Petrological insights into the storage conditions, and magmatic processes that yielded the centennial 2010 Merapi explosive eruption

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 (<10 km) is recorded by the lower anorthite plagioclase and low-Al in clinopyroxene, and perhaps also in orthopyroxene. This part of the system is probably crystal-rich and largely degassed, and is the likely source of the high-temperature fumaroles and the volcanic gas plumes that are commonly seen at Merapi. We propose that the 2006 and 2010 eruptions were driven by basically the same processes and magma types. The main difference is the much larger size of the deep and volatile-rich magma replenishment that took place in 2010, which had large effects on the kinetics and dynamics of the plumbing system and processes. In 2006, and perhaps also in most of the typical small dome-forming historical eruptions at Merapi, the direct ascent of deep and gas-rich magmas towards the surface is slowed down and partially arrested by the shallower crystal-rich zones of left-over magma from previous events. However, this was not possible in 2010, where the much larger (up to 10 times) size of the magma intrusions overwhelmed the crystal-rich eruption filter. In 2010 the deep magma probably resided for only a short time at intermediate to shallower depths which allowed it to proceed to the surface still carrying most of its deep gas cargo. The larger magma intrusion probably induced higher rates of crustal carbonate assimilation and production of additional CO2 gas at shallow depths. This contributed to the much faster than usual ascent rates and larger explosivities in 2010 than in 2006. These inferences are supported by the shorter interaction times calculated from the diffusion models of clinopyroxene compositions for the 2010 magmas, by the fact that most amphiboles are not broken down in 2010 as opposed to 2006, and also by the much shorter times of escalating monitoring signals (seismicity and deformation) in 2010 compared to 2006. A puzzling observation is that despite the multiple explosive phases of the 2010 eruption, pumiceous materials are rare, and were only found in the last part of the eruption. This contrasts with the abundant tephra layers and vesiculated deposits of older historical explosive events like 1872, and suggest that syn-eruptive processes in 2010 were also different from standard models. The rarity of expanded pumices in 2010 may be due to rapid degassing and re-welding of magma as it ascended from intermediate depths. Given the near constant bulk composition of Merapi magmas erupted in the last decades, and the similarity of textures and minerals in 2006 and 2010, our study suggests that most Merapi magmas are intrinsically capable of explosive eruptions. Here we propose that whether they do so or not mainly depends on the degree of interaction and magma mass proportions between the upper crystal-rich parts of the system (including carbonates) and the deeper and more gas-rich replenishing magmas. Older historical explosive eruptions at Merapi such as in 1872 were driven by more mafic magmas than those erupted in 2006 and 2010 and thus might be caused by different processes from those discussed here. The still unanswered and vexing questions remain as to why in 2010 a much larger amount of magma was segregated from depths and whether this will happen again in the near future. (C) 2013 Elsevier B.V. All rights reserved.