Thermodynamic Properties of Water in Granitic Melts: A Re-evaluation

Marcus Nowak (m.nowak@ mineralogie.uni-hannover.de) & Harald Behrens

Institut für Mineralogie, Universität Hannover, Welfengarten 1, 30167 Hannover, Deutschland

It is well established that dissolved water strongly effects physicochemical and kinetic properties of silicate melts. Water is incorporated into silicate melts as molecular water and as OH groups. In situ near infrared spectroscopic studies of hydrous granitic melts using combination bands of H₂O at 5200 cm^-1 and OH at 4500 cm^-1 allow the determination of the equilibrium constant of the reaction: H₂O_melt + O_melt ­> 2OH_melt if thickness and density of the melt, and absorption coefficients of the combination bands are known.

Previously published in situ spectroscopic data were based on the assumption of temperature independent molar absorptivities (Shen and Keppler, 1995; Sowerby and Keppler, 1999) or on a constant density of the melt and changes of the linear molar absorptivities with temperature, but with a constant ratio (Nowak and Behrens, 1995). Withers et al. (1999) showed that the absorption coefficients for rhyolitic glasses vary with temperature. The recently published density model for hydrous silicate melts (Ochs and Lange, 1999) allows us to resolve this controversy.

We calibrated directly the required absorption bands of the melt containing 1.3 to 5.2 wt% water properly. It turned out, that the linear molar absorptions (epsilon) of both bands indeed change dramatically from glass at room temperature to melt but are constant in the melt: (epsilon)_H2O decreases from 1.79 (0.02) to 1.32 (0.03) l mol^-1 cm^-1 and (epsilon)_OH increases from 1.56 (0.02) to 1.68 (0.02) l mol^-1 cm^-1. As a consequence, the equilibrium constants of the speciation reaction are lower than previously published values for haplogranitic melts (Nowak and Behrens, 1995). At low temperatures (500-550°C) our revised in situ speciation data match now the data of Zhang et al. (1997) who used species concentrations determined in quenched glasses, but at high temperatures (800°C) our equilibrium constant is by a factor of 2 significantly higher. The recalculated reaction enthalpy H increases from 33.6 (2.0) to 35.5 (1.2) kJ mol^-1, and the reaction entropy S decreases from 29.8 (1.2) to 28.7 (1.1) J mol^-1 K^-1.

Nowak M & Behrens H, Geochim. Cosmochim. Acta, 59, 3445-3450, (1995).

Ochs FA & Lange RA, Science, 283, 1314-1317, (1999).

Shen A & Keppler H, Am. Mineral, 80, 1335-1338, (1995).

Sowerby JR & Keppler H, Am. Mineral, 84, 1843-1849, (1999).

Witthers AC, Zhang, Y & Behrens H, EPSL, 173, 493-516, (1999).

Zhang Y, Belcher R, Ihinger PD, Wang L, Xu Z & Newman S, Geochim. Cosmochim. Acta, 61, 3089-3100, (1997).

Volatile in Rhyolitic to Basaltic Melts: Calibration of IR Spectroscopy and H₂O Solubility

Susanne Ohlhorst (ohlhorst@ mineralogie.uni-hannover.de), Harald Behrens (h.behrens@mineralogie.uni-hannover.de), Jasper Berndt (j.berndt@mineralogie.uni-hannover.de) & François Holtz (f.holtz@mineralogie.uni-hannover.de)

Institut für Mineralogie, Universität Hannover, Welfengarten 1, 30167 Hannover, Germany

Molar absorption coefficients of the near infrared combination bands at 4500 and 5200 cm^-1 assigned to OH groups and H₂O molecules, respectively, were determined for glasses of dacitic, andesitic and basaltic compositions. Compositions were chosen, so that absorption coefficients are directly applicable to natural samples of the Unzen volcano (Japan). Total water contents (range 1.5 - 6.3 wt.%) of the samples used in the calibration were determined by Karl - Fischer Titration. Different combinations of baseline types and intensity measure (peak height / area) were applied to investigate the effect of evaluation procedure on infrared spectroscopic determination of apparent species concentrations and total water. Plots of normalized absorbances are consistent with identical ratios of the absorption coefficients, (epsilon)_H2O/(epsilon)_OH, for dacitic, andesitic and basaltic compositions as well as for a rhyolitic composition (data from Withers and Behrens, 2000). A parabolic equation is proposed to predict the molar linear and integrated absorption coefficients as a function of the SiO₂ content of the glass within the range of water contents used in the calibration.

Solubility experiments of pure water were performed with basaltic to rhyolitic compositions in internally heated pressure vessels (IHPV) at 1200°C and 50 to 200 MPa. In this pressure range we observe a maximum of water solubility in dacitic melts and lower solubilities in both rhyolitic and basaltic melts (at 200 MPa 5.7 wt.% H₂O in dacite, 5.2 and 4.8 wt.% in rhyolite and basalt, respectively). The dacitic melt was equilibrated with various H₂O - CO₂ fluids at 500 MPa and 1250°C. At 500 MPa the solubility of pure water is higher in rhyolitic (Tamic et al., 2000) than in dacitic melts. In contrast, at intermediate H₂O - CO₂ mixtures the water solubility is up to 100% higher in the dacitic melt.

Withers AC & Behrens H, Phys. Chem. Minerals, in press, (2000).

Tamic N, Behrens H & Holtz F, Chem. Geol., submitted, (2000).

Solubility of Ti in Wadeite Under Crustal and Upper Mantle Conditions

Andrea Orlando (aorlando@geo.unifi.it)^{1, 2} & Yves Thibault (ythibaul@julian.uwo.ca)¹

¹ Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada N6A 5B7

² C.N.R.-C.S. per la Minerogenesi e la Geochimica Applicata - Via G. La Pira, 4 - 50121 Firenze, Italy

Wadeite (K₂ZrSi₃O₉) is an "exotic" mineral occurring in some lamproitic rocks. K₂TiSi₃O₉ is a Ti-analogue of wadeite and its occurrence in rocks is rare (Mitchell & Steele, 1992). Nevertheless, it has been suggested that comparable K-Ti silicates could be significant constituents in the mantle source of some lamproitic magmas (e.g. Edgar & Mitchell, 1997). In this scenario, it is important to know the phase relationships of these minerals under upper mantle conditions. No data exist about the extent of solid solution between wadeite and its Ti-analogue as a function of pressure but the general absence of natural intermediate members could suggest limited miscibility between K₂ZrSi₃O₉ and K₂TiSi₃O₉.

In this context, an experimental investigation was undertaken at P-T corresponding to crustal and upper mantle conditions (0.15 - 3 GPa and 770 - 1300°C) using mixtures of synthetic end-members. The results indicate complete miscibility on the K₂ZrSi₃O₉ - K₂TiSi₃O₉ join in the P-T area investigated. This is in contrast with the typical observation of natural wadeites having Zr/(Zr+Ti) > 0.85 coexisting with Ti-rich phases, mainly phlogopite. The potential effect of the presence of phlogopite on the stability of K₂ZrSi₃O₉ and K₂TiSi₃O₉ was, therefore, evaluated. At P=2 GPa and T=1200°C, TiO₂ partitions strongly into phlogopite and liquid with the coexisting wadeite showing low Ti contents, similar to those observed in nature. However, with increasing pressure from 2 to 3 GPa, the partitioning behaviour of TiO₂ changes strongly in favour of the K₂(Zr,Ti)Si₃O₉ phase, with Zr/(Zr+Ti) decreasing down to 0.6. This observation is compatible with the suggestion of K-Ti silicates as potential reservoir of K and Ti in deep enriched region of the upper mantle.

Edgar AD & Mitchell RH, Journal of Petrology, 38, 457-477, (1997).

Mitchell RH & Steele I, Canadian Mineralogist, 30, 1153-1159, (1992).