Tracking timescales of short-term precursors to large basaltic fissure eruptions through Fe-Mg diffusion in olivine
Tracking timescales of short-term precursors to large basaltic fissure eruptions through Fe-Mg diffusion in olivine
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DOI：
10.1016/j.epsl.2016.01.018
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发表时间：
2016-04-01
影响因子：
5.3
通讯作者：
Thordarson, Thor
中科院分区：
文献类型：
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作者：
Hartley, Margaret E.;Morgan, Daniel J.;Thordarson, Thor
对喷发前晶体储存和岩浆脱气时间尺度的岩石学约束为解释火山活动区的地震、大地测量和气体监测数据提供了重要的框架。我们在冰岛东部火山带拉基火山喷发的86个橄榄石大晶体中使用了铁-镁扩散计时法，以表征这次喷发前晶体储存和运输的时间尺度。大多数橄榄石的核心成分为Fo<；76，边缘成分为Fo(69)-Fo(74)，与拉基熔体接近平衡。用背散射电子图像的灰度级强度作为橄榄石成分的扩散模拟表明，Laki橄榄石最可能的Fe-Mg扩散时间为7.8天，这反映了喷发前载体熔体中橄榄石的特征停留时间。一小部分Fo&Gt；81橄榄石记录的Fe-Mg扩散时间为124天；这些晶体很可能形成于地壳中部的岩浆室，在较浅的水平被转移到存储中，然后在喷发前被带入拉基熔体。典型的铁-镁扩散时间为6-10天，比拉基火山喷发离散事件之间的平均时间间隔短，这表明在每一事件开始之前，储存的晶体在载体液体中发生了可变或脉冲的解聚。扩散时间尺度与冰岛东南部强烈而频繁的地震的历史记录相吻合，我们将其解释为与熔体退出有关的碎屑解体和从地壳岩浆储集层开始的岩墙传播，深度类似于地表6+/-3公里。我们计算了喷发前二氧化碳通量为2-6Mt d(-1)，假设拉基喷发的喷发前二氧化碳排放预算为189.6 Mt，并且在每个喷发事件之前的6-10天内二氧化碳的释放速率保持不变。我们的数据集表明，对历史性喷发前几天发生的岩浆作用时间尺度的岩石学限制，可能会增强我们预测冰岛和更远地区未来大喷发开始的能力。皇冠版权所有(C)2016年，爱思唯尔公司出版。
Petrological constraints on the timescales of pre-eruptive crystal storage and magma degassing provide an important framework for the interpretation of seismic, geodetic and gas monitoring data in volcanically active regions. We have used Fe-Mg diffusion chronometry in 86 olivine macrocrysts from the AD 1783-1784 Laki eruption on Iceland's Eastern Volcanic Zone to characterise timescales of crystal storage and transport in the lead-up to this eruption. The majority of these olivines have core compositions of Fo < 76, and rim compositions in the range Fo(69)-Fo(74) that are close to equilibrium with the Laki melt. Diffusion modelling using the greyscale intensity of backscattered electron images as a proxy for olivine composition reveals that the most probable Fe-Mg diffusion timescale for Laki olivines is 7.8 days, which reflects the characteristic olivine residence time in the carrier melt prior to eruption. A small population of Fo > 81 olivines record Fe-Mg diffusion timescales of 124 days; these crystals are likely to have formed in mid-crustal magma chambers, been transferred to storage at shallower levels and then entrained into the Laki melt prior to eruption. Typical Fe-Mg diffusion timescales of 6-10 days are shorter than the average time interval between discrete episodes of the Laki eruption, indicating variable or pulsed disaggregation of stored crystals into the carrier liquid prior to the onset of each episode. The diffusion timescales coincide with historical accounts of strong and frequent earthquakes in southeast Iceland, which we interpret as being associated with mush disaggregation related to melt withdrawal and the initiation of dyke propagation from a crustal magma reservoir at similar to 6 +/- 3 km depth to the surface. We calculate pre-eruptive CO2 fluxes of 2-6 Mt d(-1), assuming a pre-eruptive CO2 outgassing budget of 189.6 Mt for the Laki eruption and a constant rate of CO2 release in the 6-10 days preceding each eruptive episode. Our dataset indicates that petrological constraints on the timescales of magmatic processes occurring in the days leading up to historic eruptions may enhance our ability to forecast the onset of future large eruptions, both in Iceland and further afield. Crown Copyright (C) 2016 Published by Elsevier B.V.