The Archean continental crust grew via the addition of tonalite-trondhjemite-granodiorite (TTG) magmas. The chemical and isotope compositions of TTGs provide a key to constraining the processes of early crust development. The Hf isotope analysis of zircon in these rocks is of particular interest because it can provide insights into the mantle reservoirs from which the TTG source melt was extracted. Commonly, the Hf isotope composition of zircon, or rocks in general, is interpreted in terms of juvenile (DM, depleted mantle) versus evolved (felsic crust) signatures, with DM being represented by a linear evolution line back-calculated from modern-day MORB values. This reservoir is considered the main source of the TTG parental melts. However, recent studies of TTGs from various cratons documented prevailing Hf isotope signatures that are distinctly less radiogenic (e.g., Whitehouse et al., 2022). The data indicate the presence of a less-depleted mantle reservoir that may have contributed to Archean crustal growth, but it is unclear whether this represents a local or global feature. To investigate this, we performed U-Pb and Hf analysis of magmatic zircon from TTG samples ranging in age from 3.8 to 2.6 Ga from seven different cratons. The Hf isotope signatures of TTGs from some of the investigated Archean cratons indicate a juvenile source close to, or indistinguishable from, the DM evolution line. However, a significant subset of TTG complexes tracks the Hf isotope evolution of a less-radiogenic mantle reservoir, which resembles that observed in the above-mentioned study. These data could be explained "conventionally" through the contamination of DM-derived melts with the evolved crust, or through variable residence time of the TTG source. However, this would require the proportion of juvenile and evolved material to coincidentally change through time within the different cratons to maintain persistent supra-chondritic Hf isotope composition throughout the Archean. The data thus likely indicate that the Archean crust-producing mantle was isotopically bimodal with some regions having DM composition and others comprising a distinctly less depleted mantle with Hf compositions intermediate between DM and chondritic reservoirs. This mantle reservoir was a global, rather than local, phenomenon and may have been as prevalent as DM in producing continental crust during the Archean.

Reference(s)

Whitehouse, MJ, Kemp, AIS, and Petersson, A 2022, Persistent mildly supra-chondritic initial Hf in the Lewisian Complex, NW Scotland: Implications for Neoarchean crust-mantle differentiation: Chemical Geology, 606, 121001.