Geochemical characteristics of platinum-group elements in polymetallic nodules from the Northwest Pacific Ocean

Abstract

Polymetallic nodules and cobalt (Co)-rich crusts are enriched in platinum-group elements (PGEs), especially platinum (Pt) and may be important sinks of PGEs. At present, little information is available on PGEs in polymetallic nodules, and their geochemical characteristics and the causes of PGEs enrichment are unclear. Here, PGEs of polymetallic nodules from abyssal basin in the Marcus-Wake Seamount area of the Northwest Pacific Ocean are reported and compared with the published PGEs data of polymetallic nodules and Co-rich crusts in the Pacific. The total PGEs (LPGE) content of polymetallic nodules in study area is 258×10−9 in average, markedly higher than that of Clarion-Clipperton Zone (CCZ) nodules (ΣPGE=127×10−9) and lower than that of Co-rich crusts in the Marcus-Wake Seamount (ΣPGE=653×10−9), similar to that of Co-rich crusts in the South China Sea (ΣPGE=252×10−9). The CI chondrite-normalized PGEs patterns in different regions of polymetallic nodules and cobalt-rich crusts are highly consistent, with all being characterized by positive Pt and negative Pd anomalies. These results, together with those of previous studies, indicate that PGEs in polymetallic nodules and Co-rich crusts are mainly derived directly from seawater. Pt contents of polymetallic nodules from the study area are negatively correlated with water depth, and Pt/ΣPGE ratios in nodules there are also lower than those of the Co-rich crusts in the adjacent area, indicating that sedimentary water depth and oxygen fugacity of ambient seawater are the possible important controlling factors for Pt accumulation in crusts and nodules.