Crystal forensics: constraining the timescales of magmatic processes leading to volcanic eruptions

火山每天喷发，是全世界数百万人的日常危险。今天，活火山受到持续监测，以寻找即将喷发的迹象。然而，为了实现对未来喷发的准确预测，需要全面了解火山的内部运作，包括熔岩在火山中流动的时间尺度。岩浆是由液态岩石和称为晶体的固体颗粒组成的熔岩。岩浆在地壳中的迁移和最终的喷发是复杂的，然而这一过程的记录保存在晶体中。正如你最喜欢的犯罪剧使用法医科学来破案一样，我们也可以用火山喷发中的矿物成分来做同样的事情。晶体的生命不是简单的，而是复杂的，包括生长和融化、迁移和在不同岩浆体中停留的时期，直到最终晶体喷发和成分冻结。这些经验中的每一个都作为化学或纹理标记保存在水晶中，这导致了高度带状的水晶。这些水晶带可以用记录树木生长历史的年轮来处理。各个带的化学成分可以用来确定形成带的岩浆过程的指纹，我们可以利用两个相邻带之间的化学成分的差异来确定这些岩浆过程发生的时间尺度。在形成后，两个相邻带之间的成分差异立即很大，但随着元素的时间扩散(构成晶体的小颗粒的迁移)平滑了这一成分边界，这可用于计算岩浆作用的时间尺度。重要的是，不同的元素在晶体结构中以不同的速率迁移，因此可以研究在喷发前几个小时到几个月的时间尺度上发生的整个岩浆过程。然而，这些晶体中的分区发生在亚微米级(小于人类头发的宽度)。因此，这项研究将首次使用新一代高分辨率二次离子质谱仪来测量来自美国圣海伦山和新西兰塔拉纳基山的斜长石晶体的精细化学分带。这将使岩浆过程和这些过程在喷发前直接发生的时间尺度得以评估。然后可以对照最近圣海伦火山喷发之前已知的岩浆运动时间尺度来评估这些时间尺度，从而限制对未来火山喷发的更好预测模型。这将为火山喷发前的工作提供有价值的见解，并可能为生活在火山阴影下的世界各地数百万人带来巨大好处，并有助于减轻活火山的相关危险。这项研究将在布里斯托尔大学地球科学系、德国布里斯托尔明斯特大学、西澳大利亚大学、美国喀斯喀兹火山观测站和奥根大学以及新西兰惠灵顿维多利亚大学的界面分析中心联合进行。

Volcanoes erupt daily and are an everyday hazard for millions of people worldwide. Today, active volcanoes are continuous monitored looking for signs of imminent eruptions. However, in order for the accurate prediction of future eruptions to be achieved a comprehensive knowledge of the internal workings of volcanoes is required, including the timescales over which molten rock moves through the volcano. Magma is molten rock composed of liquid rock and solid particles called crystals. The migration of magma through the Earth's crust and the eventual eruption is complex, however a record of this is preserved in crystals. Just as your favourite crime drama uses forensic science to solve the crime, we can do the same with the composition of minerals from volcanic eruptions. The life of a crystal is not simple but a complex one involving periods of growth and melting, migration and periods of residence in different magma bodies until eventually the crystal is erupted and the composition frozen in. Each of these experiences are preserved in the crystal as chemical or textural markers, that results in highly zoned crystals. These crystal zones can be treated in just the same way as tree rings which record the growth history of a tree. The chemical composition of individual zones can be used to fingerprint the magmatic process that formed the zone and we can use the difference in the chemical composition between two adjacent zones to determine the timescale over which these magmatic processes occurred. Immediately after formation the compositional difference between two adjacent zones is sharp, but with time diffusion of elements (migration of small particles that the crystal is made from) smoothes this compositional boundary and this can be used to calculate the timescale of magmatic processes. Importantly, different elements migrate through the crystal structure at differ rates and therefore a whole range of magmatic processes that occur on the timescales of hours to months prior to eruption can be investigated. However, the zoning in these crystals occur on a sub-micron scale (smaller than the width of a human hair). Therefore, for the first time this study will use the new generation of high resolution Secondary Ion Mass Spectrometers permitting the measurement of fine-scale chemical zonation of plagioclase crystals from Mount St. Helen's in the USA and Mount Taranaki in New Zealand. This will allow the magmatic processes and the timescales over which these processes occurred directly prior to eruption to be assessed. These timescales can then be evaluated against the known timescales for the movement of magma prior to the recent eruptions of Mount St. Helen's allowing better models for the future prediction of volcanic eruptions to be constrained. This will provide valuable insights into the working of volcanos directly prior to eruption and could have immense benefits to the millions of people worldwide who live in the shadow of volcanoes and help mitigate the associated hazards of active volcanoes. This research will be conducted in the Department of Earth Sciences, University of Bristol in conjunction with the Interface Analysis Centre, Bristol, Muenster University, Germany, University of Western Australia, Cascades Volcano Observatory and University of Orogen in the USA and Victoria University of Wellington, New Zealand.