Other
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.1579l&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 59, Issue 8, pp.1579-1588
Other
5
Scientific paper
Diffusion profiles of tin were produced in hydrous silicate melts adjacent to cassiterite crystals; the method of Harrison and Watson (1983) was adapted to produce Sn diffusion profiles in hydrous silicate melts adjacent to cassiterite crystals at 2 kbar, 850°C, and various redox conditions, from which information on the solubility, diffusion, and speciation of Sn in silicate liquids can be obtained. The use of diffusion profiles and a hydrous, yet slightly H 2 O-undersaturated melt composition were chosen, in order to avoid the loss of Sn to the noble-element capsule walls. Such losses occurred at reduced conditions in the previous experimental studies on the solubility or partitioning of Sn in silicate liquids (±fluid phase), which probably interfered with the redox or SnO 2 activity control of those experiments. The redox conditions investigated in this study were controlled by the intrinsic or an imposed f H 2 in rapid-quench cold-seal and internally heated pressure vessels, and were measured by either the hydrogen sensor or Shaw membrane techniques. Cassiterite solubilities at 850°C and 2 kbar range from 28,000 ppm SnO 2 at FMQ-0.84 to approximately 800 ppm at FMQ+3.12, in a haplogranitic melt with a normative (anhydrous) composition of 37.2% quartz, 28.3% orthoclase, 34.1% albite, 0.4% corundum, and 5.6 wt% H 2 O. For redox conditions higher than FMQ+1.5, SnO 2 solubility is independent of f O 2 , indicating that cassiterite dissolved into the melt largely as Sn 4+ . By contrast at more reduced conditions, log SnO 2 concentration vs. log f O 2 define a slope of approximately -0.5, implying that Sn is dominantly in the 2 + valence, if Sn 2+ melt is constant. The solubilities obtained at reduced conditions in this study are an order of magnitude higher than previously published data (at a comparable P-T , composition and f H 2 of the autoclave); the low values in the previous work are attributed to the loss of Sn to the capsule walls. The diffusion of Sn is also apparently related to f O 2 , ranging from approximately 10 -8 cm 2 /s at FMQ-0.84 to 10 -9 cm 2 /s at FMQ+3.12. This is consistent with Sn 2+ behaving as a network modifier, whereas the diffusion of Sn 4+ is slower, similar to other high field strength elements. The strong f O 2 dependence of Sn solubility can be used to explain some magmatic tin deposits and also indicates that tin might prove useful as a marker of redox changes of a variety of magmatic systems.
Burgess Simon
Holtz François
Linnen Robert L.
Pichavant Michel
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