Diagenetic mobilization of Ti and formation of brookite/anatase in early Cambrian black shales, South China

Titanium (Ti) is typically hosted in detrital minerals in marine sediments and has long been considered to be immobile during diagenesis. In this study, the authigenic titania minerals, brookite and anatase, are observed in early Cambrian carbonaceous shales from the Meishucun and Zhajin sections of South China, respectively. Black shales in the Meishucun section have total organic carbon (TOC) contents from 1.6 to 3.9 wt% and HI (hydrogen index) values from 3.8 to 20 mg HC/g TOC, whereas black shales in the Zhajin section have much higher TOC (7.1–15.6 wt%) but lower HI (<2.0 mg HC/g TOC) and contain abundant bitumen (~3 vol%). Brookite in black shales from the Meishucun section crystallized invariably along cleavages of detrital biotite. This intimate association suggests that the Ti required to form titania minerals was derived from detrital biotite and that Ti was mobilized only on a nano- to micro-meter scale. In contrast, anatase aggregates in black shales from the Zhajin section are intergrown with bitumen. It is proposed that Ti in these shales was mobilized in low pH organic-rich fluids and, subsequently, preferentially precipitated as anatase with increasing pH. The mobilization of inert Ti in black shales is indicative of a possible diagenetic or post-diagenetic elemental redistribution in carbonaceous rocks. Our study further demonstrates that proxies based on geochemical component of black shales can be employed to estimate redox state of ancient oceans, but must be applied with caution.