Stratigraphy and geochemistry of Co-rich ferromanganese crusts from the Magellan seamount cluster: insights into substrate influence and growth conditions from core drill sampling

Cobalt-rich ferromanganese crusts (CFCs) are potential future sources of strategic metals. This study presents textural and geochemical profiles of CFCs drill cores from KC-8 and KC-9, ~270 km apart within the Magellan Seamount cluster, Western Pacific. CFCs grew on different substrate rocks: calcareous conglomerate, calcareous rock, basaltic breccia, and basalt. Drill core sampling enables classification by substrate type with high spatial precision. Structural and geochemical analyses assess spatial and temporal variations in growth conditions and substrate influence. Four layers (1 to 4) with distinct textures and geochemistry are identified. Upper layers (1 and 2) show minimal phosphatization whereas lower layers (3 and 4) are intensely phosphatized. Under microscopic observation (scales of a few hundred µm), both upper layers (1 and 2) exhibit a stromatolite-like columnar growth pattern. In Layer 1, however, the columns are less distinct and occur over shorter intervals than in Layer 2. Layers 3–4 are chiefly laminated; in Layer 3, voids are infilled with carbonate fluorapatite (CFA), while in Layer 4, CFA occurs sporadically, aligned parallel to laminations or along fractures. Across all four substrate types, CFCs exhibit broadly consistent depth-dependent geochemical profiles and enrichment/depletion patterns for major, minor, and trace elements, as well as consistent REE anomalies. Older phosphatized layers have higher Mn, lower Fe, and greater Ni, Zn, Mo, Cu, and REEs, while Co remains similar. Gd and Ce show persistent positive anomalies, while Y is positive in lower and negative in younger non-phosphatized layers. These trends reflect phosphatization in lower layers and increased input of detrital sediment during upper layer formation. Stratigraphic and geochemical data suggest that Magellan Seamount CFCs formed under regionally consistent elemental fluxes and phosphatization regardless of substrate, while local growth patterns such as crustal thickness and continuity were controlled chiefly by detrital sediment input and substrate stability or lithification.