Hybrid dacite magmas from Taápaca volcano in the Central Andean Volcanic Zone (18°S, northern Chile) contain sanidine crystals of unusual size (1–12 cm) and abundant mafic enclaves of variable composition throughout the entire eruptive history (1·5 Ma to recent) of the volcano. They are rich in mineral inclusions and strongly zoned in Ba with distinct growth bands separated by resorption interfaces. Resorption is followed by a sudden increase in Ba with compositional contrasts up to 2·3 wt% BaO. We argue that resorption and the sharp jumps in Ba concentration reflect distinct heating and melting events, suggesting that different growth zones formed at different temperatures. Amphibole–plagioclase thermobarometry based on mineral inclusions gives variable temperatures of ∼720–820 °C at shallow pressures (0·1–0·3 GPa) for individual growth zones. Using these temperatures for diffusion modelling, Ba profiles from X-ray scanning profiles and grey-scale gradients based on accumulated back-scattered electron images across these interfaces allow us to estimate crystal residence and reactivation times prior to eruption. This temperature control allowed the application of a ‘non-isothermal’ diffusion algorithm to obtain diffusion times for individual diffusive boundaries that range from 0·4 to 490 kyr and add up to total residence times of 9–499 kyr for different crystals from different stages of eruption. A combination of temperatures, pressure, diffusion times and R-MELTS modelling of the parent rhyodacite suggests storage conditions for the Taápaca reservoir at near eutectic composition at shallow depth (4–10 km). Temperatures never fell below the magma solidus but frequently cycled between 720 °C and 820 °C (i.e. between eruptible and non-eruptible state with crystallinity circling around ∼40–50 vol%) for tens to hundreds of thousands of years. We define this as ‘long-term transitional temperature cycling’ or LTTC storage. Frequent recharge events of basaltic andesite magma, as represented by abundant mafic enclaves, orchestrated the temperature cycling, resulted in multiple heating events that caused frequent resorptions and interrupted crystal growth, and kept the reservoir thermally ‘alive’. Recharge events became more frequent only ∼3–11 kyr before the eventual eruption that carried a particular set of sanidine megacrysts to the surface. Thus, after many earlier recharge events that did not result in eruption, a final event involved mixing at a critical recharge rate to mobilize, entrain, and erupt a particular set of megacrysts from the resident rhyodacite in a hybrid dacite host. This process, happening not more than a few centuries before an eruption, has been repeated at similar time-scales at different stratigraphic stages throughout the 1·5 Myr history of Taápaca volcano. The observed mineral zonation patterns and size of sanidine crystals from the resident magma reservoir below Taápaca volcano are identical to those observed in the megacrysts from granite intrusions that also show typical age ranges of zircon crystallization that are comparable with the residence times extracted here from Ba zonation. Taápaca sanidines thus may represent an erupted equivalent and provide ‘smoking gun’ evidence of temperature cycling during the formation of such K-feldspar megacrysts in granites.

中文翻译：

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浅层岩浆水库中的长期温度循环：来自安第斯山脉中部塔帕卡火山的萨尼丁巨晶的见解

来自安第斯中部火山带（智利北部 18°S）的 Taápaca 火山的混合英安岩岩浆在整个喷发历史（1·5 Ma 到最近）的火山。它们富含矿物包裹体，并在 Ba 中强烈分区，具有由吸收界面分隔的明显生长带。吸收之后 Ba 突然增加，组成对比高达 2·3 wt% BaO。我们认为，再吸收和 Ba 浓度的急剧上升反映了不同的加热和熔化事件，表明在不同温度下形成了不同的生长区。基于矿物包裹体的闪石-斜长石热气压测量法在浅压力（0·1-0·3 GPa）下为各个生长区提供了~720-820°C的可变温度。使用这些温度进行扩散建模，来自 X 射线扫描轮廓的 Ba 轮廓和基于这些界面上累积的背散射电子图像的灰度梯度使我们能够估计喷发前的晶体驻留和再激活时间。这种温度控制允许应用“非等温”扩散算法来获得从 0·4 到 490 kyr 范围内的各个扩散边界的扩散时间，并且对于来自不同阶段的不同晶体，总停留时间总计为 9-499 kyr的爆发。母体流纹岩的温度、压力、扩散时间和 R-MELTS 模型的组合表明 Taápaca 储层在浅层（4-10 公里）处接近共晶成分的储存条件。温度从未低于岩浆固相线，但经常在 720°C 和 820°C 之间循环（即在可喷发和非喷发状态之间，结晶度在 ∼40-50 vol% 左右循环）数万年至数十万年。我们将其定义为“长期过渡温度循环”或 LTTC 存储。以丰富的镁铁质飞地为代表的玄武质安山岩浆的频繁补给事件协调了温度循环，导致多次加热事件导致频繁的再吸收和中断晶体生长，并使储层保持热“活力”。在最终喷发将一组特定的sanidine巨晶带到地表之前，再充电事件仅在~3-11 kyr时变得更加频繁。因此，在许多没有导致喷发的早期补给事件之后，最后一个事件涉及以临界补给速率混合，以从混合英安岩宿主中的常驻流纹岩中调动、夹带和喷发一组特定的巨晶。这一过程发生在喷发前不超过几个世纪，在塔帕卡火山 1·5 Myr 历史的不同地层阶段以相似的时间尺度重复。从塔帕卡火山下方的常驻岩浆储层中观察到的矿物分带模式和山尼丁晶体的大小与在花岗岩侵入体的巨晶中观察到的相同，这也显示出典型的锆石结晶年龄范围，与从 Ba 分带中提取的停留时间相当.