成核。我们的研究结果进行了比较，从以前的研究中的非均质H2O气泡成核突出的显着特点。我们的新的实验数据在CO2为主的岩浆晶体润湿性允许通过经典的成核理论方法的表面张力的第一次评估。有关CO2气泡成核的新发现将有助于更好地理解在富钾岩浆系统的爆发和强度。
We report the first experimental study on CO2bubble nucleation and growth in silicate magmas in response to pressure release. The starting materials were nominally anhydrous trachyte and trachybasalt glasses that were CO2saturated at 1200 °C and 300 MPa for at least 48 h under oxidizing conditions. These starting glasses were doped with diverse crystals relevant to the studied melt compositions: olivine, Cr-spinel, clinopyroxene, K-feldspar and leucite. Decompression experiments were performed after an initial equilibration at 1128–1156 °C and 300 MPa for time durations ranging between 15 and 45 h. Pressure was subsequently decreased to a final value of 30 MPa with a decompression rate of 4 MPa/min. Backscattered electron microphotographs of the quenched products were collected using a scanning electron microscope.In all experiments, we observed an event of homogeneous and heterogeneous bubble nucleation, testified by the formation of vesicles with diameters up to 30 μm on the crystal rims as well as in the residual melts. In the samples containing K-feldspar and leucite, another generation of bubbles with diameter up to 130 μm is present in proximity of those crystals, suggesting the occurrence of an earlier heterogeneous nucleation event.CO2bubble nucleation and growth was investigated by the determination of the wetting angles (ϑ) between the vesicles and the crystal surfaces, which can be used as proxy for the efficiency of the crystals as nucleation sites. It was calculated that the activation energy for bubble nucleation in the studied magmas it is lowered by a factor φ up to approx. 0.64 in presence of crystals.The experimental results show several peculiarities of heterogeneous CO2bubble nucleation: i) CO2vesicles form indiscriminately on diverse crystals, in accordance to the small ϑ distribution range between 40° and 70°; ii) CO2bubbles do not nucleate preferentially on capsule walls; iii) the crystal shapes seem to not to influence bubble nucleation. Our findings are compared with data on heterogeneous H2O bubble nucleation from previous studies to highlight distinctive features.Our new experimental data on crystal wettability in CO2dominated magmas allow a first assessment of the surface tension through the classical nucleation theory approach.The new findings about CO2bubble nucleation will be useful to better understand the onset and the intensity of the eruptions in the K-rich magmatic systems.
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