Magmatic crystal records in time, space, and process, causatively linked with volcanic unrest
Magmatic crystal records in time, space, and process, causatively linked with volcanic unrest
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DOI：
10.1016/j.epsl.2018.04.025
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发表时间：
2018-07
影响因子：
5.3
通讯作者：
M. Pankhurst;D. Morgan;T. Thordarson;S. Loughlin
中科院分区：
文献类型：
--
作者：
M. Pankhurst;D. Morgan;T. Thordarson;S. Loughlin
一座火山在其过去的行为是其未来行为的指南。详细了解特定火山爆发之前的情况，以及如何实时识别这些情况，是该领域的关键问题。在这里，2010年从冰岛埃亚菲亚德拉冰盖火山侧翼喷发的岩浆的物理历史，仅使用喷发晶体的化学记录在绝对时间和空间中重建。这次重建的细节包括岩浆体的数量、几何形状、深度、相对膨胀率以及所有上述内容随时间的变化。岩石学和大地测量学（实时收集的数据）得出了同样的结论。因此，我们报告了详细的一致性，这表明通过物理化学角度确定的喷发后知识与从监测信号中获得的同喷发知识之间存在因果关系。橄榄石晶体核心（Fo 74 -87）的组成以及不平衡过程引起的每个核心周围的化学分带被证明在种群规模上形成系统模式。反环带（向富Mg方向）与其晶核的化学偏移量（≤ 2mol%Fo）恒定不变，而正环带（向富Fe方向）则趋于单一成分（≤ Fo 75）。传统的岩石学模型--例如在地壳深度范围内的多种岩浆混合--可以解释喷发岩石中存在的一系列晶核成分，但不能解释这些晶体不平衡模式，相反，我们描述了单一原始熔体如何产生成分范围广泛的晶体，并产生系统的不平衡。冷却导致晶体从水平岩浆几何形状的顶部和底部产生。晶体沉降导致熔体的不对称热分层，因此也导致熔体的成分分层，这是由于通过在底部形成晶体糊状物而进行的渐进绝缘，这一过程我们称之为“晶体雨”。至关重要的是，每个晶体的记录都是同时分步结晶和沉降的内部过程的原因和结果;不需要外部过程或材料。然后，我们使用Fe-Mg相互扩散模型从晶体中提取时间信息，并将其与成分和环带数据联合收割机结合起来。水晶雨的概念被应用，并解决了两个薄（米）的窗台，这是交错的时间和深度，并表现出不同的通货膨胀率。由于在成因物理框架内整合晶体年代学的方法可以应用于整个火山序列，因此它有可能通过确定性手段对过去和未来的岩浆和火山行为产生有价值的见解。
How a volcano has behaved throughout its past is a guide to its future behaviour. Detailed knowledge of what preceded eruptions from specific volcanoes, and how this can be recognised in real-time, are pivotal questions of this field. Here, the physical history of the magma that erupted in 2010 from the flank of Eyjafjallajökull volcano, Iceland, is reconstructed in absolute time and space using only chemical records from erupted crystals. The details of this reconstruction include the number of magma bodies, their geometry, their depth, their relative inflation rate and changes to all of the aforementioned through time. Petrology and geodesy (data gathered in real-time) arrive at the same set of conclusions. As such, we report detailed agreement, which demonstrates a causative link between knowledge determined post-eruption via a physical–chemical perspective and knowledge gained syn-eruption from monitoring signals.The composition of olivine crystal cores (∼Fo74–87), and that of the chemical zonation around each core caused by disequilibrium processes, are shown to form systematic patterns at the population scale. Reverse zonation (toward Mg rich) exhibits a constant chemical offset from its crystal core (≤2 mol % Fo), while normal zonation (toward Fe rich) converges to a single composition (∼Fo75). Conventional petrological models — for instance multiple-magma-mixing across a range of crustal depths — can explain the presence of a range of crystal core composition in the erupted rocks, but cannot explain these patterns of crystal disequilibria.Instead, we describe how a single primitive melt produces crystals over a wide range in composition and generates systematic disequilibrium. Cooling causes crystal production from both roof and floor of a horizontal magma geometry. Crystal settling causes asymmetric thermal – and therefore compositional – stratification of the melt due to progressive insulation via development of a crystal mush at the floor, a process we term “Crystal Rain”. Crucially, each crystal's record is both a cause and effect of the internal process of simultaneous fractional crystallisation and settling; no external processes or materials are required.We then extract temporal information from our crystals using Fe–Mg interdiffusion modelling, and combine it with the composition and zonation data. The concept of Crystal Rain is applied, and resolves two thin (metres) sills which are staggered in time and depth, and exhibit different inflation rates. Since the approach of integrating crystal chronology within a causative physical framework may be applied to entire volcanic successions, it has potential to yield valuable insights to past, and by inference future, magmatic and volcanic behaviours by deterministic means.