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

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 849Ti values in the saprolites range from -0.068%o to +0.076%o, with an average of 0.026%o, which is slightly lighter than the unaltered basalt (0.052 +/- 0.051%o). A significant Ti isotope fractionation exists between extracted crystalline Fe (hydr)-oxide phases and residual phases, which is up to 0.6%o. The residual phases (they account for 72% to 94% of the Ti) have light Ti isotope compositions (-0.165%o to 0.043%o), inherited from the primary weather-resistant ilmenite (-0.200 +/- 0.055%o), whereas the crystalline Fe (hydr)oxides (5% to 26% of the Ti) have heavy 849Ti values, ranging from 0.108%o to 0.540%o, transferred from the primary pyroxene phenocrysts (0.176 +/- 0.039%o) and groundmass (0.153 +/- 0.057%o). 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 849Ti gaps among secondary Ti-bearing minerals, we emphasise that the application of Ti isotopic compositions to sediment provenance studies should be undertaken with caution.