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2025-12
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Geology
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Although incomplete, the Archean rock record provides a rare opportunity to investigate early Earth's crustal evolution. Spherule layers, formed through condensation of vaporized target rock and bolide material during meteorite impacts, represent a unique window into this time. This study models target rock compositions of two Archean-Paleoproterrozoic impact events, represented by the Dales Gorge–Kuruman (~2.49 Ga) and Paraburdoo–Reivilo (~2.57 Ga) spherule layer pairs. Using immobile high-field strength elements (Nb, Ta, Zr, Hf) and selected elements (Al, Ti, Sc), which resist alteration during diagenesis, a geochemical mixing model was applied to identify the most probable target compositions, with Ir and Cr constraining the meteoritic component. Principal Component Analysis (PCA) was performed to evaluate likely endmembers, followed by a multicomponent mixing model that reproduced the observed spherule bed geochemistry. The results reveal that target rocks for both impacts reflect mixtures of plateau basalt, Archean basalt and continental crust, with no mantle component. The meteoritic component of the Dales Gorge–Kuruman pair corresponds to an enstatite chondrite, whereas the Paraburdoo–Reivilo pair reflects an ordinary chondrite source. Both impacts indicate contributions from continental crust and the lack of mantle-derived material, supporting the hypothesis that the younger Neoarchean-Paleoproterozoic impacts excavated thicker crustal targets. This is consistent with continued continental crustal growth throughout the Archean, reducing the likelihood of mantle excavation as the scale of the impacts decreased. This study demonstrates the utility of immobile elements and mixing models in reconstructing target rock compositions and highlights the progression of continental growth through the Archean into the present.
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