CO2 solubility in dacitic melts equilibrated with H2O-CO2 fluids: Implications for modeling the solubility of CO2 in silicic melts
Introduction
Volatiles dissolved in silicate melts influence dramatically the chemical and physical properties of magmas. Because of its high solubility in silicate melts, water is of particular interest for understanding properties of magmas. CO2 is the second most abundant volatile in natural magmas. Although CO2 is usually subordinate in concentration in silicic magmas, it is often the first component to reach saturation due to its low solubility in the melt. Even small amounts of dissolved CO2 in water bearing melts shift the fluid saturation limit to higher pressures and thus greater depth (Holloway, 1976). Because vesiculation, initiated by exsolution of CO2-H2O-bearing fluids from the melt, is the driving force in many eruptive situations, it is important to investigate the saturation limit of fluids in the system C-H-O.
Similar to H2O, CO2 dissolves in silicate glasses and melts in form of at least two different species, as an unreacted molecular species (CO2 molecules, hereafter referred to as CO2,mol) and as species formed by reaction between molecular species and the silicate framework (carbonate groups). Whereas water speciation in glasses depends mainly on the total water content but only weakly on anhydrous composition (e.g., Silver et al., 1990; Behrens et al., 1996), the speciation of CO2 is found to be independent on total CO2 concentration but varies strongly with anhydrous composition of the glass (Fine and Stolper, 1985; Blank and Brooker, 1994; Brooker et al., 2001). In highly polymerized rhyolitic glasses CO2 is incorporated exclusively as molecular CO2 (e.g., Blank et al., 1993; Tamic et al., 2001), whereas only carbonate is present in more depolymerized glasses (Blank and Brooker, 1994). Glasses of intermediate compositions such as dacite, andesite and phonolite contain both types of species (Blank and Brooker, 1994; King and Holloway, 2002; Morizet et al., 2002).
Although CO2 solubility data (if not specified otherwise, the term CO2 refers to the total carbon dioxide) are available for a variety of natural and synthetic melt compositions (Mysen et al., 1975, 1976; Blank and Brooker, 1994; Dixon et al., 1995; Jakobsson, 1997; Brooker et al., 1999; Tamic et al., 2001; King and Holloway, 2002; King et al., 2002; Morizet et al., 2002), general models to predict the solubility of CO2 in silicate melts of mafic to silicic compositions (Spera and Bergman, 1980; Papale, 1999; Brooker et al., 2001) have large errors. This is due mainly to the uncertainty of the input data for the thermodynamic models. Especially, the early studies on CO2 solubility in silicate melts suffer from imprecise analytical techniques.
CO2 solubility in melts equilibrated with mixed H2O-CO2 fluids was investigated only in a few studies. In the pioneering works of Mysen et al. (1975, 1976), the highest CO2 solubilities in andesitic, basaltic and albitic compositions were not found in melts in equilibrium with pure CO2 fluids but for fluids with mole fractions of CO2 (xCO2fluid) of 0.6–0.7. Mysen et al. (1975, 1976) suggested that the formation of OH groups depolymerizes the melt structure, resulting in an increase of carbonate groups in the melt. However, as discussed by Tingle (1987), there may be substantial errors with the 14C β-track autoradiography which was used by Mysen et al. (1975, 1976) to determine carbon quantitatively. In more recent studies, IR and NMR spectroscopy was used to measure CO2 speciation and concentration in glasses. Rhyolitic compositions (containing only molecular CO2) were examined by Blank et al. (1993) at 75 MPa and 850°C, and by Tamic et al. (2001) at 200 and 500 MPa and 800 and 1100°C. Basaltic compositions (containing only carbonate) were studied by Dixon et al. (1995) at 1200°C and pressures up to 98 MPa. To our knowledge, the effect of water on CO2 speciation in glasses containing both CO2,mol and carbonate groups has been systematically investigated only in the studies of Kohn and Brooker (1994) and King and Holloway (2002) using samples synthesized at higher pressures in piston cylinder apparatus. Using 13C MAS NMR spectroscopy, Kohn and Brooker (1994) found a maximum in the ratio of CO2,mol/carbonate at a water content of 1–2 wt% in albitic and jadeitic glasses. On the other hand, IR spectroscopy on andesitic glasses showed a continuous decrease of CO2,mol/carbonate with increasing water content of the melt from 0 to 3.4 wt% (King and Holloway, 2002).
In this paper, we report new experimental results on the solubility of CO2 in dacitic melts in equilibrium with mixed H2O-CO2 fluids at 100, 200 and 500 MPa and 1250°C. To understand the solubility behavior of CO2 in the melts, we have also examined the effect of water on the speciation of CO2 in the quenched dacitic glasses using IR micro-spectroscopy. However, because the quantification of carbonate concentration in the glass by IR spectroscopy was found to be too imprecise, we applied secondary ion mass spectrometry (SIMS) to measure quantitatively the total CO2 concentration in the samples. On the basis of the new solubility and speciation data for dacitic composition and data for rhyolitic composition from previous studies, a thermodynamic model is proposed to describe the dependence of the CO2 solubility in silicic melts (rhyolite to dacite) on temperature, pressure, fugacity of CO2 (fCO2) and the water content in the melt.
Section snippets
Starting material
The starting composition was a synthetic dry glass with a composition close to the bulk composition of the dacite of the Unzen Volcano, Japan (Chen et al., 1993). The homogenous dry glass was synthesized by melting oxides and carbonates at 1600°C for more than 4 h, grinding and remelting for additional 4 h at the same temperature. The composition of the glass, determined by electron microprobe, is given in Table 1 (average analysis using a Cameca microprobe, with 15 kV accelerating voltage, 5
Karl-Fischer titration (KFT)
The water content of glasses (Cwater) from the solubility experiments were measured after thermal extraction using Karl-Fischer titration (Behrens, 1995). Single wafers of 10–20 mg were wrapped tightly in a Pt foil to avoid explosion during heating. To account for unextracted water in the samples after analysis, 0.13 ± 0.07 wt% H2O were added to the quantity measured by KFT. This value was found to be typical for polymerized rhyolitic glasses (Leschik et al., 2004) as well as for depolymerized
MIR spectroscopic investigation of CO2
Figure 1a shows IR spectra of selected CO2 bearing dacitic glasses in comparison to the starting glass synthesized in air. The sharp absorption band at 2348 cm−1 is attributed to the ν3 asymmetric stretching vibration of CO2,mol dissolved in the glass (Fine and Stolper, 1985). This peak is distinct from that for free gaseous molecular CO2, which would rise to a doublet of unresolved rotational structure centered at ∼2350 cm−1 (Brooker et al., 1999). The band system at ∼1530 cm−1 and ∼1430 cm−1
Speciation of CO2
As shown in Figure 6a, the ratio of CO2,mol (measured by IR spectroscopy) to CO2,total (measured by SIMS) in dacitic glasses strongly decreases with increasing water content. Up to 30% of carbon dioxide is dissolved in molecular form in water-poor glasses but less than 3% in glasses containing more than 3 wt% water (Table 4). This trend is consistent with data of King and Holloway (2002) for andesitic glasses. On the other hand, synthesis pressure and total CO2 content appears to have a minor
Conclusions
New CO2 solubility data are presented for dacitic melts equilibrated with H2O-CO2 fluids at 1250°C and pressures of 100, 200 and 500 MPa. SIMS was calibrated to quantify the total carbon content of the quenched glasses using self-made glass standards. The relative sensitivity factor for 12C/28Si was found to decrease systematically with water content of the glasses. Due to variations in SIMS measurement conditions calibration lines have to be determined for each analytical session.
A strong
Acknowledgments
This study was supported by the Deutsche Forschungsgemeinschaft (DFG grant Ho 1337/3+7). Fruitful comments of Hans Keppler, Yang Liu and two anonymous reviewers helped to improve the manuscript.
References (63)
- et al.
Near-infrared spectroscopic determination of water species in glasses of the system MAlSi3O8 (M = Li,Na,K)An interlaboratory study
Chem. Geol.
(1996) - et al.
Solubilities of carbon dioxide and water in rhyolitic melt at 850°C and 750 bars
Earth Planet. Sci. Lett.
(1993) - et al.
Experimental determination of trace-element partitioning between pargasite and a synthetic hydrous andesitic melt
Earth Planet. Sci. Lett.
(1995) - et al.
Solubility, speciation and dissolution mechanisms for CO2 in melts on the NaAlO2-SiO2 join
Geochim. Cosmochim. Acta
(1999) - et al.
Structural controls on the solubility of CO2 in silicate melts. Part I: Bulk solubility data
Chem. Geol.
(2001) - et al.
Ion microprobe determination of water in silicate glassesMethods and applications
Chem. Geol.
(1995) - et al.
High carbon concentrations in meteoritic chondrules:A record of metal-silicate differentiation
Geochim. Cosmochim. Acta
(1998) - et al.
SIMS analysis of volatiles in silicate glasses 1. Calibration, matrix effects and comparisons with FTIR
Chem. Geol.
(2002) - et al.
CO2 solubility and speciation in intermediate (andesitic) meltsThe role of H2O and composition
Geochim. Cosmochim. Acta
(2002) - et al.
13C MAS NMRA method for studying CO2 speciation in glasses
Geochim. Cosmochim. Acta
(1991)
Structural role of CO2 and [CO3]2− in fully polymerized, sodium-aluminosilicate melts and glasses
Geochim. Cosmochim. Acta
Carbon dioxide solubility and carbon isotope fractionation in basaltic melt
Geochim. Cosmochim. Acta
Ion microprobe isotopic measurements of individual interplanetary dust particles
Geochim. Cosmochim. Acta
CO2 in haplophonolitic meltSolubility, speciation and carbonate complexation
Geochim. Cosmochim. Acta
Carbon dioxide speciation in silicate meltsA restart
Earth Planet Sci. Lett.
Compositional dependence of molar absorptivities of near-infrared OH- and H2O bands in rhyolitic to basaltic glasses
Chem. Geol.
The pressure and temperature dependence of carbon dioxide solubility in tholeiitic basalt melts
Geochim. Cosmochim. Acta
Water and the viscosity of andesitic melts
Chem. Geol.
The solubility of H2O and CO2 in rhyolitic melts in equilibrium with a mixed CO2-H2O fluid phase
Chem. Geol.
An evaluation of the carbon-14 beta track techniqueImplications for solubilities and partition coefficients determined by beta track mapping
Geochim. Cosmochim. Acta
H2O diffusion in rhyolitic melts and glasses
Chem. Geol.
Interstellar SiC in the Murchison and Murray meteoritesIsotopic compostion of Ne, Xe, Si, C and N
Geochim. Cosmochim. Acta
Experimental determination of CO2-H2O activity-composition relations at 600–1000°C and 6–14 kbar by reversed decarbonation and dehydration reactions
Am. Mineral.
Determination of water solubilities in high-viscosity meltsAn experimental study on NaAlSi3O8 and KAlSi3O8 melts
Eur. J. Mineral.
Quantification of water speciation in silicate glasses and melts by IR spectroscopy—In situ vs. quench technique
Phase Transitions.
Quantification of H2O contents in silicate glasses using IR spectroscopy—A calibration based on hydrous glasses analyzed by Karl-Fischer titration
Glass Sci. Techn.
Determination of molar absorption coefficient for the IR absorption band of CO2 in rhyolitic glasses
Am. Mineral.
A combined rapid-quench and H2-membrane setup for internally heated pressure vesselsDescription and application for water solubility in basaltic melts
Am. Mineral.
An experimental investigation of the behaviour of carbon dioxide in rhyolitic melt
Experimental studies of carbon dioxide in silicate meltsSolubility, speciation and stable isotope behavior
Rev. Mineral.
Relationship between eruption volume and neodymium isotopic composition at Unzen volcano
Nature
Cited by (78)
Formation of carbon-bearing silicate melts by melt-metacarbonate interaction at convergent plate margins
2022, Earth and Planetary Science LettersRaman spectroscopy to determine CO<inf>2</inf> solubility in mafic silicate melts at high pressure: Haplobasaltic, haploandesitic and approach of basaltic compositions
2021, Chemical GeologyCitation Excerpt :More recently, higher pressures were reached but, for mafic compositions, few studies achieve pressures higher than 1 GPa, and very few higher than 3 GPa (review in Iacono-Marziano et al., 2012). On the theoretical side, often to interpret experimental data, a first approach developed classical models based on macroscopic thermodynamics yielding a very abundant literature (e.g., Khitarov and Kadik, 1973; Dixon et al., 1995; Papale, 1999; Behrens et al., 2004; Iacono-Marziano et al., 2012). These models are essentially empirical and include parameters that are adjusted to reproduce experimental data.
Associate editor: C. Romano