Iron and zinc isotopes constrain the continental crust formation

Abstract

The origin of Earth’s earliest felsic melts from mafic crust is still controversial. Their formation likely involved hydrothermal alteration of mafic protoliths followed by partial melting. However, the specific mechanisms and conditions of anatexis remain unclear. Investigating modern analogues like plagiogranites may offer key insights into the formation and evolution of the Earth’s early juvenile continental crust. Stable isotopes of iron (Fe) and zinc (Zn) are sensitive to chemical dynamic conditions and do not evolve with time, making them highly promising tracers for deciphering the origin of plagiogranites. Here, we present high-precision Fe-Zn isotope analysis and thermodynamic modeling to explore the formation of the El-Shadli plagiogranite complex in the Arabian-Nubian Shield as an example. These plagiogranite rocks have low Sr/Y (0.4–5.3) characteristics and exhibit a range of Fe-Zn isotopic compositions, with δ⁵⁶Fe values of 0.06 ± 0.04 ‰ to 0.19 ± 0.02 ‰ (2SD) and δ⁶⁶Zn values of 0.27 ± 0.03 ‰ to 0.41 ± 0.04 ‰ (2SD). Thermodynamic modeling indicates that H₂O-saturated melting of altered oceanic crust (AOC) at shallow crustal depths (2–6 kbar) and elevated temperatures (700–900 °C) with geothermal gradients of 1200–4500 °C/GPa under a mantle plume setting was the primary mechanism driving plagiogranite formation. Sub-mantle oxygen isotopes in zircon (δ¹⁸O from 4.06 ± 0.17 ‰ to 5.09 ± 0.20 ‰) indicate that the required fluid originated from seawater and regulates the partial melting of plagioclase and amphibole during anatexis, directly influencing Fe-Zn isotope fractionation between the resulting plagiogranite melts and their residual source. Notably, the 4.02 billion years (Ga) Idiwhaa gneisses share similar Fe isotope and trace element compositions with the El-Shadli plagiogranites, suggesting that the Earth’s earliest felsic continental crust may have formed through a comparable process. Thermodynamic modeling also indicates that the Idiwhaa gneisses originated from partial melting of an AOC-like protolith under a high thermal gradient of 1270–4100 °C/GPa, possibly in the context of a mantle plume or meteorite impact. Our findings underscore the effectiveness of Fe and Zn isotopes in tracing the genesis of felsic magmas and emphasize their promise in probing the evolution of the Early Earth’s continental crust.