Thallium isotope ratios record amphibole-dominated fractionation of Earth’s earliest continental crust

Earth’s oldest continental crust is found in cratonic nuclei comprised of tonalite-trondhjemite-granodiorites (TTGs). However, the nature of the source materials that formed TTGs, as well as the timing and mechanisms of its formation remain controversial. Current models for the formation of Archean TTGs are (i) vertical subduction and partial melting of the subducting hydrated oceanic crust, (ii) partial melting at the base of plume-generated oceanic plateaux within the garnet stability field, and (iii) fractional crystallization of mafic-intermediate magmas and accumulation of plagioclase ± amphibole [1, 2, 3]. The thallium stable isotope composition of magmatic rocks identify distinct components in modern subduction zone settings [4] and thus can be exploited to address early continental crust formation. Here we report Tl isotope compositions for rocks from the Acasta Gneiss Complex (AGC) in Canada, which contains the oldest known evolved crust, with igneous crystallization ages ranging from 4.03 to 2.95 Gyr [5]. Tonalitic gneisses of the ~4 Gyr old Idiwhaa unit have ε205Tl values in the range of -4.0 ± 0.3 to -1.1 ± 0.4 that correlate with decreasing Dy/Yb and Dy/Dy*. The correlation follows indices of magmatic differentation, which is consistent with extensive amphibole fractionation at shallower depths than the garnet stability field.

[1] Foley et al. (2002) Nature 417, 637-740. [2] Zegers & Van Keken (2001) Geology 29, 1083-1086. [3] Laurent et al. (2020) Nat Geo 13, 163–169. [4] Nielsen et al. (2017) Rev Mineral Geochem 82, 759-798. [5] Bowring et al. (1989) Geology 17, 971-975.