2008 Joint Annual Meeting (5-9 Oct. 2008): Discovery of the Oldest (~3.4 Ga) Lateritic Paleosols in the Pilbara Craton, Western Australia

An alteration zone (~20-80 m thick), characterized by the abundance of pyrophyllite and the depletion of Fe and most elements, is widely developed in pre-3.4 Ga submarine basalts that occur beneath the oldest-known (~3.4 Ga) erosional unconformity (i.e., oldest land surface) in the North Pole Dome region of the Pilbara Craton, Western Australia. Previous researchers suggested this alteration zone was created by hydrothermal activity associated with submarine volcanism ~3.42 Ga ago. Our field investigations during the past three years have resulted in the discovery of more than 100 iron pods (each ~1 to ~6 m thick and ~1 to ~50 m long) within the iron-depleted pyrophyllite-rich zone at seven sites over a ~30 km expanse in the studied area. These iron pods are enriched in iron (~30 to ~90 wt% Fe2O3), composed mostly of hematite, and occur 0 - 30 m below and generally parallel to the ~3.4 Ga unconformity.

The geological, mineralogical, and geochemical characteristics of iron pods and alteration zone in the studied areas, especially the behaviors of Al, heavy metals, redox sensitive elements (Fe, U, Cr, REEs), and Th, resemble those of “groundwater-type” laterites of younger ages, such as the 2.2 Ga Hekpoort paleolaterites. Groundwater-type laterites require the following conditions for formation: (i) distinct wet/dry seasons; (ii) the development of microbial mats/vegetation on/in soils during wet seasons, which produce abundant organic acids that leach both ferric and ferrous irons from the soils and create ferrous-rich groundwater; and (iii) an abundance of oxygen molecules supplied from the atmosphere to the soils and groundwater, mostly during dry seasons, to precipitate the aqueous ferrous iron as ferric (hydr)oxides. Therefore, the ~3.4 Ga laterites suggest the very early developments of the terrestrial biosphere and an oxygen-rich atmosphere.