Experimental studies of alunite: I. 18 O- 16 O and D-H fractionation factors between alunite and water at 250-450°C

Details Experimental studies of alunite: I. 18 O- 16 O and D-H fractionation factors between alunite and water at 250-450°C Experimental studies of alunite: I. 18 O- 16 O and D-H fractionation factors between alunite and water at 250-450°C

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58..903s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 2, pp.903-916

Computer Science

4

Scientific paper

We have determined oxygen and hydrogen isotope fractionation factors between alunite and water over a temperature range of 250-450°C by reacting synthetic natroalunite with 0.7 m K 2 SO 4 -0.1 to 0.65 m H 2 SO 4 solutions to produce K-rich alunite. From 88 to 95% alkali and isotope exchange were observed in most of these experiments, and the partial equilibrium method was used to compute equilibrium fractionation factors. Least-squares fits of the data give 10 3 In alunite ( so 4 )- H 2 O = 3.09 (10 6 / T 2 ( K )) - 2.94 and 10 3 In alunite ( OH )- H 2 O = 2.28 (10 6 / T 2 ( K )) - 3.90. The intramineral 18 O- 16 O fractionation factor 10 3 In alunite ( so 4 - OH site ) is given by the expression 0.8 (10 6 / T 2 ( K )) + 0.96. The alunite-water D-H fractionation factor ranges from -19 at 450°C to -6 at 250°C and does not appear to be strongly dependent on temperature. Runs with alkali exchange in the opposite direction were used to obtain 18 O- 16 O and D-H fractionation factors between natroalunite (mol% Na = 70-75) and water at 350-450°C. These indicate that mol% Na has negligible effect on the fractionation factors over this temperature range. Measured 18 O- 16 O and D-H fractionation factors between alunite and 1.0 m KCl -0.5 m H 2 SO 4 fluids also agree within 2 with the values obtained from the K 2 SO 4 -H 2 SO 4 fluids. However, experiments with alunite and distilled water at 400°C gave a value of 10 3 In alunite ( SO 4 )- H 2 O of 0.0, compared with a value of 3.9 obtained at this temperature with K 2 SO 4 - and H 2 SO 4 -bearing fluids. This suggests that changes in fluid composition can affect alunite-water 18 O- 16 O fractionation factors. Reconnaissance experiments with fine-grained natural natroalunite demonstrate that alunite-water D-H exchange can occur by hydrogen diffusion, although this process is generally not significant in the experiments with coarser grained synthetic alunites.

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