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）通常被认为在表生环境中具有惰性，但已有研究表明，在海洋沉积物成岩过程中，钛可从其原生矿物相向新形成的含钛矿物发生再活化。然而，支撑这一转变的机制仍不甚明确。本研究在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钾质斑脱岩（火山灰）层中，识别出两种自生含钛矿物：锐钛矿与假板钛矿。为厘清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（FIB-SEM）与高分辨透射电子显微镜（HRTEM）对钾质斑脱岩样品进行分析，以阐明其矿物学变化。锐钛矿颗粒仅以单晶（粒径0.1~4.0 μm）或团聚体（粒径0.6~15.0 μm）形式与黏土矿物伴生。化学成分数据显示，钛与钙、钾、钠、铝呈正相关关系，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放钛、蒙脱石发生伊利石化密切相关。后一过程主要由成岩蚀变阶段的孔隙水化学性质所调控。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次生风化中间产物，可能代表钛铁矿的早期蚀变；而自生锐钛矿则主要在早成岩阶段的后期沉淀析出。这一发现证实了假板钛矿可在低温沉积体系中产出。通过电子能量损失谱（EELS）线扫描分析，光谱结果显示钛的价态从假板钛矿内部向新形成的针铁矿逐渐降低。通过探究成岩过程中影响钛分布的因素，我们推断：在氧化性成岩环境下，钛可被有效活化，并随后以自生纳米级二氧化钛矿物的形式再次沉淀。这意味着在利用钛元素示踪火山灰物源与沉积环境时，需格外谨慎。