Constraining magma storage conditions at a restless volcano in the Main Ethiopian Rift using phase equilibria models
Constraining magma storage conditions at a restless volcano in the Main Ethiopian Rift using phase equilibria models
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DOI：
10.1016/j.jvolgeores.2017.02.026
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发表时间：
2017-05-01
影响因子：
2.9
通讯作者：
Wade, Jon
中科院分区：
文献类型：
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作者：
Gleeson, Matthew L. M.;Stock, Michael J.;Wade, Jon
埃塞俄比亚主裂谷拥有许多过碱性火山中心，其中几个显示出最近动荡的迹象。由于该地区历史上记录的火山喷发次数很少，而且缺乏火山监测，埃塞俄比亚主裂谷的岩浆储存条件仍然受到很大限制。阿尔贝托是这些不平静的火山中心之一，确定岩浆储存条件对于评估最近动荡时期的重要性至关重要。以伊拉克利翁为例，我们探讨了岩浆储存条件，使用Rhyolite-MELTS热力学模拟软件。我们使用原始玄武岩作为起始组合物进行了~ 150个分步结晶模型，并针对一系列压力（50-300 MPa）、初始H2O含量（0.5-3重量%）和氧逸度（QFM 2-Q，FM + 1）。将这些模型预测的液体下降线与已发表的全岩数据进行比较，并与天然相组合和喷发矿物成分的新观察结果一起，提供岩浆储存条件的约束。我们发现，通过从裂谷相关的玄武岩成分中延长的（90%）等压分步结晶，而不需要显著的地壳同化。所需的分离结晶程度表明，大部分岩浆系统可能存在的高度结晶糊状物，只有一个小的流纹质熔体透镜。模型和天然样品之间的最佳协议是在低压（150 MPa），低初始H2O浓度（0.5重量%）和氧逸度附近的QFM缓冲。来自这些结果（-5.6-1公里）的岩浆储存的深度是一致的源深度从测量的地面变形建模。来自埃塞俄比亚主裂谷（Boset和Gedemsa）其他过碱性火山中心以及全球其他地点（如Pantelleria、意大利）的数据表明，这些储存条件是许多过碱性火山中心的共同特征。(C)2017爱思唯尔B. V.保留所有权利。
The Main Ethiopian Rift hosts a number of peralkaline volcanic centres, several of which show signs of recent unrest. Due to the low number of historical eruptions recorded in the region and lack of volcanic monitoring, Conditions of magma storage in the Main Ethiopian Rift remain poorly constrained. Aluto is one of these restless volcanic centres and identifying magma storage conditions is vital for evaluating the significance of recent periods of unrest. Using Aluto as a case study, we explore magma storage conditions using Rhyolite-MELTS thermodynamic modelling software. We performed-150 fractional crystallisation models using a primitive basalt as the starting composition, and for a range of pressures (50-300 MPa), initial H2O contents (0.5-3 wt%) and oxygen fugacities (QFM 2-Q,FM + 1). Predicted liquid lines of descent from these models are compared with published whole rock data and, together with new observations of natural phase assemblages and erupted mineral compositions, provide constraints on magma storage conditions.Using a statistical approach to compare empirical data and thermodynamic model outputs, we find that compositions of evolved peralkaline rhyolites from Aluto are best reproduced by protracted (90%) isobaric fractional crystallisation from a rift-related basaltic composition, without the need for significant crustal assimilation. The required extent of fractional crystallisation suggests that much of the magmatic system may exist as a highly crystalline mush with only a small lens of rhyolitic melt. The best agreement between models and natural samples is at low pressures (150 MPa), low initial H2O concentrations (0.5 wt%) and an oxygen fugacity near the QFM buffer. The depth of magma storage derived from these results (-5.6 1 km) is consistent with the source depths modelled from measured ground deformation. Data from other peralkaline volcanic centres in the Main Ethiopian Rift (Boset and Gedemsa), and other locations globally (e.g. Pantelleria, Italy) suggest that these storage conditions are a common feature of many peralkaline volcanic centres. (C) 2017 Elsevier B.V. All rights reserved.