Trace element abundances and Sr and Nd isotope compositions in bulk sediments from DSDP Legs 6 and 20

Rare earth element and Nd isotopic data for ten representative samples of Lower Cretaceous to Miocene pelagic sediments from the western Pacific indicate a wide range of compositions for sediments being subducted beneath the Mariana and Volcano arcs. All samples are enriched in light rare earth elements and show negative Eu and Ce anomalies. The values of e-Nd range from +0.6 to -7.3. These data are used to calculate the Bulk Western Pacific Sediment (BWPS), which is characterized by low Sr/Nd (10), Ba/La (13), and e-Nd (-5.2) and high 87Sr/86Sr (0.7078) compared to that of Mariana and Volcano arc lavas. This composite sediment is used to refine a mixing model for the origin of Mariana and Volcano arc melts. Some lavas from the northern Mariana Arc have Ba/La higher than that of BWPS, which indicates that a third component is required. The high Ba/La in the mantle source for these lavas is interpreted to result from multiple episodes of fluid fractionation. The mixing model indicates that a minor amount of sediment and a low proportion of metasomatic fluid fluxes the mantle source at a late stage when the subarc mantle is already highly metasomatized. This model also suggests that the mantle source for arc melts is affected more by metasomatic fluids than by melting or bulk mixing of sediments.

Rare earth elements from the Atlantic Sector

We present a Rare Earth Elements (REE) record at decadal resolution determined in the EPICA ice core drilled in Dronning Maud Land (EDML) in the Atlantic Sector of the East Antarctic Plateau, covering the transition from the last glacial age (LGA) to the early Holocene (26 600-7500 yr BP). Additionally, samples from potential source areas (PSAs) for Antarctic dust were analysed for their REE characteristics. The dust provenance is discussed by comparing the REE fingerprints in the ice core and the PSAs samples. We find a shift in REE composition at 15 200 yr BP in the ice core samples. Before 15 200 yr BP, the dust composition is very uniform and its provenance was likely to be dominated by a South American source. After 15 200 yr BP, multiple sources such as Australia and New Zealand become relatively more important, albeit South America is possibly still an important dust supplier. A similar change in the dust characteristics was observed in the EPICA Dome C ice core at around ~15 000 yr BP. A return to more glacial dust characteristics between ~8300 and ~7500 yr BP, as observed in the EPICA Dome C core, could not be observed in the EDML core. Consequently, the dust provenance at the two sites must have been different at that time.

Major, trace element and XRF data for sediments and fish tooth samples, from a shallow marine setting with varying redox conditions (Site U1356, Wilkes Land Margin, E.Antarctica)

Fossil fish teeth from pelagic open ocean settings are considered a robust archive for preserving the neodymium (Nd) isotopic composition of ancient seawater. However, using fossil fish teeth as an archive to reconstruct seawater Nd isotopic compositions in different sedimentary redox environments and in terrigenous-dominated, shallow marine settings is less proven. To address these uncertainties, fish tooth and sediment samples from a middle Eocene section deposited proximal to the East Antarctic margin at Integrated Ocean Drilling Program Site U1356 were analyzed for major and trace element geochemistry, and Nd isotopes. Major and trace element analyses of the sediments reveal changing redox conditions throughout deposition in a shallow marine environment. However, variations in the Nd isotopic composition and rare earth element (REE) patterns of the associated fish teeth do not correspond to redox changes in the sediments. REE patterns in fish teeth at Site U1356 carry a typical mid-REE-enriched signature. However, a consistently positive Ce anomaly marks a deviation from a pure authigenic origin of REEs to the fish tooth. Neodymium isotopic compositions of cleaned and uncleaned fish teeth fall between modern seawater and local sediments and hence could be authigenic in nature, but could also be influenced by sedimentary fluxes. We conclude that the fossil fish tooth Nd isotope proxy is not sensitive to moderate changes in pore water oxygenation. However, combined studies on sediments, pore waters, fish teeth and seawater are needed to fully understand processes driving the reconstructed signature from shallow marine sections in proximity to continental sources. This article is protected by copyright. All rights reserved.

Rare earth elements in standard samples of ocean Fe-Mn nodules and crusts

Contents of rare earth elements (REE) in standard samples of Fe-Mn nodules (SDO-5, 6), Fe-Mn crust (SDO-7), and red clay (SDO-9) have been determined by ICP-MS and instrumental neutron activation analysis. Reproducibility of ICP-MS was 5-6%. These results are discussed and compared with other data. It has been found that distribution of REE in the standard samples of ocean Fe-Mn ores and red clay is highly homogenous.

(Table 1) Platinum group element abundanves in ODP Leg 135 samples

Ocean Drilling Program Leg 135 backarc basin lavas are characterized by anomalously high Au contents (1.0-11.4 ppb) and strongly fractionated relative platinum group element (PGE) abundances (Pd/Ir ratio, approximately 100). The Rh and Ir contents are very low, ranging from below detection (approximately 0.02 ppb) to 0.08 ppb. The Pd and Pt contents range from <0.3 to 4 ppb. Rh, Pd, and Pt values are consistently and significantly higher in Site 836 and 839 samples relative to those from Sites 834 and 835. Major, trace, and rare earth element (REE) data suggest Sites 836 and 839 have a more pronounced arc signature than Sites 834 and 835. No correlation exists between noble metal abundance and indices of alteration or fractionation (e.g., loss on ignition (LOI), Mg#, and Cr or Ni contents), suggesting that measured values and ratios are primary and reflect characteristics of the mantle source. The evaluation of Leg 135 noble metal data with respect to potential mantle-source components is hindered by the lack of data on magmas derived from such sources. However, analyses of the limited available data for the different magma types suggest that the characteristic enrichment of Leg 135 lavas in Au, relative to Pd and Cu, cannot be derived solely from simple MORB-type or ocean-island-type mantle, or mantle depleted by a previous melt extraction event. The Au-enriched signature of the Lau basin lavas could, however, be produced through the addition of a sedimentary component from the downgoing slab. Separation of Au from the PGE occurs within oceanic hydrothermal systems and gold values of the resultant precipitates are 2-3 orders of magnitude higher than other oceanic crustal components. Even small additions of this component from the downgoing oceanic crust to a supra-subduction zone mantle melt could account for the high mean Au/Pd ratios of the Leg 135 samples (Sites 834 and 835, Au/Pd = 5.04; Sites 836 and 839, Au/Pd = 2.26).

Rare earth elements of ROV sample GeoB12338-2 from the Makran accretionary complex

Authigenic carbonates forming at an active methane-seep on the Makran accretionary prism mainly consist of aragonite in the form of microcrystalline, cryptocrystalline, and botryoidal phases. The d13Ccarbonate values are very negative (-49.0 to -44.0 per mill V-PDB), agreeing with microbial methane as dominant carbon source. The d18Ocarbonate values are exclusively positive (+ 3.0 to + 4.5 per mill V-PDB) and indicate precipitation in equilibrium with seawater at bottom water temperatures. The content of rare earth elements and yttrium (REE + Y) determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and solution ICP-MS varies for each aragonite variety, with early microcrystalline aragonite yielding the highest, cryptocrystalline aragonite intermediate, and later botryoidal aragonite the lowest REE + Y concentrations. Shale-normalised REE + Y patterns of different types of authigenic carbonate reflect distinct pore fluid compositions during precipitation: Microcrystalline aragonite shows high contents of middle rare earth elements (MREE), reflecting REE patterns ascribed to anoxic pore water. Cryptocrystalline aragonite exhibits a seawater-like REE + Y pattern at elevated total REE + Y concentrations, indicating higher concentrations of REEs in pore waters, which were influenced by seawater. Botryoidal aragonite is characterised by seawater-like REE + Y patterns at initial growth stages followed by an increase of light rare earth elements (LREE) with advancing crystal growth, reflecting changing pore fluid composition during precipitation of this cement. Conventional sample preparation involving micro-drilling of carbonate phases and subsequent solution ICP-MS does not allow to recognise such subtle changes in the REE + Y composition of individual carbonate phases. To be able to reconstruct the evolution of pore water composition during early diagenesis, an analytical approach is required that allows to track the changing elemental composition in a paragenetic sequence as well as in individual phases. High-resolution analysis of seep carbonates from the Makran accretionary prism by LA-ICP-MS reveals that pore fluid composition not only evolved in the course of the formation of different phases, but also changed during the precipitation of individual phases.

Trace element contents in zircons from oceanic crust

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the field defined by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fingerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid field, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.