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首页> 外文期刊>Contributions to Mineralogy and Petrology >Textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens, USA
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Textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens, USA

机译：1980年美国圣海伦斯火山喷发产生的斜长石斑晶的结构和化学变化

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摘要

This study presents major- and trace-element chemistry of plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens volcano. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross-correlated between crystals. The variation of Sr and Ba concentration in the melt, as calculated from the concentration in the phenocrysts using partition coefficients, suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector. In both of these melts Sr and Ba are correlated despite the abundance of plagioclase in the 1980 dacites. We propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower Sr and higher Ba, as documented in the rims of the phenocrysts. Although the cores of the phenocrysts crystallised at relatively shallow depths, they preserve the Sr and Ba of the deep-seated melts as they ascended from a deeper region. Further magma ascent resulted in microlite nucleation, which is responsible for a similar shift to even lower Sr concentration as observed in the groundmass of post-18 May 1980 samples.
机译：这项研究介绍了1980年圣海伦斯火山喷发产生的斜长石隐晶石的主要和微量元素化学性质。尽管斜长石斑晶的质地和组成发生了很大的变化，但晶体之间却存在互相关的不同片段。熔体中Sr和Ba浓度的变化（通过使用分配系数从共晶中的浓度计算得出）表明，由斜长石结晶载体抵消了成分不同的熔体结晶的核和边缘。尽管1980年的数据中斜长石含量丰富，但在这两种熔体中Sr和Ba还是相关的。我们提出岩浆上升时斜长石的快速结晶会导致熔体成分向较低的Sr和较高的Ba转移，如在晶状体的边缘中所记录。尽管表晶的核心在相对浅的深度结晶，但是当它们从更深的区域上升时，它们保留了深层熔体的Sr和Ba。进一步的岩浆上升导致微晶核的形成，这与1980年5月18日后的样品的地面质量中观察到的相似变化甚至更低。

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