Titanium mobility and formation of anatase/pseudobrookite during diagenesis of deep-marine K-bentonites

Titanium (Ti) is generally considered immobile in surface environments, but during diagenesis of marine sediments it has been shown be remobilized from its primary mineral phase to neoformed Ti-bearing minerals. However, the mechanisms underpinning this transformation remain poorly understood. Here, we identified two authigenic Ti-bearing minerals, anatase and pseudobrookite, within volcanic ash (K-bentonite) beds from the deep-water Upper Permian Dalong Formation to the Lower Triassic Luolou Formation at Dongpan in South China. To understand the diagenetic remobilization of Ti, we examined the K-bentonite samples by focused ion beam–scanning electron microscopy (FIB-SEM) and high-resolution transmission electron microscopy (HRTEM) to elucidate the changes in mineralogy. Anatase particles exclusively exist as single crystals (0.1-4.0 μm) or as agglomerates (0.6-15.0 μm) in association with clay minerals. Chemical compositional data show positive correlations of Ti with Ca, K, Na, and Al, which is consistent with the retention of Ti in Dongpan K-bentonites being related to adsorption of authigenic titania on clay minerals during devitrification of volcanic glass as well as to the release of Ti from detrital phenocryst grains and smectite illitization during diagenesis. The latter process was primarily mediated by porewater chemistry during diagenetic alteration. Notably, goethite and pseudobrookite are two intermediate weathering products of ilmenite in the Dongpan K-bentonites which may represent early alteration of ilmenite, whereas authigenic anatase may precipitate mainly during a later stage of early diagenesis. This finding supports that pseudobrookite can occur at low-temperature sedimentary system. Through a line scan with electron energy-loss spectroscopy (EELS), the spectra show a trend indicating that the valence state of Ti decreases from the interior of pseudobrookite to the newly formed goethite. By exploring the factors affecting Ti distribution during diagenesis, we infer that titanium can be effectively mobilized under oxidizing diagenetic conditions and subsequently reprecipitated as authigenic nano-sized TiO₂ minerals. This means that caution must be taken when using Ti to study the provenance and depositional environment of volcanic ash.