Experimental determination of tin partitioning between titanite, ilmenite and granitic melts using improved capsule designs.

Abstract

Investigating mineral/melt Sn partitioning at high temperatures and pressures is a difficult task because Sn is a redox-sensitive multivalent element and easily alloys with noble metal sample capsules. To obtain accurate Sn partition coefficients between titanite, ilmenite, and granitic melts, we developed single capsule Pt or Au and double capsule Pt95Rh5 (or Au)–Re designs to avoid significant Sn loss at a controlled oxygen fugacity (ƒO2). With these new capsule designs, we performed piston-cylinder experiments of Sn partitioning between titanite, ilmenite, and granitic melts. The experimental P–T–ƒO2 conditions were 0.5–1.0 GPa, 850–1000 °C and ~QFM+8 to ~QFM–4 (QFM: Quartz–Fayalite–Magnetite buffer), with ƒO2 controlled by the solid buffers of Ru–RuO2, Re–ReO2, Co–CoO, graphite, and Fe–FeO. The obtained mineral/melt Sn partition coefficients (DSnmin/melt) are 0.48–184.75 for titanite and 0.03–69.45 for ilmenite at the experimental conditions. The DSnmin/melt values are largely dependent on ƒO2 although the effects of temperature and melt composition are also observed. DSnTtn/melt strongly decreases with decreasing ƒO2, from ~46–185 at the most oxidizing conditions (Ru–RuO2 buffer), to ~2–16 at moderately oxidizing to moderately reducing conditions (Re–ReO2 to Co–CoO and graphite buffers), to < 1 at the most reducing conditions (Fe–FeO buffer). DSnIlm/melt exhibits a variation trend similar to DSnTtn/melt, but is always lower than DSnTtn/melt at a given ƒO2. These DSnmin/melt values can be applied to quantitatively assess the mineralization potential of granitic magmas. Using DSnTtn/melt, we estimate that Sn contents are ~150–400 ppm in the premineralization magmas of the tin-mineralized Qitianling plutons (South China).