An Iron Isotope Study On Laterite Weathering Profile Of Surigao,Southern Philippines

Weathering leads to the decomposition of rocks,the release and the residue of materials from continents in the low temperature surface environment.It also plays an important role in the geochemical link between the outer parts of the Earth i.e.,lithosphere,hydrosphere and biosphere.Laterites cover one third of the continents and 85%of the total soils,their characteristic red color being due to the large contents of oxidized Fe they contain.The dissolution and the leaching of iron from weathering crust into hydrosphere,due to the addition of external fluid,significantly influence the chemical composition of river waters and seawater.Furthermore,iron is an essential element of plant growth.During the process of plant growth,the roots can assimilate iron element from the soils,which will result in significant Fe isotope fractionation.It is therefore necessary to understand how laterites evolve and determine the iron isotope variations given their importance in the cycling of elements and environmental changes at the Earth's surface.On the other hand,the laterites developed in conditions such as those of the Surigao can be used as Ni deposits,which can play an important role in the global mineral resource economy.Thus,it is important to investigate the distribution of major and trace elements along the vertical profile.In order to investigate the formation of laterites and geochemical cycling of Fe during tropical weathering,we choose a typical laterite profile obtained from an equatorial rainforest,Southern Philippines.The laterite profile is 7m deep from top soil to peridotite.According to the difference of the degree of weathering,the profile can be divided into topsoil,gravel layer,homogeneous fine laterite.transition layer and protolith.The laterite samples of different layers were systematically collected.In this study,we present Fe isotope,major trace-element compositions and X-ray diffraction(XRD)analysis results of the laterite samples.XRD analyses reveal that the major Fe-bearing minerals are hematite and goethite.For the major elements,fluid-mobile elements such as Ca,K,Na and Mg are depleted in the weathered crust.Moreover,the results indicate that the highest Ni contents are found in the transition layer and the highest Cr contents are found in the bottom of homogeneous fine laterite layer.The homogeneous fine laterite layer shows a sharp increase of iron content.To evaluate quantitatively the relative mobilization of an element during chemical weathering,we calculated the percentage variations of element ratios to conservative element Ti relative to parent rock,?Ti?Fe.Most samples show distinguishable negative?Ti.Fe,showing that Fe was obviously lost from almost all of the layers.The studied profile shows a large variation in Fe2O3 concentrations(32.1-73.3wt%)and dramatic Fe loss based on ?Ti.Fe factors(?Ti,Fe?-50%to-90%)calculated from the open-system mass fraction transport function.Notably,?56Fe depicts a limited range from-0.03‰in the peridotite to +0.10‰ in the extremely weathered saprolites.These observations suggest that the lateritization process produced strong Fe loss but limited iron isotope fractionation along the whole laterite profile.Coupling the Fe isotopic compositions of the Surigao profile with pioneering studies leads one to acknowledge that Fe should have experienced a complete and in situ oxidation prior to Fe migration in the tropical high oxygen fugacity and frequent heat and humidity alternation climate.It is therefore conductive for the iron released from the rocks to precipitate as Fe3+-bearing minerals.Fe3+ is not believed to be fluid mobile in water and significant fractionation of Fe isotope can occur during biological reductive processes.Therefore,the most possible explanation of the notable Fe loss could be associated with ferric iron colloids.Fe3+ in its colloidal form has a positive charge by which it vigorously attaches to the negatively charged clay minerals and is leached together with it.To conclude,Fe transport over the history of the laterite formation and evolution may not have had a discernible effect on the iron isotope composition of the ecosystem.