Textural and mineral chemistry constraints on evolution of Merapi Volcano, Indonesia Sabrina Innocenti a, ⁎, Mary-Ann del Marmol b , Barry Voight a , Supriyati Andreastuti c , Tanya Furman a a Department of Geosciences, Pennsylvania State University, University Park, PA 16802, United States b Geology and Soil Science Department, Ghent University, Krijgslaan 281/S8, 9000 Gent, Belgium c Center for Volcanology and Geological Hazard Mitigation, Jalan Diponegoro 57, Bandung 40122, Indonesia abstract article info Article history: Received 18 May 2012 Accepted 15 January 2013 Available online xxxx Keywords: Merapi Volcano Crystal size distribution Textural analysis Mineral chemistry Dome lavas Magma storage Magma transport We analyze and compare the textures of Merapi lavas (basalts and basaltic andesites) ranging in age from Proto-Merapi through modern activity, with the goal of gaining insights on the temporal evolution of Merapi's magmatic system. Analysis of textural parameters, such as phenocryst and microphenocryst crystallinity, coupled with crystal size distribution theory, provides information about the storage and transport of magmas. We combine textural analyses with geochemical investigations for a comprehensive comparison of erupted lavas over time. The chemical analyses identify crystal growth processes in magma chambers and underline dif- ferences between sample groups. Our work suggests the occurrence of two distinct histories, presumably asso- ciated with (at least) two generally distinct types of rheological behaviors and storage/transport systems. These behaviors are associated with different plagioclase growth patterns, with both groups influenced by late-stage shallow decompression degassing-induced microlite crystallization. Both groups contain amphibole crystals that indicate an early period of mid-crustal to deep-crustal storage of water-rich magmas. Dome lavas from the 20th century eruptive activity indicate quasi-steady-state nucleation-and-growth evolution interspersed with episodes of reheating and textural coarsening, suggesting residence in magma storage at multiple depths, both >10 km, and b 10 km, while samples from the older stratigraphic history of Merapi record both repeated attainment and loss of quasi-steady-state conditions. These observations, coupled with our companion study of Merapi tephra samples, suggest that the relatively benign type of activity observed in the 20th century will be interrupted from time to time in the future by more explosive eruptions, such as that of 2010. © 2013 Published by Elsevier B.V. 1. Introduction Igneous rocks preserve the history of magmatic crystallization, and for this reason there is a high degree of interest in understanding the kinetic processes and intensive parameters that can be determined by the quantitative study of their textures. Quantitative textural analy- ses have proven useful in identifying magma chamber processes such as accumulation, magma mixing and fractionation, and in estimating residence times in magma reservoirs (Marsh, 1988, 1998; Higgins, 2011). Cooling-induced crystallization has been studied extensively for Hawaiian lava flows (Cashman and Marsh, 1988; Cashman, 1993; Cashman et al., 1999), and decompression or degassing-induced crystalli- zation has been studied in both natural systems (Hammer et al., 1999, 2000; Couch et al., 2003a) and laboratory experiments (Hammer and Rutherford, 2002; Couch, 2003; Couch et al., 2003a,b; Hammer and Rutherford, 2003). Both crystallization and degassing change the melt chemistry (Cashman and Blundy, 2000; Hammer et al., 2000; Harford et al., 2003) and magma rheology (Dingwell, 1998), and therefore can affect eruption dynamics and style (Melnik and Sparks, 2002). However, the interactions are intricate and the processes remain incompletely under- stood, despite significant recent modeling advancements (Melnik and Sparks, 1999; Barmin et al., 2002; Melnik and Sparks, 2002; Diller et al., 2006; de' Michieli Vitturi et al., 2010). Merapi volcano is an example of a relatively well documented sys- tem with a complex variety of observed behaviors (Voight et al., 2000a,b), and therefore provides an ideal opportunity to investigate the relationship between petrographic textures and mineral chemis- try in lavas produced by eruptions of different styles. The eruptions at Merapi have been well documented, and recent events have been monitored closely (Andreastuti et al., 2000; Newhall et al., 2000; Voight et al., 2000a; Gertisser and Keller, 2003a, b; Chadwick et al., 2007; Charbonnier and Gertisser, 2008; Gertisser et al., 2012; Surono et al., 2012; Chadwick et al., 2012; Costa et al., this issue). Here we quantify textures and bulk, mineral, and glass chemistries from a representative sampling of Merapi lavas of different vintage, and correlate these data to the eruptive processes. We also explore the mutual influence of geochemistry and rheological parameters in affecting the observed crystallization patterns and textures. To achieve these goals, we focus on occurrence, textures, and mineral chemistry Journal of Volcanology and Geothermal Research xxx (2013) xxx–xxx ⁎ Corresponding author. E-mail address: sabrinainnocenti@gmail.com (S. Innocenti). VOLGEO-05063; No of Pages 18 0377-0273/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jvolgeores.2013.01.006 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Please cite this article as: Innocenti, S., et al., Textural and mineral chemistry constraints on evolution of Merapi Volcano, Indonesia, Journal of Volcanology and Geothermal Research (2013), http://dx.doi.org/10.1016/j.jvolgeores.2013.01.006