U-Th-REE-Hf bearing phases in Mediterranean Sea sediments: Implications for isotope systematics in the ocean

The abundance, size and composition of micron-size U-Th-REE-Hf rich phases of marine clayey silt sediments from the Western Mediterranean Sea were determined using an automated scanning electron microscope equipped with an energy dispersive spectrometer. The minerals found in the sediment were monazite, allanite, florencite, xenotime and zircon. The size distribution and chemical composition of each phase were used to determine their contribution to the total content of the sediment in U, Th, REE, Y, Zr and Hf. Zircon accounts for most of the Zr and Hf of the sample. Xenotime and zircon account for most Y and HREE of the sample. However, the high Y-HREE-U-Th content of the zircons analyzed in this study (possibly due to xenotime overgrowth on zircon surface and alteration processes) contrasts with previous works and cannot be extended to any type of zircons. By contrast, only a small fraction (20-30%) of U, Th and LREE (such as Nd) are carried by U-Th-REE-Hf rich minerals (monazite mostly). It reflects the higher alterability of U-Th and LREE bearing phases compared to zircon and xenotime and implies that most U-Th and LREE are present in other phases at lower concentrations (possibly clays, organic matter or Fe-Mn oxides). The different size distributions of the different carriers can contribute to U-Th and LREE/HREE fractionation observed during the transport of these elements. Due to its very high physical and chemical resistance, zircon appears to be the main Hf carrier even in fine grained 'zircon free type' sediments. By storing Lu and possibly releasing extremely radiogenic Hf, xenotime has an unforeseen role in the Lu-Hf systematics in marine sediment. (authors)