Authigenic apatite in a fluvial sandstone sequence: evidence for rare-earth element mobility during diagenesis and a tool for diagenetic correlation

Apatite grains in heavy-mineral separates of samples from the Statfjord Formation (Triassic-Lower Jurassic, North Sea), exhibit variably corroded detrital cores and euhedral diagenetic overgrowths. In the overlying marine deposits of the Nansen Formation, apatite grains show no evidence of either corrosion or overgrowths. Compositions of detrital apatite in both formations are indistinguishable, indicating that the source areas responsible for Statfjord and Nansen deposition included similar apatite-bearing lithologies. Apatite corrosion in the Statfjord Formation is considered to be a response to leaching by acidic groundwaters during sediment storage on the fluvial floodplain. Any variations in the degree of corrosion that may have been generated by differences in floodplain residence time have been homogenized by sediment reworking. Laser-ablation microprobe inductively coupled plasma mass spectrometry and wavelength-dispersive electron microprobe analysis indicate that the authigenic apatite contains high abundances (up to 3.5% total) of the rare earth elements (REE), implying a degree of REE mobilization during diagenesis. REE profiles (NASC normalized) for the authigenic apatites are convex-upwards with enrichment of the MREE. The REE are considered to have been derived from dissolution of other REE-bearing heavy minerals (e.g., garnet) during alluvial storage, with possible contributions from dissolved feldspars (which may account for slight positive Eu anomalies) and REEs adsorbed onto clay surfaces. REE mobilization was probably enhanced by fluoride and/or phosphate complexes, although organic complexing related to pedogenic processes is also a possibility. Through the Statfjord Formation, authigenic apatite displays subtle variations in major-element, minor-element, and trace-element chemistry, which may find application as a tool for diagenetic correlation.