科研之友
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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 vesiculation 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.% H2O). 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.
- 出版日期2012-1-15
