Cobalt-Rich Fe-Mn Crusts in the Western Pacific Magellan Seamount Trail: Geochemistry and Chronostratigraphy

Synthesis of published and new data from Govorov and Kocebu guyots provides geochemical and chronostratigraphic constraints on hydrogenetic cobalt-rich Fe-Mn crusts from the Western Pacific Magellan Seamount Trail (MST). The history of the crusts began about 65–60 Myr ago, when the relict layer R was deposited in the Campanian–Maastrichtian and Late Paleocene along the shores of guyots. The growth of the old-generation crusts continued in the Late Paleocene–Early Eocene (Layer I-1) and in the Middle–Late Eocene (Layer I-2) in a shallow-water shelf environment. The younger layers of crusts formed in the Late Oligocene–Early Miocene (Layer I-2b), Miocene (Layer II), and Pliocene–Pleistocene (Layer III) at depths about the present sealevel. The precipitation of Fe and Mn oxyhydroxides from seawater was interrupted by several episodes of dissolution, the longest one between the old and young layers of Fe-Mn crusts (from 38 to 26.5 Ma). Fe and Mn oxyhydroxides in the crusts were affected by two global events of phosphogenesis in the Pacific: Late Eocene–Early Oligocene, from 43 to 39 Ma (Layers R, I-1, I-2) and Late Oligocene–Early Miocene, from 27 to 21 Ma (Layer I-2b). The trace element patterns in different layers of the Fe-Mn crusts are grouped using factor analysis of principal components (varimax raw) into four factors: (1) + (all REEs except Ce and La); (2) +(Ce, La, Ba, Mo, Sr, Pb); (3) +(Zr, Hf, Nb, Rb, As)/-Pb; (4) +(U, Th, Co, As, Sb, W)/-Y. The factor score diagrams highlight fields which are especially contrasting for Layers I-1, I-2 and II+III according to factors 2 and 4. Consistent REE and Y variations in Layers I-2b→II →III of the crust from Pallada Guyot correlate with gradual ocean deepening between the Late Oligocene–Early Miocene and Present when the MST guyots were submerging. Large variations in trace element contents across coeval layers may be due to hydrodynamics of currents on the guyot surfaces. Furthermore, the geochemistry of the crusts bears effects from repeated episodes of Cenozoic volcanism in the MST region of the Pacific Plate. Higher contents of Nb, Zr, As, Sb, and W in the younger layers II and III may result from large-scale volcanism, including Miocene eruptions of petit-spot volcanoes.