Formation of U-depleted rhyolite from a basanite at El Hierro, Canary Islands
Formation of U-depleted rhyolite from a basanite at El Hierro, Canary Islands
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加那利群岛 El Hierro 的贫铀流纹岩的形成
DOI：
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发表时间：
2013
影响因子：
3.5
通讯作者：
J. Martí
中科院分区：
文献类型：
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作者：
O. Sigmarsson;D. Laporte;M. Carpentier;B. Devouard;J. Devidal;J. Martí
响岩和粗面岩是碱性岩浆岩套中的长英质岩浆，是加那利群岛的特征。然而，2011年10月在最西端的耶罗岛附近的海底喷发产生了少量的流纹岩岩浆。流纹岩呈高度泡状的白色浮石，包裹在深色玄武质浮石中并与深色玄武质浮石混合。玄武质浮石的晶体相对贫乏，含有少量自形橄榄石（主要是Fo 77 -79）、单斜辉石和富铁尖晶石，而在流纹岩中发现了非常罕见的相同成分的橄榄石以及同样罕见的铁硫化物和富铁钛氧化物。氧化物中的Fe-Mg交换平衡允许计算流纹岩的平衡温度为970-890 °C，与石英熔体平衡温度约为970-890 °C一致。九百三十度。流纹岩的一个显著矿物学特征是存在圆形到扭曲的乳白色石英颗粒，这些颗粒是并入并部分溶解于岩浆中的捕虏晶。玻璃中残留的挥发物浓度的分析表明，流纹岩熔体高度脱气，而玄武质玻璃仍然有重要的卤素浓度。微量元素图案的镁铁质玻璃和它们的升高不相容元素浓度是典型的西加那利岛玄武岩。与此相反，微量元素组成的流纹岩显示出令人惊讶的低浓度的所有元素，除了最不相容的（如Rb，Ba，K和Th）。所有其他测得的LILE、HFSE和REE的浓度明显低于玄武质对应物，这可以通过副相（1%磷灰石、1%辉石和0.1%锆石）的分馏来解释。令人惊讶的是，低U浓度可能与流纹岩中的氧逸度升高有关，导致U处于六价状态，富F气体的流动导致UF 6的挥发，已知在低温下散发。结果表明，富含气体的玄武质熔体重新活化了少量停滞的流纹岩熔体，该流纹岩熔体是由约10%的富含石英的沉积物掺入到粗面岩成分的晚期分异中形成的。古老洋壳与大陆盾和年轻火山岛交界处的沉积物可能充当岩浆圈闭，因为沉积物同化作用可能改变幔源岩浆的成分。因此，石英同化作用解释了流纹岩岩浆在一个火山岛的生产特点是碱性岩浆系列从原始玄武岩粗面岩。
Phonolite and trachyte are the felsic magmas of the alkaline magma suites, which characterize the Canary Islands. The October 2011 submarine eruption off El Hierro, the westernmost island, nevertheless, produced a small volume of rhyolitic magma. The rhyolite occurred as highly vesicular, white coloured pumices enveloped in and mingled with darker coloured basanitic pumice. The basanitic pumice is relatively crystal poor with a few euhedral olivines (mostly Fo77–79), clinopyroxenes and Fe-rich spinels, whereas very rare olivine of same composition is found together with equally rare Fe-sulphide and FeTi-rich oxides in the rhyolite. The Fe–Mg exchange equilibrium in the oxides permits to calculate an equilibrium temperature of 970–890 °C for the rhyolite, in agreement with quartz-melt equilibrium at ca. 930 °C. A striking mineralogical feature of the rhyolite is the presence of rounded to contorted grains of milky quartz, which are xenocrysts incorporated and partly dissolved into the magma. Analyses of residual volatile concentrations in the glasses show that the rhyolite melt was highly degassed, whereas the basanitic glass still has important halogen concentrations. Trace element patterns of the mafic glasses and their elevated incompatible element concentrations are typical of the western Canary Island basanites. In contrast, the trace element composition of the rhyolite shows surprisingly low concentrations for all elements except the most incompatible ones (e.g. Rb, Ba, K and Th). All other measured LILE, HFSE and REE have significantly lower concentration than the basanitic counterpart that can be explained by fractionation of accessory phases (1 % apatite, 1 % sphene and 0.1 % zircon). Surprisingly, low U concentration is presumably related to elevated oxygen fugacity in the rhyolite, causing U to be in a hexavalent state, and fluxing of F-rich gas leading to volatilization of UF6, known to emanate at low temperature. The results suggest that a gas-rich basanitic melt remobilized a small volume of stagnant rhyolitic melt formed by incorporation of approximately 10 % quartz-rich sediment into a late differentiate of trachytic composition. Sediments at the interface of an old oceanic crust adjacent to a continental shield and younger volcanic island are likely to act as magma traps were sediment assimilation may alter the mantle-derived magma composition. Quartz assimilation thus explains the production of rhyolite magma in a volcanic island characterized by an alkaline magma series from primitive basanites to trachytes.