Boron isotopes in global TTGs trace the increase in deep crustal recycling in the Mesoarchean

Granitoids of the Tonalite-Trondhjemite-Granodiorite (TTG) group are a prime constituent of Archean cratons. Differences in the composition of these rocks relative to modern-day, more potassic granitoids have been proposed to reflect changes in the conditions and mechanisms of crust generation. By extension, these differences may indicate changes in the tectonic regime through geological time. Despite a continuously growing body of TTG research, consensus on TTG generation and Archean tectonic settings has not yet been reached. A remaining open question regarding TTGs is whether a reworked crustal component is present. Silicon and O isotopes have been previously employed to address this question and both isotope systems suggest that at least some TTGs indeed contain reworked material. Boron provides an alternative isotope system that can trace surface-altered material in magmatic rocks because B isotopes fractionate significantly at Earth’s surface but remain relatively unaltered at high temperatures. On modern-day Earth, the deep recycling of isotopically heavy seawater-derived B through subduction results in a diverse, but on average heavy, B isotope composition in arc granitoids. Conversely, juvenile granitoids formed in settings unrelated to subduction typically have mantle B-isotope values. These systematics are likely uniform and would apply to the Archean as well, given that Archean seawater also appears to exhibit isotopically heavy B. The B isotope system may thus be used to investigate the presence of subducted or otherwise surface-derived material in Archean granitoids. To this end, B isotopes were analyzed for a geographically and temporally spread sample set of pristine TTGs and related granitoids (n=45, from 9 different Archean terranes covering an age range of 3.78 to 2.68 Ga). This is a considerably larger and more geographically spread sample set than a B-isotope pilot study on TTGs (Smit et al., 2019), and may as such provide more globally representative results. The B isotope signature of TTGs seem to diversify over time, diverging more from mantle-derived values starting between 3.3 and 2.9 Ga. TTGs younger than 2.9 Ga exhibit up to δ¹¹B = +10.5 ± 0.2‰, and 48% of the samples have δ¹¹B values heavier than depleted mantle, whereas this is 18% for TTGs older than 3.3 Ga. The B isotope signature additionally diversifies with decreasing K₂O/Na₂O and La/Sm. Boron isotope compositions do not correlate with geochemical or petrological proxies for (post-)magmatic processes, such as weathering, metamorphism, hydrothermal alteration, or the loss of magmatic fluids, and therefore seem to be at least not significantly altered by these processes. Instead, isotopically heavy B in TTGs may be explained by the addition of a sodic and ¹¹B-rich contaminant into the TTG source. These contaminant characteristics point to seawater-altered oceanic crust, possibly introduced to the TTG source through subduction. If this is correct, the temporal trend observed in the δ¹¹B values in TTGs may reflect a shift from local and episodic to global and systematic subduction of oceanic crust in the Mesoarchean.

Smit, M.A. et al., 2019, Formation of Archean continental crust constrained by boron isotopes: Geochemical Perspectives Letters, doi:10.7185/geochemlet.1930.