Calcium and iron isotope fractionation during felsic magma differentiation

Stable calcium (Ca) and iron (Fe) isotopes could provide a new way to investigate granite petrogenesis, and their isotope fractionation mechanisms in felsic magmas have been increasingly understood through continuous efforts in recent years. However, comprehensive Ca and Fe isotope fractionation during highly fractionated magmas is still unclear. This study presents Ca and Fe isotope data for some fractionated granites from Southern Myanmar. The δ^56/54Fe values of the less fractionated Eocene granites range from 0.11 ± 0.03 ‰ to 0.23 ± 0.04 ‰. The highly fractionated Late Cretaceous and Paleocene granites clearly exhibit 0.15 ‰ and 0.42 ‰ variations in δ^56/54Fe values, respectively. These δ^56/54Fe values are negatively correlated with those of Fe₂O_3T, TiO₂ contents and (La/Yb)_N ratios, suggesting that more evolved melts are enriched in heavy Fe isotopes, primarily as a result of fractional crystallization of Fe-rich minerals enriched in light Fe isotopes (e.g., biotite and ilmenite). Some Late Cretaceous granites with low Nb/Ta and Zr/Hf ratios display relatively low δ^56/54Fe values, which may be modified by exsolved fluids enriched in light Fe isotopes. Moreover, the δ^44/40Ca values of the Late Cretaceous, Paleocene, and Eocene granites range from 0.71 ± 0.07 ‰ to 0.90 ± 0.06 ‰, 0.62 ± 0.08 ‰ to 0.89 ± 0.06 ‰, and 0.66 ± 0.06 ‰ to 0.75 ± 0.05 ‰, respectively. Most of the studied granites have relatively consistent Ca isotopic compositions with those of the continental crust. Combined with high δ^44/40Ca values (up to 0.90 ‰), the studied granites have a weakly negative correlation between δ^44/40Ca values and Eu/Eu^⁎ ratios. This evidence suggests that fractional crystallization of plagioclase with light Ca isotopes may also be a reason for Ca isotope fractionation during felsic magma differentiation, in addition to crustal magma sources and crustal contamination. Additionally, a Late Cretaceous granite with a high (Dy/Yb)_N ratio has the lowest δ^44/40Ca value (0.52 ± 0.06 ‰), possibly reflecting the presence of residual garnet in the source. The affirmation of significant Ca and Fe isotope fractionation in highly evolved melts strengthens the utility of Fe and Ca isotopes as tracers of magma differentiation.