Description: During ODP Leg 193, 4 sites were drilled in the active PACMANUS hydrothermal field on the crest of the felsic Pual Ridge to examine the vertical and lateral variations in mineralization and alteration patterns. We present new data on clay mineral assemblages, clay and whole rock chemistry and clay mineral strontium and oxygen isotopic compositions of altered rocks from a site of diffuse low-temperature venting (Snowcap, Site 1188) and a site of high-temperature venting (Roman Ruins, Site 1189) in order to investigate the water-rock reactions and associated elemental exchanges. The volcanic succession at Snowcap has been hydrothermally altered, producing five alteration zones: (1) chlorite +/- illite-cristobalite-plagioclase alteration apparently overprinted locally by pyrophyllite bleaching at temperatures of 260-310degreesC; (2) chlorite +/- mixed-layer clay alteration at temperatures of 230degreesC; (3) chlorite and illite alteration; (4) illite and chlorite +/- illite mixed-layer alteration at temperatures of 250-260degreesC; and (5) illite +/- chlorite alteration at 290-300degreesC. Felsic rocks recovered from two holes (1189A and 1189B) at Roman Ruins, although very close together, show differing alteration features. Hole 1189A is characterized by a uniform chlorite-illite alteration formed at similar to250degreesC, overprinted by quartz veining at 350degreesC. In contrast, four alteration zones occur in Hole 1189B: (1) illite chlorite alteration formed at similar to300degreesC; (2) chlorite +/- illite alteration at 235degreesC; (3) chlorite illite and. mixed layer clay alteration; and (4) chlorite illite alteration at 220degreesC. Mass balance calculations indicate that the chloritization, illitization and bleaching (silica-pyrophyllite assemblages) alteration stages are accompanied by different chemical changes relative to a calculated pristine precursor lava. The element Cr appears to have a general enrichment in the altered samples from PACMANUS. The clay concentrate data show that Cr and Cu are predominantly present in the pyrophyllites. Illite shows a significant enrichment for Cs and Cu relative to the bulk altered samples. Considerations of mineral stability allow us to place some constraints on fluid chemistry. Hydrothermal fluid pH for the chloritization and illitization was neutral to slightly acidic and relatively acidic for the pyrophyllite alteration. In general the fluids, especially from Roman Ruins and at intermediate depths below Snowcap, show only a small proportion of seawater mixing (〈10%). Fluids in shallow and deep parts of the Snowcap holes, in contrast, show stronger seawater influence. Copyright (C) 2004 Elsevier Ltd.

Description: Hydrous CaMg-carbonate was synthesized at temperatures of 40 degrees, 60 degrees and 80 degrees C in the laboratory. This material has very similar mineralogical characteristics to natural disordered dolomite from the Coorong region in South Australia. Besides the dolomite variable amounts of amorphous carbonate are present in all samples. The oxygen isotope compositions of synthesized bulk carbonate samples (e.g., amorphous carbonate plus dolomite) plot significantly lower than the Northrop and Clayton (1966) dolomite-water equilibrium. Fractionated degassing of the samples, however, revealed relatively low oxygen isotope values for fast-reacting (using 100% H3PO4) amorphous carbonate. In contrast, slow-reacting dolomite has more positive oxygen isotope values, and calculated carbonate-water oxygen isotope fractionation values are close to strongest known dolomite-water oxygen isotope fractionation published earlier on. Variations of reaction/stabilization temperatures during synthesis gave evidence for dolomite formation from hypersaline solutions by a dissolution/reprecipitation process. It is likely that amorphous carbonate has been a problem in defining the dolomite-water fractionation in the past. Moreover, dolomite-associated amorphous carbonate contents probably led to incorrect speculations about lower oxygen isotope fractionation in a so-called protodolomite-water system. Copyright (c) 2005 Elsevier Ltd.

Description: In order to develop the potential tool of diatom oxygen isotopes for paleoenvironmental studies we compared oxygen isotopes of natural marine diatoms sampled from ocean surface water, sediment traps and surface sediments with oxygen isotopic fractionations determined for laboratory diatom cultures. Freshly grown natural diatoms (phytoplankton samples and sediment trap material) and cultured diatoms reveal similar oxygen isotope fractionation factors. The fresh diatoms have 3 to 10 parts per thousand lower isotope fractionation factors than fossil (sedimentary) diatoms. A temperature-related oxygen isotope fractionation could not be established for the laboratory cultures (and the natural phytoplankton samples), and there is evidence that diatom growth rate until reaching the stationary growth state also controls the measured silica-water oxygen isotope fractionation factor. It is possible, however, that slow diatom growth in sea surface water may well lead to a temperature-dependent silica-water oxygen isotope fractionation which is the prerequisite for a use of diatom oxygen isotopes in palco-surface water studies. FTIR-spectroscopic analyses of various diatomaceous materials revealed that the ratio of integrated peak intensities for Si-O-Si/Si-OH correlates with the 3 to 10 parts per thousand delta O-18(silica) increase from fresh to fossil diatoms. Open-system (flow-through) silica dissolution experiments suggest that the diatom frustules are isotopically homogenous and that the increase in O-18 is therefore not due to dissolution of isotopically light surficial Si-OH groups. It is concluded that slow internal condensation reactions during silica maturation in surface sediments cause both an increase in the intensity ratio of Si-O-Si/Si-OH and the O-18 content of framework oxygen. These findings also indicate that the oxygen isotope compositions of marine sediment diatoms do not indicate sea surface water temperature but rather reflect variable O-18 contents of surface sediments. Copyright (C) 2001 Elsevier Science Ltd.

Description: Hydrothermal brines from the Atlantis II Deep, Red Sea, have been sampled in situ and analyzed for noble gases. The atmospheric noble gas concentrations (Ne, Aratm, Kr, Xe) in the deepest layer (LCL) are depleted by 20 to 30% relative to the initial concentrations in ambient Red Sea Deep Water without a systematic mass fractionation between the different noble gases. Sub surface boiling during the hydrothermal circulation and subsequent phase separation is shown to be a consistent explanation for the observed depletion pattern. Using a conceptual model of phase separation under sub-critical conditions, in which gases are partitioned according to Henry's Law, we reconstruct the fluid history before injection into the Atlantis II Deep: after having circulated through evaporites and young oceanic crust, where it becomes enriched in HeMORB and ArMORB, the ascending fluid boils, and the residual liquid becomes depleted in noble gas concentrations. The depleted fluid rises to the sediment surface and feeds the Atlantis II basin. The relatively low boiling degree of about 3% (i.e., the percentage of fluid removed as vapor) derived from the model indicates that the Atlantis II system represents an early stage of boiling with relatively small gas loss, in contrast to hydrothermal systems at sediment-free mid-ocean ridges.

Description: The element compositions Si, Ca and Al of up to 2 1.1 ka old sediments in about 10 in long cores from the southern basin of the Shaban and Kebrit deeps in the northern Red Sea allowed a classification of major sediment types in carbonate sands and -muds and siliceous oozes. A FeOOH-enriched sediment horizon and a few samples with high Zn values in the Kebrit core indicate a hydrothermal origin probably near the brine-sea water interface with subsequent transport of hydrothermal compounds into the deep sediments. High organic carbon contents up to 8.4% are positively correlated with the Ba concentrations, which suggests that high bioproductivity, and rapid deposition (C-14 dating suggests a sedimentation rate near 70 cm/ka) led to the formation of sapropelic sediments between 11.8 and 13.6 ka (Younger Dryas). Organic petrological observations showed that the sediment organic material largely consists of 〈20 gm-sized roundish fecal pellets (intimate mixtures of organic matter and inorganic constituents) and bituminite. Terrestrial organic matter (pollens of land plants, fusinite etc.) is very rare in the sediment cores from both deeps. Organic-geochemical investigations of kerogens and organic extracts show that a significant (hydrothermal) hydrocarbon production did not occur in near-surface sediments of the Shaban and Kebrit deeps. Rock Eval pyrolysis of kerogens characterised the organic matter to be of type II quality. The delta C-13 values of the kerogens from the most prominent sapropel in the Shaban deep indicate an enrichment of(C-12-rich) nutrients in the water column during postglacial sapropel formation in the Younger Dryas. The n-alkane spectra are dominated by short chain lengths between n-C-15 and n-C-25 Prevailing n-C-15 to n-C-25 alkanes in low mature sediments are indicative of algal and microbial source. Pristane/phytane ratios are generally low (〈 I to similar to 1) which suggests that anoxic conditions prevailed within the anaerobic brine-filled deeps for the whole time covered by the sediments. This again indicates that sapropel formation was caused by high bioproductivity in the northern Red Sea rather than episodic stagnation with better preservation of the organic matter. Long-chain alkenones and sterols are the dominating compounds of the lipid fraction. Cholesterol contents in the sediment cores reflect phases of eukaryotes production in the water column, whereas the positive correlations of dinosterol with TOC and the amounts of total extract suggests that the major organic carbon source in the northern Red Sea during postglacial high-productivity stages were dinoflagellates. Another important carbon source, however, is indicated by the occurrence of 22,29,30-trisnorhopan-21 -one (TNH). Although the formation of TNH from its precursors is not fully understood, this compound probably results from microbial. degradation of intact bacteriohopanepolyols (BHP), which can be used as indicators for bacterial abundances and phyla. TNH is most likely produced at the brine-sea water interface where sedimenting organic matter accumulates and, if the redoxcline corresponds to the density gradient, the organic matter is subjected to efficient aerobic bacterial degradation processes. However, during high bioproductivity stage (Younger Dryas) the redoxcline was probably higher in the water column and thus, a significant TNH production at the brine-sea water interface did not occur at times of sapropel formation in the northern Red Sea deeps. (C) 2007 Elsevier B.V All rights reserved.

Description: Despite the particular scientific interest in the elements with high affinity to S and O2, but found in zero-valence state in nature, the origin of these native minerals has been little explored and remains obscure. Here we describe unique Sn–Pb droplets found in a closed analcime–calcite amygdale collected from a basaltic unit cropping out at Carsaig Bay (Isle of Mull, Inner Hebrides). The droplets consist of intimate intergrowths of nearly pure Sn0 and Pb0 domains in proportion 88:12 and are enveloped in a thin, brownish film of organic composition. The occurrence of the Sn–Pb droplets in a closed amygdale, their relationship with the host analcime + calcite and their Pb isotope composition (which does not match any known anthropogenic Pb source) rule out the possibility of anthropogenic contamination and support the natural origin of the Sn–Pb alloy. The variable isotope (Pb, Sr, Nd) compositions in different members of the host basaltic sequence suggest that a parent basaltic magma was modified by crustal assimilation and post-emplacement alteration processes. Considering all possible explanations, it appears that the most likely source of Pb for the Sn–Pb alloy is a discrete basaltic unit with an isotopic composition comparable to the Antrim basalts (Northern Ireland). The amygdale phases, on the other hand, show isotopic evidence for incorporation of elements from both local basaltic and sedimentary units. The apparent isotopic disequilibrium between Sn–Pb droplets and amygdale phases indicates a complex, multi-stage fluid evolution. The occurrence of Sn–Pb droplets in organic capsules suggests that the droplets and the enveloping organic substances are co-precipitates. This implies that the transportation and deposition of Sn and Pb might have occurred through organometallic compounds. We assume interaction of seawater fluids carrying metals leached from basaltic rocks with hydrocarbons from sedimentary units as a prerequisite for the formation of the organometallic complexes. The zeolites lining the basaltic vesicles might have destabilized the migrating organo-Sn and Pb compounds causing their breakdown and precipitation of Sn–Pb alloy.

Description: The surface sediments of two mud mounds (‘‘Mound 11’’ and ‘‘Mound 12’’) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded d18Ocarbonate values ranging between 34.0 and 37.7 % Vienna standard mean ocean water (VSMOW) and d13Ccarbonate values from -52.2 to -14.2 % Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The d18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (d18Oporefluid = 0 % VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (d18Oporefluid &5 %) in Mound 11. A positive correlation between d13Ccarbonate and d18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (d13Cporefluid &0 %) and (2) bicarbonate which formed during the AOM (d13Cporefluid &-70 %). Furthermore, the d18Oporefluid composition, with values up to ?4.7 % Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boronenriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (d13CCH4 = -38 %). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s).

Description: Parasound profiles across the Shaban Deep in the Red Sea indicate turbiditic transport of surface sediments from the topographic hight (basalt ridge) into the interior of the deep. This is supported by petrographical and (isotope-) geochemical evidence in the East Basin of the Shaban Deep where the presence of variable mixtures of authochtonous and allochthonous sediment compounds had been found. The uppermost 170 cm of both sediment cores 17008-1 and 17009-3 reveal “normal” stable oxygen isotope values for the planktonic foraminifera G. ruber near -1 ‰ which is indicative for carbonate formation in Red Sea surface water around 27°C. However, below 182 cm in core 17008-1 highly variable δ 18O values for G. ruber between 0.26 and -10.68 ‰ occur which are not the result of temperature-controlled oxygen isotope fractionation between foraminiferal carbonate and Red Sea surface water. The lowest δ18O values of -10.68 ‰ measured for highly-altered foraminifera shells suggests carbonate precipitation higher than 90°C. Organic petrographical observations show a great diversity of marine-derived macerals and terrigenous organic particles. Based on petrographical investigations sediment core 17008-1 can be subdivided in intervals predominantly of authochtonous character (i.e. 1, 3, 5 corresponding to core depths 0-170 cm, 370-415 cm, 69-136 cm), and allochthonous/thermally altered character (e.g. 2, 4 corresponding to core depths 189-353 cm, 515-671 cm). Allochthonous/thermally altered material displays a wide to an extremely wide range of maturities (0.38-1.42 % Rr) and also natural coke particles were found. Similarily, the organic geochemical and pyrolysis data indicate the predominance of well-preserved, immature algal and bacterial remains with a minor contribution of land plant material. Sediments below 170 cm (core 17008-1) contain contributions of re-sedimented pre-heated material most likely from the area of the basaltic ridge. This is documented by individual coke particles reduced hydrogen indices and elevated Tmax values up to 440°C. An “oil-type” contribution (evidenced by mature biomarkers, hopene/hopane ratios, elevated background fluorescence, n-alkane distribution) is also present in the sediments which most likely originated at greater depth and impregnated the surface sediments. The heat source responsible for recrystallisation of foraminiferal carbonate and maturation of organic particles in Shaban Deep sediments most likely is attributed to modern basalt extrusions which now separate the Shaban Deep subbasins.

Description: An active seafloor hydrothermal system subjects the background sediments of the Grimsey Graben (Tjörnes Fracture Zone) to alteration that produces dissolution of the primary volcaniclastic matrix and replacement/precipitation of sulfides, sulfates, oxides, oxyhydroxides, carbonates and phyllosilicates. Three types of hydrothermal alteration of the sediment are defined on the basis of the dominant hydrothermal phyllosilicate formed: smectite, kaolinite, chlorite. The most common alteration is near‐total conversion of the volcaniclastic material to smectite (95–116°C). The dominant smectite in the deepest sediments sampled is beidellite, which is replaced by montmorillonite and an intimate mixture of di‐ and tri‐octahedral smectite up core. This gradual vertical change in smectite composition suggests an increase in the Mg supply upward, the result of sediment alteration by the ascending hydrothermal fluids mixing with descending seawater. The vertical sequence kaolinite → kaolinite‐smectite mixed‐layer → smectite from bottom to top of a core, as well as the distinct zonation across the veins (kaolinite in the central zone → kaolinite‐smectite in the rim), suggests hydrothermal transformation of the initially formed smectite to kaolinite through kaolinite‐smectite mixed‐layer (150–160°C). The cause of this transformation might have been an evolution of the fluids toward a slightly acidic pH or a relative increase in the Al concentration. Minor amounts of chamosite fill thin veins in the deepest sections of some cores. The gradual change from background clinochlore to chamosite across the veins suggests that chamosite replaces clinochlore as Fe is made available from hydrothermal dissolution of detrital Fe‐containing minerals. The internal textures, REE distribution patterns and the mode of occurrence of another magnesian phyllosilicate, kerolite, suggest that this mineral is the primary precipitate in the hydrothermal chimneys rather than an alteration product in the sediment. Kerolite precipitated after and grew on anhydrite in the chimneys. Oxygen isotope ratios are interpreted to reflect precipitation of kerolite at temperatures of 302° to 336°C. It accumulated in the hydrothermal mounds following the collapse of the chimneys and subsequent dissolution of anhydrite, thereby forming highly permeable aquifer layers underlying the vent field. Some kerolite was redeposited in the near vent field sediments by turbidity flows. The altered sediments are depleted in Mn, Rb and Sr, and enriched in U, Mo, Pb, Ba, As, Bi, Sb, Ag, Tl and Ga, as a result of leaching and precipitation, respectively. Conservative elements (Ti, Zr, Hf, Sc, Cr, Nb and Sn) are depleted or enriched in the altered sediments because of passive (precipitation or leaching of other phases) rather than active (because of their mobility) processes.