Mantle to surface degassing of carbon- and sulphur-rich alkaline magma at El Hierro, Canary Islands
Mantle to surface degassing of carbon- and sulphur-rich alkaline magma at El Hierro, Canary Islands
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DOI：
10.1016/j.epsl.2016.11.043
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发表时间：
2017-02
影响因子：
5.3
通讯作者：
M. Longpre;J. Stix;A. Klügel;N. Shimizu
中科院分区：
文献类型：
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作者：
M. Longpre;J. Stix;A. Klügel;N. Shimizu
玄武岩火山将挥发物从地幔转移到地球表面。深部挥发分通量的量化在很大程度上依赖于原始岩浆的挥发分含量的估计，其中最好的档案是由熔融包裹体。现有的数据从火山生产镁铁质碱性熔岩在一系列的构造环境表明高挥发性通量，但信息仍然稀少，特别是板内海洋岛屿。在这里，我们对来自2011-2012年加那利群岛耶罗岛海底喷发的淬冷碧玄岩熔岩气球样本中的橄榄石熔融包裹体和基质玻璃中的挥发性和微量元素浓度以及硫形态进行了测量。结果表明，该岩浆中溶解挥发分和不相容微量元素的浓度非常高，Nb含量高达1080 ppm，CO2含量高达3420 ppm，3.0 wt.% H2O和5080 ppm S。重建原始CO2含量，考虑CO2/Nb系统学和可能的CO2封存收缩气泡，达到重量百分比的水平，表明碳是加那利岛岩浆的主要成分在深度和富CO2流体的出溶开始在地幔中的压力超过1 GPa。硫浓度，硫的形态和水含量之间的相关性表明强烈的减少最初氧化的地幔岩浆，可能控制耦合H2O和S脱气。这种后期的氧化还原变化可能触发了硫化物饱和，记录在单斜辉石和绿柱石中的球状硫化物包裹体。因此，耶罗碧玄岩具有特别高的挥发物携带能力，并向环境释放至少1.3-2.1 Tg CO2和1.8-2.9 Tg S，对当地海底生态系统造成重大压力。这些结果突出了碱性海洋岛屿火山，如加那利群岛，从地幔挥发通量的重要贡献。
Basaltic volcanoes transfer volatiles from the mantle to the surface of the Earth. The quantification of deep volatile fluxes relies heavily on estimates of the volatile content of primitive magmas, the best archive of which is provided by melt inclusions. Available data from volcanoes producing mafic alkaline lavas in a range of tectonic settings suggest high volatile fluxes, but information remains sparse, particularly for intraplate ocean islands. Here we present measurements of volatile and trace element concentrations, as well as sulphur speciation, in olivine-hosted melt inclusions and matrix glasses from quenched basanite lava balloon samples from the 2011–2012 submarine eruption at El Hierro, Canary Islands. The results reveal remarkably high concentrations of dissolved volatiles and incompatible trace elements in this magma, with ∼80 ppm Nb and up to 3420 ppm CO2, 3.0 wt.% H2O and 5080 ppm S. Reconstructed primitive CO2contents, considering CO2/Nb systematics and possible CO2sequestration in shrinkage bubbles, reach weight percent levels, indicating that carbon is a major constituent of Canary Island magmas at depth and that exsolution of a CO2-rich fluid begins in the mantle at pressures in excess of 1 GPa. Correlations between sulphur concentration, sulphur speciation and water content suggest strong reduction of an initially oxidised mantle magma, likely controlled by coupled H2O and S degassing. This late-stage redox change may have triggered sulphide saturation, recorded by globular sulphide inclusions in clinopyroxene and ulvöspinel. The El Hierro basanite thus had a particularly high volatile-carrying capacity and released a minimum of 1.3–2.1 Tg CO2and 1.8–2.9 Tg S to the environment, causing substantial stress on the local submarine ecosystem. These results highlight the important contribution of alkaline ocean island volcanoes, such as the Canary Islands, to volatile fluxes from the mantle.