Rare-earth elements and isotope ratios at DSDP Leg 82 Holes

We report 48 analyses of rare-earth elements (REE) and 15 143Nd/144Nd and 87Sr/86Sr analyses for basalts from the eight holes drilled during Leg 82. Discrete and distinct REE patterns and 143Nd/144Nd ratios characterize the eight holes, with little variation observed downhole except in Holes 561 and 558, thus suggesting dominantly long-term temporal and large-scale spatial variations in the mantle source of these basalts beneath the Mid-Atlantic Ridge over the last 35 Ma of its spreading activity. There is a good inverse correlation between 143Nd/144Nd and (La/Sm)EF with one exception in Hole 558 (approximately 35 Ma), the latter suggesting a recent (35 Ma) light REE depletion event, perhaps caused by dynamic or fractional melting. Short-term temporal and small-scale spatial mantle source variability is also evident in Hole 561 (approximately 18 Ma), which has rapid fluctuations in REE patterns and 143Nd/144Nd ratios (suggesting rapid transfer of magma from the time of melting) and is evidence contrary to the presence of a well-mixed magma chamber at this particular site and time. The mantle source variations noted can be interpreted within two extreme models. The first model invokes a convecting mantle depleted in large ion lithophile elements (LILE) and containing lumps (or veins) of LILE-enriched material of various shapes and sizes, passively and randomly distributed throughout. A second more restrictive model considers the interaction of fixed mantle plumes and the LILE-depleted asthenosphere flowing towards a migrating Mid- Atlantic Ridge (MAR) axis. With the exception of Hole 558 and the uncertainties of reconstructions of absolute plate movements in the region, the observed variations can be explained by two hot spots; the nearly ridge-centered Azores hot spot (plume) and another hot spot located beneath the African plate that may be affecting the source of basalts currently erupting at the MAR axis at 35°N and which, in the past, would have produced the New England chain of seamounts on the North American plate and (later) the Atlantis-Great Meteor chain on the African plate. Basalts erupted south of the Hayes Fracture Zone have not been affected by either of these two hot spots over the last 35 Ma and appear to have been continuously derived from the LILE-depleted source. Subaxial flow downridge from the Azores plume appears to have started 9 Ma, on the basis of the southward converging V-shaped time-transgressive ridges branching from the Pico and Corves Island, or not earlier than 16 Ma, on the basis of the geochemical results. Variations within Hole 558 remains unexplained by the latter model, unless we hypothesize a third hot spot.

Concentrations of rare earth elements in pore waters of IODP Holes 323-U1345A and 323-U1345B

We studied the diagenetic behavior of rare earth elements (REEs) in a highly productive passive margin setting of the Bering Sea Slope. Site U1345 was drilled during the Integrated Ocean Drilling Program Expedition 323 at a water depth of 1008 m currently in the center of an oxygen minimum zone. Pore water concentrations of fourteen REEs were determined down to ~ 140 meters below the seafloor (mbsf). The REE concentrations were higher in the pore water than the deep seawater, indicating that there was significant liberation from the sediments during diagenesis. There was a major peak at ~ 10 mbsf that was more pronounced for the heavy REE (HREE); this peak occurred below the sulfate-methane transition zone (6.3 mbsf) and coincided with high concentrations of dissolved iron and manganese. At ~ 2 mbsf, there was a minor peak in REE and Mn contents. Below ~ 40 mbsf, the REE concentration profiles remained constant. The Ce anomaly was insignificant and relatively constant (PAAS-normalized Ce/Ce = 1.1 ± 0.2) throughout the depth profile, showing that the Ce depleted in seawater was restored in the pore water. HREE-enrichment was observed over the entire 140 m except for the upper ~ 1 m, where a middle REE (MREE)-bulge was apparent. REE release in shallow depths (2-4 mbsf) is attributed to the release of light REEs (LREEs) and MREEs during the organoclastic reduction of Mn oxides in anoxic sediments. The high HREE concentrations observed at ~ 10 mbsf can be attributed to the reduction of Fe and Mn minerals tied to anaerobic oxidation of methane or, less significantly, to ferromagnesian silicate mineral weathering. The upward diffusion flux across the sediment-water interface was between 3 (for Tm) and 290 (for Ce) pmol/m**2/y.