Geochemical and isotopic composition of sediments and pore waters from ODP Leg 164 sites

Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sediments that overlay the Blake Ridge Diapir on the Carolina Rise (Ocean Drilling Program [ODP] Site 996). Active chemosynthetic communities at this site are apparently fed by fluid conduits extending beneath a bottom-simulating reflector (BSR). Gas hydrates occur at several depth intervals in these near-surface sediments. The carbonate nodules are composed of rounded to subangular intraclasts and carbonate cemented mussel shell fragments. Electron microprobe and X-ray diffraction (XRD) investigations show that aragonite is the dominant authigenic carbonate. Authigenic aragonite occurs both as microcrystalline, interstitial cement, and as cavity-filling radial fibrous crystals. The d13C values of the authigenic aragonite vary between -48.4 per mil and -30.5 per mil (Peedee belemnite [PDB]), indicating that carbon derived from 13C-depleted methane is incorporated into these carbonates. The d13C of pore water sum CO2 values are most negative in the upper 10 mbsf, near the sediment/water interface (-38 per mil ± 5 per mil), but noticeably more positive below 25 mbsf (+5 per mil ± 6 per mil). Because carbonates derive their carbon from HCO3-, dissimilarities between the d13C values of carbonate precipitates recovered from greater than 10 mbsf and d13C values of the associated pore fluids suggests that these carbonates formed near the seafloor. Differences of about 1 per mil in the oxygen isotopic composition of carbonate precipitates from different depths are possibly related to changes in bottom-water conditions during glacial and interglacial time periods. Measurements of the strontium isotopic composition on 13 carbonate samples show 87Sr/86Sr values between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the approximate age of their host sediment. Furthermore, the 87Sr/86Sr values of six pore-water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater (87Sr/86Sr = 0.709175), and to 87Sr/86Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source. The 87Sr/86Sr values of the authigenic carbonates at Site 996 are not consistent with the Sr isotopic values predicted for carbonates precipitated from fluids transported upward along fault conduits extending through the base of the gas hydrate-stability zone. Based on our data, we see no evidence of continuing carbonate diagenesis with depth. Therefore, with the exception of their seafloor expression as carbonate crusts, fossil vent sites will not be preserved. Because these authigenic features apparently form only at the seafloor, their vertical distribution and sediment age imply that seepage has been going on in this area for at least 600,000 yr.

Geochemistry of the lower sheeted dike complex of ODP Holes 137-504B and 140-504B

Sixty-three samples representing 379 m of sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program Site 504B have been analyzed for major and selected trace elements by X-ray fluorescence. The samples range from microcrystalline aphyric basalts to moderately phyric (2%-10% phenocrysts) diabase that are typically multiply saturated with plagioclase, olivine, and clinopyroxene, in order of relative abundance. All analyzed samples are classified as Group D compositions with moderate to slightly elevated compatible elements (MgÆ-value = 0.65% ± 0.03%; Al2O3 = 15.5% ± 0.8%; CaO = 13.0% ± 0.3%; Ni = 114 ± 29 ppm), and unusually depleted levels of moderate to highly incompatible elements (Nb < 1 ppm; Zr = 44 ± 7 ppm; Rb < 0.5 ppm; Ba ~ 1 ppm; P2O5 = 0.07% ± 0.02%). These compositions are consistent with a multistage melting of a normal ocean ridge basaltic mantle source followed by extensive fractionation of olivine, plagioclase, and clinopyroxene. Leg 140 aphyric to sparsely phyric (0%-2% phenocrysts) basalts and diabases are compositionally indistinguishable from similarly phyric samples at higher levels in the hole. An examination of the entire crustal section, from the overlying volcanics through the sheeted dikes observed in Leg 140, reveals no significant trends indicating the enrichment or depletion of Costa Rica Rift Zone source magmas over time. Similarly, significant trends toward increased or decreased differentiation cannot be identified, although compositional patterns reflecting variable amounts of phenocryst addition are apparent at various depths. Below ? 1700 mbsf to the bottom of the Leg 140 section, there is a broadly systematic pattern of Zn depletion with depth, the result of high-temperature hydrothermal leaching. This zone of depletion is thought to be a significant source of Zn for the hydrothermal fluids depositing metal sulfides at ridge-crest hydrothermal vents and the sulfide-mineralization zone, located in the transition between pillow lavas and sheeted dikes. Localized zones of intense alteration (60%-95% recrystallization) are present on a centimeter to meter scale in many lithologic units. Within these zones, normally immobile elements Ti, Zr, Y, and rare-earth elements are strongly depleted compared with "fresher" samples centimeters away. The extent of compositional variability of these elements tends to obscure primary igneous trends if the highly altered samples are not identified or removed. At levels up to 40% (or possibly 60%) recrystallization, Ti, Zr, and Y retain their primary signatures. Although the mechanisms are unclear, it is possible that these intense alteration zones are a source of Y and rare-earth elements for the typically rare-earth-element-enriched hydrothermal vent fluids of mid-ocean ridges.

Geochemistry and age determination of sediments from teh Rainbow hydrothermal plume

A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO3) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of "hydrothermal" elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr?1, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores' surface mixed layers are presented which, typically, are an order of magnitude greater than "background" authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time - perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.

Isotopic composition of various forms of sulfur in barite crusts from the Red Sea rift

Barite crusts were formed by an intermittent hydrothermal vent with output temperature from 85 to 465°C. Principal sources of supply of sulfate sulfur are sea water, evaporites, and tholeiitic basalts of the Red Sea rift. Sulfides and sulfates were formed in conditions of isotope disequilibrium with respect to sulfur because rate of precipitation of sulfur compounds from hydrothermal solution was high compared with rate of isotope exchange.

Isotopic and chemical compositions of rocks and minerals from the TAG hydrothermal mound, ODP Site 957

Strontium- and oxygen-isotopic measurements of samples recovered from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound during Leg 158 of the Ocean Drilling Program provide important constraints on the nature of fluid-rock interactions during basalt alteration and mineralization within an active hydrothermal deposit. Fresh Mid-Ocean Ridge Basalt (MORB), with a 87Sr/86Sr of 0.7026, from the basement beneath the TAG mound was altered at both low and high temperatures by seawater and altered at high temperature by near end-member black smoker fluids. Pillow breccias occurring beneath the margins of the mound are locally recrystallized to chlorite by interaction with large volumes of conductively heated seawater (>200°C). The development of a silicified, sulfide-mineralized stockwork within the basaltic basement follows a simple paragenetic sequence of chloritization followed by mineralization and the development of a quartz+pyrite+paragonite stockwork cut by quartz-pyrite veins. Initial alteration involved the development of chloritic alteration halos around basalt clasts by reaction with a Mg-bearing mixture of upwelling, high-temperature (>300°C), black smoker-type fluid with a minor (<10%) proportion of seawater. Continued high-temperature (>300°C) interaction between the wallrock and these Mg-bearing fluids results in the complete recrystallization of the wallrock to chlorite+quartz+pyrite. The quartz+pyrite+paragonite assemblage replaces the chloritized basalts, and developed by reaction at 250-360°C with end-member hydrothermal fluids having 87Sr/86Sr ~0.7038, similar to present-day vent fluids. The uniformity of the 87Sr/86Sr ratios of hydrothermal assemblages throughout the mound and stockwork requires that the 87Sr/86Sr ratio of end-member hydrothermal fluids has remained relatively constant for a time period longer than that required to change the interior thermal structure and plumbing network of the mound and underlying stockwork. Precipitation of anhydrite in breccias and as late-stage veins throughout most of the mound and stockwork, down to at least 125 mbsf, records extensive entrainment of seawater into the hydrothermal deposit. 87Sr/86Sr ratios indicate that most of the anhydrite formed from ~2:1 mixture of seawater and black smoker fluids (65%±15% seawater). Oxygen-isotopic compositions imply that anhydrite precipitated at temperatures between 147°C and 270°C and require that seawater was conductively heated to between 100°C and 180°C before mixing and precipitation occurred. Anhydrite from the TAG mound has a Sr-Ca partition coefficient Kd ~0.60±0.28 (2 sigma). This value is in agreement with the range of experimentally determined partition coefficients (Kd ~0.27-0.73) and is similar to those calculated for anhydrite from active black smoker chimneys from 21°N on the East Pacific Rise. The d18O (for SO4) of TAG anhydrite brackets the value of seawater sulfate oxygen (~9.5?). Dissolution of anhydrite back into the oceans during episodes of hydrothermal quiescence provides a mechanism of buffering seawater sulfate oxygen to an isotopically light composition, in addition to the precipitation and dissolution of anhydrite within the oceanic basement during hydrothermal recharge at the mid-ocean ridges.