The Solubilities of High Field Strength Element Minerals as a Function of Water Content in Peralkaline and Subaluminous Granitic Melts

Robert Linnen (rlinnen@uwaterloo.ca)

Department of Earth Sciences, University of Waterloo, Waterloo, ON, N2L 3G1, Canada

It is not clear how the water contents of melts effect the solubilities of high field strength element (HFSE) minerals, but it is important to know these effects in granitic melts because: 1) granites commonly crystallize at water-undersaturated conditions, and; 2) HFSE accessory phases control much of the trace element distribution in granitic rocks. An experimental investigation was therefore undertaken to determine the solubilities of columbite, tantalite, zircon, hafnon, rutile and wolframite in peralkaline and subaluminous granitic melts as a function of the water content of the melt.

Experiments were conducted in TZM autoclaves at 1035°C and 200 MPa and solubilities were determined for melts with nominally dry, 1, 2, 3, 4, and 6 wt% H₂O. The solubility of each mineral in the subaluminous melt increase dramatically with the first wt% H₂O. For example the solubility product of columbite increases from 0.4 x10^-4 mol²/kg² for the nominally anhydrous composition to 21 x10^-4mol²/kg² for the subaluminous melt with 1 wt% H₂O. For subaluminous melts with >1 wt% H₂O, columbite solubility is nearly independent of the water content of the melt. Similar behaviour is observed for tantalite, wolframite, hafnon and rutile solubilities in subaluminous melts. By contrast, no increase in solubility is observed for the first wt% H₂O in peralkaline melts. The solubility product of columbite is 675 x10^-4 mol²/kg² for the nominally anhydrous composition and 618 x10^-4mol²/kg² for the 6 wt% H₂O melt composition. Tantalite and hafnon solubilities in peralkaline melts behave similarly. These data suggest that solubilities, and hence activity coefficients of HFSE, are independent of water contents for peralkaline and subaluminous granitic melts with >1 wt% H₂O. Therefore, neither the crystallization of accessory phases, nor the partitioning of HFSE should be affected by the water content of the melt, except for very dry subaluminous compositions.

The Solubility of Cuprite in Aqueous Chloride and Acetate Solutions between 50^oC and 250^oC

Weihua Liu (liu@mail.earth.monash.edu.au), Derry C. McPhail (bear@mail.earth.monash.edu.au) & Joël Brugger (joelb@mail.earth.monash.edu.au)

Department of Earth Sciences, Monash University, Clayton, VIC 3168, Australia

Identifying metal aqueous species in hydrothermal brines and measuring reliable thermodynamic properties of those species is very important for predicting ore transport and deposition in geological systems. Unfortunately the existing knowledge about such properties of copper is limited and controversial. mineral solubility experiment is used in this study to determine the aqueous speciation of Cu(I) chloride and acetate complexes between 50°C and 250°C and saturated water vapor pressures.

Cuprite (cuprous oxide) was chosen in order to avoid redox reactions in the experiments. The pH of solution is buffered by acetic acid -sodium acetate, and chloride concentrations are using NaCl (0.01 m to 2 m). The experiments are conducted in evacuated silica glass tubes, which are placed in a water bath (50°C) or laboratory oven (150°C and 250°C). Preliminary experiments indicated complexing of Cu(I) with acetate so the concentration of total acetate was also varied in subsequent experiments to identify the important Cu(I)-acetate species. Cu(II) concentrations are negligible based on the lack of color in the quenched experimental solutions and UV-Vis absorbance spectra of the quenched solutions..

Measured copper concentrations vary between 0.0001 m and 0.2 m. Copper concentration increases with increasing temperature, decreasing pH and increasing chloride concentration. Logarithms of the equilibrium constants (log K) of copper complexes were fitted by non-linear regression to the measured copper concentrations and chloride concentration. Preliminary interpretation of our results shows that CuCl_(aq), CuCl₂^- (dominant) at least CuAc_(aq) and were present in our experiments. Our results show that copper- acetate complexing is much stronger than theoretical estimation (Shock and Koretsky, 1993). The derived log K values of copper chloride complexes are similar with recently published experimental derived values (Xiao et al., 1998) but disagree with theoretical values (Sverjensky et al. 1997). Our results lead to predict much higher copper concentrations.

Shock EL & Koresky CM, Geochim et Cosmochim Acta, 57, 4899-4922, (1993).

Sverjensky DA, Shock EL & Helgeson HC, Geochim et Cosmochim Acta, 61, 1359-1421, (1997).

Xiao Z, Gammons CH & Williams-Jones AE, Geochim et Cosmochim Acta, 62, 2949-2964, (1998).

Fe-Mg Interdiffusion in Magnesiowüstite

Stephen Mackwell (mackwell@uni-bayreuth.de)

Bayerisches Geoinstitut, Universitaet Bayreuth, D-95440 Bayreuth, Germany

We have performed a series of interdiffusion experiments on magnesiowüstite samples at 1 bar pressure and temperatures from 1600 to 1700 K, over a range of oxygen fugacities using mixed carbon monoxide/dioxide and hydrogen/carbon dioxide gases. The interdiffusion couples utilized a single crystal of MgO lightly pressed against a single crystal of magnesiowüstite with FeO/(MgO+FeO) between 0.08 and 0.30. The interdiffusion coefficient was calculated using the Boltzmann-Matano analysis as a function of iron content, oxygen fugacity, temperature, and water fugacity. For the entire range of conditions and compositions tested, the interdiffusion coefficient varied as the oxygen fugacity with an exponent of 0.18, and the activation energy for interdiffusion was determined to be 183 kJ/mol. These dependencies are consistent with interdiffusion mediated by unassociated cation vacancies. For the limited range of water activities that could be investigated using mixed gases at 1 bar pressure, no effect of water on interdiffusion could be observed. The dependence of the interdiffusion coefficient on iron content decreased with increasing iron concentration at constant oxygen fugacity and temperature. Thus, the interdiffusion coefficient could be described using a single power law dependence on iron content with an exponent of 1.7. As the interdiffusion is mediated by vacancies, we were able to calculate cation vacancy diffusivities in magnesiowüstite, which agree well with previous calculations based on conductivity measurements.