Hafnium isotope evidence for a transition in the dynamics of continental growth 3.2 Gyr ago

Abstract

Earth's lithosphere probably experienced an evolution towards the modern plate tectonic regime, owing to secular changes in mantle temperature. Radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time, with some estimates suggesting that over 70% of the present continental crustal reservoir was extracted by the end of the Archaean eon. Patterns of crustal growth and reworking in rocks younger than three billion years (Gyr) are thought to reflect the assembly and break-up of supercontinents by Wilson cycle processes and mark an important change in lithosphere dynamics. In southern West Greenland numerous studies have, however, argued for subduction settings and crust growth by arc accretion back to 3.8 Gyr ago, suggesting that modern-day tectonic regimes operated during the formation of the earliest crustal rock record. Here we report in situ uranium-lead, hafnium and oxygen isotope data from zircons of basement rocks in southern West Greenland across the critical time period during which modern-like tectonic regimes could have initiated. Our data show pronounced differences in the hafnium isotope-time patterns across this interval, requiring changes in the characteristics of the magmatic protolith. The observations suggest that 3.9-3.5-Gyr-old rocks differentiated from a >3.9-Gyr-old source reservoir with a chondritic to slightly depleted hafnium isotope composition. In contrast, rocks formed after 3.2 Gyr ago register the first additions of juvenile depleted material (that is, new mantle-derived crust) since 3.9 Gyr ago, and are characterized by striking shifts in hafnium isotope ratios similar to those shown by Phanerozoic subduction-related orogens. These data suggest a transitional period 3.5-3.2 Gyr ago from an ancient (3.9-3.5 Gyr old) crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3.2 Gyr ago that involved juvenile crust generation by plate tectonic processes.

The zircon archive of continent formation through time

“…Although a Hadean onset has been hypothesized, the evidence for this remains equivocal (see previous section). Naeraa et al (2012) propose that the pattern exhibited by zircon data from SW Greenland indicates reworking of crust from 3.9 to 3.2 Ga, and that significant juvenile signatures with fluctuating trends after 3.2 Ga correspond to the onset of a geodynamic regime featuring arc formation and accretion (see Fig. 6).…”

Formation and Evolution of the Continental Crust

“…Several authors have concluded that these signatures are absent in Archean or even Proterozoic terranes, and that plate tectonics had not operated during these periods. Stern (2008Stern ( , 2013, Hamilton (2008Hamilton ( , 2011 1.8-2.7 Ga Bédard (2006), Rollinson (2010), Brown (2007), Keller and Schoene (2012) by 2.7 Ga Condie and Kröner (2008), O'Neill et al (2007), Korenaga (2006), Davies (2007), van Hunen and Moyen (2012) 3.0 Ga Benn (2006), Cawood et al (2006), Pease et al (2008), Richardson and Shirey (2008), Polat (2012), Dhuime et al (2012), Naeraa et al (2012) 3.3-3.5 Ga Zegers and van Keken (2001), , Smithies et al (2007), Van Kranendonk (2007), Griffin et al (2013) 4.3 Ga Harrison et al (2008) Stern (2008,2013) has adopted what is perhaps the most extreme position. He argues in a series of papers (and an excellent blog; Stern, 2013) that there was "a progression of tectonic styles from active Archean tectonics and magmatism to something similar to plate tectonics at ˜1.9 Ga to sustained, modern style plate tectonics … beginning in Neoproterozoic time" (Stern, 2008, page 265).…”

The Earth expansion theory and its transition from scientific hypothesis to pseudoscientific belief

“…However, the new plate tectonics theory had only a superficial similarity with continental drift, because all previous mobilist and fixist geodynamic theories became obsolete after the revolutionary advances that occurred in geophysics occurred during the 1960s. Plate tectonics quickly became the fundamental framework for Earth sciences, considered valid at least since 3-3.5 Ga (Condie and Kröner, 2008;Naeraa et al, 2012). In its more than 50 years of existence plate tectonics has amply proved its ability to explain within a single context the most diverse geological phenomena, and it is supported by a great deal of evidence.…”