(Table 1) Sulfur and oxygen isotopes from two anhydrite samples recovered from DSDP Hole 70-504B

The first anhydrite reported from oceanic basalts occurs in altered basalts drilled during DSDP Leg 70 from Hole 504B. Anhydrite has been identified in several samples, two of which were studied in detail. Anhydrite in Sample 504B-40-3 (130-135 cm), which was acquired at 310 meters sub-basement, occurs in a dolerite at the center of a vug rimmed by saponite and calcite. Red iron-hydroxide-rich alteration halos occur from 0 to 310 meters sub-basement; primary sulfides in these halos are oxidized, and the rocks have lost large amounts of sulfur. The anhydrite in this sample has a d34S value of 18.5 per mil, and it is interpreted to have formed from a fluid containing a mixture of seawater sulfate (20.9 per mil) and basaltic sulfur (0 per mil) released during the oxidation of primary sulfides. Anhydrite in Sample 504B-48-3 (14-18 cm), which was found at 376 meters sub-basement, occurs intergrown with gyrolite at the center of a 1-cm-wide vein that is rimmed by saponite and quartz. At sub-basement depths below 310 meters to the bottom of the Leg 70 section (562 m sub-basement), the rocks exhibit the effects of anoxic alteration with common secondary pyrite. Anhydrite in Sample 504B-48-3 (14-18 cm) has a d34S value of 36.7 per mil, and it is interpreted to have formed from seawater-derived fluids enriched in 34S through sulfate reduction. Temperatures of alteration calculated from oxygen isotope data range from 60 to 100°C. Sulfate reduction may have occurred in situ, or elsewhere at higher temperature, possibly deeper in the crust. The secondary mineral paragenetic sequence indicates a progressive decrease in Mg and increase in Ca in the circulating fluids. This eventually led to anhydrite formation late in the alteration process.

(Table 2) Carbonate chemistry and isotopes from northern Cascadia margin samples

Most authigenic carbonates previously recovered from the Cascadia slope have 87Sr/86Sr signatures that reflect shallow precipitation in equilibrium with coeval seawater. There is also evidence for carbonate formation supported by fluids that have been modified by reactions with the incoming Juan de Fuca plate (87Sr/86Sr = 0.7071; Teichert et al., 2005, doi:10.1016/j.epsl.2005.08.002) or with terrigenous turbidites (87Sr/86Sr = 0.70975 to 0.71279; Sample et al., 1993, doi:10.1130/0091-7613(1993)021<0507:CCICFF>2.3.CO;2). We report on the strontium isotopic composition of carbonates and fluids from IODP Site U1329 and nearby Barkley Canyon (offshore Vancouver Island), which have strontium isotope ratios as low as 0.70539. Whereas the strontium and oxygen isotopic compositions of carbonates from paleoseeps in the uplifted Coast Range forearc indicate formation in ambient bottom seawater, several samples from the Pysht/Sooke Fm. show a 87Sr-depleted signal (87Sr/86Sr = 0.70494 and 0.70511) similar to that of the anomalous Site U1329 and Barkley Canyon carbonates. Our data, when analyzed in the context of published elemental and isotopic composition of these carbonates (Joseph et al., 2012, doi:10.1016/j.palaeo.2013.01.012 ), point to two formation mechanisms: 1) shallow precipitation driven by the anaerobic oxidation of methane (AOM) with d13C values as low as -50 per mil and contemporaneous 87Sr/86Sr seawater ratios, and 2) carbonate precipitation driven by fluids that have circulated through the oceanic crust, which are depleted in 87Sr. Carbonates formed from the second mechanism precipitate both at depth and at sites of deep-sourced fluid seepage on the seafloor. The 87Sr-depleted carbonates and pore fluids found at Barkley Canyon represent migration of a deep, exotic fluid similar to that found in high permeability conglomerate layers at 188 mbsf of Site U1329, and which may have fed paleoseeps in the Pysht/Sooke Fm. These exotic fluids likely reflect interaction with the 52-57 Ma igneous Crescent Terrane, which supplies fluids with high calcium, manganese and strontium enriched in the non-radiogenic nucleide. Tectonic compression and dehydration reactions then force these fluids updip, where they pick up the thermogenic hydrocarbons and 13C-enriched dissolved inorganic carbon that are manifested in fluids and carbonates sampled at Barkley Canyon and at Site U1329. The Crescent Terrane may have sourced cold seeps in this margin since at least the late Oligocene.

Geochemical characterization and the voltammetric investigation of hydrothermal Iron speciation of samples gained during SONNE cruise SO229

Hydrothermal vent fluids are highly enriched in iron (Fe) compared to ambient seawater, and organic ligands may play a role in facilitating the transport of some hydrothermal Fe into the open ocean. This is important since Fe is a limiting micronutrient for primary production in large parts of the world's surface ocean. We have investigated the concentration and speciation of Fe in several vent fluid and plume samples from the Nifonea vent field, Coriolis Troughs, New Hebrides Island Arc, South Pacific Ocean using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-AdCSV) with salicylaldoxime (SA) as the artificial ligand. Our results for total dissolved Fe (dFe) in the buoyant hydrothermal plume samples showed concentrations up to 3.86 µM dFe with only a small fraction between 1.1 and 11.8% being chemically labile. Iron binding ligand concentrations ([L]) were found in µM level with strong conditional stability constants up to logKFeL,Fe3+ of 22.9. Within the non-buoyant hydrothermal plume above the Nifonea vent field, up to 84.7% of the available Fe is chemically labile and [L] concentrations up to 97 nM were measured. [L] was consistently in excess of Felab, indicating that all available Fe is being complexed, which in combination with high Felab values in the non-buoyant plume, signifies that a high fraction of hydrothermal dFe is potentially being transported away from the plume into the surrounding waters, contributing to the global oceanic Fe budget.

(Table 2) Chlorine geochemistry and mineralogy of altered oceanic crust samples

Bulk chlorine concentrations and chlorine stable isotope compositions were determined for hydrothermally altered basalt (extrusive lavas and sheeted dikes) and gabbro samples (n = 50) from seven DSDP/ODP/IODP drill sites. These altered oceanic crust (AOC) samples span a range of crustal ages, tectonic settings, alteration type, and crustal depth. Bulk chlorine concentrations range from < 0.01 wt.% to 0.09 wt.%. In general, higher chlorine concentrations coincide with an increase in temperature of alteration and amphibole content. d37Cl values of whole rock AOC samples range from -1.4 to +1.8 per mil. High d37Cl values (>=~0.5 per mil) are associated with areas of higher amphibole content. This observation is consistent with theoretical calculations that estimate amphibole should be enriched in 37Cl compared to co-existing fluid. Negative to near zero d37Cl values are found in areas dominated by clay minerals. Chlorine geochemistry is a rough indicator of metamorphic grade and mineralogy. AOC is a major Cl host in the subducting oceanic lithospheric slab. Here we show that bulk chlorine concentrations are ~3 times higher than previous estimates resulting in a greater contribution of Cl to the mantle.