Research paper Experimental solidification of an andesitic melt by cooling Gianluca Iezzi a,b, ⁎, Silvio Mollo b , Guglielmo Torresi a,c , Guido Ventura b , Andrea Cavallo b , Piergiorgio Scarlato b a Dipartimento DIGAT, Università G. d'Annunzio, Via Dei Vestini 30, I-66013 Chieti, Italy b Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy c Institut für Mineralogie, Universität Hannover, Hannover, Germany abstract article info Article history: Received 22 October 2010 Received in revised form 24 January 2011 Accepted 28 January 2011 Edited: D.B. Dingwell Keywords: Andesitic melt Experimental solidification Nucleation Crystal coarsening Disequilibrium phase Glass-forming ability (GFA) Solidification experiments at (a) five different cooling rates (25, 12.5, 3, 0.5 and 0.125 °C/min) between 1300 and 800 °C, and (b) variable quenching temperatures (1100, 1000, 900 and 800 °C) at a fixed cooling rate of 0.5 °C/min were performed on an andesitic melt (SiO 2 = 58.52 wt.% and Na 2 O+K 2 O = 4.43 wt.%) at air conditions from high superheating temperature. The results show that simultaneous and duplicated experiments with Pt-wire or Pt-capsule produce identical run-products. Preferential nucleation on Pt- containers or bubbles is lacking. Plagioclase and Fe–Ti oxide crystals nucleate firstly from the melt. Clinopyroxene crystals form only at lower cooling rates (0.5 and 0.125 °C/min) and quenching temperatures (900 and 800 °C). At higher cooling rates (25, 12.5 and 3 °C/min) and quenching temperature (1100 °C), plagioclase and Fe–Ti oxide crystals are embedded in a glassy matrix; by contrast, at lower cooling rates (0.5 and 0.125 °C/min) and below 1100 °C they form an intergrowth texture. The crystallization of plagioclase and Fe–Ti oxide starts homogeneously and then proceeds by heterogeneous nucleation. The crystal size distribution (CSD) analysis of plagioclase shows that crystal coarsening increases with decreasing cooling rate and quenching temperature. At the same time, the average growth rate of plagioclases decreases from 2.1 × 10 -6 cm/s (25 °C/min) to 5.7 × 10 -8 cm/s (0.125 °C/min) and crystals tend to be more equant in habit. Plagioclases and Fe–Ti oxides depart from their equilibrium compositions with increasing cooling rate; plagioclases shift from labradorite–andesine to anorthite–bytownite. Therefore, kinetic effects due to cooling significantly change the plagioclase composition with remarkable petrological implications for the solidification of andesitic lavas and dikes. The glass-forming ability (GFA) of the andesitic melt has been also quantified in a critical cooling rate (R c ) of ~ 37 °C/min. This value is higher than those measured for latitic (R c ~ 1 °C/min) and trachytic (R c b 0.125 °C/min) liquids demonstrating that little changes of melt composition are able to significantly shift the initial nucleation behavior of magmas and the following solidification paths. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The transition from a silicate melt to a fully solidified magmatic rock is an important phase transformation occurring on the Earth. The melt to rock transition involves vitrification and/or crystallization, two processes related to the melt composition and to temperature/ pressure variation (Dowty, 1980; Lofgren, 1980; Kirkpatrick, 1981; Cashman, 1991; Lasaga, 1997; Hammer, 2008). Dynamic crystallization experiments carried out to investigate the nucleation behavior of silicate melts mostly concentrated on peridotitic and basaltic liquids (Conte et al., 2006; Hammer, 2006; Pupier et al., 2007; Schiavi et al., 2009 and references therein). Conversely, few data are available for intermediate and evolved compositions (Swanson, 1977; Naney and Swanson, 1980; Couch, 2003; Hammer, 2004; Iezzi et al., 2008); crystallization data for andesitic melts are instead completely lacking (Iezzi et al., 2009). Importantly, recent data on latitic and trachytic liquids demonstrated that small compositional differences have important effects on the nucleation behavior of silicate melts (Iezzi et al., 2008). The aim of this study is to investigate the crystallization behavior of an andesitic melt under dynamic cooling conditions. Experiments were performed under variable cooling rates and final quenching temperatures at the oxygen fugacity of air, which is the appropriate fugacity for magmas at shallowest crustal levels (e.g. dikes; Burgisser and Scaillet, 2007) and emplacing lava flows or domes (Burkhard, 2005a, 2005b). The results allow us to (i) investigate the nucleation and growth of plagioclase and Fe–Ti oxide in the andesitic melts, (ii) obtain information on the ability of such melts to crystallize, (iii) shed light on the textural and compositional (disequilibrium) features observed in the outer portions of aphyric and degassed andesitic lavas and dikes, and (iv) constrain physical models for the emplacement of andesitic magmas. Chemical Geology 283 (2011) 261–273 ⁎ Corresponding author at: Dipartimento DIGAT, Università G. d'Annunzio, Via Dei Vestini 30, I-66013 Chieti, Italy. Tel.: +39 0871 3556147; fax: +39 0871 3556047. E-mail address: g.iezzi@unich.it (G. Iezzi). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.01.024 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo