Germanium isotopes and Ge/Si fractionation under extreme tropical weathering of basalts from the Hainan Island, South China

•Heavy Ge isotopes are preferentially leached away during basalt weathering.•Ge concentration, Ge/Si ratio and δ74Ge value of the solid weathering products are positively correlated.•The fractionation factor (Δ74Gesolid-dissolved) was estimated to be −1.38 ± 0.28‰ (2σ).•Solid weathering products are a sink for light Ge isotopes in surficial environment.