Late Cretaceous to Miocene Phosphatization of a Co-Rich Fe–Mn Crust from Govorov Guyot, Western Pacific: Constraints from Calcareous Nannofossils and U–Pb Dated Carbonate Fluorapatite

Cobalt-rich ferromanganese (Fe–Mn) crusts from the Magellan Seamount Trail (MST), western Pacific Ocean, record long-lived hydrogenetic mineralization interrupted by dissolution, sedimentation, phosphatization, and renewed crust growth. However, age assignments for phosphatized early crust layers remain uncertain because calcareous nannofossils commonly occur in carbonate fluorapatite (CFA) veinlets, pores, cavities, and fractures rather than in the primary Fe–Mn oxyhydroxide matrix. Here, we combine calcareous nannofossil biostratigraphy with in situ U–Pb LA-ICP-MS dating of CFA to constrain the multistage history of a four-layer Co-rich Fe–Mn crust from the eastern slope of Govorov Guyot. The section comprises, from bottom to top, Layer I-1, sublayer I-2b, and Layers II, and III. Calcareous nannofossil assemblages indicate Late Paleocene–Early Eocene fossil-bearing phosphatized carbonate material in Layer I-1, a Late Oligocene–Early Miocene assemblage in sublayer I-2b, Middle Miocene carbonate material in the lower part of Layer II, a Late Miocene–Early Pliocene assemblage in the II–III transition, and Pliocene–Pleistocene assemblages in Layer III. This age sequence in the young part of the crust is consistent with the established MST chronostratigraphic and biostratigraphic scheme. U–Pb dating of CFA reveals Late Cretaceous and Miocene phosphatization events. CFA from the phosphatized carbonate substrate beneath Layer I-1 yielded an age of 85.8 ± 4.7 Ma, consistent with the previously documented Turonian–Coniacian phosphatization event involving the oldest known Pacific CFA at Govorov Guyot. CFA veinlets within Layer I-1 yielded ages of 75.3 ± 2.6 Ma and 73.1 ± 4.9 Ma, documenting Late Cretaceous phosphatization of the old Fe-Mn crust section. CFA from the phosphatized sublayer I-2b yielded an age of 15.8 ± 2.3 Ma, recording an additional Early–Middle Miocene phosphatization event. The dated CFA veinlets demonstrate that the Fe–Mn matrix of Layer I-1 predates the 75–73 Ma phosphatization event and is therefore ~20 Myr older than the maximum age inferred from calcareous nannofossil biostratigraphy. The chronostratigraphic scheme and generalized section of MST Co-rich Fe–Mn crusts should therefore be complemented by a texturally controlled U–Pb chronology of phosphatization events, particularly for the Late Cretaceous crust succession.