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首页> 外文期刊>Contributions to Mineralogy and Petrology >In situ observations of bubble growth in basaltic, andesitic and rhyodacitic melts
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In situ observations of bubble growth in basaltic, andesitic and rhyodacitic melts

机译：玄武岩，安山岩和流纹岩熔体中气泡生长的原位观察

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摘要

Bubble growth strongly affects the physical properties of degassing magmas and their eruption dynamics. Natural samples and products from quench experiments provide only a snapshot of the final state of volatile exsolution, leaving the processes occurring during its early stages unconstrained. In order to fill this gap, we present in situ high-temperature observations of bubble growth in magmas of different compositions (basalt, andesite and rhyodacite) at 1,100 to 1,240 °C and 0.1 MPa (1 bar), obtained using a moissanite cell apparatus. The data show that nucleation occurs at very small degrees of supersaturaturation (<60 MPa in basalt and andesite, 200 MPa in rhyodacite), probably due to heterogeneous nucleation of bubbles occurring simultaneously with the nucleation of crystals. During the early stages of exsolution, melt degassing is the driving mechanism of bubble growth, with coalescence becoming increasingly important as exsolution progresses. Ostwald ripening occurs only at the end of the process and only in basaltic melt. The average bubble growth rate (G_R) ranges from 3.4 × 10~(-6) to 5.2 × 10~(-7) mm/s, with basalt and andesite showing faster growth rates than rhyodacite. The bubble number density (N_B) at nucleation ranges from 7.9 × 10~4 mm~(-3) to 1.8 × 10~5 mm~(-3) and decreases exponentially over time. While the rhyodacite melt maintained a well-sorted bubble size distribution (BSD) through time, the BSDs of basalt and andesite are much more inhomogeneous. Our experimental observations demonstrate that bubble growth cannot be ascribed to a single mechanism but is rather a combination of many processes, which depend on the physical properties of the melt. Depending on coalescence rate, annealing of bubbles following a single nucleation event can produce complex bubble size distributions. In natural samples, such BSDs may be misinterpreted as resulting from several separate nucleation events. Incipient crystallization upon cooling of a magma may allow bubble nucleation already at very small degrees of supersaturation and could therefore be an important trigger for volatile release and explosive eruptions.
机译：气泡的生长强烈影响脱气岩浆的物理性质及其喷发动力学。淬火实验中的天然样品和产物仅提供了挥发性溶出物最终状态的快照，从而使在早期阶段发生的过程不受限制。为了填补这一空白，我们提供了在现场高温观察到的各种成分（玄武岩，安山岩和流纹岩）在1,100至1,240°C和0.1 MPa（1巴）的岩浆中气泡生长的结果，这些样品是使用莫桑石电池装置获得的。数据表明，成核发生在很小的过饱和度（玄武岩和安山岩中<60 MPa，流纹岩中200 MPa），这可能是由于气泡异质成核同时发生的。在析出的早期阶段，熔体脱气是气泡生长的驱动机制，随着析出的进行，聚结变得越来越重要。奥斯特瓦尔德熟化仅发生在过程的最后，并且仅发生在玄武质熔体中。平均气泡生长速率（G_R）为3.4×10〜（-6）至5.2×10〜（-7）mm / s，其中玄武岩和安山岩的生长速度快于流纹岩。成核时的气泡数密度（N_B）为7.9×10〜4 mm〜（-3）至1.8×10〜5 mm〜（-3），并随时间呈指数下降。尽管流纹岩熔体随时间保持了良好的气泡尺寸分布（BSD），但玄武岩和安山岩的BSD却更加不均匀。我们的实验观察表明，气泡的增长不能归因于单一机制，而是许多过程的结合，这取决于熔体的物理性质。根据聚结速率，在单个成核事件之后对气泡进行退火会产生复杂的气泡尺寸分布。在自然样品中，此类BSD可能会被误解为是由几个单独的成核事件引起的。岩浆冷却后的初期结晶可能已经使气泡成核在很小的过饱和度下发生，因此可能是挥发物释放和爆炸性爆发的重要触发因素。

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