Geochemistry and Nd-Pb isotope ratios of sediments from the Lesser Antilles arc

The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north-south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions. Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd-Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. (1985, doi:10.1016/0016-7037(85)90082-1) and confirm their relatively uniform isotopic compositions (i.e., 206Pb/204Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable (206Pb/204Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3. The north-south isotopic change known along the Lesser Antilles arc can be explained by the observed geographical changes in the composition of the subducted sediments. About 1% contamination of the mantle wedge by Site 543 sediments explains the composition of the northern islands while up to 10% sediments like those of Site 144 is required in the source of the southern island lavas. The presence of black shales in the subducted pile provides a satisfactory explanation for the very low Delta8/4 values that characterize the Lesser Antilles arc.

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.

Geochemistry of Lesser Antilles lavas

We present new U-series disequilibrium and radiogenic isotope data for 7 mafic lavas from the Lesser Antilles arc. These are combined with published data in an internally consistent model that quantitatively estimates the amount of sediment and fluid added to the source of the Lesser Antilles arc system. Some lavas form an array consistent with bulk sediment addition (0.2-2%) whereas others appear to require addition of 0.4-2% sediment melt, particularly in the south of the arc. Evidence for both bulk sediment and sediment melt addition can be found within both the northern and central sections of the arc suggesting a thermal structure whereby the upper portions of the subducted sediment pile lie close to their solidus beneath much of the arc. Addition of up to 5% fluid derived from altered oceanic crust to these sediment enriched mantle wedge source regions can simulate the majority of the lavas on a plot of 207Pb/204Pb versus Ce/Pb. By taking into account the range in calculated wedge compositions and allowing for some mobility of Th in the fluid, the same model can also account for much of the observed range of U-Th-Ra disequilibria, especially if the eclogitic residue contains trace amounts of rutile. The implication of this more complex model is that the time scales for fluid addition and differentiation could be significantly shorter than those estimated in some previous studies.

Cenozoic calcareous nannofossils from the Lesser Antilles Forearc Region

During Leg 110 of the Ocean Drilling Program, sediment was recovered from six sites in the vicinity of the Lesser Antilles Forearc. Hole 671B, drilled near the toe of the Barbados deformation front, was the first-ever penetration of the decollement between the underthrusting Atlantic Plate and the off scraped Barbados accretionary prism. Stratigraphic repetitions in sequence associated with tectonic movement along the decollement zone, first observed on DSDP Leg 78A, were further documented at four ODP Leg 110 sites. A significant biostratigraphic inversion is present at Site 671 at 128 mbsf in which upper Miocene sediments rest atop lower Pleistocene strata. Smaller repetitions in sequence are recorded at Sites 671, 673, 674, and 676. Leg 110 sediments range from middle Eocene to early Pleistocene in age. Pliocene/Pleistocene assemblages are generally well preserved; however, Miocene assemblages have undergone extensive dissolution at all Leg 110 sites. Paleogene sediments are sometimes recrystallized and the nannofossils contained within exhibit a range in preservation from poor to good.

Abundance and habitat covariates of lesser florican and spiny-tailed lizard in Kachchh, India (2008-09)

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