Geochemistry of pelagic and hemipelagic sediments of ODP Leg 126 sites

Chemical analyses were performed on major, minor, and rare-earth elements of pelagic and hemipelagic sediments of the forearc, arc, and backarc sites of the Izu-Bonin Arc, Ocean Drilling Program Leg 126. Analyses of the hemipelagic and pelagic sediments of this area indicate that the chemical composition of this arc is highly affected by the chemical composition of rocks of this arc as a source of sediments. The Oligocene sediments, which are characterized by high MgO contents, reflect the chemical composition of the Paleogene volcanic rocks of the immature arc. Moreover, the late Miocene to Quaternary sediments with low MgO contents are attributed to the composition of the present arc. We also suggest that the sedimentation rates and topography of the sedimentary basin affect the MnO and SiO2 contents of pelagic and hemipelagic sediments.

Trace element geochemistry of zircons frim in situ ocean lithoshere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ~ 0.7, a SiO2 ~ 1.0, pressure ~ 2 kbar); intra-sample variation is typically ~60-15°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680-740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.

Geochemistry of selected Ferrar samples from Northern Victoria Land and George V Land during GANOVEX VIII

Newly sampled basaltic andesites and andesites from the tholeiitic Ferrar Supergroup of northern Victoria Land and George V Land, Antarctica, are attributed to the known low-Ti and high-Ti series. Aside from known sparsely distributed high-Ti extrusives, a high-Ti sill was found in the Alamein Range outside the Rennick Graben. Low-Ti lavas, sills and dikes display wide petrographical, mineral and geochemical variations, reflecting extensive in-situ differentiation. High-Ti rocks from Litell Rocks are homogeneous with respect to mineralogy and geochemistry, minor deviations are shown by the sampled sill. Chilled margins of low-Ti sills, dikes and lava flows exhibit nearly constant bulk-rock chemistry (mg# ~60) within the studied area. Compared to chilled margins from Tasmanian sills, the striking uniformity of the pre-emplacement chemistry of Ferrar magmas over large distances supports the magma transport model of Elliot et al. (1999, doi:10.1016/S0012-821X(99)00023-0). In the area investigated, compositional variations within the low-Ti series, caused by in-situ differentiation, increase towards the Wilson-Bowers Terrane boundary, possibly displaying the asymmetrical distribution of outcrops over this area. Absence of Ferrar occurrences east of the Bowers Terrane remains a matter of palaeo-geodynamic discussion. Besides, the secondary mineralogy of extrusives from Litell Rocks and Monument Nunataks exhibits noticeable differences, which indicates an elevated thermal gradient in the vicinity of Litell Rocks compared to Monument Nunataks during the Cretaceous.

Chemical and boron isotopic compositions of volcanics from ODP Sites 135-834 to 135-840

The influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was investigated using ion microprobe measurements of B/Be and boron isotope ratios (11B/10B) to document the source and relative volumes of the fluids released from the subducting oceanic plate. We analyzed young lavas from eight different islands along the Tonga-Kermadec Arc, as well as glass shards in volcanic sediments from Ocean Drilling Program (ODP) Site 840, which record the variations in the chemistry of Tonga magmatism since 7 Ma. B/Be is variable (5.8-122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from around 3 to 4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for arc lavas (18-607). These values are too high to be related simply to a sediment influence on petrogenesis. Together with very high d11B values (-11.6 to +37.5) for the same shards and lavas these data indicate that most of the B is derived from fluid escaped from the subducting altered Pacific oceanic crust, rather than from sediment. High d11B values also reflect large degrees of isotopic fractionation in this cold fast subduction zone. Lower d11B values noted in the Kermadec Arc (17 to -4.4) are related to the influence of sediment eroded from New Zealand and slower convergence. High fluid flux (B/Be) is synchronous in Tonga and the Marianas at 3 to 4 Ma and may be related to acceleration of the Pacific Plate just prior to this time. The timing of maximum B/Be at 3 to 4 Ma correlates with maximum light rare earth (LREE) and high field strength element depletion. This suggests maximum degrees of partial melting at this time. Although thinning of the arc lithosphere during rifting to form the Lau Basin is expected to influence the arc geochemistry, variable aqueous fluid flux from the subducting plate alone appears capable of explaining boron and other trace element systematics in the Tonga-Kermadec Arc with no indication of slab melting.

Major, trace and rare earth element concentrations of sediments from ODP Leg 207 sites

The Cretaceous and Paleogene sediments recovered during Ocean Drilling Program Leg 207 can be divided into three broad modes of deposition: synrift clastics (lithologic Unit V), organic matter-rich, laminated black shales (Unit IV), and open-marine chalk and calcareous claystones (Units III-I). The aim of this study is to provide a quantitative geochemical characterization of sediments representing these five lithologic units. For this work we used the residues (squeeze cakes) obtained from pore water sampling. Samples were analyzed for bulk parameters (total inorganic carbon, total organic carbon, and S) and by X-ray fluorescence for major (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) and selected minor (As, Ba, Co, Cr, Cu, Mo, Ni, Pb, Rb, Sr, U, V, Y, Zn, and Zr) elements. Inductively coupled plasma-mass spectrometry analyses for rare earth elements (REEs) were performed on acid digestions of the squeeze cake samples from Site 1258. The major element composition is governed by the mixture of a terrigenous detrital component of roughly average shale (AS) composition with biogenous carbonate and silica. The composition of the terrigenous detritus is close to AS in Units II-IV. For Unit I, a more weathered terrigenous source is suggested. Carbonate contents reach >60 wt% on average in chalks and calcareous claystones of Units II-IV. The SiO2 contribution in excess of the normal terrigenous-detrital background indicates the presence of biogenous silica, with highest amounts in Units II and III. The contents of coarse-grained material (quartz) are enhanced in Unit V, where Ti and Zr contents are also high. This indicates a high-energy depositional environment. REE patterns are generally similar to AS. A more pronounced negative Ce anomaly in Unit IV may indicate low-oxygen conditions in the water column. The Cretaceous black shales of Unit IV are clearly enriched in redox-sensitive and stable sulfide-forming elements (Mo, V, Zn, and As). High phosphate contents point toward enhanced nutrient supply and high bioproductivity. Ba/Al ratios are rather high throughout Unit IV despite the absence of sulfate in the pore water, indicating elevated primary production. Manganese contents are extremely low for most of the interval studied. Such an Mn depletion is only possible in an environment where Mn was mobilized and transported into an expanded oxygen minimum zone ("open system"). The sulfur contents show a complete sulfidation of the reactive iron of Unit IV and a significant excess of sulfur relative to that of iron, which indicates that part of the sulfur was incorporated into organic matter. We suppose extreme paleoenvironmental conditions during black shale deposition: high bioproductivity like in recent coastal upwelling settings together with severe oxygen depletion if not presence of hydrogen sulfide in the water column.

Pb-, Sr-, and Nd-isotopic composition of basaltic rocks from the Lau Basin

New Pb, Sr, and Nd isotope data are presented for 64 samples from the six backarc sites drilled during Leg 135. Systematic changes in Pb and Sr compositions illustrate significant isotopic variations between and within sites as well as provide two key pieces of information. First, a recent influx of asthenosphere with Indian Ocean mantle affinities has occurred and has successfully displaced older "Pacific" asthenosphere from the mantle underlying the backarc region. Second, clear evidence exists for mixing between these two asthenospheric end-members and at least one "arc-like" component. The latter was not the same as most material currently erupting in the Tofua Arc, but it must have had a more radiogenic Pb-isotope signature, perhaps similar to rocks analyzed from the islands of Tafahi, and Niuatoputapu. A comparison between the isotopic variations and the tectonic setting of the drill sites reveals consistent and important information regarding the mantle dynamics beneath the evolving backarc basin. We propose a model in which the source of upwelling magmas changes from Pacific to Indian Ocean asthenosphere with the propagation of seafloor spreading, a model with important implications for the rate of mantle influx into this region. Although the chemistries of backarc magmas have been profoundly influenced by this process, an additional consequence is the advection of Indian Ocean asthenosphere into the sub-arc mantle source. The isotopic compositions of arc rocks from the vicinity have been reevaluated on the basis of the proposed mantle advection model. We suggest that the slab-derived flux of trace elements into the arc wedge has remained relatively uniform with time (i.e., ~40 Ma), so that the change in arc chemistry results from mantle source substitution, rather than from differences in the composition of the downgoing plate.

Geochemistry of Lesser Antilles forearc sediments

Oceanic sediments contain the products of erosion of continental crust, biologic activity and chemical precipitation. These processes create a large diversity of their chemical and isotopic compositions. Here we focus on the influence of the distance from a continental platform on the trace element and isotopic compositions of sediments deposited on the ocean floor and highlight the role of zircons in decoupling high-field strength elements and Hf isotopic compositions from other trace elements and Nd isotopic compositions. We report major and trace element concentrations as well as Sr and Hf isotopic data for 80 sediments from the Lesser Antilles forearc region. The trace-element characteristics and the Sr and Hf isotopic compositions are generally dominated by detrital material from the continental crust but are also variably influenced by chemical or biogenic carbonate and pure biogenic silica. Next to the South American continent, at DSDP Site 144 and on Barbados Island, sediments, coarse quartz arenites, exhibit marked Zr and Hf excesses that we attribute to the presence of zircon. In contrast, the sediments from DSDP Site 543, which were deposited farther away from the continental platform, consist of fine clay and they show strong deficiencies in Zr and Hf. The enrichment or depletion of Zr-Hf is coupled to large changes in Hf isotopic compositions (-30 < epsilon-Hf < +4) that vary independently from the Nd isotopes. We interpret this feature as a clear expression of the "zircon effect" suggested by Patchett and coauthors in 1984. Zircon-rich sediments deposited next to the South American continent have very low epsilon-Hf values inherited from old zircons. In contrast, in detrital clay-rich sediments deposited a few hundred kilometers farther north, the mineral fraction is devoid of zircon and they have drastically higher epsilon-Hf values inherited from finer, clay-rich continental material. In the two DSDP sites, average Hf isotopes are very unradiogenic relative to other oceanic sediments worldwide (epsilon-Hf = -14.4 and -7.4) and they define the low Hf end member of the sedimentary field in Hf-Nd space. Their compositions correspond to end members that, when mixed with mantle, are able to reproduce the pattern of volcanic rocks from the Lesser Antilles. More generally, we find a relationship between Nb/Zr ratios and the vertical deviation of Hf isotope ratios from the Nd-Hf terrestrial array and we suggest that this relationship can be used as a tool to distinguish sediment input from fractionation during melting during the formation of arc lavas.

Chemical composition of bottom sediments from the Baltic Sea

According to geochemical analyses carbonaceous sediments from deep basins of the Baltic Sea containing 3-5% of organic carbon are enriched in some metals such as Cu, Mo, Ni, Pb, Zn, V, and U relative to shallow-water facies of the Bay of Finland. These metals also enrich (relative to background values in clayey rocks) ancient carbonaceous shales, where the average Cu and V contents are slightly higher and that of Mo, Pb, and Zn lower than in deep-sea carbonaceous sediments of the Baltic Sea. In addition, the deep-sea carbonaceous sediments of the Baltic Sea are enriched (but less notably than ancient shales) in Ag, As, Bi, and Cd. These data confirm previous assumptions that carbonaceous sediments accumulating now in seas and oceans can be considered as recent analogs of ancient metalliferous shales.

Geochemistry of ODP Hole 135-839B lavas

Four petrographic lava types occur, ranging from aphyric to moderately phyric clinopyroxene-olivine tholeiitic basalts (Unit 1); olivine-clinopyroxene picritic basalts, sparsely to strongly olivine-phyric (Unit 3-type); olivine-clinopyroxene basalts (clinopyroxene dominant) (Unit 4); and moderately to strongly phyric two-pyroxene-plagioclase basaltic andesites (Unit 9-type). The olivine phyric lavas contain forsteritic olivines (extending to Fo92), and very magnesian Cr-rich spinels similar to those occurring in boninitic lavas. The basaltic andesites are mineralogically and petrographically indistinguishable from the modern Tofua Arc basaltic andesites, one notable feature being the highly calcic cores in plagioclase phenocrysts (up to An95). The forsteritic olivines, the Cr-spinels, and the calcic plagioclases are unlikely to have been precipitated in the lava compositions in which they occur, and are thought to have been incorporated from highly primitive melts by way of mixing processes (as advocated by Allan, this volume). Notwithstanding the evidence for mixing, the major element chemistries of the Unit 1- and Unit 9-type lavas are shown to be consistent with the derivation of the Unit 9-type basaltic andesites by means of fractional crystallization, through magmas of similar chemistry to Unit 1. Some trace element discrepancies in the modeling, and the relative volcanic stratigraphy of Site 839, however, preclude a direct liquid line of descent between the actual recovered units. Trace element data as well as TiO2 and Na2O data clearly illustrate the arc-like affinities of the magmas, with strong highfield-strength element depletion and large-ion-lithophile element enrichment. The abundance patterns are very close to those of the Tofua and Kermadec arc magmas, and also Valu Fa. Pb-, Sr-, and Nd-isotopic compositions indicate closest affinities with a "Pacific" MORB source, apparently characteristic of the western, older part of the Lau Basin. A subduction-related isotopic contribution is, however, inferred. The sources of the Site 839 magmas are thus inferred to be similar to, but less depleted geochemically, than those of the modern Tofua Arc magmas. The Site 839 sequence is interpreted as an older remnant of a volcanic construct of the "proto-Tofua arc", originally developed adjacent to the Tonga Ridge. Opening of the eastern Lau Basin, because of southward migrating propagators, has split and isolated the sequence, leaving it stranded within the modern Lau Basin.