Method Li, Si and Fe P

When rainwater interacts with rocks, a whole suite of chemical reactions takes place, leading to the release of some of the most soluble chemical elements (such as sodium) into solutions, and to the storage of insoluble elements (such as aluminum and iron) into "secondary weathering phases" that form soils. In tectonically quiescent tropical environments these secondary phases have the possibility to accumulate over millions of years and form deep weathering profiles called laterites.
As a consequence, laterites are several-million-years old products of the transfer of chemical elements between rock-forming minerals, waters contained in soil pores, soil-forming phases such as clays and oxides, and possibly organisms living on top of (plants) or within (microbes) the soil. The complexity and entanglement of these transfers make it a formidable challenge to understand and quantify the processes and rates at which laterites have formed over geological time scales.
This complexity can be overcome using the fractionation of the stable isotopes of the elements involved in the processes governing laterite formation. Indeed, each element has specific chemical properties that determine its behavior at the Earth surface (solubility during dissolution of rock-forming minerals, affinity with secondary phases, utility for plant and microbe physiology...). Therefore, all the isotopes of a given element - an isotope being a "variant" of an element, that is characterized by the number of neutrons in its nucleus, hence by its mass, whereas elements are distinct by their number of protons - react almost in the same way during a process. They react almost in the same way because the slight mass difference between two isotopes of an element leads to a small difference in their reactivity, which results in so-called stable isotope fractionation. Isotope fractionation gives rise to differences in the relative abundance of the stable isotopes of an element between different compartments of a soil (minerals, horizons, plants, waters...) that can in turn be used to infer the governing processes and gauge the rates at which these processes occur.

Although the difference in isotope abundance between two compartments is very small (typically between a few % and 0.01%, depending on the element considered), these isotope signatures can be measured very precisely thanks to cutting-edge analytical techniques such as plasma-source mass spectrometry.
MC-ICP-MS Neptune Plus. Credit : PARI Platform(
We use the stable isotope signatures of one metalloid (silicon, Si) and two metals (iron, Fe and lithium, Li) to better understand laterite formation. Silicon is a major element implied in the structure of minerals of rocks and soils, and also a bio-essential element. During its incorporation in clays, the light isotopes of Si (28Si and 29Si) are preferentially incorporated compared to the heavy Si isotopes (30Si). This isotope fractionation is strongest when the clay forms fast, which typically occurs when water percolates quickly through the soil profile. This allows us to establish a link between the isotope signature of clays in the soil, and rainfall at the time of the formation of these clays.
Sketch for the evolution of the studied lateritic profile from the deposition of the Alter do Chão sedimentary formation (35-65 Ma) to present day.
Lithium is a trace element contained in all minerals, that has two isotopes (6Li and 7Li), the lightest of which preferentially participates to the formation of clays and oxides during soil formation. As lithium is highly mobile during water-rock interactions, its concentration in the residual solids is low. The recorded isotope signature in laterites is thus impacted by the isotope fractionation associated with the formation of clays (as for Si isotopes) but also by a non-negligible atmospheric input via rainfall or dust deposition.

The chemical elements in rivers draining these highly weathered provinces can be considered, in first approximation, as the complementary phase of the secondary weathering minerals present in soils. To constrain the current weathering processes acting in Amazonian tropical region, we measured the stable isotope ratio of Li and Si in river waters draining the whole Rio Negro basin.
Sampling map of rivers waters draining the Rio Negro basin.
This watershed is mostly covered by lateritic profiles but podzols, which are soils formed at the ultimate stage of chemical weathering, can be found in the upstream and northern parts of the basin. Lithium and Si isotope compositions defined a clear trend between two contrasted systems. Waters draining podzols and laterites overlying sediments have the same signature than the bedrock, typical of a congruent weathering regime (complete dissolution of the rock without any precipitation of secondary minerals in soils). Rivers draining laterites developed over magmatic rocks (Guyana Shield) are rich in heavy Li and Si isotopes, meaning that formation of secondary minerals is still operating today and thus that laterites can be considered as active geological objects.