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首页> 外文期刊>Journal of Volcanology and Geothermal Research >An analog investigation of magma fragmentation and degassing: Effects of pressure,volatile content, and decompression rate
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An analog investigation of magma fragmentation and degassing: Effects of pressure,volatile content, and decompression rate

机译：岩浆破碎和脱气的模拟研究：压力，挥发物含量和减压速率的影响

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摘要

We conducted analog experiments using gum rosin-acetone (GRA) mixtures to approximate behavior of magmas containing dissolved and exsolved volatiles. The main objective was to investigate the role of pressure upon fragmentation behavior for GRA mixtures undergoing slow and fast decompressions. We found that under conditions of slow decompression (22-80 Pa s~(-1)), the GRA mixture never fragmented, exhibiting characteristic styles of surface degassing at different pressures. Under fast decompression (620-2400 kPa s~(-1)), fragmentation behavior was controlled principally by the initial volatile content of the GRA mixture, as well as by the extent and rate of decompression. Low-viscosity GRA mixtures with high levels of acetone fragmented efficiently at high final pressures, while viscous mixtures containing less acetone did not fragment except when subjected to small final pressures, high pressure differentials, and rapid decompression. Based on our observations, we propose that GRA mixtures near the fragmentation threshold are disrupted in a ductile fashion by intense vesicula-tion and bubble movements, while mixtures subject to elevated pressure differentials which are significantly higher than this threshold fragment in a brittle manner due to foam disruption. In non-fragmenting GRA mixtures, a form of stick-slip behavior was observed, the dynamics of which varied with the rheology of the mixture and the pressure differential applied during the decompression. In low-viscosity GRA mixtures, gas transfer occurred mainly in the form of gas bubbles and slugs which moved vertically upward through the material, creating permeable pathways to promote degassing and disruption of the mixture by bubble bursts. In viscous GRA mixtures dominated by foam development, vertical permeability gradients were observed, with a strongly degassed brittle foam at the top and a less degassed ductile foam beneath. Our observations have implications for submarine volcanism, volcanoes on Venus, and lava dome eruptions. For submarine volcanism, our experiments show that explosive fragmentation of magma may not be limited by water depth if the volatile content of the magma is sufficiently high (i.e., 4-6 wt.% H_2O). Our experimental results also suggest that the high-pressure atmospheric conditions of Venus (9300 kPa) are not an impediment to the explosive eruption of high volatile content magmas, which casts doubt on the interpretation of large-volume pancake lava domes on Venus as the high-pressure equivalent of terrestrial ignimbrite eruptions. Together with our experimental results, observations of actively growing lava domes demonstrate that explosive fragmentation is promoted by rapid, large-volume collapses of a dome, and by unroofed magma which is comparatively volatile-rich and vesicular.
机译：我们使用松香-丙酮（GRA）混合物进行了模拟实验，以近似估算含有溶解和溶解挥发物的岩浆的行为。主要目标是研究压力对经历缓慢和快速减压的GRA混合物的破碎行为的作用。我们发现，在缓慢减压（22-80 Pa s〜（-1））的条件下，GRA混合物不会碎裂，在不同压力下表现出表面脱气的特征。在快速减压（620-2400 kPa s〜（-1））下，碎裂行为主要由GRA混合物的初始挥发物含量以及减压的程度和速率控制。具有高含量丙酮的低粘度GRA混合物在较高的最终压力下会有效地破碎，而含有较少丙酮的粘性混合物不会破碎，除非最终压力小，高压差和快速减压。根据我们的观察，我们提出接近碎片阈值的GRA混合物会由于强烈的囊泡作用和气泡运动而以延展的方式被破坏，而混合物的压差却由于该脆性而明显高于该阈值碎片，泡沫破裂。在无碎片的GRA混合物中，观察到了一种粘滑行为，其动力学随混合物的流变性和减压过程中施加的压差而变化。在低粘度GRA混合物中，气体转移主要以气泡和小块的形式发生，这些气泡和小块在材料中垂直向上移动，从而形成可渗透的通道，以促进混合物的脱气和由于气泡破裂而破坏混合物。在以泡沫形成为主的粘性GRA混合物中，观察到了垂直渗透率梯度，顶部是高度脱气的脆性泡沫，而下方是脱气性较小的延性泡沫。我们的观察结果对海底火山活动，金星上的火山和熔岩穹顶爆发有影响。对于海底火山活动，我们的实验表明，如果岩浆的挥发性含量足够高（即4-6 wt。％H_2O），则岩浆的爆炸性破碎可能不受水深的限制。我们的实验结果还表明，金星的高压大气条件（9300 kPa）不会阻碍高挥发性含量岩浆的爆发性喷发，这使人们对金星上的大块煎饼熔岩穹顶解释为高等效于地面火成岩爆发。连同我们的实验结果，对活跃的熔岩穹顶的观察表明，爆炸的碎片是由穹顶的快速，大体积塌陷以及非挥发岩浆（相对挥发物丰富且呈囊状）促进的。

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  • 来源
    《Journal of Volcanology and Geothermal Research》 |2012年第15期|p.12-23|共12页
  • 作者

    John Stix; Jeremy C. Phillips;

  • 作者单位

    Department of Earth & Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A7;

    Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, United Kingdom;

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