Were ancient granitoid compositions influenced by contemporaneous atmospheric and hydrosphere oxidation states?

Geologists are acquainted with the principle of uniformitarianism, which holds that present day processes are the key to those that operated in the past. But the extent this applies to the processes driving the growth and differentiation of the continental crust throughout the Earth history remains a major controversy in earth sciences. An important part of the discussion circles around the predominance of sodium-enriched tonalite-trondhjemite-granodiorite (TTG) upper continental crustal rocks in Archean times, compared to dominant potassiumenriched granite-granodiorite (GG) associations in the post-Archean upper continental crust. The transition between TTG and GG dominated upper crust occurred approximately around the Archean ⁄Proterozoic (A ⁄P) boundary (i.e. 2400–2500 Ma) and has been suggested to mark the most pronounced compositional change of the earth s crust in the geological record (Condie, 1993). In recent years it has become evident that the A ⁄P boundary is also associated with the rise of oxygen in the hydrosphere and atmosphere. Molybdenum, sulfur and carbon isotopes indicate the rise of oxygen occurred 2700– 2350 Ma (Farquhar et al., 2000; Holland, 2006; Anbar et al., 2007; Wille et al., 2007) possibly indicating a yet poorly understood feedback loop between environmental conditions and compositional evolution of the upper continental crust. General consensus exists that plate tectonic theory explains the mechanism leading to the present-day GGtype upper crust. For the origin of the TTGs, however, the importance of this mechanism, and of plate tectonics in general, has been questioned and strongly modified or entirely different crust forming processes have been proposed for the Archean (e.g. Condie, 2005; Martin et al., 2005; Hamilton, 2007). Accordingly, understanding the dominance of TTG rocks in the Archean and their reduced importance after the A ⁄P boundary will provide important insights into the evolution of continental crust formation mechanism through time. Previous work on the origin of the TTG rocks dominantly focused on their trace element characteristics and more specifically on their high Sr ⁄Y and fractionated MREE ⁄HREE, which generally have been interpreted to indicate the involvement of garnet in their formation (Hanson et al., 1971; Arth and Hanson, 1972). As garnet could be either a residual or cumulative magmatic phase involved in three different location ⁄processes in a subduction system, three different explanations have been put forward to explain TTGs (Martin, 1986; Drummond and Defant, 1990; Smithies, 2000; Müntener et al., 2001; Kleinhanns et al., 2003; Alonso-Perez et al., 2009). However, none of these models provides a conclusive explanation for the major element difference between Archean TTG and post-Archean GG. Based on an extensive compilation of experimental melt compositions, Moyen and Stevens (2006), have inferred that the Na concentration of the melt is dependent on the pressure of melting, with high pressure melts have high Na concentration. The K concentration of the melts however are largely dependent on the source composition and the degree of melting (Moyen and Stevens, 2006). In Fig. 1, the Na ⁄K systematics of partial melting experiments is compared to the Na ⁄K of their respective source. The Na ⁄K in the melt is strongly dependent on the Na ⁄K of the source whereas the pressure and temperature effects are less significant. This observation questions the popular interpretation that the switch from TTG to GG type rocks coincides with transition from subduction of hot shallow dipping slabs in the Archean to steeper dipping colder ones in the post Archean. Alternatively, the Na ⁄K of the source of the granitoids could have varied through time. One possibility is that the overall maturity of the sedimentary record changed at the A ⁄P boundary (Engel et al., 1974), which ultimately must reflect changes in the ABSTRACT