Petrological insights into the storage conditions, and magmatic processes that yielded the centennial 2010 Merapi explosive eruption
Petrological insights into the storage conditions, and magmatic processes that yielded the centennial 2010 Merapi explosive eruption
复制标题
DOI：
10.1016/j.jvolgeores.2012.12.025
复制
发表时间：
2013-07-01
影响因子：
2.9
通讯作者：
Pallister, John S.
中科院分区：
文献类型：
--
作者：
Costa, Fidel;Andreastuti, Supriyati;Pallister, John S.
为了了解使2010年的默拉皮喷发比上个世纪大多数穹顶形成的喷发更大和更具爆炸性的过程，我们调查了2006年和2010年喷发的岩浆的地球化学、岩石学和喷发前条件。2010年的年轻岩为斜长岩、二辉石玄武岩，具参议院结构，少量无反应角闪石、铁钛氧化物，以及罕见的橄榄石和黑云母晶体。主体岩石成分、矿物共生和结构类似于2006年爆炸性小得多的年轻地块。其中一个关键的区别是，2010年的大多数角闪石没有击穿反应圈，而2006年的角闪石大部分是反应的。我们获得了大面积(1 cm(2))和单晶的主次元素X射线分布图、背散射电子图像、电子探针分析和跨晶体的成分分布。数据显示，2006年和2010年的样品在不同的空间尺度上都是不均匀的，辉石和角闪石之间存在大量的反应结构，溶解结构和晶体大小、形貌和成分变化很大。这些特征记录了开放系统的岩浆过程，包括碳酸盐岩的同化作用，以及默拉皮管道系统各部分之间的相互作用，包括脱气的浅层岩浆系统和深层更热、更易挥发的富含岩浆的岩浆侵入。样品的岩石学复杂性使解开默拉皮岩浆喷发前的条件成为岩石学难题。我们应用了五种不同的地温压力计，并用MILTS算法进行了热力学模拟，我们认为在Merapi以下至少有三个结晶带或环境。一些角闪石和高铝单斜辉石暗示了约30(+/-3)km深的油气藏。这里是来自默拉皮的高铝玄武岩安山岩产生的地方，可能是更原始岩浆的富水分馏作用产生的。这种深部岩浆挥发分丰富，当它们开始向地表移动时，处于近液相线条件(>；=4-6wt.%H2O，>；=0.15 wt.%SO2，以及未知数量的二氧化碳，约1050摄氏度)。另一种角闪石在约13(+/-2)千米处记录了第二个结晶带。在这里，高铝单斜辉石也可能与富钙斜长石共生。石灰石的同化也可能发生在这一水平上，正如在一些晶体的核心中发现的非常富钙的斜长石所记录的那样。在这个位置，熔体的水含量必须保持足够高，以稳定闪石(4-6wt.%H2O)，但二氧化碳和二氧化硫可能已经在脱气，并有助于监测系统在地表观察到的气体变化。最后，系统中较浅的部分(
To understand the processes that made the 2010 eruption of Merapi much larger and more explosive than most dome-forming eruptions of the past century, we investigated the geochemistry, petrology, and pre-eruptive conditions of magmas erupted in 2006 and 2010. The juvenile rocks of 2010 are plagioclase, two-pyroxene basaltic andesites with senate textures and minor amounts of reaction-free amphibole, Fe-Ti oxides, and rare crystals of olivine and biotite. The bulk-rock composition, mineral paragenesis, and textures are similar to those of juvenile blocks from the much less explosive eruption of 2006. One of the key differences is that most amphiboles in 2010 don't have breakdown reaction rims, whereas those of 2006 are largely reacted. We acquired >80 X-ray distribution maps of major and minor elements of large areas (>1 cm(2)) and single crystals, backscattered electron images, electron microprobe analyse, and compositional traverses across crystals. The data reveal that both the 2006 and 2010 samples are heterogeneous at various spatial scales, with numerous reaction textures between pyroxenes and amphiboles, dissolution textures, and large variations of crystal sizes, morphologies, and compositions. These features record open-system magmatic processes involving the assimilation of carbonate rocks, and interactions between various parts of Merapi's plumbing system, including a degassed shallow magma system and deep hotter and more volatile rich magma intrusions.The petrological complexity of the samples makes unraveling the pre-eruptive conditions of Merapi magmas a petrological puzzle. We applied five different geothermobarometers and performed thermodynamic modeling with the MELTS algorithm, and we propose that there are at least three crystallization zones or environments below Merapi. A deep reservoir at about 30 (+/-3) km depth is suggested by some amphiboles and high-Al clinopyroxenes. Here is where the high-Al basaltic andesites from Merapi are generated probably by water-rich fractionation of more primitive magmas. Such deep magmas are volatile-rich and at near-liquidus conditions (>= 4-6 wt.% H2O, >= 0.15 wt.% SO2, and an undetermined amount of CO2, at about 1050 degrees C) when they start moving towards the surface. A second crystallization zone is recorded by another type of amphibole at about 13 (+/-2) km. Here high-Al clinopyroxene may also grow together with Ca-rich plagioclase. Assimilation of limestone may also occur at this level as recorded by the very Ca-rich plagioclases found in the cores of some crystals. At this location the water content of the melt must remain high enough to stabilize amphibole (4-6 wt.% H2O) but CO2 and SO2 are probably already degassing and contribute to gas changes observed by the monitoring system at the surface. Finally, a shallower part of the system (