Abstract

The paper reports results of an experimental study of amphibole crystallization from the highly magnesian andesite melt of Shiveluch volcano, Kamchatka. The experiments were carried out in IHPV at 300 MPa and 940–980°С in iron-saturated platinum capsules, using rapid quenching and temperature oscillations (in some experiments). The redox state of iron in the system was measured before and after the experiments using Mössbauer spectroscopy. The maximum size of the experimental amphibole crystals (up to 200 μm) was close to those of natural amphibole phenocrysts in the volcanic rocks of Shiveluch volcano. The experimental data show that the content of octahedrally coordinated Al (Al⁶) in the amphibole considerably varies with small variations in the intensive parameters (P, T, and \(f{{{\text{O}}}_{2}}\)) and composition of the melt, and the maximum Al⁶ concentration can be evaluated only by using a reasonably large dataset of amphibole analyses. A modified 13eCNK method is suggested to calculate the values of Al⁶ and Fe³⁺/Fe²⁺ with regard for the Ti concentration and the probable partial transfer of Mg into site B in high-Mg amphibole. Calculations with this modified technique yield lower Fe³⁺/Fe²⁺ and higher Al⁶ values. Our experimental data show that the temperature of amphibole liquidus crystallization decreases from about 990°C to 960°C when the oxygen fugacity drops from NNO + 1.5 to NNO + 0.4. In view of this, the transition from amphibole-bearing to anhydrous mineral assemblage in the magmas of Shiveluch volcano might have been caused by variations of the oxygen fugacity but not water. The application of our geobarometer to amphiboles from Shiveluch volcano (extrusions Krasnaya and Karan) yields the highest pressure estimate of above 1 GPa, corresponding to the P–T conditions of the melting of garnet-bearing amphibolite in the lower crust.

中文翻译：

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堪察加半岛Shiveluch火山高镁质熔体闪石的结晶实验研究

摘要

这篇论文报道了从堪察加半岛Shiveluch火山的高度镁质安山岩熔体中闪石结晶的实验研究结果。实验是在IHPV中于300 MPa和940–980°С下在铁饱和的铂金胶囊中进行的，使用了快速淬灭和温度振荡（在某些实验中）。在实验之前和之后使用Mössbauer光谱法测量系统中铁的氧化还原状态。在Shiveluch火山的火山岩中，实验性闪石的最大尺寸（最大200μm）接近于天然闪石的隐晶。实验数据表明，闪石中八面体配位的Al（Al ⁶）含量随强度参数（P，T和\（f {{{\ text {O}}} _ {2}} \））和熔体的组成以及最大的Al ⁶浓度只能通过使用相当大的闪石分析数据集进行评估。建议使用一种改进的13eCNK方法来计算Al ⁶和Fe ³⁺ / Fe ²⁺的值，这些值涉及Ti的浓度以及Mg可能部分转移到高Mg闪石中的B位。使用这种改进技术的计算得出较低的Fe ³⁺ / Fe ²⁺和较高的Al ⁶价值观。我们的实验数据表明，当氧逸度从NNO + 1.5降至NNO + 0.4时，闪石液相线结晶温度从990°C降至960°C。有鉴于此，Shiveluch火山岩浆从含闪石的过渡到无水矿物组合的转变可能是由氧逸度的变化引起的，而不是由水引起的。将我们的气压计应用于Shiveluch火山的闪石（挤压Krasnaya和Karan）可产生最高1 GPa以上的最高压力估算值，这对应于下地壳中带有石榴石的闪石熔融的P – T条件。