Megacrystals track magma convection between reservoir and surface
Megacrystals track magma convection between reservoir and surface
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巨晶体追踪储层和地表之间的岩浆对流
DOI：
10.1016/j.epsl.2014.12.022
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发表时间：
2015
影响因子：
5.3
通讯作者：
P. Kyle
中科院分区：
文献类型：
--
作者：
Y. Moussallam;C. Oppenheimer;B. Scaillet;I. Buisman;Christine Kimball;N. Dunbar;A. Burgisser;C. Schipper;J. Andújar;P. Kyle
活火山通常由位于地壳上部的岩浆库提供燃料。在这样的岩浆房的热梯度和/或成分梯度的发展可能会导致强烈的对流，从理论模型和火山岩中记录的岩浆混合的证据推断。根据观测到的地表气体和热排放以及模拟模型的实验，还推断在许多持续活跃的火山的管道中存在双向流动。然而，迄今为止，还缺乏关于这种外汇流动的更直接的证据。本文分析了南极洲埃里伯斯火山熔岩湖喷发出的斜长长石巨晶的显著振荡环带。一个全面的方法，相平衡相结合，溶解度实验和熔融包裹体和纹理分析表明，化学配置文件最好的解释为一个更深的水库和熔岩湖之间的岩浆运输在表面上的多个事件的结果。单个晶体在管道系统中反复上下移动，距离可达几公里，这可能是大量岩浆流夹带的结果。因此，我们的研究结果证实了岩浆管道中的双向流动模型。它们还意味着水库和管道中的流动状态截然不同，前者具有强烈的对流（规则的对流周期为150天，速度为10.5 mm s-1），后者具有更复杂的交换流和再夹带循环。我们估计，典型的，1厘米宽的晶体应该至少有14年的历史，并可以记录几个（从1到3）完整的循环之间的水库和熔岩湖通过管道。这种持久的响岩岩浆的再循环可能是持续的CO2 fluxing，这表明镁铁质岩浆在下地壳的积累体积更显着比内的editarian演化的岩浆。
Active volcanoes are typically fed by magmatic reservoirs situated within the upper crust. The development of thermal and/or compositional gradients in such magma chambers may lead to vigorous convection as inferred from theoretical models and evidence for magma mixing recorded in volcanic rocks. Bi-directional flow is also inferred to prevail in the conduits of numerous persistently-active volcanoes based on observed gas and thermal emissions at the surface, as well as experiments with analogue models. However, more direct evidence for such exchange flows has hitherto been lacking. Here, we analyse the remarkable oscillatory zoning of anorthoclase feldspar megacrystals erupted from the lava lake of Erebus volcano, Antarctica. A comprehensive approach, combining phase equilibria, solubility experiments and melt inclusion and textural analyses shows that the chemical profiles are best explained as a result of multiple episodes of magma transport between a deeper reservoir and the lava lake at the surface. Individual crystals have repeatedly travelled up-and-down the plumbing system, over distances of up to several kilometers, presumably as a consequence of entrainment in the bulk magma flow. Our findings thus corroborate the model of bi-directional flow in magmatic conduits. They also imply contrasting flow regimes in reservoir and conduit, with vigorous convection in the former (regular convective cycles of ∼150 days at a speed of ∼0.5 mm s−1) and more complex cycles of exchange flow and re-entrainment in the latter. We estimate that typical, 1-cm-wide crystals should be at least 14 years old, and can record several (from 1 to 3) complete cycles between the reservoir and the lava lake via the conduit. This persistent recycling of phonolitic magma is likely sustained by CO2fluxing, suggesting that accumulation of mafic magma in the lower crust is volumetrically more significant than that of evolved magma within the edifice.