Effects of phosphatization on the mineralogical and geochemical composition of marine ferromanganese crusts from the JiaXie Guyot in the Western Pacific: Constraints from high resolution analysis

The marine crusts comprise ferromanganese sediments formed through the precipitation of dissolved elements in seawater. These crusts serve as a record elements signal from paleo-ocean and are extensively utilized in paleo-oceanography. Previous researches have examined the chemical composition of the phosphatized crust layers using powder chemistry and leaching experiments. However, there exist divergent viewpoints regarding the influence of phosphatization on element behavior. In this study, we employed a combination of analytical techniques including scanning electron microscope (SEM), powder X-ray diffractometer (XRD), TESCAN Integrated Mineral Analyzer (TIMA), laser ablation and inductively coupled plasma time of flight mass spectrometry combined system (LA-ICP-TOF), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). These methods were utilized to achieve mineral phase quantification, precise depiction of element distribution, and in-situ profile analysis of elements within the phosphatized crust layers from the JiaXie Guyot in the Western Pacific. The mineral phase analysis shows enormous carbonate fluorapatite (CFA) precipitate, which replace calcite in the old phosphatized layer. Additionally, vernadite experiences partial dissolution. Furthermore, a small amount of hematite, goethite, todorokite, zeolite, barite, as well as trace amounts chalcopyrite, sphalerite, galena, ilmenite, and pyrite are formed. The element distributions suggest potential outward migration of Fe, V, Zr, Zn and Cu, while Ce and Pb are locally enriched. In the phosphatized crust, some elements exhibit depletion in the old crust layer compared to the young crust layer, with Fe > Si > V>Zn > Ti ≥ Pb, while others are enriched in the order crust layer, such as Y>Pd > Ce > Pt. The study proposes that FeOOH·nH₂O and associated adsorbed elements like Si, Ce, Y, V, Zn, Ti and Pb dissolve in vernadite and migrate outward during phosphatization, with weak phosphatization influence on Mn oxide. In addition, the phosphate-rich seawater might contribute additional elements such as Y, Ce and Pd to the crust layers. Thus, this study highlights the elemental distributions, migration patterns observed, and significance of presence for critical metals between old and young crust layers from the influence of phosphatization based on high resolution analysis. Particularly, it acknowledges the contribution of phosphate-rich seawater to the elemental composition of crust layers.