146,147Sm-142,143Nd systematics of the Acasta Gneisses: New insights on the formation of Earth’s oldest rocks

Reconstructing the earliest evolution of the silicate Earth remains a major geological challenge because of the scarcity and incomplete preservation of rocks older than 3.8 Ga. Isotope variations produced by short-lived radionuclides, such as 146Sm, have the potential to trace the early formation of distinct chemical reservoirs within the silicate Earth, and at the same time can also help decipher the petrogenesis of the oldest-preserved cratonic units.

Here, we present coupled 146,147Sm-142,143Nd systematics of layered gneisses from the Acasta Gneiss Complex (AGC) in northwest Canada. In addition, U-Pb dates of zircon extracted from individual layers and whole-rock major and trace element data are used to constrain the emplacement ages of the gneiss protoliths and their petrogenetic history. Mafic and felsic layers yield average µ142Nd values of +2.7 and −7.5 ppm (±2.5 ppm, 2 S.D.), respectively, documenting the existence of both Hadean mantle and crustal components within the sources of the AGC. Within individual layers, zircon U-Pb data form bands along concordia suggesting ancient Pb loss, with major clusters at 3.5–3.6 Ga for the felsic layers and 3.65–3.75 Ga for one mafic layer. Using these ages, the felsic layers yield consistently negative ε143Ndi values of about −5, while the mafic layers have near-chondritic ε143Ndi. Variations in ε143Ndi and µ142Nd are tightly correlated, suggesting correct age assignment and intactness of the long-lived 147Sm-143Nd system since zircon crystallization. Thus, the studied samples likely record primary 142,143Nd variations, implying that multiple sources were involved in the formation of the AGC protoliths.