Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Abstract (EDOC: 13890)
Mg isotope analyses of mantle rocks and minerals revealed large variations. Systematic differences in δ26Mg found between minerals from mantle xenoliths offer the potential for this isotopic system as an isotope geothermometer. However, results from other studies reveal a more complicated picture, where isotopic variability was found in similar samples (San Carlos olivines). To date, the cause for the observed variability remains uncertain. We explore the role of inter-mineral Mg isotope fractionation at upper mantle conditions. Experiments were done at 2 GPa and 00°C for 167 to 185 hours to investigate isotope partitioning between forsterite (Fo) and enstatite (En). Mg isotopes of coexisting En and Fo were analysed by MC-ICP-MS. The experiments were complemented by analyses of olivine and orthopyroxene from a SC xenolith and by theoretical estimations of equilibrium inter-mineral Mg isotope fractionation, based on vibrational frequency calculations and minimization of the total free energy of the solids. First experimental results revealed no Mg isotope fractionation between En and Fo beyond analytical precision at 00°C, 2 GPa (∆26MGEn-Fo = -0.02‰±0.12 (2SD); -0.03‰±0.).This result is consistent with our measurement on the SC xenolith (∆26Mgopx-ol = -0.08‰±0.11) and is close to the theoretical estimate (0.21‰). The absence of equilibrium inter-mineral Mg isotope fractionation suggests that the variability observed in various mantle-derived rocks is caused by other processes, such as diffusion-induced kinetic isotope fractionation as previously observed for Li isotopes. This is supported by a co-variation between Li and Mg isotopes found by others in a global peridotite dataset, implying both Li and Mg isotopes are affected by the same process. Hence, a combined Mg and Li isotope approach has the potential to better understand processes leading to the observed isotopic variability.
(2009): An experimental and theoretical investigation of Mg isotope fractionation in the Earth´s mantle. GV 99th Annual Meeting (Göttingen 2009).