Massive magnesium depletion and isotope fractionation in weathered basalts

Magnesium isotopes have been shown to fractionate significantly during continental weathering, however, the degree and direction of fractionation varies from one study to the next, and the main factors that control Mg isotope fractionation during weathering have yet to be delineated. Here, we report Mg contents and isotopic compositions for two ~10m deep drill cores through bauxites developed on Columbia River Basalts (CRBs). Samples from these two drill cores have very low MgO contents (0.12-0.25wt.%) and variable but high δ²⁶Mg values (from -0.1 to up to +1.8, the heaviest isotopic composition ever reported for Mg) relative to the average values of fresh CRBs (δ²⁶Mg=-0.23±0.07 and MgO=5.9wt.%). These features reflect the near complete loss of Mg in the isotopically heavy regolith. The most highly weathered bauxites at the tops of the profiles have the lowest δ²⁶Mg values, reflecting the addition of isotopically light eolian dust. Excluding these samples, δ²⁶Mg in bauxites displays a weak, positive correlation with gibbsite abundance, suggesting that gibbsite preferentially retains ²⁶Mg in the bauxites. The integrated Mg isotopic fractionation factors between bauxite and fluid (δ²⁶Mg_{bauxite-fluid}), inferred from Rayleigh fractionation calculations, vary from 0.05‰ to 0.4‰, which is similar to the range inferred from previous studies of igneous rock weathering profiles, though none of these previous studies found the extremely heavy δ²⁶Mg and massive Mg depletion seen here. Collectively, studies of weathering profiles suggest loss of isotopically light Mg from the continents, which must contribute to the very light δ²⁶Mg seen in river water and seawater.