The combined effects of f_o2 and melt composition on SnO₂ solubility and tin diffusivity in haplogranitic melts

verfasst von

Robert L. Linnen, Michel Pichavant, François Holtz

Abstract

The diffusion profile method has been employed to measure tin diffusion coefficients and SnO₂ solubility in water-saturated, peralkaline to peraluminous haplogranitic melts at 850°C, 2 kbar, and log f_O2, conditions ranging from FMQ - 0.57 to FMQ + 3.49. At reduced conditions cassiterite is highly soluble and tin is present dominantly as a Sn²⁺ species, whereas at oxidized conditions SnO₂ is much less soluble, and tin is present dominantly as a Sn⁴⁺ species. There is a strong melt composition control on SnO₂ 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_O2, of FMQ - 0.57 cassiterite solubility increases from 2.78 to 4.11 wt% SnO₂ for melt with Al/(Na + K) compositions (A.S.I.) of 1.0 and 1.2, respectively. At oxidized conditions SnO₂ solubility is ~500 ppm for both the A.S.I. 1.0 and 1.2 compositions. By comparison SnO₂ solubilities in the most peralkaline composition investigated range from 3.94 wt% to ~10 wt% SnO₂, for the most oxidized to the most reduced conditions, respectively. Thermodynamic modelling of the data indicates that the Sn⁴⁺/ΣSn ratio in the melt is also at a minimum at the subaluminous composition, ranging from ~0.4 at log f_O2, of FMQ + 3.49 to ~0.01 at FMQ - 0.57. Over the same log f_O2, range the Sn⁴⁺/ΣSn 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_O2 and melt composition. The effective binary diffusion coefficient of tin at reduced conditions is approximately 10^-75 cm²/sec for the peraluminous compositions and 10^-8.2 cm²/sec for the peralkaline compositions. At oxidized conditions these values decrease to approximately 10^-8.2 and 10^-9.0 cm²/sec, respectively. These are interpreted to reflect relatively fast diffusion where Sn²⁺ is the dominant valence and tin in this case behaves similar to a network modifier and relatively slow diffusion where Sn⁴⁺ is dominant and tin likely has a lower coordination number. Alternatively, the coordination of Sn²⁺ and Sn⁴⁺ is the same, but the bond strengths are different. At fixed f_O2, the faster diffusivity in the peraluminous compositions reflects the lower Sn⁴⁺/Sn²⁺ ratio. The fact the Sn⁴⁺ /Sn²⁺ ratio in melts varies greatly with f_O2, 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.

Externe Organisation(en)

Universität Bayreuth
Centre national de la recherche scientifique (CNRS)

Typ

Artikel

Journal

Geochimica et Cosmochimica Acta

Band

60

Seiten

4965-4976

Anzahl der Seiten

12

ISSN

0016-7037

Publikationsdatum

12.1996

Publikationsstatus

Veröffentlicht

Peer-reviewed

Ja

ASJC Scopus Sachgebiete

Geochemie und Petrologie

Elektronische Version(en)

https://doi.org/10.1016/S0016-7037(96)00295-5 (Zugang: Unbekannt)