Why was iron lost without significant isotope fractionation during the lateritic process in tropical environments?

Abstract

To investigate the formation of laterites and Fe cycling during tropical weathering, this study presents Fe isotope and major trace-element compositions of a laterite profile obtained from an equatorial rainforest, Southern Philippines. The lateritic profile is 7m deep from top soil to less-weathered peridotites. X-ray diffraction analyses reveal that the major Fe-bearing minerals are hematite and goethite. The profile shows a large variation in Fe2O3 concentrations (32.1–73.3wt%) and dramatic Fe loss based on τTi,Fe factors (τTi,Fe≈−50% to −90%) calculated from the open-system mass fraction transport function. Notably, δ56Fe depicts a small range from −0.03‰ in the peridotite to +0.10‰ in the extremely weathered saprolites.The small Fe isotopic fractionation and significant Fe loss provide important insights into Fe cycling during extreme weathering of peridotites in a tropical climate. Variations in Fe content and δ56Fe can be modeled by a Rayleigh distillation process with apparently small fractionation factors of 56Fe/54Fe between the saprolite and fluid (103lnαsaprolite–fluid) of 0.01 to 0.20, much smaller than those experimentally determined for reductive dissolution of goethite (103lnαgoethite-Fe(II)≈1.2; Icopini et al., 2004) and hematite (103lnαhematite-Fe(II)≈1.3; Beard et al., 2003). These observations suggest that Fe should have experienced a complete and in situ oxidation prior to Fe migration and Fe was probably transferred in the form of colloidal substances. Fe transport over the history of the laterite formation and evolution may not have had a discernible effect on the Fe isotopic composition of the ecosystem.