What drives Fe depletion in calc-alkaline magma differentiation: Insights from Fe isotopes

Abstract The continental crust is strongly depleted in iron relative to mid-oceanic ridge basalt, broadly identical the calc-alkaline magmas, suggesting that differentiation key continent formation. However, it remains contentious as what drives Fe depletion during magmatic crust. two competing hypotheses for differentiation—magnetite versus garnet (± amphibole) fractionation—predict contrasting isotopic fractionation pathways evolved melts because magnetite preferentially depletes ferric, isotopically heavy whereas does opposite. We report whole-rock isotope data suites of igneous rocks from central Andes, which represent magmas traversing normal and thickened arc crust, respectively. show a strong trend consistently high δ56Fe values (0.14‰ ± 0.02‰, 1 standard deviation [SD]), while those are less variable (0.10‰ 0.05‰, 1SD). Andean both heavier than Mariana (Pacific Ocean) differentiate along tholeiitic (Fe-enriching) paths. These results confirm fractionation/retention primary driver highlight role crustal thickening generating magmas.