Germanium-Silicon Fractionation During Weathering of Basalt and Granite: Examples from the Tropics

Silicate weathering processes and terrestrial Si cycling fractionate Ge/Si ratios, leading to elevated ratios in soils and correspondingly low ratios in streamwaters. We have studied weathering at tropical sites developed on basaltic (Hawaiian Islands) and granitic (Luquillo, Puerto Rico) parent materials to elucidate the processes controlling Ge/Si fractionation in soils. Ge/Si ratios in soils developed on Hawaiian basalt (molar Ge/Si ∼2.5\times10^-6) range from 2.5 to 35\times 10^-6. Young ( ∼2 ka) soils that have lost little Si (relative to immobile Nb) are relatively unfractionated, while most older soils have Ge/Si ratios ∼8 to 10\times 10^-6. We attribute this fractionation to partitioning of Ge into secondary aluminosilicates (allophane). Old, highly weathered soils that have lost >90% of initial Si can have Ge/Si ratios up to 35\times 10^-6. These extreme Ge/Si ratios are influenced by a Ge-enriched non-silicate secondary phase, tentatively identified as a Ti-oxide. Although authigenic marine Fe-oxyhydroxides clearly scavenge Ge from seawater, our study of a soil redox gradient on Maui shows that Ge is not mobilized by reductive dissolution of pedogenic Fe-oxyhydroxides. Secondary Fe-minerals apparently play an insignificant role in Ge/Si fractionation in soils. Ge/Si ratios in granitic soils are also elevated relative to parent material but display strong mineralogical control. Individual primary minerals in the quartz diorite Rio Blanco stock in Puerto Rico (Ge/Si = 2.0\times 10^-6) range from 0.5\times 10^-6 (quartz) to 6.6\times 10^-6 (hornblende). Incongruent weathering of plagioclase (Ge/Si = 1.5\times 10^-6) strongly partitions Ge into kaolinite (Ge/Si = 4.9 to 6.1\times 10^-6.) Soil and saprolite Ge/Si ratios range from 2.6 to 3.6\times 10^-6 reflecting relative amounts of residual quartz and biotite, and neoformed kaolinite. Streamwater Ge/Si ratios at the Hawaii and Puerto Rico sites are almost always lower than local bedrock, and vary between 0.2\times 10^-6 and 2.3\times 10^-6 reflecting sampling of distinct Si sources by changing hydrologic flowpaths. At the Puerto Rico site, low Ge/Si ratios at baseflow reflect Si released by incongruent weathering of plagioclase to form kaolinite. Increasing Ge/Si during storm flow reflects Si released from weathering of biotite and kaolinite in shallow soils. In Hawaii, this pattern is overprinted by Si cycled through plant phytoliths, which carry low Ge/Si ratios ( ∼0.1 \times 10^-6.) Our results demonstrate that Ge/Si can be a useful tracer of both silicate weathering processes and the terrestrial biological silica cycle.