Long-term temperature cycling in a shallow magma reservoir: insights from sanidine megacrysts at Taápaca volcano, Central Andes
Long-term temperature cycling in a shallow magma reservoir: insights from sanidine megacrysts at Taápaca volcano, Central Andes
复制标题
浅层岩浆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体的见解
DOI：
10.1093/petrology/egab010
复制
发表时间：
2021
影响因子：
3.9
通讯作者：
G. Wörner
中科院分区：
文献类型：
--
作者：
S. S. Rout;Magdalena Blum;G. Wörner
中安第斯火山带塔帕卡火山混合英安岩岩浆（18 °，N。智利）含有不寻常的大小（1至12厘米）透长石晶体和丰富的镁铁质包体的组成变量在整个喷发历史（1.5马到最近）的火山。它们富含矿物包裹体和强烈的Ba分带，具有由吸收界面分隔的明显生长带。再吸收之后，突然增加的Ba与组成对比高达2.3重量%的BaO。我们认为，吸收和急剧跳跃的Ba浓度反映不同的加热和熔化事件，这表明不同的生长区形成在不同的温度。基于矿物包裹体的角闪石-斜长石温压计给出了单个生长带在浅压力（0.1 - 0.3 GPa）下的温度变化范围为720 - 820 ° C。使用这些温度的扩散建模，Ba-配置文件从X射线扫描配置文件和灰度梯度的基础上积累的BSE图像在这些接口允许估计晶体驻留和再激活时间之前喷发。这种温度控制允许应用“非等温”扩散算法，以获得范围从0.4到490千公里的各个扩散边界的扩散时间，并添加到9至499千公里的总停留时间为不同的晶体从不同阶段的喷发。温度，压力，扩散时间和R-熔体建模的母流纹英安岩的组合表明存储条件的Taápaca水库在近共晶组合物在浅深度（4 - 10公里）。温度从未低于岩浆固相线，但经常在720 °C和820 °C之间循环，即在可喷发和不可喷发状态之间循环，结晶度约为40 - 50体积%，持续数万至数十万年。我们将其定义为“长期过渡温度循环”或LTTC存储。丰富的镁铁质包体为代表的玄武安山岩岩浆的频繁补给事件，精心策划的温度循环，导致多个加热事件，造成频繁的再吸收和中断晶体生长，并保持水库热“活”。再充事件变得更加频繁，只有2003 - 11千万年前，最终爆发，进行了一个特定的一套透长石巨晶的表面。因此，在许多早期的充电事件，并没有导致喷发，最后一个事件涉及混合在一个关键的充电率动员，夹带，并从混合英安岩主机的居民流纹英安岩爆发一组特定的巨晶。这个过程发生在喷发前不超过几个世纪，在塔帕卡火山1.5 My的漫长历史中，在不同的地层阶段以类似的时间尺度重复。观察到的矿物分带模式和大小透长石晶体的居民岩浆水库塔帕卡火山下面的花岗岩侵入体，也显示出典型的锆石结晶的年龄范围，这是可比的驻留时间提取在这里从Ba分带的巨晶中观察到的是相同的。因此，塔帕卡透长岩可能代表了一个喷发的等价物，并提供了花岗岩中这种钾长石巨晶形成过程中温度循环的“确凿证据”。
Hybrid dacite magmas from Taápaca volcano in the Central Andean volcanic zone (18ᵒ, N. Chile) contain sanidine crystals of unusual size (1 to 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 thermo-barometry based on mineral inclusions gives variable temperatures of ∼720 – 820 ᵒC 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 BSE images across these interfaces allow 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 ky and add up to total residence times of 9 to 499 ky for different crystals from different stages of eruption. A combination of temperatures, pressure, diffusion times and R-melts modeling 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 ky 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 timescales at different stratigraphic stages throughout the 1.5 My long 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 which are comparable to 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.
2008 年 5 月智利 Chaitén 喷发的火山灰地层和喷发量
影响因子：
3.5
作者：
Alfano F
通讯作者：
Alfano F
镁铁质岩浆包体的生存和岩浆补给的时间
影响因子：
5.2
作者：
Ruprecht, Philipp;Simon, Adam C.;Fiege, Adrian
通讯作者：
Fiege, Adrian