Geochemistry of sediments beneath the Tonga-Kermadec arc

The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga-Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2-10, (b) the top 1-22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be-Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ~800 °C during the breakdown of amphibole +/- plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10-15% partial melts of depleted mantle after it has been enriched by the addition of 0.2-2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ~800 °C by the time it attains ~ 80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partialmelting of the altered oceanic crust.

Porphyrin geochemistry of DSDP Hole 71-511

Late Jurassic-early Cretaceous black shales and an overlying sequence of Albian-Campanian zeolitic claystones from the Falkland Plateau (DSDP/IPOD Leg 71, Site 511) were analyzed for tetrapyrrole pigment type and abundance. The "black shale" sequence was found to be rich in DPEP-series dominated free-base, nickel (Ni) and, to a lesser extent, vanadyl (V = 0) porphyrins. A low level of organic maturity (i.e. precatagenesis) is indicated for these strata as nickel chelation by free-base porphyrins is only 50-75% complete, proceeding down-hole to 627 meters sub-bottom. Electronic and mass spectral data reveal that the proposed benzo-DPEP (BD) and tetrahydrobenzo-DPEP (THBD) series are present in the free-base and Ni species, as well as the more usual occurrence in V = 0 porphyrin arrays. Highly reducing conditions are suggested by an abundance of the PAH perylene, substantial amounts of the THBD/BD series and a redox equilibrium between free-base DPEP and 7,8-dihydro-DPEP series, which exist in a 7:1 molar ratio. The Albian-Campanian claystone strata were found to be tetrapyrrolepoor, and those pigments present were typed as Cu/Ni highly dealkylated (C26 max.) etioporphyrins, thought to be derived via redeposition and oxidation of terrestrial organic matter (OM). Results from the present study are correlated to our past analyses of Jurassic-Cretaceous sediments from Atlantic margins in an effort to relate tetrapyrrole quality and quantity to basin evolution and OM sources in the proto-Atlantic.

Chemical and isotope compositions of main phases of metalliferous sediments from holes of DSDP Leg 92

The carbonate-free metalliferous fraction of thirty-nine sediment samples from four DSDP Leg 92 sites has been analyzed for 12 elements, and a subset of 16 samples analyzed for Pb isotopic composition. The main geochemical features of this component are as follows: i) very high concentrations of Fe and Mn, typically 25-39% and 5-14%, respectively; ii) Al and Ca contents generally less than 2% and 5%, respectively; iii) high Cu (1000-2000 ppm), and Zn and Ni (500-1000 ppm) values; and iv) Co and Pb concentrations of 100-250 ppm. In terms of element partitioning within the metalliferous fraction, amorphous to poorly crystallized oxide-oxyhydroxides removed by the second leach carry virtually all of the Mn, and about 90% of the Ca, Sr and Ni. The well-crystallized goethite-rich material removed by the third leach carries the majority of Fe, Cu, and Pb. These relations hold for sediments as young as ~1-2 Ma, indicating early partitioning of hydrothermal Fe and Mn into separate phases. Calculated mass accumulation rates (MAR) for Fe, Mn, Cu, Pb, Zn and Ni in the bulk sediment show the same overall trends at three of the sites, with greatest MAR values near the basement, and a general decrease in MAR values towards the tops of the holes (for sediments deposited above the lysocline). These relations strongly support the concept of a declining hydrothermal contribution of these elements away from a ridge axis. Nevertheless, MAR values for these metals up to ~200 km from the ridge axis are orders of magnitude higher than on abyssal seafloor plains where there is no hydrothermal influence. Mn/Fe ratios throughout the sediment column at two sites indicate that the composition of the hydrothermal precipitates changed during transport through seawater, becoming significantly depleted in Mn beyond ~200-300 km from the axis, but maintaining roughly the same proportion of Fe. Most of the Pb isotope data for the Leg 92 metalliferous sediments form approximately linear arrays in the conventional isotopic plots, extending from the middle of the field for mid-ocean ridge basalts toward the field for Mn nodules. The array of data lying between these two end-members is most readily interpreted in terms of simple linear mixing of Pb derived from basaltic and seawater end-member sources. The least radiogenic sediments reflect the average Pb isotope composition of discharging hydrothermal solutions and ocean-ridge basalt at the EPR over the ~4-8 Ma B.P. interval. Pb in sediments deposited up to 250 km from the axis can be almost entirely of basaltic-hydrothermal origin. Lateral transport of some basaltic Pb by ocean currents appears to extend to distances of at least 1000 km west of the East Pacific Rise.