Feeding large eruptions : crystallisation, mixing and degassing in Icelandic magma chambers
Feeding large eruptions : crystallisation, mixing and degassing in Icelandic magma chambers
复制标题
供给大喷发：冰岛岩浆室中的结晶、混合和脱气
DOI：
--
复制
发表时间：
2009
期刊：
影响因子：
--
通讯作者：
E. Passmore
中科院分区：
文献类型：
--
作者：
E. Passmore
冰岛横跨大西洋中脊，位于地幔热点上。这种构造环境产生了大量的拉斑岩浆。冰岛的火山活动集中在三个新火山扩张脊上。在全新世期间，冰岛东南部的东部火山带（EVZ）是火山活动最活跃的地区，并发生了几次大喷发（bbbb6公里），其中包括有记录以来唯一的洪水玄武岩型喷发，即1783-84年的拉基火山喷发。本研究对三次大喷发进行了采样：1783-84年拉基火山喷发（15.1公里）；3000 - 4000 yBP Thjórsárdalur喷发（可能bbbb40 km）；和~8,600 yBP Thjórsá喷发（>21 km）。这些喷发的产物已经使用一系列分析技术进行了分析，其具体目的是研究岩浆储层中的结晶、脱气和混合过程，这些过程为大规模喷发提供了动力。拉基火山喷发一直是这项研究的重点。来自熔岩流不同部分的样品显示出微量元素浓度和比例的细微变化。这种成分变化不能完全用分数结晶过程来解释，而是受到晶体积累的强烈控制，因为整个岩石中不相容的微量元素浓度与样品中晶体的质量分数呈线性负相关。然而，简单的晶体积累模型无法解释成分的变化，一种解释是，均匀的拉基熔体混合了不同比例的晶体糊状物，其中含有自己的液体。热压计算结果表明，喷发的拉基液在1 ~ 3kb处与橄榄石、斜长石和辉石处于平衡状态。大部分携带的晶体都太原始，不可能从喷出的液体中结晶，气压计计算表明斜辉石在3-7 kb处结晶。因此，拉基玄武岩样品中的大部分大晶体都是在与拉基载体熔体相同的岩浆管道系统中生长的反结晶，但与之没有直接的化学平衡。这一发现得到了以下事实的证实：镁含量过高的橄榄石晶体与拉基全岩组成的化学平衡存在熔融包裹体，其平均La/Yb值在误差范围内与全岩值相同。镁含量最高（Fo86）的橄榄石晶体与镁含量最低（Fo<74）的橄榄石晶体的熔体包裹体中La/Yb值的差异较大，表明拉基岩浆在同步结晶和广泛混合之前的初始变异性，这使得载体熔体组成均匀化。与整个岩石组成相比，熔体包裹体中La/Yb的保存范围很广，并且来自同一晶体的不同包裹体的La/Yb值范围也很广，这表明熔体包裹体包裹和火山喷发期间淬火之间的时间尺度很短。熔体包裹体研究也揭示了拉基岩浆在橄榄石结晶开始时的溶解挥发性含量，尽管大部分H2O浓度几乎肯定是通过与宿主晶体或周围岩浆的低压扩散交换而重置的。熔体包裹体中挥发物与不相容元素的行为比较表明，在橄榄石结晶过程中，CO2脱气，而S、F和Cl没有脱气。根据熔体包裹体挥发性浓度对喷发期间大气总挥发性负荷的新估计表明，SO2和HCl负荷与以前的估计相当，但HF负荷更高。质量平衡计算表明，与拉基全岩组成化学平衡的橄榄石中所含熔体包裹体的H2O和CO2浓度分别比岩浆储层未发生喷发前脱气时的预期浓度低~50%和~93%，这意味着喷发前岩浆中H2O和CO2的脱气作用一定是显著的。Thjórsá熔岩流比Laki熔岩流的成分变化更大，其平均主微量元素组成不同。Thjórsá整体岩石组成的变化可以用来源变化和分数结晶的结合来解释，并且与Laki不同，不受晶体积累的强烈控制。这篇论文是献给宝琳·帕斯莫尔的，她从未看到它完成
Iceland straddles the Mid-Atlantic Ridge and overlies a mantle hotspot. This tectonic setting produces voluminous tholeiitic magmas. Volcanism in Iceland is focussed along three neovolcanic spreading ridges. During the Holocene, the Eastern Volcanic Zone (EVZ) in southeast Iceland has been the most volcanically active and has been the site of several large (>6 km) eruptions, including the only floodbasalt type eruption in recorded history, the 1783-84 Laki eruption. Three eruptions of large volume have been sampled for this study: the 1783-84 Laki eruption (15.1 km); the 3,000-4,000 yBP Thjórsárdalur eruption (probably >4 km); and the ~8,600 yBP Thjórsá eruption (>21 km). The products of these eruptions have been analysed using a range of analytical techniques, with the specific aim of investigating crystallisation, degassing and mixing processes in the magma reservoirs that feed large eruptions. The Laki eruption has been the particular focus of this study. Samples from different parts of the lava flow show fine-scale variations in trace element concentrations and ratios. This compositional variation is not fully explained by fractional crystallisation processes, but is strongly controlled by crystal accumulation as whole-rock incompatible trace element concentrations show a linear, negative correlation with the mass fraction of crystals in the sample. Simple crystal accumulation models, however, fail to explain the compositional variation, and one explanation is that the homogeneous Laki melt mixed with varying proportions of a crystal mush that contained its own liquid. The results of thermobarometry calculations indicate that the erupted Laki liquid was in equilibrium with olivine, plagioclase and augite at 1-3 kb. Most of the crystals carried by the flow are too primitive to have crystallised from the erupted liquid and barometry calculations indicate that clinopyroxene crystallised at 3-7 kb. The majority of the large crystals hosted in the Laki basalt samples are therefore antecrysts that grew within the same magma plumbing system as the Laki carrier melt but are not in direct chemical equilibrium with it. This finding is verified by the fact that olivine crystals that are too magnesian to be in chemical equilibrium with the Laki whole-rock composition contain melt inclusions with average La/Yb values that are the same within error as the whole-rock values. The wide range of La/Yb values in melt inclusions hosted in the most magnesian (Fo86) olivine crystals in comparison to the least magnesian (Fo<74) indicates the initial variability of the Laki magma prior to concurrent crystallisation and extensive mixing, which acted to homogenise the carrier melt composition. The preservation of a wide range of La/Yb within the melt inclusions in comparison to the whole-rock composition, and a range of La/Yb values in different inclusions from the same crystal, indicates short timescales between melt inclusion entrapment and quenching during eruption. Melt inclusion studies also reveal the dissolved volatile content of the Laki magma at the onset of olivine crystallisation, although the majority of H2O concentrations have almost certainly been reset by low pressure diffusive exchange with the host crystal or surrounding magma. Comparison of the behaviour of volatiles with that of incompatible elements in the melt inclusions indicates that CO2 was degassing during olivine crystallisation, but S, F and Cl were not. New estimates of total volatile loading to the atmosphere during the eruption based on melt inclusion volatile concentrations show SO2 and HCl loading comparable to previous estimates, but higher HF loading. Mass balance calculations show that the observed H2O and CO2 concentrations of melt inclusions hosted in olivines in chemical equilibrium with the Laki whole-rock composition are ~50% and ~93% lower respectively than would be expected if no pre-eruptive degassing of the magma reservoir had occurred, meaning that pre-eruptive degassing of H2O and CO2 from the magma must have been significant. Lava flows from Thjórsá are more compositionally variable than those from Laki, and have different average major and trace element compositions. Compositional variation within the Thjórsá whole-rock composition is explained by a combination of source variation and fractional crystallisation, and, unlike Laki, is not strongly controlled by crystal accumulation. This thesis is dedicated to Pauline Passmore, who never saw it finished
影响因子：
5
作者：
J. Maclennan
通讯作者：
J. Maclennan