MG ISOTOPE FRACTIONATION IN BASALTIC MELT UNDER THERMAL GRADIENT IN NATURAL SETTINGS

Experiments have shown that chemical and thermal diffusion in silicate melts can fractionate isotopes(Richter et al., 2009). However, these isotopic fractionations caused by thermal gradient have not been documented in a natural system(Dauphas et al., 2010). It is perplexing since thermal gradient commonly exits in natural igneous systems. In order to resolve this discrepancy, theoretical analysis and related calculation were performed hear. Firstly, we demonstrate that the isotopic fractionation by Soret diffusion (also called Richter effect, defined as δ₁ hear) obtained from laboratory experiments is suitable to natural setting. While basaltic melt under temperature gradient in a natural setting is a dynamic system, in which the temperature of cold end is always less than the liquidus of the bulk melt and crystallization occurs continuously at the cold end(Lesher and Walker, 1988). Steady state is never reached in natural setting and experimental results cannot directly be applied to natural magma system. Due to the continuous crystallization at the cold end, element diffusion should occur and kinetic isotopic fractionation caused by mass diffusion also happens. The corresponding kinetic Mg isotopic fractionation is defined as δ₂ (caused by thermal migration) and δ₃ (caused by Soret diffusion). Secondly, we calculated the δ₁, δ₂ and δ₃of cooling komatiitic magma in Alexo area, Canada, which is interpreted as result of Soret diffusion under thermal gradient(Bouquain et al., 2009). The results showed that the total Mg isotopic fractionation could be zero or an undetectable value.

References

Bouquain, S., Arndt, N. T., Hellebrand, E., and Faure, F., 2009, Crystallochemistry and origin of pyroxenes in komatiites: Contributions to Mineralogy and Petrology, v. 158, no. 5, p. 599-617.

Dauphas, N., Teng, F. Z., and Arndt, N. T., 2010, Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine: Geochimica Et Cosmochimica Acta, v. 74, no. 11, p. 3274-3291.

Lesher, C. E., and Walker, D., 1988, Cumulate Maturation and Melt Migration in a Temperature Gradient: JOURNAL OF GEOPHYSICAL RESEARCH, v. 93, p. 10295-10311.