Interpreting H 2 O and CO 2 Contents in Melt Inclusions: Constraints from Solubility Experiments and Modeling Gordon Moore Dept of Chemistry & Biochemistry Arizona State University

Outline Focus: Review recent H 2 O-CO 2 solubility experimental data, and to review and assess solubility models for natural melts used in the interpretation of melt inclusion measurements. Review of experimental H 2 O-CO 2 solubility data: Brief review of a “good” solubility experiment, experimental apparatus, and analytical techniques used. Solubility data for pure and mixed H 2 O-CO 2 fluids. Review and assessment of H 2 O-CO 2 solubility models for natural silicate melts: Models for pure and mixed H 2 O-CO 2 fluids Compositionally specific models (e.g. rhyolitic, basaltic) General compositionally dependent models Limitations of model use Application of compositionally dependent H 2 O-CO 2 solubility models to melt inclusion data.

Scope of review To review solubility determinations and modeling relevant to natural melts and the interpretation of melt inclusion data.To review solubility determinations and modeling relevant to natural melts and the interpretation of melt inclusion data. Work done since “Volatiles in Magmas” Rev. Mineral, v.30, 1994.Work done since “Volatiles in Magmas” Rev. Mineral, v.30, Only natural melt compositions (i.e. excludes haploid melts and simple synthetic systems).Only natural melt compositions (i.e. excludes haploid melts and simple synthetic systems). For detailed information on H 2 O- CO 2 in silicate melts in general, read the “bible”.For detailed information on H 2 O- CO 2 in silicate melts in general, read the “bible”.

The “good” solubility experiment 1.Relatively large sample volume (to accommodate large molar volume of fluid and enough sample to analyze). 2.Rapid quench from run temperature to form crystal-free, glassy sample (difficult for hydrous melts). 3.Near-hydrostatic pressure conditions to minimize run failure and error in run pressure estimate (solid media apparatus). 4.Precise characterization of volatile content of run product and composition of fluid (mixed fluid experiments).

Experimental Apparatus: Rapid quench cold seal (to MPa, max T~1100°C; Ihinger, 1991; Larsen and Gardner, 2004).Rapid quench cold seal (to MPa, max T~1100°C; Ihinger, 1991; Larsen and Gardner, 2004). Rapid quench internally heated pressure vessel (to 500 MPa, ~1200°C; Holloway et al, 1992; DiCarlo et al, 2006)Rapid quench internally heated pressure vessel (to 500 MPa, ~1200°C; Holloway et al, 1992; DiCarlo et al, 2006) Large volume piston cylinder (>300 MPa; to 1600°C; Baker, 2004; Moore et al, 2008)Large volume piston cylinder (>300 MPa; to 1600°C; Baker, 2004; Moore et al, 2008) Basalt Rhyolite Ni-NiO H 2 O-CO 2 fluid Moore et al, 2008

Analytical techniques for H 2 O and CO 2 measurement Determining glass H 2 O-CO 2 contents (see Ihinger et al, 1994): Bulk techniques (primary): 1.High T vacuum manometry (H 2 O and CO 2 ) 2.Karl-Fischer titration (H 2 O only) 3.Elemental Analyzer (CO 2 only) Microbeam techniques for both H 2 O and CO 2 (secondary): 1.Fourier-transform Infra-red spectroscopy (FTIR) 2.Secondary ion mass spectrometry (SIMS) 3.Raman spectroscopy Determining fluid composition (H 2 O-CO 2 fluids only) : 1.Mass balance/gravimetry Simple, but error related to fluid mass and scale precision (20-100% error reported). 2.Low T vacuum manometry Requires a vacuum line, precise to ± 10 micromoles of fluid (5-10% relative error). Two critical measurements: H 2 O-CO 2 content of melt AND fluid composition

Summary of solubility data for pure H 2 O and CO 2 in natural melts Pure H 2 O (Table 1): Greater than 30 different melt compositionsGreater than 30 different melt compositions ~44-78 wt% SiO 2 ; peralkaline to peraluminous Broad range in P and TBroad range in P and T 0.1 to 500 MPa; °C; good coverage for most compositions Pure CO 2 (Table 1): Only 7 compositionsOnly 7 compositions mostly mafic compositions (32-55 wt% SiO 2 ); rhyolites studied earlier Dominated by high P (> 1000 MPa) and T (> 1200°C)Dominated by high P (> 1000 MPa) and T (> 1200°C) Due to low solubility of CO 2 and increased solidus T; not extremely useful for understanding melt inclusion measurements

General H 2 O solubility behavior Relatively large dissolved H 2 O contents (1-8 wt%; to mol%) at magmatic P-T conditions.Relatively large dissolved H 2 O contents (1-8 wt%; to mol%) at magmatic P-T conditions. Strong postive P dependence, with weaker negative T dependence.Strong postive P dependence, with weaker negative T dependence. Total H 2 O solubility has a significant compositional dependence (e.g. Moore et al, 1998; Behrens & Jantos, 2001).Total H 2 O solubility has a significant compositional dependence (e.g. Moore et al, 1998; Behrens & Jantos, 2001). Less data on mafic compositions due to higher T and difficulty quenching H 2 O-rich mafic melts to glass.Less data on mafic compositions due to higher T and difficulty quenching H 2 O-rich mafic melts to glass. Dissolved H 2 O content in silicic melts (Behrens & Jantos, 2001) as a function of alkali/alumina. Figure 4

General pure CO 2 solubility behavior Low dissolved concentration ( ’s ppm) at fluid-saturated magmatic P-T conditions.Low dissolved concentration ( ’s ppm) at fluid-saturated magmatic P-T conditions. Strong P dependence, negative T dependence.Strong P dependence, negative T dependence. Strong compositional dependence (e.g. Dixon, 1997), but much less data overall relative to H 2 O solubility.Strong compositional dependence (e.g. Dixon, 1997), but much less data overall relative to H 2 O solubility. Dominates fluid saturation behavior of magmas.Dominates fluid saturation behavior of magmas. Two infra-red active species: carbonate (mafic) and molecular CO 2 (silicic).Two infra-red active species: carbonate (mafic) and molecular CO 2 (silicic). Mixed speciation in intermediate composition melts such as dacite and andesite (Behrens et al, 2004; King et al, 2002).Mixed speciation in intermediate composition melts such as dacite and andesite (Behrens et al, 2004; King et al, 2002). DaciteAndesite

Solubility data for mixed H 2 O + CO 2 fluids in natural melts Most important for melt inclusion interpretation, yet only 8 new studies (see Table 2).Most important for melt inclusion interpretation, yet only 8 new studies (see Table 2). Good coverage for calc-alkaline rhyolite melts, but mafic and intermediate studies are sparse, as are alkaline compositions.Good coverage for calc-alkaline rhyolite melts, but mafic and intermediate studies are sparse, as are alkaline compositions. Silicic: ~ MPa, °C (e.g. Tamic et al, 2001) Mafic and intermediate: ~ MPa, up to 1400°C (e.g. Dixon et al, 1995; Botcharnikov et al, 2005, 2006, 2007). Difficult experimental solubility measurements:Difficult experimental solubility measurements: Fluid composition measurement (low T manometry or weight-loss method). Dissolved CO 2 measurements can be problematic in mixed volatile bearing glasses: multiple speciation in intermediate melts (Behrens et al, 2004; King & Holloway, 2002).multiple speciation in intermediate melts (Behrens et al, 2004; King & Holloway, 2002). potential matrix effects in calibrations of secondary techniques such as SIMS and FTIR (Behrens et al, 2004; Moore and Roggensack, 2007).potential matrix effects in calibrations of secondary techniques such as SIMS and FTIR (Behrens et al, 2004; Moore and Roggensack, 2007).

General H 2 O + CO 2 solubility behavior Simple linear solubility dependence as a function of fluid composition at low P.Simple linear solubility dependence as a function of fluid composition at low P. Less than 150 MPa for basalts (Dixon et al, 1995; Botcharnikov et al, 2005), 200 MPa and lower for rhyolite (Tamic et al, 2001) and dacite (Behrens et al, 2004). More complex, non-linear dependence for both H 2 O and CO 2 at higher P conditions.More complex, non-linear dependence for both H 2 O and CO 2 at higher P conditions. CO 2 speciation changes with H 2 O content (molecular CO 2 decreases with increasing H 2 O)CO 2 speciation changes with H 2 O content (molecular CO 2 decreases with increasing H 2 O) Figure from Liu et al, 2005 (filled squares and circles from Tamic et al, 2001; triangles, Blank et al, 1993; open symbols, Fogel and Rutherford, 1990) Figure from Behrens et al (2004) Rhyolite 500 MPa 200 MPa Dacite

Compositional dependence of H 2 O + CO 2 solubility Dissolved CO 2 is stabilized by H 2 O in melt (non-Henrian), particularly at high P.Dissolved CO 2 is stabilized by H 2 O in melt (non-Henrian), particularly at high P. Strong dependence of CO 2 and H 2 O content on melt compositionStrong dependence of CO 2 and H 2 O content on melt composition E.g. dissolved CO 2 increases w/ increasing CaO, Na 2 O, K 2 O, etc (Dixon, 1997; Roggensack & Moore, 2008) Any H 2 O-CO 2 solubility model needs to take these complexities into account.Any H 2 O-CO 2 solubility model needs to take these complexities into account. X H2O (fluid)~0.45 P ~ 400 MPa T = 1200°C Figure from Roggensack & Moore (2008)

Modeling the solubility of H 2 O-CO 2 in natural melts Types of models: 1.Regular solution (single composition; Silver and Stolper, 1985) 2.Empirical 3.Compositionally dependent (includes comp dependent regular solution of Papale, 1997, 1999; Papale et al, 2006) Limitations and caveats: Extrapolation beyond range of data (P, T, or compositionally)Extrapolation beyond range of data (P, T, or compositionally) Interpretation of fit parameters (e.g. partial molar volume of H 2 O and CO 2 )Interpretation of fit parameters (e.g. partial molar volume of H 2 O and CO 2 ) Extrapolation leads to significant error when inverting melt inclusion volatile contents to obtain saturation pressure!

Adventures in solubility model extrapolation Figure comparing rhyolite-H 2 O solubility models from Behrens & Jantos, (2001). Note good fit of Moore model to data up to 200 MPa, and instability when extrapolated above 300 MPa.Note good fit of Moore model to data up to 200 MPa, and instability when extrapolated above 300 MPa. Figure showing the compositional variable (PI) from the basalt-CO 2 solubility model of Dixon (1997). Note that calc-alkaline basalts have significantly different CaO/Al 2 O 3 (strong effect on CO 2 solubility).Note that calc-alkaline basalts have significantly different CaO/Al 2 O 3 (strong effect on CO 2 solubility). Some give zero or negative PI values.Some give zero or negative PI values. Basis for Newman & Lowenstern (2002) VolatileCalc H 2 O-CO 2 model that is widely used for melt inclusions.Basis for Newman & Lowenstern (2002) VolatileCalc H 2 O-CO 2 model that is widely used for melt inclusions. Pressure extrapolation Compositional extrapolation

Melt compositional variation in melt inclusions and H 2 O + CO 2 solubility models How significant is compositional variation in melt inclusion suites? ( See Fig 10 for ref’s) Only 2 compositionally dependent mixed H 2 O-CO 2 solubility models available: 1.VolatileCalc (Newman & Lowenstern, 2002) Rhyolite: regular solution model for calc-alkaline rhyolite (Silver et al, 1990; Blank et al, 1993). Note: No melt compositional dependence for H 2 O or CO 2 solubility. Basalts: regular solution model w/ compositional dependence for CO 2 calibrated by alkali-rich basalts (Dixon et al, 1995; Dixon, 1997). No compositional dependence for H 2 O in model. 2.Papale et al (2006) Uses most C-O-H solubility measurements to calibrate a compositionally dependent regular solution model across a broad range of P, T, and melt composition. Recently made available for general use by Dr. Mark Ghiorso. (

Comparison of VolatileCalc and Papale to silicic solubility data Rhyolite data from Tamic et al (2001) Dacite data from Behrens et al (2004) Calculated fluid compositions and saturation pressures for rhyolite (77 wt% SiO 2 ) and dacite (66 wt% SiO 2 ) versus experimental values Good agreement for both VolatileCalc and Papale with the rhyolite data.Good agreement for both VolatileCalc and Papale with the rhyolite data. Note failure of VC to estimate the dacite fluid compositions and pressures (no compositional dependence), while Papale matches data quite well.Note failure of VC to estimate the dacite fluid compositions and pressures (no compositional dependence), while Papale matches data quite well.

Comparison of VolatileCalc and Papale models to basaltic solubility data 1.VolatileCalc Calcic and calc alkaline basalt data (45-53 wt% SiO 2 ) from Moore et al (2006) and Moore et al (2008). Some of data beyond the stated 500 MPa limit of VC, but majority is at or below. Systematic overestimation of saturation pressure and underestimation of mole fraction of H 2 O in fluid. Significant error (up to 50%) in pressure estimate due to extrapolation of the compositional parameter used for CO 2 solubility. The model is unable to account for the higher CO 2 solubility in calc-alkaline compositions.

Comparison of VC and Papale to basaltic experimental data Papale et al (2006) 2.Papale et al (2006) Basalt data same as for VC, andesite (57 wt% SiO 2 ) from Botcharnikov et al (2007) For calculated saturation pressures and fluid compositions, values scatter around 1:1 line. Up to 30% error in pressure estimates. Large amount of scatter in fluid composition estimates (systematic error for calcic basalt). Possibly due to error in fluid measurements used to calibrate model. Best model currently available that can account for broad melt compositional variation over magmatic P-T range.

Application of VC and Papale et al (2006) to basaltic melt inclusions Significant error in pressure using VC for calc-alkaline basalts.Significant error in pressure using VC for calc-alkaline basalts. Isobars and degassing paths do not account for melt composition variation (49-52 wt% SiO 2 ; wt% CaO).Isobars and degassing paths do not account for melt composition variation (49-52 wt% SiO 2 ; wt% CaO). Cerro Negro MI data from Roggensack (2001) Isobars and degassing paths calculated using VolatileCalc for Cerro Negro inclusions.

Application of Papale et al (2006) to basaltic melt inclusions Calculated minimum saturation pressures versus calculated fluid composition, measured CO 2 and H 2 O content of Cerro Negro melt inclusions using Papale et al (2006). More precise pressure estimates for calc-alkaline melts (usually lower estimated P).More precise pressure estimates for calc-alkaline melts (usually lower estimated P). Accounts for solubility dependence on compositional variation in melt inclusions.Accounts for solubility dependence on compositional variation in melt inclusions. Recast data allows identification of pressure regions critical to fluid/melt evolution of the magma ( MPa).Recast data allows identification of pressure regions critical to fluid/melt evolution of the magma ( MPa). Theoretical degassing behavior (e.g. open vs closed) in a compositionally variable system is not easily visualized.Theoretical degassing behavior (e.g. open vs closed) in a compositionally variable system is not easily visualized.