Kinetics of heterogeneous bubble nucleation in rhyolitic melts: implications for the number density of bubbles in volcanic conduits and for pumice textures
Kinetics of heterogeneous bubble nucleation in rhyolitic melts: implications for the number density of bubbles in volcanic conduits and for pumice textures
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DOI：
10.1007/s00410-008-0313-1
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发表时间：
2008-12-01
影响因子：
3.5
通讯作者：
Kannewischer, I.
中科院分区：
文献类型：
--
作者：
Cluzel, N.;Laporte, D.;Kannewischer, I.
我们进行了减压实验，以模拟上升的斑晶流纹质岩浆在火山管道。起始材料是在氧化条件下在200 MPa-800 ℃下无气泡的水饱和的水：它们含有6.0 wt%溶解的H2O和密集的赤铁矿晶体群（8.7 +/- 2 x 10（5）mm（-3））。在恒定温度（800 ° C）下，压力从饱和值降低到范围为99至20 MPa的最终值;减压速率为1，000或27.8 kPa/s。在所有的实验中，我们观察到一个单一的非均质气泡成核的事件开始于压力PN等于63 +/- 3 MPa的1,000 kPa/s系列，和69 +/- 1 MPa的27.8 kPa/s系列。低于P-N时，液体中水的过饱和度迅速降低到0.1wt%，成核速率下降，气泡数密度（BND）稳定在对减压速率非常敏感的值：27.8 kPa/s时为80 mm（-3），1，000 kPa/s时为5，900 mm（-3）。这种行为就像以前描述的流纹岩-H2O系统中的均匀气泡成核的情况下，在上升的岩浆中的泡囊的数值模拟的行为。在27.8和1，000 kPa/s下测得的水过饱和度相似，这意味着更快的减压速率不会导致更大的平衡偏离。我们的实验结果表明，在火山管道中上升的酸性至中等岩浆中的BND将取决于减压速率垂直棒dP/dt垂直棒和斑晶的数量密度，特别是磁铁矿微斑晶（1-100 mm（-3））的数量密度，它是唯一能够显著降低气泡成核所需的水过饱和度的矿物种类。在喷溢喷发的情况下，预测BND非常低（约为1 mm（-3））（垂直bar dP/dt垂直bar约为0.1 kPa/s）。高BND（最大10（7）mm（-3））和双峰气泡尺寸分布的情况下，预计爆炸喷发：（1）相对小的气泡数密度（1-100 mm（-3））将首先在管道垂直杆的下部成核（dP/dt垂直杆约为10 kPa/s），无论是在高压下磁铁矿或在较低的压力下石英和长石（或通过液体中的均质成核）和（2）然后，接近破碎水平垂直条dP/dt垂直条接近10 - 3 kPa/s的极端减压速率）将触发导致大量小气泡的主要成核事件，通常直径为几微米到几十微米，这是大多数浮岩的特征。
We performed decompression experiments to simulate the ascent of a phenocryst-bearing rhyolitic magma in a volcanic conduit. The starting materials were bubble-free rhyolites water-saturated at 200 MPa-800 degrees C under oxidizing conditions: they contained 6.0 wt% dissolved H2O and a dense population of hematite crystals (8.7 +/- 2 x 10(5) mm(-3)). Pressure was decreased from the saturation value to a final value ranging from 99 to 20 MPa, at constant temperature (800 degrees C); the rate of decompression was either 1,000 or 27.8 kPa/s. In all experiments, we observed a single event of heterogeneous bubble nucleation beginning at a pressure PN equal to 63 +/- 3 MPa in the 1,000 kPa/s series, and to 69 +/- 1 MPa in the 27.8 kPa/s series. Below P-N, the degree of water supersaturation in the liquid rapidly decreased to a few 0.1 wt%, the nucleation rate dropped, and the bubble number density (BND) stabilized to a value strongly sensitive to decompression rate: 80 mm(-3) at 27.8 kPa/s vs. 5,900 mm(-3) at 1,000 kPa/s. This behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite-H2O system and in numerical simulations of vesiculation in ascending magmas. Similar degrees of water supersaturation were measured at 27.8 and 1,000 kPa/s, implying that a faster decompression rate does not result in a larger departure from equilibrium. Our experimental results imply that BNDs in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate vertical bar dP/dt vertical bar and the number density of phenocrysts, especially the number density of magnetite microphenocrysts (1-100 mm(-3)), which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation. Very low BNDs (approximate to 1 mm(-3)) are predicted in the case of effusive eruptions vertical bar dP/dt vertical bar approximate to 0.1 kPa/s). High BNDs (up to 10(7) mm(-3)) and bimodal bubble size distributions are expected in the case of explosive eruptions: (1) a relatively small number density of bubbles (1-100 mm(-3)) will first nucleate in the lower part of the conduit vertical bar dP/dt vertical bar approximate to 10 kPa/s), either at high pressure on magnetite or at lower pressure on quartz and feldspar (or by homogeneous nucleation in the liquid) and (2) then, extreme decompression rates near the fragmentation level vertical bar dP/dt vertical bar approximate to 10 3 kPa/s) will trigger a major nucleation event leading to the multitude of small bubbles, typically a few micrometers to a few tens of micrometers in diameter, which characterizes most silicic pumices.