Abstract

Despite agreement that calc-alkaline volcanism occurs at subduction zones and is responsible for the genesis of continental landmasses, there is no consensus on the source of the Fe-depleted signature hallmark to calc-alkaline volcanism. In this study, we utilize mafic tephras collected from Buldir Volcano to address the genesis of strongly calc-alkaline volcanic rocks (those with a low Tholeiitic Index; ≤0·7) in a segment of the western Aleutian Arc to determine if the eruptions are plausibly part of a liquid line of descent, if they are mixtures of crustal melts and parental magmas, or if they are mixtures of melts of the mantle and the subducting slab. We conducted a series of H₂O-saturated phase equilibrium experiments (1175–1000°C; 100 MPa) in a rapid-quench cold-seal (MHC) apparatus on the most primitive natural lava from Buldir (9·34 wt % MgO) at oxidizing conditions near the Re–ReO₂ buffer. We confirmed that all experiments equilibrated 0·3 ± 0·23 log units above the Re–ReO₂ buffer (ΔQFM ∼ +2·8) using X-ray Absorption Near Edge Structure (XANES) spectroscopy. Chromite is the liquidus phase, followed by olivine, then plagioclase, then clinopyroxene, and finally hornblende. Once clinopyroxene saturates, spinel composition shifts to magnetite. We compared our experimental results to the major element geochemistry and petrology of six tephras (51·9–54·8 wt % SiO₂) from Buldir collected during the 2015 field season of the GeoPRISMS shared platform. Tephras contain olivine + plagioclase + clinopyroxene + spinel ± hornblende; plagioclase comprises most of the crystalline volume, followed by either olivine or hornblende. Spinel is ubiquitous; with Cr-rich spinel inclusions in olivine and hornblende, and magnetite in the groundmass.Variations in phenocryst assemblages and compositions between samples can be attributed to differences in pre-eruptive temperatures, where hotter samples are devoid of hornblende, and contain Fo-rich olivine and plagioclase with lower An-contents, owing to the position of the mineral-in curves at fluid-saturated conditions. Experimental glasses match the depletion in FeO^T observed in the tephra whole rock compositions. The continuous depletion in FeO^T is attributable to saturation of spinel as a liquidus phase (initially as chromite) and continuous crystallization through the experimental series (changing to magnetite at colder temperatures). In contrast to the natural samples, the experiments show enrichment in TiO₂ with decreasing MgO, suggesting that differentiation did not occur at 100 MPa on Buldir. The TiO₂ depletion in volcanic rocks from Buldir can be accounted for if hornblende crystallization occurs close to the liquidus of a parental magma; a condition that is met at higher pressures and hydrous conditions.The emerging picture for Buldir Island is that (1) oxidizing conditions are required to drive the observed depletions in FeO^T via crystallization of spinel, and (2) elevated H₂O contents and high pressures are required to saturate hornblende close to the liquidus to reproduce the entire suite of major elements. Our study provides a mechanism to generate the calc-alkaline trends observed at Buldir without requiring mixing of slab and mantle melts. We conclude that calc-alkaline volcanic rocks with extremely low Tholeiitic Indices (0·7), like those from Buldir, cannot be generated in absence of high oxygen fugacity, even at high pressure and/or elevated water pressures.

中文翻译：

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高氧逸度结晶过程中钙碱性岩浆的生成：来自阿拉斯加西部阿留申弧线的博尔德火山的特非拉斯的实验和岩石学研究

摘要

尽管人们一致认为钙碱性火山活动发生在俯冲带，并且是大陆陆块的成因的原因，但对于钙碱性火山活动的贫铁签名来源，目前尚无共识。在这项研究中，我们利用从Buldir火山收集的铁镁质特弗拉斯来解决阿留申弧段西部强烈钙钙碱性火山岩（那些具有低Tholeiitic指数；≤0·7的火山岩）的成因，以确定火山喷发是否为如果它们是地壳熔体和母岩浆的混合物，或者它们是地幔和俯冲板块的熔体混合物，则可能是下降液体线的一部分。我们进行了一系列的H ₂在B矿床最原始的天然熔岩（9·34 wt％MgO）上，在Re附近的氧化条件下，在快速骤冷冷封（MHC）装置中进行O饱和相平衡实验（1175-1000°C; 100 MPa） –ReO ₂缓冲区。我们证实，所有实验均使用X射线吸收近边缘结构（XANES）光谱平衡了Re-ReO ₂缓冲液上方的0·3±0·23对数单位（ΔQFM〜+ 2·8）。铬铁矿是液相线相，其次是橄榄石，然后是斜长石，然后是斜辉石，最后是角闪石。Clinopyroxene饱和后，尖晶石成分就会转变为磁铁矿。我们将实验结果与六种特弗拉（51·9–54·8 wt％SiO ₂）收集自Bouldir，该数据是在GeoPRISMS共享平台的2015野外采集的。提弗拉斯含有橄榄石+斜长石+斜柏基+尖晶石±角闪石; 斜长石占据了大部分的晶体体积，其次是橄榄石或角闪石。尖晶石无处不在；橄榄石和角闪石中富含Cr的尖晶石夹杂物，地层中含有磁铁矿。样品之间的表晶组合和成分的变化可归因于喷发前温度的差异，其中较热的样品不含角闪石，并且含有富Fo的橄榄石由于矿物饱和曲线在流体饱和条件下的位置，因此具有较低An含量的斜长石和斜长石。实验眼镜可满足FeO ^T的消耗在提法拉观察到整个岩石成分。FeO ^T的持续消耗归因于尖晶石的液相饱和状态（最初是亚铬铁矿）和整个实验系列的连续结晶（在较冷的温度下变为磁铁矿）。与天然样品相反，实验表明，随着MgO的减少，TiO ₂富集，这表明在100 MPa的Buldir上没有发生分化。TiO ₂如果角闪石的结晶发生在母岩浆的液相附近，则可以解释布尔迪尔火山岩的枯竭。Buldir岛的新情况是：（1）氧化条件需要通过尖晶石的结晶来驱动FeO ^T中观察到的耗尽，以及（2）H ₂升高需要O含量和高压以使接近液相线的角闪石饱和，以重现整套主要元素。我们的研究提供了一种机制，可以生成在Buldir观察到的钙碱性趋势，而无需混合板状和地幔熔体。我们得出的结论是，在没有高氧气逸度的情况下，即使在高压和/或升高的水压下，也无法生成具有极低的生硬指数（0·7）的钙碱性火山岩。