Mass-dependent fractionation of titanium stable isotopes during intensive weathering of basalts

Abstract

The titanium (Ti) isotope compositions of bulk saprolites, parent-rock minerals, extracted crystalline Fe (hydr)-oxide phases and residual phases (clay minerals and Fe-Ti oxides) from the Neogene tholeiitic basalt weathering profile on Hainan Island, China, were measured in order to (1) test the hypothesis that Ti isotope compositions are immune to water–rock interactions and (2) elucidate Ti isotope fractionation mechanisms during extreme weathering processes. Relative to the OL-Ti isotope standard, the δ49Ti values in the saprolites range from −0.068‰ to +0.076‰, with an average of 0.026‰, which is slightly lighter than the unaltered basalt (0.052±0.051‰). A significant Ti isotope fractionation exists between extracted crystalline Fe (hydr)-oxide phases and residual phases, which is up to 0.6‰. The residual phases (they account for 72% to 94% of the Ti) have light Ti isotope compositions (−0.165‰ to 0.043‰), inherited from the primary weather-resistant ilmenite (−0.200±0.055‰), whereas the crystalline Fe (hydr)-oxides (5% to 26% of the Ti) have heavy δ49Ti values, ranging from 0.108‰ to 0.540‰, transferred from the primary pyroxene phenocrysts (0.176±0.039‰) and groundmass (0.153±0.057‰). The Ti isotope composition of this weathering profile was controlled by the formation and dissolution of titaniferous minerals. Given that physical sorting as a result of long-distance transport and deposition changes the mineral assemblages of sediments and combined with significant δ49Ti gaps among secondary Ti-bearing minerals, we emphasise that the application of Ti isotopic compositions to sediment provenance studies should be undertaken with caution.