Formation of authigenic titania during the alteration of volcanic glasses in modern deep-sea environments

Abstract

Titanium (Ti), a typical rock-forming element, is traditionally considered to be lithophile, incompatible, and fluid-immobile, and is recognized as a significant proxy for understanding the oceanic environments, such as the content of terrestrial materials in modern marine sediments. However, recent investigations have revealed the production of various titania minerals through the alteration of volcanic rocks. Considering the occurrence of abundant volcanic rocks (e.g., volcanoclastics, seamounts, and oceanic crusts) in the oceanic environments, the activities of Ti may have been underestimated. Our study provides novel findings on the occurrence and growth mechanism of authigenic titania in the completely altered volcanic glasses collected from Line and Marshall seamounts by using multiple microanalytical methods. Both specimens primarily consist of mixed-layer illite/smectite (I/S), phillipsite, and carbonate-bearing fluorapatite, revealing that the volcanic glasses underwent palagonization processes. Scanning electron micrographs reveal numerous spherical Ti-rich shells with thicknesses of 450–1000 nm, enveloping spherical mixed-layer I/S aggregates with diameters of dozens to hundreds of microns. From the center to the edge of the spherule-shell structures, the concentration of Ti significantly increased. Further analysis using transmission electron microscopy confirms the presence of authigenic Fe-bearing titania particles within these Ti-rich shells. These titania shells likely form as a result of the migration of dissolved Ti(IV) ions and titania nanoparticles, which fill, nucleate, and aggregate at the interfaces of microspherules originally present in the volcanic glasses, ultimately coalescing into micron-sized particles through oriented attachment. Additionally, numerous titania nanoparticles, with aggregate sizes of 4–32 nm, were observed in the pore spaces of the mixed-layer I/S aggregates, likely be those trapped during Ti migration. Brookite is the dominant titania phase in both the titania shells and the smectite pores, with rutile and anatase occasionally present in the titania shells, implying the continuous mineral phase evolution during the growth of titania particles. The occurrence and distribution of titania suggest short-distance migration of Ti during palagonization processes. These findings offer new insight into the brookite formation and the Ti mobilization in basic and oxygen-rich marine environments, which is crucial for understanding the marine geochemical behaviors of Ti.