Iron solid-phase differentiation controls isotopic fractionation during lateritic weathering of basalt

Abstract

Laterites are important iron (Fe) reservoirs impacting the biogeochemical cycle of Fe at Earth’s surface, and Fe oxides are critical Fe-host minerals in laterites that affect the geochemical behavior of Fe. Whether and how the differentiation of Fe minerals controls Fe isotope fractionation during laterization, however, remains unclear. In this study, the mineralogy, element concentrations, and Fe isotope compositions are reported for a basalt-derived lateritic profile on Hainan Island, China, to investigate the role of Fe solid-phase differentiation in controlling Fe isotope fractionation during lateritic weathering. 57Fe Mössbauer spectroscopy at 13 K is conducted to quantitatively identify the solid-phase Fe, including FeIII-(oxy)hydroxides, FeIII-oxides, and organic/silicate-bound FeIII. The results from soil samples in the lower section show that RMZr,Fe values are positive and that δ56Fe values (−0.01‰ to 0.07‰; 0.05 ± 0.05‰ on average) are identical to those in parent basalt (0.03 ± 0.04‰). The limited variation in δ56Fe values is likely caused by the compaction effect and/or the downward transport of Fe from the upper soils. In contrast, RMZr,Fe values in soils from the upper section are generally negative, and their δ56Fe values (0.01‰ to 0.29‰) display zigzag variations and are linearly correlated with the amount of Fe present in hematite and goethite. The variation in δ56Fe value is likely caused by the atomic distribution of Fe in hematite and goethite, with heavy Fe preferentially incorporated into hematite rather than goethite. Based on the linear regression analysis, we calculated the apparent Fe isotope fractionation factor between hematite and goethite as 0.99 ± 0.18‰ (SD). A comparison of published Fe isotope data shows that the variation in δ56Fe value of silicate-derived soils and rivers may have a potential link with climatic factor (mostly temperature). Our study highlights the importance of Fe-mineral differentiation in controlling Fe isotope fractionation in laterite systems.