Multi-Stage Magma Evolution in Intra-Plate Volcanoes: Insights From Combined in situ Li and Mg–Fe Chemical and Isotopic Diffusion Profiles in Olivine
Multi-Stage Magma Evolution in Intra-Plate Volcanoes: Insights From Combined in situ Li and Mg–Fe Chemical and Isotopic Diffusion Profiles in Olivine
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DOI：
10.3389/feart.2020.00201
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发表时间：
2020-06
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影响因子：
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通讯作者：
L. Steinmann;Martin Oeser;I. Horn;S. Weyer
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文献类型：
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作者：
L. Steinmann;Martin Oeser;I. Horn;S. Weyer
了解岩浆演化和上升的时间尺度对于解释活火山的地球物理监测信号至关重要。在这项研究中，我们探索了由岩浆橄榄石晶体记录的扩散驱动的Li浓度和同位素分带剖面揭示岩浆演化过程的时间尺度的可能性。锂是一种快速扩散的元素，可能为研究喷发前不久岩浆上升过程中岩浆成分的变化提供了机会。测定了来自法国中部火山地块两个地区的橄榄石的锂化学和同位素剖面，这两个地区以前曾被研究过其铁-镁同位素体系。同位素和化学分布的联合研究使区分晶体生长和扩散事件成为可能。在晶体核心区的δ7Li值极低至−30.7‰(相对于常用的锂同位素标准IRMM-16)，而在晶体边缘(δ7Li∼8-10‰)的值随着浓度的增加而升高(∼3-1μg/g)到边缘(12-6μg/g)。化学和同位素剖面的形状和取向表明，它们主要是在岩浆分异过程中Li在橄榄石颗粒内和内部扩散产生的。虽然镁-铁同位素和常量元素分布由单一扩散事件模拟(Oeser等人，2015年)，但Li的浓度和同位素分布表明发生了第二次扩散事件，而镁-铁交换扩散对没有记录到这一事件。第一次扩散事件被解释为反映了橄榄石晶体在岩浆室中的滞留。由于铁-镁交换扩散的扩散系数被很好地确定，这一事件的时间尺度很可能是用镁-铁同位素交换扩散模型(Oeser等人，2015年)最好地量化的。这一事件可能也产生了在橄榄石岩芯中观察到的低δ7Li。因此，比较镁-铁和锂的分布长度可以用来确定锂在所研究的橄榄石晶体中不太为人所知的扩散系数。这项研究的结果表明，通常在天然橄榄石中观察到的低锂浓度下的锂扩散可能没有之前认为的那么快。第二次扩散事件可能代表与岩浆上升和/或熔岩侵位后岩浆冷却有关的短暂事件，如脱气。这样的过程只会影响锂，与难熔元素铁和镁不同，锂在脱气过程中是挥发的。研究结果表明，根据不同的扩散速率和物理化学性质，结合空间分辨Li和Mg-Fe的化学和同位素扩散剖面，是模拟岩浆系统甚至多阶段演化过程的有力工具。
Understanding the timescales of magma evolution and ascent is essential for interpreting geophysical monitoring signals from active volcanoes. In this study, we explore the potential of diffusion-driven Li concentration and isotope zoning profiles recorded by magmatic olivine crystals to unravel time scales of magma evolution processes. Lithium is a fast-diffusing element and may provide the opportunity to investigate changes in magma composition during magma ascent, shortly before eruption. Lithium chemical and isotopic profiles were determined in olivines from two localities in the Massif Central volcanic region (France) that have previously been investigated for their Fe–Mg isotope systematics. The combined investigation of isotopic and chemical profiles makes it possible to distinguish between crystal growth and diffusion events. Extremely low δ7Li-values down to −30.7‰ (relative to the commonly used Li isotope standard IRMM-16) in the crystal core regions and elevated values at crystal rims (δ7Li ∼8 to 10‰), along with increasing concentrations from cores (∼3 to 1 μg/g) toward rims (12 to 6 μg/g) were found. The shape and orientation of both the chemical and isotopic profiles indicate that they were dominantly generated by Li diffusion into and within the olivine grains during magmatic differentiation. While Mg–Fe isotope and major element profiles have been modeled by a single diffusion event (Oeser et al., 2015), concentration and isotope profiles of Li indicate that a second diffusion event took place, that was not recorded by the Mg–Fe exchange diffusion couple. The first diffusion event was interpreted as reflecting the residence of the olivine crystals in a magma chamber. As diffusion coefficients for Fe–Mg exchange diffusion are very well determined, the time scales of this event are likely best quantified by Mg–Fe isotopic exchange diffusion modeling (Oeser et al., 2015). This event probably also generated the low δ7Li observed in olivine cores. Comparing the length of the Mg–Fe and Li profiles could thus be used to determine the less well-known diffusion coefficients of Li in the studied olivine crystals. The findings of this study indicate that Li diffusion at low Li concentration levels, as typically observed in natural olivine, may be not as fast as previously thought. The second diffusion event might represent a short-lived event, such as degassing, related to the ascent of the magma and/or magma cooling after emplacement of the lava. Such a process would only affect Li, which, in contrast to the refractory elements Fe and Mg, is volatile during degassing. The findings of this study show that, according to their different diffusion rates and physiochemical properties, the combined use of spatially resolved Li and Mg–Fe chemical and isotopic diffusion profiles, is a powerful tool to model even multi-stage evolution processes in magmatic systems.