Computer Science
Scientific paper
Dec 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.4965l&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 60, Issue 24, pp.4965-4976
Computer Science
7
Scientific paper
The diffusion profile method has been employed to measure tin diffusion coefficients and SnO 2 solubility in water-saturated, peralkaline to peraluminous haplogranitic melts at 850°C, 2 kbar, and log f O 2 conditions ranging from FMQ - 0.57 to FMQ + 3.49. At reduced conditions cassiterite is highly soluble and tin is present dominantly as a Sn 2+ species, whereas at oxidized conditions SnO 2 is much less soluble, and tin is present dominantly as a Sn 4+ species. There is a strong melt composition control on SnO 2 solubility; solubilities are at a minimum at the subaluminous composition, increase strongly with alkali content in peralkaline compositions and weakly with Al content in peraluminous compositions. In the case of the latter, this increase can only be distinguished at reduced conditions, e.g., at a log f O 2 of FMQ - 0.57 cassiterite solubility increases from 2.78 to 4.11 wt% SnO 2 for melt with Al/(Na + K)compositions (A.S.I.) of 1.0 and 1.2, respectively. At oxidized conditions SnO 2 solubility is ~ 500 ppm for both the A.S.I. 1.0 and 1.2 compositions. By comparison Sn0 2 solubilities in the most peralkaline composition investigated range from 3.94 wt% to -10 wt% Sn02, for the most oxidized to the most reduced conditions, respectively. Thermodynamic modelling of the data indicates that the Sn 4+ / Sn ratio in the melt is also at a minimum at the subaluminous composition, ranging from ~ 0.4 at log f O 2 of FMQ + 3.49 to ~0.01 at FMQ - 0.57. Over the same log foZ range the Sn 4+ / Sn ratio for the A.S.I. 0.6 composition ranges from ~0.98 to ~0.4 and for the A.S.I. 1.25 composition, from ~0.8 to ~0.02. Tin diffusivity is dependent on both f O 2 and melt composition. The effective binary diffusion coefficient of tin at reduced conditions is approximately 10 -7.5 cm 2 /sec for the peraluminous compositions and 10 -8.2 cm 2 /sec for the peralkaline compositions. At oxidized conditions these values decrease to approximately 10 -8.2 and 10 -9.0 cm 2 /sec, respectively. These are interpreted to reflect relatively fast diffusion where Sn 2+ is the dominant valence and tin in this case behaves similar to a network modifier and relatively slow diffusion where Sn 4+ is dominant and tin likely has a lower coordination number. Alternatively, the coordination of Sn 2+ and Sn 4+ is the same, but the bond strengths are different. At fixed f O 2 the faster diffusivity in the peraluminous compositions reflects the lower Sn 4+ /Sn 2+ ratio. The fact the Sn 4+ /Sn 2+ ratio in melts varies greatly with f O 2 at redox conditions near FMQ suggests that the partitioning behaviour of tin possibly changes during the evolution of an igneous suite in general and of a peraluminous granite suite in particular.
Holtz François
Linnen Robert L.
Pichavant Michel
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