Li isotope geochemical study on weathering of granite in Longnan, Jiangxi Province, South China

Abstract:

Li isotopes have been demonstrated to be a powerful tool in silicate weathering research from the continental scale to that of catchments and weathering profiles. In this study, we focus on Li isotopic behavior during granite weathering and pedogenic processes. Study profiles are developed on a granite intrusion in Longnan, South China, under a subtropical monsoon climate. Two soil profiles (JLN-S4 on the ridgetop and JLN-S3 on the midslope; both 120cm deep) and one weathering profiles (JLN-S1 on the toe slope; 1100cm deep) along a hill slope were selected to represent the granite weathering process in near surface condition. Bulk samples of these profiles were analyzed for major element and Li content, and δ⁷Li value. CIA (chemical index of alteration) values were determined.

The uppermost soil profile (JLN-S4) samples have CIA values between 97.6 and 98.3, with δ⁷Li values from -2.50 to -3.90‰; JLN-S3 profile samples have lower CIA values varing from 94.4 to 96.1 and δ⁷Li values from -0.73‰ to -1.55‰. The weathering profile JLN-S1 at the toe slope has a generally increasing CIA values (in the range of 57.8~92.4) upward in the profile. The δ⁷Li values variation with CIA in JLN-S1, shows a two staged relationship: δ⁷Li increasing from -14.92‰ to 0.66‰ when CIA increases from 57.8 to 74.6 as the first stage (from the bottom to 370cm depth), and no obvious relationship between CIA andδ⁷Li（-4.74~3.62‰） was observed in the second stage (CIA higher than 74.6, 370cm depth above). Li contents in JLN-S4 vary between 3.84 and 15.03 μg/g, in JLN-S3 between 1.43~8.03 μg/g, and in JLN-S1 between 6.95~22.27μg/g. In the first stage defined by δ⁷Li and CIA relationship, Li contents decrease with increasing CIA. However, in the second stage, Li abundance decreases upward at the beginning and turns into an increacing tendency (at 175cm depth) to the surface. Two granite bedrock samples have δ⁷Li value of -0.23 and 0.39‰, and Li contents of 38.58 and 34.22μg/g.

Our study indicates that Li isotope composition of residual material is dominanted by the extent of granite weathering. However complex processes, including adsorption and desorption of clay minerals, fluid flow and interaction, and atmospheric input control the Li isotopic composition of residual materials during deep weathering and pedogenic processes.