Magmatic differentiation of peralkaline granites: Constraints from iron isotope fractionation between Fe-bearing minerals

Abstract

Iron isotopes have been found useful in tracing magmatic processes of calc-alkaline granitic magmas, but its application in peralkaline granitic systems is hampered by the lack of information regarding the Fe isotope fractionation factors between alkali-rich ferromagnesian silicate minerals and FeTi oxides. To better understand the behavior of Fe isotopes during peralkaline magma differentiation, we carried out high-precision Fe isotope analyses on peralkaline and associated metaluminous high-silica granite rocks and Fe-bearing minerals separated from the rocks in Zhoushan archipelago, southeast China. The Fe-bearing mineral show a large dispersion in Fe isotope compositions, with δ⁵⁶Fe ranging from 0.03 ‰ to 0.70 ‰, following the sequence of K-feldspar ≥ magnetite > aegirine > arfvedsonite > ilmenite. The δ⁵⁶Fe differences between the mineral pairs are relatively consistent. Based on the magmatic temperatures defined by quartz-zircon oxygen isotopic geothermometer, the temperature-dependent equilibrium Fe isotope fractionation functions between following mineral pairs are obtained: Δ⁵⁶Fe_{aegirine-arfvedsonite} = 0.20 (± 0.07) × 10⁶/T², Δ⁵⁶Fe_{magnetite-arfvedsonite} = 0.38 (± 0.06) × 10⁶/T², and Δ⁵⁶Fe_{magnetite-aegirine} = 0.16 (± 0.04) × 10⁶/T². The bulk peralkaline granites have variable but generally high δ⁵⁶Fe values ranging from 0.28 ± 0.03 ‰ to 0.62 ± 0.04 ‰, with a mean of 0.42 ± 0.09 ‰ (1SD), which are higher than those of the associated metaluminous granitic samples (δ⁵⁶Fe = 0.22 ± 0.05 ‰, 1SD). Furthermore, δ⁵⁶Fe values of the peralkaline granites are negatively correlated with Sm/Yb and MnO, consistent with removal of isotopically light Fe-enriched arfvedsonite, implying that peralkaline granites experienced extensive magma differentiation regardless whether they were derived from differentiation of mantle-derived basaltic melts or partial melting of curst sources. Our results highlight a large Fe isotope fractionation between alkali ferromagnesian silicates and oxides, confirming Fe isotopes as a potential tool in tracking the differentiation processes of peralkaline magmas.