Using (U-Th)/He dating of iron oxides to constrain past weathering episodes

Abstract

<p>Laterites are deep weathering profiles which develop under tropical and subtropical climatic conditions. Their geochemical and mineralogical composition is strongly influenced by the climatic condition under which they form, whereas the composition of the parental material plays a subordinate role. Due to their long-term stability throughout the intertropical zone, they are archives of past climates. It has been proposed that laterites formed episodically throughout the Earth’s history in periods with very favorable climate, but little is known about the duration, frequency and spatial extent of such weathering events. As different climatic signals are potentially superposed in old and constantly exposed laterites, disentangling of the paleoclimatic information preserved in these archives is a complex task. Since laterites are mainly composed of clay minerals, iron (oxyhydr)oxides and aluminum (oxy)hydroxides, most conventional geochronological methods cannot be applied to those formations. However, hematite and goethite, which are the main components of the ferruginous duricrust that is present in most lateritic profiles, can be dated using the (U-Th)/He method.</p> <p>This contribution presents an approach which couples (U-Th)/He geochronology of supergene hematite and goethite with high resolution mineralogical and geochemical methods. Our data derives from the South American Guiana shield which has been tectonically stable and in tropical latitudes since the Cretaceous - conditions favorable for the development of thick lateritic profiles. Two coupled data sets from lateritic duricrusts from the northeastern and eastern Guiana shield reveal the existence of several important weathering events during the Cenozoic. Mineralogical and geochemical characteristics of the dated material allow us a deepened insight into weathering processes and conditions by placing mineralogical and geochemical processes into a temporal framework and thereby enables us to extract paleoclimatic information from lateritic iron duricrusts. The results from the eastern part of the shield indicate the existence of three discrete weathering events during the Late Cretaceous / Early Paleocene, the Oligocene and the Middle Miocene that can be related to seasonally contrasted  tropical climate. By contrast, the results from the northeastern Guiana shield are dominated by a Late Neogene weathering event which shows characteristics of more humid climatic conditions.</p>