Extreme lithium isotopic fractionation during continental weathering revealed in saprolites from South Carolina

The lithium concentration and isotopic composition of two saprolites developed on a granite and diabase dike from South Carolina have been measured in order to document the behavior of lithium isotopes during continental weathering. Both saprolites show a general trend of decreasing δ7Li with increasing weathering intensity, as measured by both bulk density and the chemical index of alteration (CIA). The saprolite developed on the granite is isotopically lighter than the fresh igneous rock (δ7Li=−6.8‰ to +1.4‰ vs. +2.3‰, respectively), and is generally depleted in lithium. These observations are consistent with leaching of lithium via Rayleigh distillation during progressive weathering; most saprolites fall on a Rayleigh distillation curve corresponding to an apparent fractionation factor (α) of 0.997. However, two samples have higher lithium contents than the fresh granite and thus point to additional processes affecting lithium in the saprolite (e.g., sorption of lithium on to clay minerals). The saprolite profile developed on the diabase dike shows highly variable δ7Li values, ranging down to extremely light compositions (−20‰). Previous work has identified a chemical and mineralogical discontinuity at a depth of 2 m, but our lithium data show a marked discontinuity at 6 m depth. Saprolite samples at or above 6 m depth are highly weathered (CIA=88–95), depleted in lithium (having <50% of the original diabase lithium) and isotopically light (−10‰ to −20‰ vs. −4.3‰ for the unweathered diabase). Most of the data are consistent with leaching of lithium via Rayleigh distillation during intense weathering, with apparent α values of 0.995 to 0.980. Samples experiencing lower apparent α values tend to have higher kaolinite/smectite ratios, suggesting a mineralogical control on isotopic fractionation. However, the lightest sample (at −20‰) is only slightly depleted in lithium and would require extremely low α values to explain via Rayleigh distilliation. This extreme composition remains enigmatic. Saprolite samples below 6 m have highly variable δ7Li (−5‰ to −14‰) and, importantly, lithium concentrations that are higher than that of the unweathered diabase (up to 2.4 times the concentration of the fresh diabase). These deeper saprolites thus cannot be explained by Rayleigh distillation. A positive correlation between δ7Li and Li concentration suggests these samples formed by mixing between an isotopically light saprolite and heavy, groundwater lithium, and we thus suggest that the 6-m discontinuity may mark the position of a paleo water table.