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.钛（Ti）通常被认为在地表环境中具有惰性，但海洋沉积物成岩作用过程中，钛已被证实可从其原生矿物相再活化至新形成的含钛矿物中。然而，支撑这一转变的机制仍不甚明确。本次研究于华南东攀地区深水相上二叠统大隆组至下三叠统罗楼组的火山灰（钾质斑脱岩）层中，识别出两种自生含钛矿物：锐钛矿与假板钛矿。为阐明钛的成岩再活化机制，我们采用聚焦离子束-扫描电子显微镜（focused ion beam–scanning electron microscopy, FIB-SEM）与高分辨透射电子显微镜（high-resolution transmission electron microscopy, HRTEM）对钾质斑脱岩样品开展分析，以明确矿物学变化特征。锐钛矿颗粒仅以单晶（0.1~4.0 μm）形式存在，或与黏土矿物伴生形成团聚体（0.6~15.0 μm）。化学成分数据显示，钛与钙、钾、钠、铝呈显著正相关，这表明东攀地区钾质斑脱岩中钛的滞留与火山玻璃脱玻化过程中自生二氧化钛在黏土矿物表面的吸附，以及成岩作用期间碎屑斑晶颗粒释钛与蒙脱石伊利石化过程密切相关；其中蒙脱石伊利石化过程主要受控于成岩蚀变阶段的孔隙水化学环境。值得注意的是，针铁矿与假板钛矿是东攀钾质斑脱岩中钛铁矿的两种中间风化产物，可能代表钛铁矿的早期蚀变产物；而自生锐钛矿则主要在早成岩作用的后期阶段沉淀生成。该发现证实假板钛矿可在低温沉积体系中产出。通过电子能量损失谱（electron energy-loss spectroscopy, EELS）线扫描分析，谱图呈现出钛的价态从假板钛矿内部向新形成的针铁矿逐渐降低的变化趋势。通过探究成岩过程中影响钛分布的关键因素，我们推断钛可在氧化性成岩条件下有效活化，并随后以自生纳米级二氧化钛矿物的形式再次沉淀。这意味着在利用钛示踪火山灰的物源与沉积环境时，需谨慎对待。