TEMPORAL EVOLUTION OF CONTINENTAL CRUST COMPOSITION: IMPLICATIONS FOR THE START OF PLATE TECTONICS (Invited Presentation)

Geochemical analyses of fine-grained terrigenous sedimentary rocks have been one of the main means by which the composition of ancient crust that was exposed to weathering and erosion (generally considered continental crust) has been determined over Earth history. The bulk composition of that crust, whether dominated by mafic or felsic igneous lithologies, may link to processes of its generation. Subduction is considered the most likely means by which to generate large volumes of felsic rocks, whereas a crust dominated by mafic rock types may reflect a different tectonic regime such as plume-driven magmatism and so-called ‘stagnant lid’ tectonics. Recent studies of such sediments have arrived at different conclusions regarding whether there has been significant change in the average composition of crust during the Late Archean. On the basis of elevated transition metals contents and elemental ratios that track igneous differentiation, the upper continental crust was suggested to undergo a change from a mafic-dominated bulk composition to felsic-dominated bulk composition during the Late Archean (Dhuime et al., 2015; Tang et al., 2016; Smit and Mezger, 2017; Chen et al., 2019). By contrast, stable Ti isotopes of shales, which also track igneous differentiation, as well as Al/Ti and Zr/Ti ratios were interpreted to reflect a felsic crust back to 3.5 Ga; albeit with a higher proportion of komatiite (Greber et al., 2017; Greber and Dauphas, 2019). New insights into how Ti isotopes fractionate between tholeiitic and calc-alkaine igneous suites though show that Ti isotopes cannot be uniquely interpreted in terms of major element composition (Deng et al., 2019). This talk will review the current debate surrounding crust compositional evolution.

References: Deng et al., 2019, PNAS, 116; Dhuime et al., 2015, Nat. Geosci. 8; Greber et al., 2017, Science, 357; Greber and Dauphas, 2019, GCA; Chen et al., 2019, GCA (in press); Smit and Mezger, 2017, Nat. Geosci. 10; Tang et al., 2016, Science, 351.