Experimental constraints on pre-eruptive water contents and changing magma storage prior to explosive eruptions of Mount St Helens volcano
Experimental constraints on pre-eruptive water contents and changing magma storage prior to explosive eruptions of Mount St Helens volcano
复制标题
圣海伦斯火山爆发前对喷发前含水量和岩浆储存变化的实验限制
DOI：
10.1007/bf00298703
复制
发表时间：
1995
影响因子：
3.5
通讯作者：
Haraldur Sigurdsson
中科院分区：
文献类型：
--
作者：
J. Gardner;M. Rutherford;S. Carey;Haraldur Sigurdsson
在过去的4000年里，圣海伦火山喷发出了成分多样的英质岩浆。这些英安岩中的相组合及其组成提供了有关喷发前熔体组成、与熔体平衡的相和岩浆温度的信息。从这些资料可以推断出许多英安岩的喷发前压力和水逸性。这是通过在850°C和一系列压力和水流度下进行热液实验，并将结果与780°C和920°C的实验结果相结合来完成的，以覆盖英安岩平衡条件的可能范围。将自然相组合和成分与实验结果进行比较，推断岩浆喷发前最可能的条件。然后根据推断的条件估计英安岩熔体中溶解的水的含量。在过去的4000年里，英安岩的含水量变化很大，从3.7%到6.5%不等。在4000年到3000年前，英安岩趋向于水饱和，含有5.5%到6.5%的水。然而，从那时起，英安岩明显处于水不饱和状态，含水量低于5.0%。这些英安岩往往更热，更富镁性，安山岩和玄武岩岩浆喷发。这些变化可以通过长英安岩和玄武岩之间的不同数量的混合来解释，以产生更热、更干燥、更基性的英安岩和安山岩。英安质岩浆的岩浆储存区域在4000年里也发生了显著变化，在很短的时间内，甚至可能是两年，地壳中向较浅的层次转移。这些变化可能与上覆顶板岩石的破裂有关，这是在大规模喷发期间岩浆撤退的结果。
Compositionally diverse dacitic magmas have erupted from Mount St Helens over the last 4000 years. Phase assemblages and their compositions in these dacites provide information about the composition of the pre-eruptive melt, the phases in equilibrium with that melt and the magmatic temperature. From this information pre-eruptive pressures and water fugacities of many of the dacites have been inferred. This was done by conducting hydrothermal experiments at 850°C and a range of pressures and water fugacities and combining the results with those from experiments at temperatures of 780 and 920°C, to cover the likely range in equilibration conditions of the dacites. Natural phase assemblages and compositions were compared with the experimental results to infer the most likely conditions for the magmas prior to eruption. Water contents disolved in the melts of the dacites were then estimated from the inferred conditions. Water contents in the dacites have varied greatly, from 3.7 to 6.5 wt.%, in the last 4000 years. Between 4000 and about 3000 years ago the dacites tended to be water saturated and contained 5.5 to 6.5 wt.% water. Since then, however, the dacites have been significantly water-undersaturated and contained less than 5.0 wt.% water. These dacites have tended to be hotter and more mafic, and andesitic and basaltic magmas have erupted. These changes can be explained by variable amounts of mixing between felsic dacite and basalt, to produce hotter, drier and more mafic dacites and andesites. The magma storage region of the dacitic magmas has also varied significantly during the 4000 years, with shifts to shallower levels in the crust occurring within very short time periods, possibly even two years. These shifts may be related to fracturing of overlying roof rock as a result of magma with-drawal during larger volume eruptions.