Extremely enrichment of Li-7 in highly weathered saprolites developed on granite from Huizhou, southern China

摘要

Lithium isotope is potentially useful tracer of continental weathering. However, the factors affecting Li isotope composition in highly weathered saprolites are still largely unclear. In the present study, Li and Nd isotope compositions in saprolites developed on granite from Huizhou, southern China, were analyzed and Li isotope composition in quartz samples separated from the saprolites was determined. The Nd isotope composition of saprolites (epsilon(Nd) =-6.1 +/- 0.4, 1a) was almost identical to that of parent granite (epsilon(Nd) =-5.7), suggesting the eolian deposition in this profile is negligible. The Li-87 value in saprolites varied greatly from-7.7 parts per thousand to +14.0 parts per thousand. Below a depth of 3 m, almost all saprolites were isotopically lighter than the parent granite (+1.0 parts per thousand). However, above 3 m, Li-87 values were higher in saprolites (+2.2%o to +14.0 parts per thousand, average + 7.6 parts per thousand) than in the parent granite and showed a significant increasing trend toward the surface. Moreover, the delta Li-7 value showed a negative correlation with the CIA value below 3 m, but a positive correlation above 3 m. Compared with the parent granite, quartz separates had a higher Li concentration (1.1-28.9 mg/kg, average 9.5 mg/kg) and delta Li-7 value (+12.1 parts per thousand to +13.9 parts per thousand). As weathering progressed, the formation of secondary minerals (such as kaolinite) led to the incorporation of lighter Li-6, which may have contributed significantly to the low Li-87 value in saprolites below 3 m. However, this mechanism could not explain the relative enrichment of heavy Li-7 in the upper layer saprolites. The relative enrichment of quartz may contribute significantly to the increase of delta Li-7 in saprolites. The direct evidence was that Li was abundant and distinctly isotopically heavier in quartz separates. Moreover, quartz content correlated positively with Li concentration (R-2 = 0.90, p < 0.01) and Li-87 value (R-2 = 0.90, p < 0.01) in the upper layer saprolites. The results showed that a 10% increase in Li due to quartz enrichment (delta Li-7 = -+13 parts per thousand) resulted in a +1.3 parts per thousand increase in Li-87 in the saprolites. Our results highlight that relative enrichment of quartz may result in isotopically heavier Li in highly weathered saprolites developed on granite, which may help to explain the higher Li-87 values detected near the surface layer of weathering profiles.