Processes controlling the distribution of Ti and Al in weathering profiles, siliciclastic sediments and sedimentary rocks

ABSTRACT The ratio of Ti to Al varies greatly in primary (igneous) source rocks. These elements are considered to be relatively immobile in most weathering regimes, so that TiO2:Al2O3 ratios have been used to investigate the provenance of sediments and sedimentary rocks. In modern weathering profiles that have undergone moderate weathering, Ti:Al ratios remain fairly constant so that siliciclastic sediments derived from them should have Ti:Al ratios like that of the source materials. Under extreme weathering conditions, however, the upper (most weathered) parts of profiles commonly show a marked increase in Ti:Al ratio that is thought to be due to preferential translocation of Al-rich phases. Under steady-state weathering conditions, one particular level of the profile may act as the main source of detrital material and may thus control the Ti:Al ratio of sediments produced from it. In a glacio-fluvial setting (minimum weathering, moderate sorting) there is considerable variation in Ti content but Al is nearly constant. This is due to concentration of Ti-bearing mafic minerals, such as biotite, in the fine fraction. In modern sediments derived from moderately weathered and sorted material, the Ti:Al distribution reflects mixing of Al- and Ti- enriched fine-grained materials with sands that are depleted in these elements. When a plot of TiO2 vs. Al2O3 is constructed for a suite of such sands and muds, the resultant trend differs significantly from that shown by a similar suite of unweathered glacio-fluvial sediments. Comparison of these results with data from sedimentary rock suites suggests that trends shown by plots of TiO2 vs. Al2O3 may provide insight into weathering and depositional history, in addition to their use as provenance indicators.