ORIGINAL PAPER H 2 O–CO 2 solubility in mafic alkaline magma: applications to volatile sources and degassing behavior at Erebus volcano, Antarctica Kayla Iacovino • Gordon Moore • Kurt Roggensack • Clive Oppenheimer • Philip Kyle Received: 15 October 2012 / Accepted: 4 April 2013 / Published online: 19 April 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract We present new equilibrium mixed-volatile (H 2 O–CO 2 ) solubility data for a phonotephrite from Erebus volcano, Antarctica. H 2 O–CO 2 -saturated experiments were conducted at 400–700 MPa, 1,190 °C, and *NNO ? 1 in non-end-loaded piston cylinders. Equilibrium H 2 O–CO 2 fluid compositions were determined using low-temperature vacuum manometry, and the volatile and major element compositions of the glassy run products were determined by Fourier transform infrared spectroscopy and electron microprobe. Results show that the phonotephrite used in this study will dissolve *0.8 wt% CO 2 at 700 MPa and a fluid composition of X H 2 O *0.4, in agreement with pre- vious experimental studies on mafic alkaline rocks. Fur- thermore, the dissolution of CO 2 at moderate to high X fluid H 2 O in our experiments exceeds that predicted using lower- pressure experiments on similar melts from the literature, suggesting a departure from Henrian behavior of volatiles in the melt at pressures above 400 MPa. With these data, we place new constraints on the modeling of Erebus melt inclusion and gas emission data and thus the interpretation of its magma plumbing system and the contributions of primitive magmas to passive and explosive degassing from the Erebus phonolite lava lake. Keywords Degassing  Alkaline volcanism  Volatile solubility  Carbon dioxide  H 2 O Introduction Dissolved volatiles, notably H 2 O and CO 2 , strongly affect the physical properties of magmas, including density and viscosity, thereby influencing magma ascent and eruptive behavior (Lange 1994). Additionally, volcanoes play a key role in carbon exchange between the deep Earth and the atmosphere (Dasgupta 2013). For these reasons, experi- mental studies determining the solubilities of pure H 2 O (e.g., Behrens and Jantos 2001; Botcharnikov et al. 2006, 2007; Carroll and Blank 1997; Moore et al. 1998; Lesne et al. 2011a), pure CO 2 (e.g., Botcharnikov et al. 2006; Brooker et al. 2001, Jendrzejewski et al. 1997; Lesne et al. 2010; Morizet et al. 2002; Thibault and Holloway 1994), and to a lesser extent mixed H 2 O–CO 2 systems (e.g., Behrens et al. 2004; Botcharnikov et al. 2006, 2007; Dixon et al. 1995; Jakobsson 1997; Lesne et al. 2011b; Vetere et al. 2011) have been conducted for silicate melts over a range of compositions and experimental P–T conditions. Mixed-volatile solubility data are particularly important for the interpretation of melt inclusion volatile contents and their application to geobarometry and interpretation of magma degassing histories (Wallace 2003). The solubility of H 2 O and CO 2 can be strongly dependent upon melt composition (Dixon 1997; Papale et al. 2006), and Communicated by T. L. Grove. K. Iacovino  K. Roggensack School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA Present Address: K. Iacovino (&)  C. Oppenheimer Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, UK e-mail: ki247@cam.ac.uk G. Moore Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA P. Kyle Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA 123 Contrib Mineral Petrol (2013) 166:845–860 DOI 10.1007/s00410-013-0877-2