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

• Published in: Chemical geology 2004-11, Vol.212 (1), p.45-57
• Main Authors: Rudnick, Roberta L., Tomascak, Paul B., Njo, Heather B., Gardner, L. Robert
• Format: Article
• Language: English
• Subjects: Continental weathering; Lithium isotopes; Saprolite; South Carolina
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2004.08.008

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 δ 7Li 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 ( δ 7Li=−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 δ 7Li 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