Microtextural and Chemical Heterogeneity of Polymetallic Nodules Revealed by X-ray Computed Tomography and Geochemical Mapping: Insights from the Clarion-Clipperton Zone and the Cook Islands EEZ

Polymetallic nodules grow through the successive concentric agglomeration of ferromanganese oxides around a nucleus, incorporating critical trace metals such as Co, Cu and Ni. Individual layers alternate through changes in the relative influence of hydrogenetic and diagenetic processes. Linking the internal growth structures with the elemental distribution is essential to reconstruct and compare the heterogeneous formation history of different nodule localities. Here, we integrate three-dimensional high-resolution X-ray computed tomography (μCT) with geochemical mapping to more effectively compare the microtextural and chemical heterogeneity of nodules from the Clarion-Clipperton Zone (CCZ) with nodules from the Cook Islands Exclusive Economic Zone (CI EEZ). CCZ nodules showed an alternation between hydrogenetic and early-diagenetic growth with micro-scale lateral heterogeneity, including a discontinuous diagenetic dendritic layer reflecting local redox conditions. In contrast, CI EEZ nodules generally showed hydrogenetic growth, with the co-precipitation of hydrogenetic Ti at low Mn/Fe ratios (< 0.5). High Mn/Fe ratios (> 5) could be intimately linked with early diagenetic massive and dendritic textures with relatively higher amounts of Ni and Cu. A Mn/Fe ratio < 2.5 was typically noted for hydrogenetic laminae relatively enriched in Co. Mottled textures with intermediate Mn/Fe ratios were considered to be oxic-early diagenetic in origin and transitional between hydrogenetic and diagenetic conditions. This study demonstrates the pronounced influence of local redox conditions on the growth of polymetallic nodules. The novelty of combining high-resolution μCT and geochemical mapping proved to be indispensable to characterize the often overlooked micro-scale textural and chemical heterogeneity of individual layers in polymetallic nodules.