Isotopic fractionation of the major elements of molten basalt by chemical and thermal diffusion

Abstract

Samples produced in piston cylinder experiments were used to document the thermal isotopic fractionation of all the major elements of basalt except for aluminum and the fractionation of iron isotopes by chemical diffusion between a natural basalt and rhyolite. The thermal isotopic fractionations are summarized in terms of a parameter Ω i defined as the fractionation in per mil per 100 °C per atomic mass units difference between the isotopes. For molten basalt we report Ω Ca = 1.6, Ω Fe = 1.1, Ω Si = 0.6, Ω O = 1.5. In an earlier paper we reported Ω Mg = 3.6. These fractionations represent a steady state balance between thermal diffusion and chemical diffusion with the mass dependence of the thermal diffusion coefficient being significantly larger than the mass dependence of the chemical diffusion coefficients for isotopes of the same element. The iron isotopic measurements of the basalt–rhyolite diffusion couple showed significant fractionation that are parameterized in terms of a parameter β Fe = 0.03 when the ratio of the diffusion coefficients D 54 and D 56 of 54Fe and 56Fe is expressed in terms of the atomic mass as D 54/ D 56 = ( 56 / 54 ) β Fe . This value of β Fe is smaller than what we had measured earlier for lithium, magnesium and calcium (i.e., β Li = 0.215, β Ca = 0.05, β Mg = 0.05) but still significant when one takes into account the high precision with which iron isotopic compositions can be measured (i.e., ±0.03‰) and that iron isotope fractionations at magmatic temperatures from other causes are extremely small. In a closing section we discuss technological and geological applications of isotopic fractionations driven by either or both chemical and thermal gradients.