Dating of anatase-forming diagenetic reactions in Rotliegend sandstones of the North German Basin

Diagenetic conditions controlling authigenic formation of anatase and in situ LA-ICP-MS U–Pb isotope dating of this phase are studied in Upper Rotliegend II sandstones from two wells in NW-Germany. Anatase grew after breakdown of detrital Ti-phases (e.g. ilmenite) although local scale Ti transport and mobilization from detrital clay-hematite coats or condensed hydrocarbons cannot be ruled out. The anatase-forming reaction marks the change from a regime of reducing conditions imposed by first hydrocarbon generation to oxidizing conditions, probably caused by influx of a fluid from evaporitic Zechstein rocks. This change in fluid influx is interpreted as response to enhanced normal faulting and halokinesis during accelerated burial in Triassic times. Isotope data of U and Pb indicate incorporation of crustal common Pb from Mesozoic pore fluids but precisely mark diagenetic growth of anatase at 224.3 + 5.1/− 5.6 Ma in a Tera-Wasserburg plot. Locally, U–Pb isotope signatures are consistent with either additional incorporation of U and Pb from detrital precursor phases or with uptake of uranogenic Pb from hydrocarbons generated prior to 224 Ma. Anatase is shown to be a valuable authigenic phase suitable for U–Pb chronometry of diagenetic events, which appears to be unaffected by protracted burial and temperatures exceeding temperatures of crystallization.