1 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet2 Institut für Geologie Universität Bern3 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet
A laterite profile from Central Madagascar
We performed a mineral, geochemical and Cr–Sr–Pb isotope study on a laterite profile developed on ca. 540 Ma old tonalitic bedrock in Madagascar with special emphasis on the behavior of chromium during tropical weathering. The observed strong depletions of Ca, Si, and P, and enrichment of Fe and Al, in the soil, relative to bedrock and underlying saprolite, are the characteristic features pertinent to laterites. The enrichment of Fe in topsoil horizon can be correlated with enrichment of P, and the redox sensitive elements Mn and Cr, and indicates redistribution of these elements related to oxidation–reduction processes. The slight scatter of 87Sr/86Sr, and 206,207,208Pb/204Pb values in the profile is one consequence of such redistribution processes. Our results are compatible with a two stage process during alteration: (1) an incipient alteration stage characterized mainly by a pervasive, effective and complete loss of sulfides accompanied by patchy alteration constrained along fissures and cracks, and (2) a main stage of soil formation (actual laterization) with loss of Na, K, Ca, and Si, and accumulation of Fe and Mn relative to the unaltered bedrock. This two stage evolution is also depicted by Cr concentrations and δ53Cr values along the studied profile, where the first incipient alteration stage caused a pronounced depletion of Cr particularly hosted by sulfides, and where the second alteration (laterization) led to redistribution and a small increase in Cr concentration in the uppermost portions of the soil profile relative to stage one altered saprolite. This gain in Cr is accompanied by decreasing δ53Cr values and can be explained by partial immobilization (possibly by adsorption/coprecipitation on/with Fe-oxy-hydroxides) of mobile Cr(III) during upward transport in the weathering profile. The negatively fractionated δ53Cr values measured in the weathering profile relative to the unaltered tonalitic bedrock characterized by a high temperature magmatic inventory Cr isotope signature are consistent with loss of a positively fractionated Cr(VI) pool formed during weathering. The predicted existence of a former, positively fractionated and mobile chromium pool has been experimentally constrained in circumneutral and basic leachates of powdered tonalite bedrock where δ53Cr of + 0.21 to + 0.48‰ was measured. Our results show that mobilization of chromium is effective under highly oxidative conditions, which in well drained sulfide-bearing parent bedrocks potentially lead to both, acid dissolution of sulfide-hosted Cr and redox-promoted mobilization of Cr(VI) from silicates during later stages of weathering under basic pH conditions.