Genesis of REY-rich deep-sea sediments in the Tiki Basin, eastern South Pacific Ocean: Evidence from geochemistry, mineralogy and isotope systematics

Abstract

Deep-sea sediments in the eastern South Pacific Ocean were recently found to contain high concentrations of rare-earth elements and yttrium (REY). To understand the genesis of REY enrichment in the sediments, we performed detailed element and isotope geochemical analyses on samples from gravity core S028GC23 recovered from the Tiki Basin in the eastern South Pacific Ocean.The REY-rich sediments in core S028GC23 are dark-brown to black zeolite clays, with REY (∑REY) contents of 1136–2213 ppm (average of 1857 ppm). The ∑REY contents in the sediments are positively correlated with P2O5 and negatively correlated with Ce anomalies. The REY-rich sediments display obvious Ce depletion and positive Y anomalies in the post-Archean Australian shale (PAAS)-normalized REY diagrams. Bioapatite fossils contain the highest REY concentrations (average of 8921 ppm) among the constituents, indicating that they are the primary hosts of REY. The REY enter through the roots and diffuse to the tops in the bioapatite fossils. Fe–Mn micronodules are divided into two different types based on Ce and Y anomalies in the PAAS-normalized REY patterns, and they have average REY concentrations of 781 ppm and 937 ppm, respectively. The phillipsite has low REY contents (average of 106 ppm) and is associated with a low sedimentation rate. REY patterns and Sr-Nd isotopic signatures show that seawater was the main ore-forming fluids during the formation of REY-rich sediments. The sedimentation rate of the sediments calculated by the 230Thex activities was 0.8 mm/Ka. The REY accumulation process can be explained by two-stage fluid-bioapatite fossil interactions and small contributions of Fe-Mn micronodules in an oxidized environment with slow sedimentation rate.