Lithospheric controls on Earth evolution

The Archean eon represents an important time in the evolution of our planet, during which tectonic activity was vigorous and the rate of crustal recycling extremely energetic [Hawkesworth and Kemp, 2006]. Although apparently inhospitable to complex forms of life, the Archean Earth was shaped by geological processes, which prepared the ground for the establishment of a complex biosphere-hydrosphere-atmosphere at ~2.0 Ga [Kump and Barley, 2007]. Therefore, a more comprehensive understanding of Archean cratonic architecture may provide crucial insights into the geodynamic and ecological evolution of our planet. Spatially distributed felsic crustal rocks (granitoids/volcanics) from the Yilgarn Craton of Western Australia were analysed for U-Pb zircon geochronology, Lu-Hf zircon and Sm-Nd whole-rock isotopes. Using this data, a number of 'time-slices' (isotopic contour maps constrained by U-Pb zircon age) were plotted. The isotope maps show a network of lithospheric blocks of varying age and genesis which represent the intra-cratonic architecture of the Yilgarn Craton. Within this architecture, there are multiple Earth systems which vary in space and time. The major systems under first-order control appear to be heat flux, magmatism, sedimentary environment and isostacy. Whether an area is juvenile (eHf>0) or evolved (eHf<0) indicates the level of mantle input and by proxy heat input into the crust. In turn, isotopic nature can be used as a proxy for lithospheric thickness, with evolved blocks having more extensive vertical accretion and addition of plume head restites than juvenile regions. These thickness contrasts are believed to control the localisation of plume melts and subsequent volcanism into shallower, juvenile, craton-margins [Begg et al., 2010]. Thickness variations also control the level of isostatic equilibrium of a 'block' relative to the geoid, with implications for emergence, topography and subsequent depositional and ecological environments. Spatial variations in Banded Iron Formation (BIF) facies also correlate with architectural variations. Granular facies BIF found proximal to recently active volcanic sites occurs in more juvenile regions, while thicker, finely-laminated BIF formed over abyssal plains occur in evolved regions. Subsequent affects of these primary processes include continental emergence, regional metamorphism, development of ecological niches and the localisation of large mineral systems. This work demonstrates that Archean Cratons have an intra-cratonic architecture which formed a first-order control on a multitude of early Earth systems and subsequently the development of the hydrosphere-biosphere-atmosphere and the evolution of life. Begg, G.C., et al. (2010), Lithospheric, Cratonic, and Geodynamic Setting of Ni-Cu-PGE Sulfide Deposits, Economic Geology, 105, 1057-1070. Hawkesworth, C.J., and A.I.S. Kemp (2006), Evolution of the continental crust, Nature, 443, 811-817. Kump, L.R., and M.E. Barley (2007), Increased subaerial volcanism and the rise of atmospheric oxygen 2.5 billion years ago, Nature, 448, 1033-1036.