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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Equilibrium and disequilibrium degassing of a phonolitic melt (Vesuvius AD 79 'white pumice') simulated by decompression experiments
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Equilibrium and disequilibrium degassing of a phonolitic melt (Vesuvius AD 79 'white pumice') simulated by decompression experiments

机译：减压实验模拟的胶状熔体（维苏威火山AD 79“白色浮石”）的平衡和不平衡脱气

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Equilibrium and disequilibrium degassing of a volatile phase from a magma of K-phonolitic composition was investigated to assess its behavior upon ascent. Decompression experiments were conducted in Ar-pressurized externally heated pressure vessels at superliquidus temperature (1050℃), in the pressure range 10-200 MPa using pure water as fluid phase. All experiments were equilibrated at 200 MPa and then decompressed to lower pressures with rates varying from 0.0028 to 4.8 MPa/s. Isobaric saturation experiments were performed at the same temperature and at 900-950℃ to determine the equilibrium water solubility in the pressure range 30-250 MPa. The glasses obtained from decompression experiments were analyzed for their dissolved water content, vesicularity and bubble size distribution. All decompressed samples presented a first event of bubble nucleation at the capsule-melt interface. Homogeneous bubble nucleation in the melt only occurred in fast-decompressed experiments (4.8 and 1.7 MPa/s), for ΔP ≈ 100 MPa. For these decompression rates high water over-saturations were maintained until a rapid exsolution was triggered at ΔP > 150 MPa. For slower rates (0.0028, 0.024, 0.17 MPa/s) the degassing of the melt took place by diffusive growth of the bubbles nucleating at the capsule-melt interface. This process sensibly reduced water over-saturation in the melt, preventing homogeneous nucleation to occur. For decompression rates of 0.024 and 0.17 MPa/s low water over-saturations were attained in the melt, gradually declining toward equilibrium concentrations at low pressures. A near-equilibrium degassing path was observed for a decompression rate of 0.0028 MPa/s. Experimental data combined with natural pumice textures suggest that both homogeneous and heterogeneous bubble nucleations occurred in the phonolitic magma during the AD 79 Vesuvius plinian event. Homogeneous bubble nucleation probably occurred at a depth of ～ 3 km, in response to a fast decompression of the magma during the ascent.
机译：研究了由K胶质岩浆组成的岩浆对挥发相的平衡和不平衡脱气，以评估其上升时的行为。以纯水为液相，在超液相线温度（1050℃），压力范围为10-200 MPa的Ar加压外部加热压力容器中进行了减压实验。所有实验均在200 MPa下平衡，然后以0.0028至4.8 MPa / s的速率减压至较低压力。在相同温度和900-950℃下进行等压饱和实验，以确定在30-250 MPa压力范围内的平衡水溶性。分析从减压实验获得的玻璃的溶解水含量，囊泡性和气泡尺寸分布。所有减压样品在胶囊-熔体界面处呈现出气泡成核的第一事件。熔体中的均匀气泡成核仅发生在快速减压实验（4.8和1.7 MPa / s）中，ΔP≈100 MPa。对于这些减压速率，维持高水过饱和度，直到在ΔP> 150 MPa时触发快速溶解为止。对于较慢的速率（0.0028、0.024、0.17 MPa / s），通过在胶囊-熔体界面成核的气泡的扩散生长来进行熔体的脱气。此过程可明显减少熔体中的水过饱和，从而防止发生均相成核。对于0.024和0.17 MPa / s的减压速率，在熔体中实现了低水过饱和度，并在低压下逐渐趋向平衡浓度。减压速率为0.0028 MPa / s时，观察到接近平衡的脱气路径。实验数据与天然浮石质地相结合，表明在公元79年维苏威火山夹层事件中，均质和非均质的气泡成核都发生在回音岩浆中。在上升过程中，由于岩浆的快速减压，均质气泡成核可能发生在〜3 km的深度。

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