Gallium isotope constraints on the intense weathering of basalt

Abstract

Gallium (Ga) isotopes are potentially advantageous for characterizing the surficial biogeochemical cycles of Ga and tracking the geochemical behavior of the monoisotope element aluminum (Al) during chemical weathering. To test the potential of Ga and its isotopes to trace continental weathering, we studied Ga isotope compositions of a strongly-weathered latosol profile developed from basalt on the Leizhou Peninsula, Guangdong Province, South China. In the profile, δ71GaSRM-994 values of latosol samples decrease from ∼0.69‰ in unweathered basalts at the bottom to ∼0.51‰ in shallow sections, while the Ga concentration increases from ∼19.9 µg/g in the bedrock to ∼42.7 µg/g in soil, indicating significant enrichment of Ga and its isotope fractionation triggered by weathering processes. Bulk samples analysis and sequential leaching of selected weathering products suggest that the majority of Ga in these samples occurs in the crystal lattice of Al-rich and Fe-rich minerals, with the light Ga isotope (69Ga) enriched in latosol samples relative to the coexisting solution. Interestingly, our results show δ71Ga values of latosols display a bigger variation (about 3–4 times of its analytical uncertainty) than their Ga/Al ratios (about 1.5 times of its analytical uncertainty) throughout the profile, indicating δ71Ga values may be a more sensitive proxy to track the loss of Al3+ and involvement of Fe3+ during weathering process. Based on a simple Rayleigh model, the upper limit of Ga isotope fractionation between solution and weathered basalt (Δ71Gasolution-weathered rocks) is estimated to be ∼1.50‰, implying that heavy Ga isotopes may be enriched in surface river systems. This study highlights the potential of Ga elemental and isotope geochemistry to trace continental weathering and global Ga cycling.