The presence of supracrustal Mg isotope signature in plume-derived felsic magmas

The origin of Earth’s felsic protocrust remains enigmatic, with Iceland’s plume-thickened crust serving as a key analogue. We report Mg isotope systematics (δ²⁶Mg) across Hekla volcano’s basalt-rhyolite suite, revealing unprecedented variations from -0.20‰ to +0.77‰, distinctly heavier than oceanic igneous rocks (-0.47‰ to -0.06‰). While fractional crystallization explains δ²⁶Mg trends within basalt-andesite and dacite-rhyolite suites, the ~0.8‰ jump at intermediate compositions requires alternative processes. The exceptionally high δ²⁶Mg in dacites, coupled with Th/U, and O-Li isotope systematics, fingerprints melting of hydrothermally altered mafic crust, likely recycled via plume-driven isostatic subsidence. These findings demonstrate that supracrustal signatures found in felsic magmas can emerge without plate tectonics, reshaping our understanding of early continental crust formation.