Extreme magnesium isotope fractionation during bauxite formation on the Columbia River Basalts

The behavior of magnesium isotopes during intense weathering of continental basalt is investigated by analyses of two ~10 m deep drill cores through bauxite developed on Columbia River Basalts (CRBs) in western Oregon and Washington, United States. XRD analyses reveal that these cores consist of gibbsite, hematite, +/- halloysite, kaolinite, goethite and maghemite; quartz, which is not present in fresh basalt, occurs only at the top of the cores and its abundance decreases progressively with depth; no quartz is observed below 5 m depth in either core. Both profiles display strong Mg depletion (up to 99%) relative to fresh basalt and one profile shows re-enrichment of Mg near the surface. δ²⁶Mg values in bauxites are extremely high (up to +1.7) relative to the fresh basalts, which have mantle-like δ²⁶Mg of -0.24 ± 0.07. The Mg isotopic fractionation in these bauxites is unlikely to be caused by kinetic fractionation via chemical diffusion (as suggested for lithium isotopes for a different weathering profile by Teng et al. (1)) because Richter et al. (2) found no measureable Mg isotopic fractionation associated with Mg diffusion in water. Moreover, due to the intense weathering, Mg isotopic fractionation in these drill cores should not be influenced by dissolution of basalts. Therefore, it is likely that the observed extreme Mg isotopic fractionation is associated with secondary mineral formation. However, δ²⁶Mg tends to lower values towards the surface in both cores, opposite the trend that is expected to be produced by progressive leaching of the basalt accompanied by secondary mineral formation. Both the presence of quartz and less radiogenic Nd isotopic compositions at the tops of the profiles suggest that eolian material has been added to the top few meters of these weathering profiles, causing the Mg isotopic composition to be lighter at the surface. Moreover, both Mg concentration and δ²⁶Mg in bauxites influenced by eolian addition show correlations with quartz contents. Overall, δ²⁶Mg is positively correlated with gibbsite contents in bauxites, suggesting gibbsite controls the very heavy δ²⁶Mg values in these drill cores. Excluding bauxites influenced by eolian addition, we model Mg behavior through Rayleigh distillation and infer fractionation factors between bauxite and fluid (Δ²⁶Mg_{bauxite-fluid}) from 0.1 to 0.35‰. Collectively, these data indicate that water-soluble elements such as Mg experienced extreme leaching during weathering, accompanied by significant isotopic fractionation in gibbsite. There has also been mass addition at the surface that lowered δ²⁶Mg towards values more typical of the original basalt.