Gordon Moore Dept of Chemistry & Biochemistry Arizona State University

1. Interpreting H2O and CO2 Contents in Melt Inclusions: Constraints from Solubility Experiments and Modeling Gordon Moore Dept of Chemistry & Biochemistry Arizona State University

2. Outline • Focus:Review recent H2O-CO2 solubility experimental data, and to review and assess solubility models for natural melts used in the interpretation of melt inclusion measurements. • Review of experimental H2O-CO2 solubility data: • Brief review of a “good” solubility experiment, experimental apparatus, and analytical techniques used. • Solubility data for pure and mixed H2O-CO2 fluids. • Review and assessment of H2O-CO2 solubility models for natural silicate melts: • Models for pure and mixed H2O-CO2 fluids • Compositionally specific models (e.g. rhyolitic, basaltic) • General compositionally dependent models • Limitations of model use • Application of compositionally dependent H2O-CO2 solubility models to melt inclusion data.

3. Scope of review • 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. • Only natural melt compositions (i.e. excludes haploid melts and simple synthetic systems). • For detailed information on H2O-CO2 in silicate melts in general, read the “bible”.

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

5. Experimental Apparatus: • Rapid quench cold seal (to 200-300 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) • Large volume piston cylinder (>300 MPa; to 1600°C; Baker, 2004; Moore et al, 2008) H2O-CO2 fluid Basalt Rhyolite Ni-NiO Moore et al, 2008

6. Analytical techniques for H2O and CO2 measurement Two critical measurements: H2O-CO2 content of melt AND fluid composition • Determining glass H2O-CO2 contents (see Ihinger et al, 1994): • Bulk techniques (primary): • High T vacuum manometry (H2O and CO2) • Karl-Fischer titration (H2O only) • Elemental Analyzer (CO2 only) • Microbeam techniques for both H2O and CO2 (secondary): • Fourier-transform Infra-red spectroscopy (FTIR) • Secondary ion mass spectrometry (SIMS) • Raman spectroscopy • Determining fluid composition (H2O-CO2 fluids only) : • Mass balance/gravimetry • Simple, but error related to fluid mass and scale precision (20-100% error reported). • Low T vacuum manometry • Requires a vacuum line, precise to ± 10 micromoles of fluid (5-10% relative error).

7. Summary of solubility data for pure H2O and CO2 in natural melts • Pure H2O (Table 1): • Greater than 30 different melt compositions • ~44-78 wt% SiO2; peralkaline to peraluminous • Broad range in P and T • 0.1 to 500 MPa; 800-1250°C; good coverage for most compositions • Pure CO2 (Table 1): • Only 7 compositions • mostly mafic compositions (32-55 wt% SiO2); rhyolites studied earlier • Dominated by high P (> 1000 MPa) and T (> 1200°C) • Due to low solubility of CO2 and increased solidus T; not extremely useful for understanding melt inclusion measurements

8. General H2O solubility behavior • Relatively large dissolved H2O contents (1-8 wt%; to 20-25 mol%) at magmatic P-T conditions. • Strong postive P dependence, with weaker negative T dependence. • Total H2O 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 H2O-rich mafic melts to glass. Figure 4 Dissolved H2O content in silicic melts (Behrens & Jantos, 2001) as a function of alkali/alumina.

9. General pure CO2 solubility behavior • Low dissolved concentration (100-1000’s ppm) at fluid-saturated magmatic P-T conditions. • Strong P dependence, negative T dependence. • Strong compositional dependence (e.g. Dixon, 1997), but much less data overall relative to H2O solubility. • Dominates fluid saturation behavior of magmas. • Two infra-red active species: carbonate (mafic) and molecular CO2 (silicic). • Mixed speciation in intermediate composition melts such as dacite and andesite (Behrens et al, 2004; King et al, 2002). Dacite Andesite

10. Solubility data for mixed H2O + CO2 fluids in natural melts • 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. • Silicic: ~20-500 MPa, 800-1100°C (e.g. Tamic et al, 2001) • Mafic and intermediate: ~20-700 MPa, up to 1400°C (e.g. Dixon et al, 1995; Botcharnikov et al, 2005, 2006, 2007). • Difficult experimental solubility measurements: • Fluid composition measurement (low T manometry or weight-loss method). • Dissolved CO2 measurements can be problematic in mixed volatile bearing glasses: • 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).

11. General H2O + CO2 solubility behavior • 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 H2O and CO2 at higher P conditions. • CO2 speciation changes with H2O content (molecular CO2 decreases with increasing H2O) Rhyolite 500 MPa 200 MPa 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) Dacite Figure from Behrens et al (2004)

12. Compositional dependence of H2O + CO2 solubility • Dissolved CO2 is stabilized by H2O in melt (non-Henrian), particularly at high P. • Strong dependence of CO2 and H2O content on melt composition • E.g. dissolved CO2 increases w/ increasing CaO, Na2O, K2O, etc (Dixon, 1997; Roggensack & Moore, 2008) • Any H2O-CO2 solubility model needs to take these complexities into account. XH2O(fluid)~0.45 P ~ 400 MPa T = 1200°C Figure from Roggensack & Moore (2008)

13. Modeling the solubility of H2O-CO2 in natural melts Types of models: • Regular solution (single composition; Silver and Stolper, 1985) • Empirical • 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) • Interpretation of fit parameters (e.g. partial molar volume of H2O and CO2) Extrapolation leads to significant error when inverting melt inclusion volatile contents to obtain saturation pressure!

14. Compositional extrapolation Pressure extrapolation Adventures in solubility model extrapolation • Figure comparing rhyolite-H2O 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. • Figure showing the compositional variable (PI) from the basalt-CO2 solubility model of Dixon (1997). • Note that calc-alkaline basalts have significantly different CaO/Al2O3 (strong effect on CO2 solubility). • Some give zero or negative PI values. • Basis for Newman & Lowenstern (2002) VolatileCalc H2O-CO2 model that is widely used for melt inclusions.

15. Melt compositional variation in melt inclusions and H2O + CO2 solubility models • How significant is compositional variation in melt inclusion suites? (See Fig 10 for ref’s) • Only 2 compositionally dependent mixed H2O-CO2 solubility models available: • 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 H2O or CO2 solubility. • Basalts: regular solution model w/ compositional dependence for CO2 calibrated by alkali-rich basalts (Dixon et al, 1995; Dixon, 1997). No compositional dependence for H2O in model. • 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. • (http://ctserver.ofm-research.org/Papale/Papale.php)

16. Comparison of VolatileCalc and Papale to silicic solubility data • Calculated fluid compositions and saturation pressures for rhyolite (77 wt% SiO2) and dacite (66 wt% SiO2) versus experimental values • 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. Rhyolite data from Tamic et al (2001) Dacite data from Behrens et al (2004)

17. Comparison of VolatileCalc and Papale models to basaltic solubility data • VolatileCalc • Calcic and calc alkaline basalt data (45-53 wt% SiO2) 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 H2O in fluid. • Significant error (up to 50%) in pressure estimate due to extrapolation of the compositional parameter used for CO2 solubility. The model is unable to account for the higher CO2 solubility in calc-alkaline compositions.

18. Comparison of VC and Papale to basaltic experimental data Papale et al (2006) • Papale et al (2006) • Basalt data same as for VC, andesite (57 wt% SiO2) 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.

19. Application of VC and Papale et al (2006) to basaltic melt inclusions Isobars and degassing paths calculated using VolatileCalc for Cerro Negro inclusions. • Significant error in pressure using VC for calc-alkaline basalts. • Isobars and degassing paths do not account for melt composition variation (49-52 wt% SiO2; 9.5-13 wt% CaO). Cerro Negro MI data from Roggensack (2001)

20. Application of Papale et al (2006) to basaltic melt inclusions Calculated minimum saturation pressures versus calculated fluid composition, measured CO2 and H2O content of Cerro Negro melt inclusions using Papale et al (2006). • More precise pressure estimates for calc-alkaline melts (usually lower estimated P). • 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 (150-250 MPa). • Theoretical degassing behavior (e.g. open vs closed) in a compositionally variable system is not easily visualized.