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Compositional and pressure controls on calcium and magnesium isotope fractionation in magmatic systems

Abstract : Stable isotope fractionation in magmatic systems depends on equilibrium isotope fractionation between different phases. However, current equilibrium stable isotope theory mostly assumes ideal crystal structures and simple chemical compositions, but it is unclear how the pressure and complex compositional variations in natural microscopic mineral structures affect inter-mineral stable isotope fractionation and thus control stable isotope fractionation in macroscopic magmatic systems. Here, we calculate the Casingle bondO and Mgsingle bondO bond lengths controlled by pressure and compositional variations of coexisting garnet (Grt) and clinopyroxene (Cpx) in the Roberts Victor eclogites from the Kaapvaal Craton and use these data as a proof of concept to interpret their inter-mineral Ca and Mg isotopic compositions (Δ44/40CaGrt-Cpx and Δ26MgGrt-Cpx). Our results show that the Casingle bondO difference between Grt and Cpx (ΔCasingle bondOGrt-Cpx) shows a significant increase with CaO content from 3.4 to 13.6 wt.% in Grt and with pressure from 2.9 to 6.9 GPa. ΔCasingle bondOGrt-Cpx has an excellent negative correlation with inter-mineral Ca isotope fractionation corrected for temperature effect (Δ44/40CaGrt-Cpx × T2/106), indicating that inter-mineral Ca isotope fractionation is controlled by pressure and compositional variations of the Grt through effects on the bond lengths. Inter-mineral Mg isotope fractionation corrected for temperature effect (Δ26MgGrt-Cpx × T2/106) in these eclogites shows a negative correlation with pressure but no obvious correlations with the mineral compositions, suggesting the dominant role of pressure effect in addition to temperature. The Mgsingle bondO bond length of Grt increases by about 0.02 Å with increasing CaO content in Grt of these eclogites, implying a mild compositional effect on inter-mineral Mg isotope fractionation. The results suggest that pressure and compositional variations in minerals control the equilibrium stable isotope fractionation between minerals. Utilizing the Ca isotope fractionation factor controlled by crystal chemistry of garnet, our modelling indicates that partial melting of eclogite in the mantle could not significantly fractionate Ca isotopes and, therefore, that low δ44/40Ca values in previously reported basalts cannot be attributed to the involvement of eclogite in their sources. This suggests that crystal chemistry exerts major controls on isotope fractionation in magmatic systems in addition to temperature.
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Chunfei Chen, Jin-Xiang Huang, Stephen Foley, Zaicong Wang, Frédéric Moynier, et al.. Compositional and pressure controls on calcium and magnesium isotope fractionation in magmatic systems. Geochimica et Cosmochimica Acta, Elsevier, 2020, 290, pp.257-270. ⟨10.1016/j.gca.2020.09.006⟩. ⟨insu-03029807⟩

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