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首页> 外文期刊>Journal of Volcanology and Geothermal Research >A 3100-year History Of Argon Isotopic And Compositional Variation At El Chichon Volcano
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A 3100-year History Of Argon Isotopic And Compositional Variation At El Chichon Volcano

机译：El Chichon火山具有3100年的氩同位素和组成变化的历史

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El Chichon volcano has produced at least 11 eruptive events in the past 8000 years, all of which display a relatively constant trachyandesitic composition. The constancy of the eruptive products attests to the tapping of a long-lived magma chamber and suggests a system held in steady state between the influx of basaltic magma and differentiation of resident magma. We have sampled eruptive products from eight of these events, subjecting their dominant phenocryst phases (plagioclase and hornblende) to argon isotopic analysis in order to investigate the dynamics of a steady state magmatic system. Plagioclase from the older eruptions, 1500 yr BP, 1600 yr BP, 2000 yr BP and 3100 yr BP, display variable enrichment of ~(40)Ar (excess argon), whereas hornblende from the same eruptions shows little or no enrichment. In contrast, both plagioclase and hornblende phenocrysts from the younger eruptions in 1982, 550 yr BP, 900 yr BP and 1250 yr BP have argon isotopic ratios near-atmospheric values. Isochron analysis of these mineral phases rules out xenocrystic contamination as the source of excess argon, while mafic enclaves and isotopic and compositional zoning evidence frequent recharge events, suggesting the source of this argon is most likely the same as the source of this basaltic magma; correlation with Cl points to melt/vapor inclusions as the primary host for Ar within the phenocrysts. Argon isotopic variations point to a disturbance of the system between 1500 yr BP and 1250 yr BP, while compositional evidence for a major mafic input is present in the 900 yr BP eruption, indicating a complex relationship between recharge and isotopic signature of eruptive products. The amount of excess argon within a plagioclase phenocryst are a function of variations in melt and vapor inclusion abundance, time elapsed between melt/vapor inclusion entrapment and eruption, variations in Ar abundance in melt (itself a function of vapor exsolution timing) and time variations in Ar isotopic composition of the me subdued behavior of hornblende is due to slower diffusion and minimal inclusions.
机译：在过去的8000年中，厄尔尼诺基钦火山（El Chichon）火山至少产生了11次喷发事件，所有这些事件都显示出相对恒定的曲安山岩成分。喷发产物的恒定性证明了长期存在的岩浆腔室的开裂，并暗示了在玄武岩浆流入与常驻岩浆分化之间保持稳定状态的系统。我们从其中的八个事件中取样了喷发产物，并对它们的主要隐晶相（斜长石相和角闪石相）进行了氩同位素分析，以研究稳态岩浆系统的动力学。来自1500年BP，1600年BP，2000年BP和3100年BP的较早喷发的斜长石显示〜（40）Ar（过量氩）的可变富集，而来自同一喷发的角闪石则几乎没有或没有富集。相比之下，来自1982年年轻喷发，550年BP，900年BP和1250年BP的斜长石和角闪石的表型都具有接近大气压的氩同位素比。对这些矿物相的等时分析排除了异晶污染作为过量氩气的来源，而镁铁质飞地，同位素和成分分区则表明频繁的补给事件，这表明这种氩气的来源很可能与该玄武岩浆的来源相同。与Cl的相关性表明，熔融/气相夹杂物是苯晶中Ar的主要主体。氩同位素变化表明该系统在1500年BP和1250年BP之间受到干扰，而900年BP喷发中存在主要镁铁质输入的组成证据，表明补给与爆发产物的同位素特征之间存在复杂的关系。斜长石伏晶石中过量的氩气量是熔体和蒸气夹杂物丰度变化，熔体/蒸气夹杂物截留和喷发之间经过的时间，熔体中Ar丰度变化的本身（其本身是蒸气释放时间的函数）和时间变化的函数熔体中的Ar同位素组成;角闪石的行为减弱是由于扩散较慢，夹杂物最少。

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