Rubidium isotope fractionation during intense weathering of basaltic rocks: Implication for tracing weathering intensity

Rubidium isotope system has been advocated as an emerging proxy for silicate weathering intensity, primarily based on systematical isotope fractionations during moderate granite weathering through adsorption-desorption processes by secondary clay minerals. However, the magnitude and mechanism of Rb isotope fractionation during chemical weathering of basaltic rocks (an alternatively important silicate rock in the Earth’s crust), especially under intense weathering conditions, are still lacking. In this study, we present Rb isotope compositions (δ87Rb) from a gabbro weathering profile at Leizhou Peninsula, which forms under tropical humid monsoon climate. The upper saprolites are characterized by a positive linear correlation between the Th (a least-mobile element that can indicate aeolian accretion) and Rb contents, and the consistency of the δ87Rb values (–0.15 ± 0.09‰; 2SD, n = 30) with the Chinese loess (–0.11 ± 0.04‰) and the lower fresh gabbros (–0.11 ± 0.08‰; 2SD, n = 2), suggesting that the Rb is dominantly hosted in extraneous aeolian dusts while the geologic Rb derived from parent rocks has been completely removed during intense chemical weathering. The δ87Rb values of the middle semi-weathered gabbros (from –0.07‰ to 0.83‰) are positively fractionated relative to the fresh gabbros, and show a decreasing trend with increasing weathering intensity, which are mainly caused by progressive desorption of previously adsorbed isotopically heavy Rb from secondary clay minerals. On this basis, we suggest to use detrital δ87Rb records or seawater δ87Rb records reconstructed from reliable marine archives to trace continental silicate weathering intensity on a regional or global scale, respectively.