Reconstructing the deep CO2 degassing behaviour of large basaltic fissure eruptions
Reconstructing the deep CO2 degassing behaviour of large basaltic fissure eruptions
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DOI：
10.1016/j.epsl.2014.02.031
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发表时间：
2014-05-01
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.;Maclennan, John;Thordarson, Thor
从火山喷发中脱气的挥发物的质量通常是通过比较未脱气的玻璃熔体包裹体中的挥发浓度和脱气的基质玻璃中的挥发浓度来估计的。然而，熔体包裹体容易发生后包裹化，包括通过宿主橄榄石晶格的扩散H+损失，这降低了包裹体的H2O含量，以及由于包裹体壁上的冷却和结晶，二氧化碳脱气成气泡。这种气泡在公元1783-1784年冰岛东南部拉基火山喷发时以橄榄石为宿主的熔体包裹体中非常常见。我们用显微拉曼光谱测定了这些气泡中的CO2含量，用SIMS测定了玻璃中的CO2浓度。我们的结果表明，90%的包裹体总二氧化碳可以被隔离到气泡中，这表明测量熔体包裹体中蒸气气泡和玻璃相的组成是重要的。我们用Nb代替熔体包裹体中未脱气的CO2含量，重建了拉基岩浆的深部脱气路径。Laki熔融包裹体(3.8-364)中CO2/Nb的显著变化可以用铺设的岩浆系统中的同时结晶和CO2脱气来解释。我们计算了单个熔体包裹体的CO2损失量，假定富冰岛地幔的CO2/Nb约为435，亏损地幔的CO2/Nb约为171。饱和压力最大的熔融包裹体在包裹体捕获之前损失的二氧化碳最少。在任何给定的饱和压力下，最丰富的熔体包裹体损失的二氧化碳最多，而最贫化的包裹体损失的二氧化碳很少。因此，初始CO2浓度较高的浓缩原生熔体有助于研究岩浆系统的深层脱气行为，因为在结晶和脱气过程中记录了一系列熔体包裹体饱和压力。初始二氧化碳浓度较低的贫化熔体包裹体仍处于蒸气不饱和的浅层水平，不能用于限制深层脱气行为。从铺设的岩浆中累积释放的二氧化碳质量是由压力和结晶程度决定的。利用一种最新的岩石学方法，考虑了原始熔体的多样性和二氧化碳封存到汽泡中，我们计算出从铺设的岩浆中释放出的二氧化碳总量为304Mt。Crown Copyright(C)2014由Elsevier B.V.出版。这是CC许可下的一篇开放获取文章
The mass of volatiles degassed from volcanic eruptions is often estimated by comparing the volatile concentrations in undegassed glassy melt inclusions with the volatile concentrations in the degassed matrix glass. However, melt inclusions are prone to post-entrapment modification, including diffusive H+ loss through the host olivine crystal lattice which lowers the H2O content of the inclusion, and the degassing of CO2 into a bubble in response to cooling and crystallisation on the inclusion walls. Such bubbles are very common in olivine-hosted melt inclusions from the AD 1783-1784 Laki eruption, southeast Iceland. We have determined the CO2 content of these bubbles using micro-Raman spectroscopy, and the CO2 concentration in the glass by SIMS. Our results show that > 90% of the total inclusion CO2 may be sequestered into the bubble, which demonstrates the importance of measuring the compositions of both vapour bubbles and the glass phase in melt inclusions. We reconstruct the deep degassing path of the Laki magma by using Nb as proxy for the undegassed CO2 content of the melt inclusions. The substantial CO2/Nb variation in the Laki melt inclusions (3.8-364) can be explained by concurrent crystallisation and CO2 degassing in the Laid magmatic system. We calculate the amount of CO2 lost from individual melt inclusions, assuming CO2/Nb approximate to 435 for enriched Icelandic mantle and CO2/Nb approximate to 171 for depleted mantle. Melt inclusions with the greatest saturation pressures have lost the least CO2 prior to inclusion trapping. At any given saturation pressure, the most enriched melt inclusions have lost the most CO2, while the most depleted inclusions have lost very little CO2. Enriched primary melts with high initial CO2 concentrations are therefore useful for investigating deep degassing behaviour in magmatic systems because a range of melt inclusion saturation pressures are recorded during crystallisation and degassing. Depleted melt inclusions with low initial CO2 concentrations remain vapour-undersaturated to shallow levels and cannot be used to constrain deep degassing behaviour. The cumulative CO2 mass release from the Laid magma is determined as a function of pressure and extent of crystallisation. Using an updated petrologic method that takes into account the diversity of primary melts and CO2 sequestration into vapour bubbles, we calculate the total mass of CO2 exsolved from the Laid magma to be 304 Mt. Crown Copyright (C) 2014 Published by Elsevier B.V. This is an open access article under the CC BY license