Experimental constraints on the mobility of Rhenium in silicate liquids

摘要

The volatization. of Rhenium (Re) from melts of natural basalt, dacite and a synthetic composition in the CaO-MgO-Al2O3-SiO2 system has been investigated at 0.1 MPa and 1250-1350 degrees C over a range of fO(2) conditions from logfO(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 degrees C in air is logD(Re) = -7.2 +/- 0.3 cm(2)/s and increases to logD(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 logD(Re)(reducing) = -7.6 +/- 0.2 cm(2)/s and to logD(Re)(dacite) = -8.4 +/- 0.2 cm(2)/s in dacitic melt. In the CMAS composition, logD(Re)(CMAS) = -7.5 +/- 0.1. The diffusivity of Re is comparable to Ar and CO2 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. (c) 2006 Elsevier Inc. All rights reserved.