Magnesium isotope fractionation in silicate melts by chemical and thermal diffusion

Two types of laboratory experiments were used to quantify magnesium isotopic fractionations associated with chemical and thermal (Soret) diffusion in silicate liquids. Chemical diffusion couples juxtaposing a molten natural basalt (SUNY MORB) and a molten natural rhyolite (Lake County Obsidian) were run in a piston cylinder apparatus and used to determine the isotopic fractionation of magnesium as it diffused from molten basalt to molten rhyolite. The thermal diffusion experiments were also run in a piston cylinder apparatus but with a sample made entirely of molten SUNY MORB displaced from the hotspot of the assembly furnace so that the sample would have a temperature difference of about 100-200 C from one end to the other. The chemical diffusion experiments showed fractionations of Mg-26/Mg-24 by as much as 7 parts per thousand, which resulted in an estimate for the mass dependence of the self-diffusion coefficients of the magnesium isotopes corresponding to D-26Mg/D-24Mg = (24/26)(beta) with beta= 0.05. The thermal diffusion experiments showed that a temperature difference of about 100 C resulted in the MgO, CaO, and FeO components of the basalt becoming slightly enriched by about 1 wt% in the colder end while SiO2 was enriched by several wt% in the hotter end. The temperature gradient also fractionated the magnesium isotopes. A temperature difference of about 150 degrees C produced an 8 parts per thousand enrichment of Mg-26/Mg-24 at the colder end relative to the hotter end. The magnesium isotopic fractionation as a function of temperature in molten basalt corresponds to 3.6 x 10(-2)parts per thousand/degrees C/amu. (c) 2007 Elsevier Ltd. All rights reserved.