MODELING TARGET ROCK COMPOSITIONS OF TWO ARCHEAN IMPACT EVENTS

The Archean rock record provides a rare opportunity to investigate early Earth's crustal evolution, yet it is incomplete and largely inaccessible. Spherule layers, which form through the condensation of vaporized target rock and bolide material during meteorite impacts, represent a unique window into this time. This study models the target rock compositions of two Archean impact events represented by the two correlated spherule layer pairs: the Dales Gorge Spherule Layer (DGSL) and Kuruman Spherule Layer (KSL) (~2.49 Ga), and the Paraburdoo Spherule Layer (PSL) and Reivilo Spherule Layer (RSL) (~2.57 Ga). Using immobile high-field strength elements (Nb, Ta, Zr, Hf) and selected major elements (Al, Ti, Sc), which resist alteration during diagenesis, this work applies a geochemical mixing model to identify the most probable target compositions. The meteoritic component was constrained by using Ir and Cr concentrations. Principal Component Analysis (PCA) was performed to determine the likely endmembers, after which a multicomponent mixing model was applied, which reproduces the geochemistry within the spherule bed. The results reveal that target rock compositions for both impacts are a mix of plateau basalt, Archean basalt, and continental crust, with no mantle mix, along with their respective meteoritic components. Notably, both impacts indicate the presence of continental crust and the absence of mantle-derived material, which supports the hypothesis that the younger Neoarchean impacts excavated thicker crustal targets. This is consistent with continued continental crustal growth throughout the Archean, reducing the likelihood of mantle excavation during smaller impacts. This study demonstrates the utility of immobile elements and mixing models in reconstructing target rock compositions and highlights the growth of Earth's crust through the Archean into the present.