Potassium Isotope Evidence for Origin of Archean TTG Rocks From Seawater-Hydrothermally Altered Oceanic Crust

Abstract

Tonalite-trondhjemite-granodiorite (TTG) rocks constitute a crucial part of the Archean continental crust, yet their origins remain contentious. It is critical to decipher their source nature and hydration mechanism. This paper presents a study of whole-rock K stable isotopes in well-preserved ca. 3.51–3.42 Ga TTG rocks and associated mafic rocks from the Barberton granitoid-greenstone terrane (BGGT) in the Kaapvaal Craton, South Africa. The results show for the first time a substantial δ⁴¹K variation from −0.69 ± 0.07‰ to −0.32 ± 0.05‰ (2SD) for the Paleoarchean mafic rocks, exceeding the present mantle δ⁴¹K range from −0.6 to −0.3‰. This variation can be well explained by the seawater-hydrothermal alteration at different temperatures. Similarly, the Paleoarchean TTGs exhibit a wide δ⁴¹K range from −0.55 ± 0.04‰ to 0.07 ± 0.08‰ (2SD). In combination with available zircon δ¹⁸O values of 5.07–6.02‰, it is evident that Archean TTGs would be derived from partial melting of the seawater-hydrothermally altered oceanic crust (AOC). The distinct K-O isotope signatures demonstrate that the hydration of Archean mafic crust is caused by the hydrothermal alteration at mid-ocean ridges during seafloor spreading. The variable K-O isotope compositions in the Archean TTGs signify a series of processes that are dominated by the seawater-hydrothermally altered AOC in a Wilson cycle from divergence through convergence to rifting of Archean oceanic plates. This offers a viable mechanism for TTG petrogenesis and the growth of continental crust in the Archean.