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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Using Amphibole Phenocrysts To Track Vapor Transfer During Magma Crystallization And Transport: An Example From Mount St. Helens, Washington
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Using Amphibole Phenocrysts To Track Vapor Transfer During Magma Crystallization And Transport: An Example From Mount St. Helens, Washington

机译：使用角闪石透晶石来追踪岩浆结晶和运输过程中的蒸汽转移：华盛顿圣海伦斯山的一个例子

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摘要

In order to evaluate and further constrain models for volatile movement and vapor enrichment of magma stored at shallow levels, amphibole phenocrysts from 2004-2005 Mount St. Helens dacite were analyzed for major and selected trace elements (Li, Cu, Zn, Mn, and REE) and Li isotopes. Several recent studies have examined fluid-mobile trace element abundances in phencryst phases and melt inclusions as a means of tracking volatile movement within subvolcanic magmatic systems, and high Li contents in plagioclase phenocrysts from 1980 and 2004 Mount St. Helens dacites have been interpreted as evidence that shallow magma was fluxed by a Li-bearing vapor phase prior to eruption. In amphibole phenocrysts, Zn and Mn behave compatibly, correlating to FeO~* and Al_2O_3, and show no systematic change with time. In contrast, Li and Cu abundances in amphibole vary by up to 3 orders of magnitude (7.6-1140 μg/g and 1.7 to 94 μ/g, respectively), and do not generally correlate with either major or trace elements. However, they do correlate moderately well (R~2=0.54,95% confidence) with each other and show systematic temporal variations that are opposite to those observed for plagioclase, precluding a simple 1-step diffusion model for Li enrichment. We propose a Diffusion-Crystallization Multi-Stage (DCMS) model to explain the temporal variations and co-variations of Li and Cu. In early erupted dacite (October-December 2004) profiles of Li isotopes in conjunction with measured Li intensities and core-to-rim increases in Li concentration are characteristic of Li diffusion into the amphiboles, consistent with prior models of plagioclase enrichment. In amphiboles from 2005 dacite, average Li and Cu concentrations are high (～260～660 μg/g and～29-45μg/ g, respectively) and in contrast to amphiboles from earlier-erupted dacite, correlate weakly with Al_2O_3 wt.%. Amphibole Al_2O_3 concentrations are an indicator of pressure, with high-Al amphiboles crystallizing at higher pressures, and we suggest that Li and Cu are partitioned into a fluid phase during ascent and crystallization of the magma so that amphiboles crystallizing at lower pressure have correspondingly lower Li and Cu concentrations. However, low Li and Cu in amphiboles from the dacite at the start of the eruption also require crystallization from a low Li-Cu bearing melt or residence times long enough for amphiboles to re-equilibrate with a Li-Cu depleted melt. Estimated residence times suggest that amphiboles in early dacite could have been present since the end of the 1980-1986 eruptive episode at Mount St. Helens.
机译：为了评估和进一步限制浅层储藏岩浆的挥发性运动和蒸气富集的模型，分析了2004-2005年圣海伦斯山粉晶闪闪发光的主要和选定的微量元素（Li，Cu，Zn，Mn和REE）和Li同位素最近的一些研究已经研究了在隐晶相和熔体包裹体中的流动痕量元素的丰度，以追踪火山岩下岩浆系统内的挥发性运动，1980年和2004年斜长石隐晶岩中的高Li含量已被解释为证据在喷发之前，浅层岩浆被含锂的汽相通入。在闪石的表晶中，Zn和Mn的行为相容，与FeO〜*和Al_2O_3相关，并且没有随时间变化的系统性变化。相反，闪石中的Li和Cu丰度变化最多3个数量级（分别为7.6-1140μg/ g和1.7至94μ/ g），并且通常与主要元素或痕量元素都不相关。但是，它们之间确实具有良好的相关性（R〜2 = 0.54， 95％置信度），并且显示出与斜长石观察到的相反的系统时间变化，从而排除了用于Li富集的简单的1步扩散模型。我们提出了扩散结晶多阶段（DCMS）模型来解释Li和Cu的时间变化和协变。在早期喷发的辉绿岩中（2004年10月至12月），Li同位素的分布与测得的Li强度以及Li浓度从核心到边缘的增加是Li扩散到闪石中的特征，这与斜长石富集的先前模型是一致的。在2005年闪锌矿的闪石中，平均Li和Cu浓度较高（分别为〜260〜660μg/ g和〜29-45μg/ g），与早期喷出的闪石的闪石相比，与Al_2O_3 wt。％的相关性较弱。闪石的Al_2O_3浓度是压力的指标，高Al的闪石在较高的压力下结晶，我们建议在岩浆的上升和结晶过程中将Li和Cu分配到液相中，以便在较低压力下结晶的闪石相应地降低Li和铜浓度。但是，在喷发开始时，来自闪锌矿的闪石中的低锂和铜还需要从低锂铜熔体中结晶出来，或者停留时间足够长，以使闪石与贫锂铜熔体重新平衡。估计的滞留时间表明，自1980-1986年圣海伦斯火山爆发以来，早在达菲岩中就已存在闪石。

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