Fossil Bioapatites with Extremely High Concentrations of Rare Earth Elements and Yttrium from Deep-Sea Pelagic Sediments

Detailed geochemical and mineralogical insights into
some of the
richest rare earth elements and yttrium (REY)-containing bioapatites
from ocean-floor sediments have been provided by combining laser ablation
inductively coupled plasma mass spectrometry (LA-ICP-MS), electron
probe microanalysis (EPMA), Rietveld X-ray diffraction analysis, and
Ce L3-edge high energy-resolution X-ray absorption near
edge structure (HR-XANES) spectroscopy. Bioapatites at 1.94 and 4.70
m below the seafloor (mbsf) of the Clarion-Clipperton Zone (CCZ) of
the Pacific Ocean have 26,600 (RSD = 15.7%, n = 20)
and 30,300 (RSD = 14.6%, n = 10) mg/kg (mg/kg) total
REY, respectively, and bioapatites at 2.28 and 6.95 mbsf of the Peru
Basin have 15,500 (RSD = 15.6%, n = 20) and 15,700
(RSD = 17.8%, n = 29) mg/kg total REY, respectively.
All bioapatite specimens have a variety of isomorphic substitutions
in all atomic positions of the crystallographic structure. The average
crystallochemical formula of bioapatites at 6.95 mbsf of the Peru
Basin is [(PO4)2.71­(SiO4)0.04­(CO3,SO4)0.25]­[Ca4.57­Na0.29­Y0.04]­[F0.87­Cl0.21]. All other substituents are below
0.04 atoms per formula unit. HR-XANES provides the first direct evidence
for trivalent Ce in sediment apatites. The strong negative geochemical
anomaly of Ce in fossil bioapatites is well explained by the occurrence
of four valent Ce-MnO2 and CeO2 within the sediment
and in seafloor ferromanganese nodules.