2001 flank eruption of the alkali- and volatile-rich primitive basalt responsible for Mount Etna's evolution in the last three decades
2001 flank eruption of the alkali- and volatile-rich primitive basalt responsible for Mount Etna's evolution in the last three decades
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DOI：
10.1016/j.epsl.2004.09.036
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发表时间：
2004-11-30
影响因子：
5.3
通讯作者：
Burton, M
中科院分区：
文献类型：
--
作者：
Métrich, N;Allard, P;Burton, M
自 20 世纪 70 年代初以来，埃特纳火山喷发活动的增强伴随着喷发熔岩的选择性地球化学变化，其中碱金属逐渐增加，其来源仍存在争议。在此，我们基于对 2001 年 7 月至 8 月高爆炸性侧翼喷发期间挤出的异常贫斜长石原始玄武岩的橄榄石晶体中所捕获的熔体包裹体的化学和溶解挥发物含量的详细研究，进一步深入了解了这一近期演化的起源。两种类型的熔岩沿着南北断裂系统同时喷发。来自上部喷口（2950-2700 m）的粗面玄武岩只是通过中央火山管道的破裂而被排出。它们与山顶火山口熔岩相同，含有贫镁橄榄石 (Fo(70-72))，具有演化的、挥发性较差的熔体包裹体，代表浅开放管道脱气过程中的后期结晶。相比之下，贫斜长石玄武岩（占总量的 80%）通过较低的喷口（2550-2100 m）挤出，这是由于更原始且富含挥发性的岩浆横向侵入沉积基底而形成的。这种原始熔体在稀有的 Fo(82.4-80.5) 骨架橄榄石中得到了最好的保存，这些橄榄石存在于 2550 m 喷口最强烈活动的火山岩矿床中。其 H2O（3.4 wt.%）、CO2（0.11 至 0.41 wt.%）、S（0.32 wt.%）、Cl（0.16 wt.%）和 F（0.094 wt.%）的高溶解含量表明其封闭系统从大约 400 MPa 上升至 250 MPa（大约 12 至 6.5 km 深度 b.s.l.）。然而，具有常见反向分带（核心 Fo(76-78) 和边缘 Fo(78-80)）和爆裂包裹体的自形橄榄石斑晶的优势表明，大多数喷发的玄武岩源自于喷发前被上升的原始熔体侵入的同一岩浆的稍微更进化的结晶体。这些橄榄石中保存下来的少数包裹体表明，该岩浆体在喷发前储存在大约 5 公里深的地方，与地震数据一致。我们认为，2001 年的侧翼喷发导致了埃特纳火山浅层管道系统的前部逐渐超压，这是由于富含挥发性成分的原始玄武岩可能提前几个月开始涌入。我们发现，与所有 1970 年代之前的埃特涅火山熔岩（1.4 wt.% K2O，S/Cl=1.5）相比，这种玄武岩的碱含量要高得多（2.0 wt.% K2O），并且具有更高的 S/Cl (2.0)，但 Cl/K 和 Cl/F 比更低，夹带橄榄石异晶中的熔体包裹体进一步证明了这一点。将这些新的观察结果与之前发表的数据相结合，我们认为 2001 年的玄武岩代表了一种新的富碱碱性端元，为埃特纳火山提供了营养，其中只有少量在 1974 年外围喷发期间以及最近的短暂阵发性顶峰事件中被挤出。在过去的三十年里，这种新的岩浆逐渐与填充管道系统的以前的贫钾粗面玄武岩混合并取代，导致逐渐挤出更原始和富含碱的熔岩。它的地球化学奇点不可能是由浅层地壳污染造成的。相反，他们认为在埃特纳火山下方最近的岩浆成因过程中，存在着富含碱但缺乏氯的弧型成分。 (C) 2004 Elsevier B.V. 保留所有权利。
Since the early 1970s enhanced eruptive activity of Mount Etna has been accompanied by selective geochemical changes in erupted lavas, among which a gradual increase of alkalis whose origin is still debated. Here we provide further insight into the origin of this recent evolution, based on a detailed study of the chemistry and dissolved volatile content of melt inclusions trapped in olivine crystals of unusual plagioclase-poor primitive basalt that was extruded during a highly explosive flank eruption in July-August 2001. Two types of lava were erupted simultaneously along a N-S fracture system. Trachybasalts from the upper vents (2950-2700 m) were simply drained out by fracturing of the central volcanic conduit. They are identical to summit crater lavas and contain Mg-poor olivines (Fo(70-72)) with evolved and volatile-poor melt inclusions that represent late-stage crystallisation during shallow open conduit degassing. In contrast, plagioclase-poor basalt (80% of total) extruded through the lower vents (2550-2100 m) derived from lateral dyke intrusion of a more primitive and volatile-rich magma across the sedimentary basement. This primitive melt is best preserved in rare Fo(82.4-80.5) skeletal olivines present in lapilli deposits from the most powerful activities at the 2550 m vent. Its high dissolved contents of H2O (3.4 wt.%), CO2 (0.11 to 0.41 wt.%), S (0.32 wt.%), Cl (0.16 wt.%) and F (0.094 wt.%) point to its closed system ascent from similar to400 to 250 MPa (similar to12 to 6.5 km depth b.s.l.). However, the predominance of euhedral olivine phenocrysts with common reverse zoning (cores Fo(76-78) and rims Fo(78-80)) and decrepited inclusions shows that most of the erupted basalt derived from a slightly more evolved, crystallizing body of the same magma that was invaded by the uprising primitive melt prior to erupting. The few preserved inclusions in these olivines indicate pre-eruptive storage of that magma body at about 5 km depth b.s.l., in coherence with seismic data.We propose that the 2001 flank eruption resulted front gradual overpressuring of Etna's shallow plumbing system due to the influx of volatile-rich primitive basalt that may have begun several months in advance. We find that this basalt is much richer in alkalis (2.0 wt.% K2O) and has higher S/Cl (2.0) but lower Cl/K and Cl/F ratios than all pre-1970s Etnean lavas (1.4 wt.% K2O, S/Cl=1.5), as further exemplified by melt inclusions in entrained olivine xenocrysts. Combining these new observations with previously published data, we argue that the 2001 basalt represents a new alkali-rich basic end-member feeding Mt. Etna, only few amount of which had previously been extruded during a 1974 peripheral eruption and, more recently, during brief paroxysmal summit events. Over the last three decades this new magma has progressively mixed with and replaced the former K-poorer trachybasalts filling the plumbing system, leading to extrusion of gradually more primitive and alkali-richer lavas. Its geochemical singularities cannot result from shallow crustal contaminations. Instead, they suggest the involvement of an alkali-richer but Cl-poorer arc-type component during recent magma genesis beneath Etna. (C) 2004 Elsevier B.V. All rights reserved.