Iron isotope fractionation during transcrustal magmatic differentiation: Implications for continental crustal formation in subduction zones

Earth’s continental crust is hypothesized to originate from mantle melting in subduction zones. However, mantle melts are typically Fe-rich, with light Fe isotopes (δ⁵⁶Fe = 0.05‰), unlike the Fe-depleted and heavy Fe isotope (δ⁵⁶Fe can be up to 0.2‰) compositions of continental crust. The mechanisms responsible for this paradox remain elusive. Here, we report Fe isotope data from a suite of co-magmatic Middle Triassic continental arc rocks in the Ailaoshan tectonic zone (Yunnan, SW China). Gabbroic diorites have light Fe isotopes (δ⁵⁶Fe = 0.04‰−0.09‰), while granodiorites and normal-arc granites have slightly heavier values (δ⁵⁶Fe = 0.07‰−0.14‰). Adakitic granites, with high Sr/Y (61−183) and La/Yb (47−90) ratios, exhibit the heaviest δ⁵⁶Fe (0.16‰−0.19‰). Rayleigh fractionation modeling results suggest that fractionation of isotopically light Fe minerals (e.g., olivine, pyroxene, and amphibole) can reproduce the Fe isotopic variations in most samples, but fails to explain the high Sr/Y, La/Yb, and δ⁵⁶Fe values of adakitic granites, which would require Fe-rich garnet fractionation. Notably, the different Fe isotopes of the adakitic granites and non-adakitic rocks reveal two distinct transcrustal magmatic differentiation pathways: lower crustal, high-pressure, garnet-dominated fractionation; and middle−upper crustal, amphibole-dominated fractionation. The heavy Fe isotopic and Fe-depleted characteristics of the Ailaoshan magmatic arc suites are more comparable to the calc-alkaline arc magmas from normal to thick crust arcs (e.g., the Andean arc), but remarkably different from the tholeiitic arc magmas from thin crust arcs (e.g., the Mariana arc). This study highlights the role of amphibole and garnet fractionation in magmatic calc-alkaline differentiation. We propose that a thickened crust would permit a longer magmatic differentiation column, thus facilitating felsic and Fe-depleted continental crustal formation. The fractionated Fe-rich, garnet-bearing arc cumulates with light Fe isotopes are density-unstable and may be recycled into the mantle by lower crustal foundering.