Donovan Amy, Blundy Jon, Oppenheimer Clive, Buisman Iris
1Department of Geography, King's College London, London, UK.
3Department of Geography, University of Cambridge, Cambridge, UK.
Contrib Mineral Petrol. 2018;173(1):1. doi: 10.1007/s00410-017-1425-2. Epub 2017 Nov 27.
The 2011 eruption of Nabro volcano, Eritrea, produced one of the largest volcanic sulphur inputs to the atmosphere since the 1991 eruption of Mt. Pinatubo, yet has received comparatively little scientific attention. Nabro forms part of an off-axis alignment, broadly perpendicular to the Afar Rift, and has a history of large-magnitude explosive silicic eruptions, as well as smaller more mafic ones. Here, we present and analyse extensive petrological data obtained from samples of trachybasaltic tephra erupted during the 2011 eruption to assess the pre-eruptive magma storage system and explain the large sulphur emission. We show that the eruption involved two texturally distinct batches of magma, one of which was more primitive and richer in sulphur than the other, which was higher in water (up to 2.5 wt%). Modelling of the degassing and crystallisation histories demonstrates that the more primitive magma rose rapidly from depth and experienced degassing crystallisation, while the other experienced isobaric cooling in the crust at around 5 km depth. Interaction between the two batches occurred shortly before the eruption. The eruption itself was likely triggered by recharge-induced destabilisation of vertically extensive mush zone under the volcano. This could potentially account for the large volume of sulphur released. Some of the melt inclusions are volatile undersaturated, and suggest that the original water content of the magma was around 1.3 wt%, which is relatively high for an intraplate setting, but consistent with seismic studies of the Afar plume. This eruption was smaller than some geological eruptions at Nabro, but provides important insights into the plumbing systems and dynamics of off-axis volcanoes in Afar.
2011年，厄立特里亚的纳布罗火山喷发，产生了自1991年皮纳图博火山喷发以来向大气中输入火山硫量最大的一次喷发，但相对而言受到的科学关注较少。纳布罗火山是偏离轴的火山排列的一部分，大致垂直于阿法尔裂谷，有着大规模爆炸性硅质火山喷发以及规模较小的镁铁质火山喷发的历史。在此，我们展示并分析了从2011年喷发期间喷出的粗面玄武质火山灰样本中获取的大量岩石学数据，以评估喷发前的岩浆储存系统，并解释大量硫排放的原因。我们发现，这次喷发涉及两批结构不同的岩浆，其中一批比另一批更原始，硫含量更高，而另一批的水含量更高（高达2.5重量%）。对脱气和结晶历史的模拟表明，更原始的岩浆从深部迅速上升并经历了脱气结晶，而另一批岩浆在约5千米深处的地壳中经历了等压冷却。两批岩浆在喷发前不久发生了相互作用。喷发本身可能是由补给引发的火山下方垂直延伸的岩浆 mush 区失稳触发的。这可能是大量硫释放的原因。一些熔体包裹体是挥发性不饱和的，这表明岩浆的原始水含量约为1.3重量%，对于板内环境来说相对较高，但与对阿法尔地幔柱的地震研究结果一致。这次喷发规模小于纳布罗的一些地质喷发，但为阿法尔偏离轴火山的管道系统和动力学提供了重要见解。
