Abstract Felsic pyroclastic deposits of overall low crystallinity erupted from caldera volcanoes frequently display internal gradients in composition and crystal content. The chemical gradients in the melt phase are consistent with differentiation paths and mineral/melt element partitioning predicted from the observed phenocryst assemblage. The same units typically show crystal-scale evidence for generation of eruptible magma by thermal rejuvenation of initially static, high-crystallinity (>50%) mush. Deposit-scale zoning can be reconciled with system rejuvenation by a model in which a high-crystallinity cumulate mush beneath its cognate supernatant liquid is melted by more mafic recharge to produce denser, remobilized magma of accumulative composition. In this model, zoning is a symptom of thermal rejuvenation, but requires a fusible cumulate, as provided by alkali feldspar-rich assemblages. We review existing data and present new whole-rock, glass and mineral compositions for nine examples of zoned felsic units, across the spectrum of alumina- and silica-saturation. We show that whole rocks and glasses in the late-erupted or least-evolved parts of these examples are strongly enriched in Ba (and, less consistently, Sr), and exhibit positive Eu anomalies, providing unequivocal evidence for the production of eruptible magma through melting of alkali feldspar cumulate mush. Hence, felsic volcanic rocks of low to moderate crystallinity may nonetheless have chemical signatures characteristic of cumulates. However, many other zoned tuffs show more subdued Ba-Eu enrichment and lack positive Eu anomalies. Enrichment of Eu and Ba in the melt is modulated by the extent and degree of equilibration during melting, and may be a transient signal that is quickly suppressed by new feldspar growth occurring between melting and eruption. In cases of very highly evolved rhyolites, the prior depletion of Ba, Sr and Eu by extensive alkali feldspar fractionation precludes development of any strong enrichment, although the patterns of relative variation are similar to those in the examples discussed here.

中文翻译：

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成分分区长英质凝灰岩中的铕和钡富集：通过累积熔化确定化学和物理梯度起源的吸烟枪

摘要 从火山口火山喷发的整体低结晶度的长英质火山碎屑沉积物经常表现出成分和晶体含量的内部梯度。熔融相中的化学梯度与从观察到的斑晶组合预测的分化路径和矿物/熔融元素分配一致。相同的单元通常显示出通过初始静态、高结晶度 (>50%) 糊状物的热再生产生可喷发岩浆的晶体级证据。沉积规模分区可以通过一个模型与系统复兴相协调，在该模型中，在其同源上清液下方的高结晶度堆积糊状物被更多的镁铁质补给熔化，以产生更致密的、再流动的累积成分的岩浆。在这个模型中，分区是热再生的一个症状，但需要一个可熔的累积，由富含碱长石的组合提供。我们回顾了现有数据，并为九个分区的长英质单元示例提供了新的全岩、玻璃和矿物成分，涵盖了氧化铝和二氧化硅饱和度的范围。我们表明，在这些例子的晚喷发或演化最少的部分，整个岩石和玻璃都富含 Ba（以及不太一致的 Sr），并表现出正的 Eu 异常，为可喷发岩浆的产生提供了明确的证据。碱长石熔炼成糊状。因此，低至中等结晶度的长英质火山岩可能仍具有堆积物的化学特征。然而，许多其他分区凝灰岩显示出更柔和的 Ba-Eu 富集并且缺乏正的 Eu 异常。熔体中 Eu 和 Ba 的富集受熔化过程中平衡程度和程度的调节，并且可能是一种瞬态信号，在熔化和喷发之间发生的新长石生长迅速抑制。在高度演化的流纹岩的情况下，通过广泛的碱长石分馏预先消耗 Ba、Sr 和 Eu 排除了任何强烈富集的发展，尽管相对变化的模式与此处讨论的示例中的模式相似。