Decoupled Zircon Si–O Isotopes Tracing the Supracrustal Silicification and Komatiitic-Derived Fluids in the Source of TTGs

The tonalite–trondhjemite–granodiorite suites (TTGs) are key components of the Archean continental crust and therefore crucial to the understanding of the evolution of the early Earth. Here, we present in situ zircon Si–O isotope data of TTGs from Barberton. Results show that the 3.45–3.42 Ga (Group 1) and 3.24–3.23 Ga TTGs (Group 2) have elevated δ³⁰Si_melt values but mantle-like δ¹⁸O_zrc values, whereas the 3.23–3.22 Ga TTGs (Group 3) have coupled elevated δ³⁰Si_melt and δ¹⁸O_zrc values relative to mantle-derived rocks. We suggest that the Group 1 and 2 TTGs had a silicified source that was affected by low-δ¹⁸O fluid released from the komatiitic rocks. The low-δ¹⁸O fluid decreased the δ¹⁸O_zrc values of Group 1 and 2 TTGs but had negligible influence on their δ³⁰Si_melt values. The Group 3 TTGs were generated solely from the silicified source, as the low-δ¹⁸O fluid had become exhausted at that time.