Experimental constraints on the mobility of Rhenium in silicate liquids

Abstract

The volatization of Rhenium (Re) from melts of natural basalt, dacite and a synthetic composition in the CaO–MgO–Al 2O 3–SiO 2 system has been investigated at 0.1 MPa and 1250–1350 °C over a range of fO 2 conditions from log fO 2 = −10 to −0.68. Experiments were conducted using open top Pt crucibles doped with Re and Yb. Analysis of quenched glasses by laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) normal to the melt/gas interface showed concentration profiles for Re, to which a semi-infinite one-dimensional diffusion model could be applied to extract diffusion coefficients ( D). The results show Re diffusivity in basalt at 1300 °C in air is log D Re = −7.2 ± 0.3 cm 2/s and increases to log D Re = −6.6 ± 0.3 cm 2/s when trace amounts of Cl were added to the starting material. At fO 2 conditions below the nickel–nickel oxide (NNO) buffer Re diffusivity decreases to log D Re reducing = − 7.6 ± 0.2 cm 2 / s and to log D Re dacite = − 8.4 ± 0.2 cm 2 / s in dacitic melt. In the CMAS composition, log D Re CMAS = − 7.5 ± 0.1 . The diffusivity of Re is comparable to Ar and CO 2 in basalt at 500 MPa favoring its release as a volatile. Our results support the contention that subaerial degassing is the cause of lower Re concentrations in arc-type and ocean island basalts compared to mid-ocean ridge basalts.