The effects of phosphatization on the mineral associations and speciation of Pb in ferromanganese crusts

The older layers of thick ferromanganese (FeMn) crusts from the central Pacific Ocean have undergone diagenetic phosphatization, during which carbonate fluorapatite (CFA) filled fractures and pore space and replaced carbonates. The effects of phosphatization on individual trace metal concentrations, speciation, and phase associations in FeMn crusts remain poorly understood yet may be important to metal enrichment mechanisms, paleoceanography, and extractive metallurgy. This study examines the concentrations, speciation, and mineral phase associations of Pb in phosphatized and nonphosphatized layers within three Pacific Ocean crusts using standard chemical and mineralogical techniques, in addition to bulk X-ray absorption spectroscopy (XAS) and microfocused X-ray fluorescence (μ-XRF) mapping. Our findings challenge the conclusions of previous works, which reported that most Pb in phosphatized crusts was associated with the CFA-dominated residual phase after sequential leaching experiments. These results were interpreted as an indication of Pb transfer from the oxide phases to diagenetic CFA during phosphatization. However, our results reveal an inverse correlation of Pb with Ca and P in the μ-XRF mapping of in situ FeMn crust sections and bulk chemistry. Furthermore, XAS measurements reveal that Pb speciation in bulk phosphatized FeMn crust layers is very similar to nonphosphatized layers, indicating only minor, if any, change in speciation during diagenetic phosphatization. Small differences in the EXAFS spectra for one highly phosphatized layer indicate that minor amounts of Pb may have been altered during phosphatization, but the new Pb-bearing phase was not unequivocally identified. Taken together, our results demonstrate that Pb speciation is not significantly affected by phosphatization.