Experimental determination of equilibrium magnesium isotope fractionation between spinel, forsterite, and magnesite from 600 to 800 ºC

Abstract

Magnesium isotopes are potentially powerful tracers for high-temperature geochemical processes if relevant fractionation factors are known. However, experimental data for Mg isotope fractionation are lacking at high temperatures. We performed experiments at 600, 700, and 800 °C and 1 GPa to establish the equilibrium magnesium isotope partitioning between forsterite (Mg_2SiO_4) and magnesite (MgCO_3) and between spinel (MgAl_2O_4) and magnesite, making use of the carbonate as an isotope exchange medium to overcome sluggish diffusion-limited magnesium isotope exchange between spinel and forsterite. Using the three-isotope method, the magnitudes of exchange between forsterite and magnesite, and between spinel and magnesite, were determined at the three temperatures for varying lengths of time, allowing equilibrium isotope partitioning to be established. Results are as follows: Δ^(26)Mg_(Fo–Mgs) = 0.04 ± 0.04‰ at 800 °C, 0.11 ± 0.10‰ at 700 °C, and 0.44 ± 0.10‰ at 600 °C; and Δ^(26)Mg_(Spl–Mgs) = 0.90 ± 0.28‰ at 800 °C, 1.10 ± 0.27‰ at 700 °C, and 1.73 ± 0.38‰ at 600 °C. From these experimentally determined equilibrium fractionation values, we derive the temperature-dependent equilibrium fractionation between spinel and forsterite by difference, yielding Δ^(26)Mg_(Spl–Fo) values of 0.86 ± 0.29‰ at 800 °C, 0.99 ± 0.29‰ at 700 °C, and 1.29 ± 0.39‰ at 600 °C. These data agree well with first-principles estimates of equilibrium magnesium isotope fractionation between spinel and forsterite at high temperatures. The data allow the calculation of an experimentally determined equation for the temperature dependence of ^(26)Mg/^(24)Mg fractionation between spinel and olivine: Δ^(26)Mg_(Spl–Fo)=0.96(±0.21)×10^6/T^2. This first high-T experimental calibration of Mg isotope fractionation of mantle minerals is consistent with expectations based on the crystal chemical environment of Mg in these phases.

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