Geochemistry of sands and sandstones in ODP Leg 126 sites

During Ocean Drilling Program Leg 126, six sites were cored in a young backarc rift basin and its flanks (rift onset 1.1-3.56 Ma) and in the forearc basin of the Izu-Bonin Arc. In the backarc area, strata are younger than about 4.5 Ma, whereas in the forearc, ages are about 0-31 Ma in sections punctuated by important Miocene unconformities. Bulk chemical analyses of volcaniclastic turbidite sands and sandstones, derived directly from the arc, were obtained from 271 atomic absorption analyses (major elements), 253 XRF analyses (trace elements) and 16 ICP-MS analyses (trace and rare-earth elements). Of the 271 samples, 78 come from the backarc area and the remainder from the forearc. The sands and sandstones reflect the igneous compositions of their sources. Most are formed of materials derived from subalkaline, low-K andesites, and dacites, although compositions range from basalt to rhyolite. Basic and acid andesites are predominant in Oligocene rocks; in contrast, Pliocene-Pleistocene sediments were derived from acid andesitic to rhyolitic sources. The oldest sandstones, estimated to have an age of about 31 Ma, were derived from an arc tholeiitic, not boninitic, source. The 26-31 Ma sandstones furthest to the north, at Sites 787 and 792, have higher relative concentrations of Ti, Zr, and Y than do those at southern Site 793. Data from younger samples indicate that, for more than 30 m.y., the average composition of volcaniclastic sediments and volcanism near Aoga Shima was more basic than to the south, near Sumisu Jima. Using the sandstones as igneous proxies, we conclude that magmas erupted along the arc have become more depleted in light-rare-earth elements (LREE) with time. There was a major change in rare-earth-element (REE) concentrations in the late Oligocene, from essentially flat patterns (normalized La/Yb about 1-1.5) to LREE-depleted patterns (normalized La/Yb about 0.5). At the same time, Zr/Y ratios decreased from about 2-4 to about 1.5-2.5. These changes may reflect a shift in provenance, or changes in the composition of the mantle wedge beneath the arc. In the backarc area, lithic clasts and glass shards of rift-facies basalt are present in sediments as old as 2.35-3.15 Ma. Two samples of mafic sand from the backarc basin have flat REE patterns (normalized La/Yb about 1.0), like some of the <1-Ma rift lavas and unlike pre-rift sand and sandstone samples. These possibly represent the local effects of sedimentary mixing of detritus from arc and backarc eruptions because no evidence from the arc itself exists to suggest a recent change in the REE content of magmas.

Contents of P2O5 and REE in phosphorites from shelves and Pacific seamounts

Recent phosphorites from the Namibian shelf are characterized by low REE contents, depletion in REE compared to host sediments and sharp deficiency of lanthanum and europium. In Late Quaternary and Pre-Quaternary phosphorites from ocean shelves REE contents and patterns in general are the same as in host sediments. Phosphorites from seamounts are enriched in REE compared to shelf phosphorites and their patterns are close to one of seawater. Behavior of REE in shelf phosphorites is determined by the fact that in early stages of phosphorite formation REE are associated not primarily with phosphate, but with organic matter and terrigenous impurities. Only in the later stages of diagenesis phosphate begins to play a leading role in concentration of REE. In metasomatic phosphorites on seamounts concentration of REE depends on age and depth of these rocks, i.e. it is determined by duration and conditions of contact with sea water.

Geochemistry of oceanic flood basalts from the Caribbean plate

The thick oceanic crust of the Caribbean plate appears to be the tectonized remnant of an eastern Pacific oceanic plateau that has been inserted between North and South America. The emplacement of the plateau into its present position has resulted in the obduction and exposure of its margins, providing an opportunity to study the age relations, internal structure and compositional features of the plateau. We present the results of 40Ar-39Ar radiometric dating, major-, trace-element, and isotopic compositions of basalts from some of the exposed sections as well as drill core basalt samples from Leg 15 of the Deep Sea Drilling Project. Five widely spaced, margin sections yielded ages ranging from 91 to 88 Ma. Less well-constrained radiometric ages from the drill cores, combined with the biostratigraphic age of surrounding sediments indicate a minimum crystallization age of ~90 Ma in the Venezuelan Basin. The synchroneity of ages across the region is consistent with a flood basalt origin for the bulk of the Caribbean plateau i.e., large volume, rapidly erupted, regionally extensive volcanism.. The ages and compositions are also consistent with plate reconstructions that place the Caribbean plateau in the vicinity of the Galápagos hotspot at its inception. The trace-element and isotopic compositions of the ~90 Ma rocks indicate a depleted mantle and an enriched, plume-like mantle were involved in melting to varying degrees across the plateau. Within the same region, a volumetrically secondary, but widespread magmatic event occurred at 76 Ma, as is evident in Curacao, western Colombia, Haiti, and at DSDP Site 152/ODP Site 1001 near the Hess Escarpment. Limited trace-element data indicate that this phase of magmatism was generally more depleted than the first. We speculate that magmatism may have resulted from upwelling of mantle, still hot from the 90 Ma event, during lithospheric extension attending gravitational collapse of the plateau, andror tectonic emplacement of the plateau between North and South America. Still younger volcanics are found in the Dominican Republic (69 Ma) and the Quepos Peninsula of Costa Rica (63 Ma). The latter occurrence conceivably formed over the Galápagos hotspot and subsequently accreted to the western edge of the plateau during subduction of the Farallon plate.

Geochemical and Nd isotopic investigations of sediments from the South China Sea

Secular variations in geochemistry and Nd isotopic data have been documented in sediment samples at ODP Site 1148 in the South China Sea. Major and trace elements show significant changes at ca. 29.5 Ma and 26-23 Ma, whereas epsilon-Nd values show a single change at ca. 26-23 Ma. Increases in Al/Ti, Al/K, Rb/Sr, and La/Lu ratios and a decrease in the Th/La ratio of the sediments beginning at 29.5 Ma are consistent with more intense chemical weathering in the source region. The abrupt change in Nd isotopes and geochemistry at ca. 26-23 Ma coincides with a major discontinuity in the sedimentology and physical properties of the sediments, implying a drastic change in sedimentary provenance and environment at the drill site. Comparison of the Nd isotopes of sediments from major rivers flowing into the South China Sea suggests that pre-27 Ma sediments were dominantly derived from a southwestern provenance (Indochina-Sunda Shelf and possibly northwestern Borneo), whereas post-23 Ma sediments were derived from a northern provenance (South China). This change in provenance from southwest to north was largely caused by ridge jumping during seafloor spreading of the South China Sea, associated with a southwestward expansion of the ocean basin crust and a global rise in sea level. Thus, the geochemical and Nd isotopic changes in the sediments at ODP Site 1148 are interpreted as a response to a major plate reorganization in SE Asia at ca. 25 Ma.

(Table 1) Geochemistry of Nauru Basin basalts from the lower portion of DSDP Hole 61-462A

Atomic-absorption spectrophotometry and instrumental neutron activation analysis were used to determine concentrations of SiO2, Al2O3, FeOt, MgO, CaO, Na2O, K2O, MnO, La, Ce, Sm, Eu, Tb, Yb, Lu, Sc, Co, Cr, Th, Hf, and Ta for 14 basalt samples from the lower portion of Hole 462A in the Nauru Basin. The basalts are similar to normal midocean ridge basalt (MORB) for the elements analyzed, and light rare-earth elements (LREE) are depleted relative to heavy rare-earth elements (HREE). Two samples are extensively altered to smectites and show significant reductions in Al2O3, CaO, MnO, Na2O, REE, Sc, Co, and Hf and gains in MgO and FeOt relative to unaltered samples. The increase in MgO and decrease in CaO indicate that alteration was caused by hydrothermal solutions.

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 element contents in deposits and minerals of the ocean floor

Current understanding of rare earth element (REE) geochemistry in the ocean is given in the book. Chemical properties determining REE migration ability in natural processes, sources of REE in the ocean, behavior of REE in river-sea mixing zones, fractionation of dissolved and particulate REE in ocean waters under aerobic and anaerobic conditions, distribution of REE in terrigenous, authigenic, hydrothermal and biogenic sediment components (clay, bone detritus, barite, phillipsite, Fe- and Mn-oxyhydroxides, Fe-Ca hydroxophosphate, diatoms and foraminiferas) are under consideration.