Germanium-silicon fractionation in the weathering environment

We present a detailed study of germanium behavior in the soil weathering environment as an important step toward using the Ge/Si system as a tracer of silicate weathering processes in both modern and ancient environments. Intensely weathered soils developed on Hawaiian basalts have bulk soil Ge/Si ratios 2 to 10 times higher than fresh basalt (e.g., 10 to 25 μmol/mol vs. 2.5 μmol/mol). Soil Ge concentrations increase with Si, and decrease with Fe, suggesting that Ge sequestration is related to accumulation of secondary soil silicates, rather than retention in soil Fe oxy-hydroxides. Sequential extractions of these soils suggest that Ge/Si fractionation takes place by Ge sequestration during the initial precipitation of secondary soil aluminosilicates (principally allophane). Further Si loss and changes in mineralogy as these soils age result in little additional Ge/Si fractionation. Ge/Si ratios in granitic soils and saprolites are strongly influenced by relative weathering rates of primary minerals. Kaolinite has a Ge/Si ratio (5.9 μmol/mol) higher than the plagioclase from which it forms (3.1 μmol/mol), whereas accumulation of primary quartz (Ge/Si 0.5 μmol/mol) prevents granitic soils from attaining high Ge/Si ratios. Laboratory synthesis of allophane confirms that Ge is preferentially partitioned into the solid phase upon precipitation of secondary aluminosilicates from solution.