Experimental determination of the effect of Cr on Mg isotope fractionation between spinel and forsterite

We report the results of spinel-magnesite Mg isotope exchange experiments at 600, 700, and 800 °C and 1 GPa to establish the equilibrium Mg isotope partitioning between magnesite (MgCO3) and spinel as a function of Cr substitution for Al in the spinel phase. We used the three-isotope method to obtain equilibrium fractionation factors between MgAlCrO4 and magnesite and MgCr2O4 and magnesite. The experimentally-determined temperature-dependent Mg isotope fractionations are Δ26MgMgAlCrO4- Mgs = 0.96 ± 0.22 × 106/T2 and Δ26MgMgCr2O4- Mgs = 0.55 ± 0.08 × 106/T2 (2 s.e.). When combined with the previous experimentally determined fractionation between forsterite and magnesite (Macris et al., 2013), the corresponding Mg isotope fractionations between these two Cr-bearing spinel compositions and forsterite are Δ26MgMgAlCrO4- Fo = 0.84 ± 0.23 × 106/T2 and Δ26MgMgCr2O4- Fo = 0.43 ± 0.10 × 106/T2. The experimentally determined isotopic fractionation relationship between magnesiochromite and magnesite agrees with theoretical predictions based on the crystal chemical environment of Mg in these minerals. By combining these new results with the existing experimental calibration of equilibrium Mg isotope exchange between pure spinel and forsterite, we arrive at the temperature-dependent Mg isotope fractionation between spinel and forsterite as a function of Cr concentration in the spinel:Δ26MgMgAl1 - xCrx2O4- Fo=-0.67±0.26x+1.10±0.23106/T2