Crystals and melt inclusions record deep storage of superhydrous magma prior to the largest known eruption of Cerro Machín volcano, Colombia

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Silvia Castilla, Megan E Newcombe, Philip M Piccoli, Liam D Peterson
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引用次数: 0

Abstract

Cerro Machín, a volcano located in the northern segment of the Andes, is considered one of the most dangerous volcanoes in Colombia with an explosive record that involves at least five plinian events. Prior studies focused on the last dome-building eruption have suggested the presence of a water-rich mid-crustal magma reservoir. However, no direct volatile measurements have been published and little work has been completed on the explosive products of the volcano. Here, we study the largest known eruption of Cerro Machín volcano which occurred 3600 yr BP producing dacitic pyroclastic fall deposits that can be traced up to 40 km from the vent. Lapilli pumice clasts have a mineral assemblage of plagioclase, amphibole, quartz, and biotite phenocrysts, with accessory olivine, Fe-Ti oxides, and apatite. The occurrence of Fo89-92 olivine rimmed by high Mg# amphibole and the established high-water contents in the magma imply the presence of magma near or at water saturation at pressures >~500 MPa. Measurements of up to 10.7 wt% H2O in melt inclusions hosted in plagioclase and quartz in the 3600 years BP eruption products support the idea that Cerro Machín is a remarkably water-rich volcanic system. Moreover, this is supported by measurements of ~103 – 161 ppm H2O in plagioclase phenocrysts. The application of two parameterizations of water partitioning between plagioclase and silicate melt allows us to use our water in plagioclase measurements to estimate equilibrium melt water contents of 5 ± 1 – 11 ± 2 wt% H2O, which are in good agreement with the water contents we measured in melt inclusions. Results of amphibole geobarometry are consistent with a magma reservoir stored in the mid-to-lower crust at a modal pressure of 700 ± 250 MPa, corresponding to a depth of ~25 km. Minor crystallization in the shallow crust is also recorded by amphibole barometry and calculated entrapment pressures in melt inclusions. Amphibole is present as unzoned and zoned crystals. Two populations of unzoned amphibole crystals are present, the most abundant indicate crystallization conditions of 853 ± 26 °C (1 se; standard error), and the less abundant crystallized at an average temperature of 944 ± 24 °C (1 se). Approximately 18% of the amphibole crystals are normally or reversely zoned, providing evidence for a minor recharge event that could have been the trigger mechanism for the explosive eruption. Plagioclase crystals also show normal and reverse zoning. The moderate Ni concentrations (<1600 μg/g) in the high-Fo olivine xenocrysts suggest that Cerro Machín primary magmas are generated by inefficient interaction of mantle peridotite with a high-silica melt produced by slab melting of basaltic material. Some sediment input is also suggested by the high Pb/Th (>2.2), Th/La (0.3 – 0.4), and low La/Th (<13; relative to mantle array) ratios. Whole rock chemistry reveals heavy rare earth element (HREE) depletion and Sr enrichment that likely formed during the crystallization of garnet and amphibole in the upper part of the mantle or lower portion of the crust, promoting the formation of water-rich dacitic magma that was then injected into the middle-to-lower crust. Textural and compositional differences in the crystal cargo that erupted during dome-building and plinian events support the idea that large volumes of magma recharge lead to effusive eruptions, while only small recharge events are needed to trigger plinian eruptions at Cerro Machín.
晶体和熔融包裹体记录了哥伦比亚 Cerro Machín 火山已知最大规模喷发前超水岩浆的深层储存情况
Cerro Machín 火山位于安第斯山脉北段,被认为是哥伦比亚最危险的火山之一,其爆炸记录涉及至少五次陨石事件。之前针对最后一次穹隆喷发的研究表明,该火山存在一个富含水的地壳中层岩浆库。但是,没有发表过直接的挥发性测量结果,对火山爆炸产物的研究也很少。在这里,我们研究了 Cerro Machín 火山已知的最大一次喷发,这次喷发发生在公元前 3600 年,产生的白云母火成碎屑沉积物可追溯到距离喷口 40 公里的地方。Lapilli浮石碎屑的矿物组合包括斜长石、闪石、石英和生物晶,以及橄榄石、铁钛氧化物和磷灰石。Fo89-92橄榄石被高镁闪石环绕,岩浆中的高含水量也已确定,这意味着岩浆在压力>~500兆帕时接近或达到了水饱和状态。在公元前 3600 年的喷发产物中,斜长石和石英所含的熔融包裹体中测量到的 H2O 含量高达 10.7 wt%,这支持了 Cerro Machín 是一个非常富水的火山系统的观点。此外,斜长石表晶中约 103 - 161 ppm 的 H2O 测量值也支持这一观点。通过对斜长石和硅酸盐熔体之间水分配的两个参数化的应用,我们可以利用斜长石中的水含量测量结果来估计熔体的平衡水含量为 5 ± 1 - 11 ± 2 wt% H2O,这与我们在熔体包裹体中测量到的水含量非常吻合。闪石测地结果与中下地壳的岩浆储层一致,其模态压力为 700 ± 250 兆帕,相当于约 25 千米的深度。闪石测压和熔融包裹体的计算夹带压力也记录了浅地壳的少量结晶。闪石以非带状和带状晶体的形式存在。非分区闪石晶体有两个种群,最丰富的结晶条件为 853 ± 26 °C(1 se;标准误差),较少的结晶条件为平均温度 944 ± 24 °C(1 se)。约有 18% 的闪石晶体呈正常或反向分带,这为小规模补给事件提供了证据,该事件可能是爆炸性喷发的触发机制。斜长石晶体也显示出正常或反向分带。高Fo橄榄石异长晶中的中等镍浓度(<1600 μg/g)表明,Cerro Machín原生岩浆是由地幔橄榄岩与玄武岩物质板块熔化产生的高硅熔体低效相互作用产生的。Pb/Th(>2.2)、Th/La(0.3 - 0.4)和La/Th(<13;相对于地幔阵列)比率较高,这也表明有沉积物进入。全岩化学显示重稀土元素(HREE)贫化和锶富集,这很可能是在地幔上部或地壳下部的石榴石和闪长岩结晶过程中形成的,促进了富含水的达开特岩浆的形成,然后注入中下地壳。在穹隆形成和陨石事件期间喷发的晶体货物在质地和成分上的差异支持了这样一种观点,即大量岩浆补给导致喷发,而在 Cerro Machín 只需要少量补给事件就能引发陨石喷发。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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