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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Contributions to mineralogy and petrology 87 (1984), S. 149-169 
    ISSN: 1432-0967
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts. 1) Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage. 2) Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H2O, increase of Fe3+/FeT, and possibly some losses of Ca and Mg occurred during this stage. 3a) Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H2O, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost. 3b) Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe3+/FeT and H2O occurred. 4) Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H2O, and CO2 in the rocks. 5) A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage. Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-0581
    Keywords: Accretion model ; oceanic crust ; downhole measurements ; hydrothermalism ; mid-ocean ridge
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Downhole measurements recorded in the context of the Ocean Drilling Program in Hole 504B, the deepest hole drilled yet into the oceanic crust, are analyzed in terms of accretion processes of the upper oceanic crust at intermediate spreading-rate. The upper part of the crust is found to support the non steady-state models of crustal accretion developed from seafloor observations (Kappel and Ryan, 1986; Gente, 1987). The continuous and vertical nature of borehole measurements provides stratigraphic and structural data that cannot be obtained solely from seafloor studies and, in turn, these models define a framework to analyze the structural, hydrological, and mineralogical observations made in the hole over the past decade. Due to the observed zonation with depth of alteration processes, and its relation to lava morphologies, the 650-m-thick effusive section penetrated in Hole 504B is postulated to be emplaced as the result of two main volcanic sequences. Massive lava flows are interpreted as corresponding to the onset of these sequences emplaced on the floor of the axial graben. The underlying lava made of structures with large porosity values and numerous cm-scale fractures is thus necessarily accreted at the end of the previous volcanic episode. On top of such high heterogeneous and porous intervals, the thick lava flows constitute crustal permeability barriers, thereby constraining the circulation of hydrothermal fluids. Accreted in the near vicinity of the magma chamber, the lower section is that exposed to the most intense hydrothermal circulation (such as black smokers activity). Once capped by a massive flow at the onset of the second volcanic phase, the lower interval is hydrologically separated from ocean-waters. A reducing environment develops then below it resulting, for example, in the precipitation of sulfides. Today, whereas the interval corresponding to the first volcanic episode is sealed by alteration minerals, the second-one is still open to fluid circulation in its upper section. Thus, upper part of the volcanic edifice is potentially never exposed to fluids reaching deep into the crust, while the lower one is near the ridge axis. Considering that most of the extrusives are emplaced within a narrow volcanic zone, the first unit extruded for a given vertical cross-section is necessarily emplaced at the ridge-axis. In Hole 504B, the 250-m-thickTransition Zone from dikes to extrusives is interpreted as the relict massive unit flooding the axial graben at the onset of the first volcanic sequence, and later ruptured by numerous dikes. Further from the axis, the same massive unit constitutes a potential permeability cap for vertical crustal sections accreted earlier. Also, the upper 50 meters of the basement might be considered as the far-end expression of massive outpours extruded near the ridge-axis.
    Type of Medium: Electronic Resource
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  • 3
    Publication Date: 2022-05-25
    Description: Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 268 (2008): 110-123, doi:10.1016/j.epsl.2008.01.010.
    Description: The subsurface biosphere in the basaltic ocean crust is potentially of major importance in affecting chemical exchange between the ocean and lithosphere. Alteration of the oceanic crust commonly yields secondary pyrite that are depleted in 34S relative to igneous sulfides. Although these 34S depleted sulfur isotope ratios may point to signatures of biological fractionation, previous interpretations of sulfur isotope fractionation in altered volcanic rocks have relied on abiotic fractionation processes between intermediate sulfur species formed during basalt alteration. Here, we report results for multiple-S isotope (32S,33S,34S) compositions of altered basalts at ODP Site 801 in the western Pacific and provide evidence for microbial sulfate reduction within the volcanic oceanic crust. In-situ ion-microprobe analyses of secondary pyrite in basement rocks show a large range of δ34S values, between –45‰ and 1‰, whereas bulk rock δ34S analyses yield a more restricted range of –15.8 to 0.9‰. These low and variable δ34S values, together with bulk rock S concentrations ranging from 0.02% up to 1.28% are consistent with loss of magmatic primary mono-sulfide and addition of secondary sulfide via microbial sulfate reduction. High-precision multiple-sulfur isotope (32S/33S/34S) analyses suggest that secondary sulfides exhibit mass-dependent equilibrium fractionation relative to seawater sulfate in both δ33S and δ34S values. These relationships are explained by bacterial sulfate reduction proceeding at very low metabolic rates. The determination of the S-isotope composition of bulk altered oceanic crust demonstrates that S-based metabolic activity of subsurface life in oceanic basalt is widespread, and can affect the global S budget at the crust-seawater interface.
    Description: Alt's contribution was supported by NSF OCE-0424558 and OCE-0622949. Rouxel's contribution was supported by NSF OCE-0622982 and Frank and Lisina Hoch Endowed Fund. Ono thanks Agouron Institute and NSF OCE-0753126 for funding. This research used samples and/or data provided by the Ocean Drilling Program. The ODP is sponsored by the US National Science Foundation (NSF) and participating countries under the management of Joint Oceanographic Institutions (JOI).
    Keywords: Sulfur isotopes ; Seafloor weathering ; Deep biosphere ; Oceanic crust ; Sulfur cycle
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
    Format: application/pdf
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  • 4
    Publication Date: 2022-05-25
    Description: Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Lithos 227 (2015): 1-20, doi:10.1016/j.lithos.2015.03.015.
    Description: Serpentine seamounts located on the outer half of the pervasively fractured Mariana forearc provide an excellent window into the forearc devolatilization processes, which can strongly influence the cycling of volatiles and trace elements in subduction zones. Serpentinized ultramafic clasts recovered from an active mud volcano in the Mariana forearc reveal microstructures, mineral assemblages and compositions that are indicative of a complex polyphase alteration history. Petrologic phase relations and oxygen isotopes suggest that ultramafic clasts were serpentinized at temperatures below 200 °C. Several successive serpe ntinization events represented by different vein generations with distinct trace element contents can be recognized. Measured Rb/Cs ratios are fairly uniform ranging between 1 and 10, which is consistent with Cs mobilization from sediments at lower temperatures and lends further credence to the low-temperature conditions proposed in models of the thermal structure in forearc settings. Late veins show lower fluid mobile element (FME) concentrations than early veins, suggesting a deacreasing influence of fluid discharge from sediments on the composition of the serpentinizing fluids. The continuous microfabric and mineral chemical evolution observed in the ultramafic clasts may have implications as to the origin and nature of the serpentinizing fluids. We hypothesize that opal and smectite dehydration produce quartz-saturated fluids with high FME contents and Rb/Cs between 1 and 4 that cause the early pervasive serpentinization. The partially serpentinized material may then be eroded from the basal plane of the suprasubduction mantle wedge. Serpentinization continued but the interacting fluids did not carry the slab-flux signature, either because FME were no longer released from the slab, or due to an en route loss of FMEs. Late chrysotile veins that document the increased access of fluids in a now fluid-dominated regime are characterized by reduced trace element contents with a slightly increased Rb/Cs ratio near 10. This lack of geochemical slab signatures consistently displayed in all late serpentinization stages may indicate that the slab-derived fluids have been completely reset (i.e. the FME excesses were removed) by continued water-rock reaction within the subduction channel. The final stage of diapiric rise of matrix and clasts in the conduits is characterized by brucite-dominated alteration of the clasts from the clast rim inward (independent of the intra-clast fabric relations), which corresponds to re-equilibration with alkaline, low-silica activity fluids in the rising mud.
    Description: This study was funded through a grant of the DFG to WB (BA 1605/5-1).
    Keywords: Serpentinization ; Polyphase alteration ; Mud volcano ; Fluid mobile elements recycling ; Hydrated mantle wedge ; Forearc peridotites ; Subduction zone
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
    Format: application/pdf
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  • 5
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    Unknown
    PANGAEA
    In:  Supplement to: Ono, Shuhei; Keller, Nicole S; Rouxel, Olivier J; Alt, Jeffrey C (2012): Sulfur-33 constraints on the origin of secondary pyrite in altered oceanic basement. Geochimica et Cosmochimica Acta, 87, 323-340, https://doi.org/10.1016/j.gca.2012.04.016
    Publication Date: 2023-05-12
    Description: Low temperature alteration of oceanic basement rocks is characterized by net gain of sulfur, which commonly yields low d34S values, suggesting involvement of microbial sulfate reduction. In order to test whether secondary sulfide minerals are consistent with a biogenic source, we apply high precision multiple sulfur isotope analysis to bulk rock sulfide and pyrite isolates from two contrasting types of altered oceanic basement rocks, namely serpentinized peridotites and altered basalts. Samples from two peridotite sites (Iberian Margin and Hess Deep) and from a basalt site on the eastern flank of the Juan de Fuca Ridge yield overlapping d34S values ranging from 0 per mil to -44 per mil. In contrast, sulfides in the basalt site are characterized by relatively low D33S values ranging from -0.06 per mil to 0.04 per mil, compared to those from peridotite sites (0.00 per mil to 0.16 per mil). The observed D33S signal is significant considering the analytical precision of 0.014 per mil (2 sigma). We present a batch reaction model that uses observed d34S and D33S relationships to quantify the effect of closed system processes and constrain the isotope enrichment factor intrinsic to sulfate reduction. The estimated enrichment factors as large as 61 per mil and 53 per mil, for peridotite and basalt sites respectively, suggest the involvement of microbial sulfate reduction. The relatively high D33S values in the peridotite sites are due to sulfate reduction in a closed system environment, whereas negative D33S values in the basalt site reflect open system sulfate reduction. A larger extent of sulfate reduction during alteration of peridotite to serpentinite is consistent with its higher H2 production capacity compared to basalt alteration, and further supports in-situ microbial sulfate reduction coupled with H2 production during serpentinization reactions.
    Keywords: Integrated Ocean Drilling Program / International Ocean Discovery Program; IODP
    Type: Dataset
    Format: application/zip, 2 datasets
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  • 6
    Publication Date: 2023-05-12
    Keywords: Belssbank; CIPW Norm; Diopside; Event label; Ilmenite; Kaalvallei; Magnetite; MULT; Multiple investigations; Nepheline; Olivine; ORDINAL NUMBER; Plagioclase; Pyroxene; Sample code/label; South Africa; Total
    Type: Dataset
    Format: text/tab-separated-values, 54 data points
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  • 7
    Publication Date: 2023-05-12
    Keywords: Belssbank; Calculated; Chromium(III) oxide; Event label; Garnet; Iron 2+/Iron total ratio; Iron 2+/Iron total ratio, standard deviation; Iron oxide, FeO; Kaalvallei; MULT; Multiple investigations; ORDINAL NUMBER; Pyroxene; Sample code/label; Sodium oxide; South Africa; Standard deviation; Temperature, calculated; Δδ18O; δ18O; δ18O, standard deviation; δ56/54Fe; δ56/54Fe, standard deviation; δ57/54Fe; δ57/54Fe, standard deviation; δ57/56Fe; δ57/56Fe, standard deviation; δ57Fe; δ57Fe, standard deviation
    Type: Dataset
    Format: text/tab-separated-values, 226 data points
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  • 8
    Publication Date: 2023-05-12
    Keywords: ALV-4055; ALVIN; AT11-20; Atlantis (1997); DEPTH, sediment/rock; East Pacific Rise; Extract; Sample code/label; Sample mass; Submersible Alvin; Δδ33S; Δδ36S; δ34S
    Type: Dataset
    Format: text/tab-separated-values, 24 data points
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  • 9
    Publication Date: 2023-05-12
    Keywords: Aluminium oxide; Aluminium oxide, standard deviation; Belssbank; Calcium oxide; Calcium oxide, standard deviation; Electron microprobe (EMP); Elements, total; Elements, total, standard deviation; Event label; Iron oxide, FeO; Iron oxide, FeO, standard deviation; Kaalvallei; Magnesium oxide; Magnesium oxide, standard deviation; Manganese oxide; Manganese oxide, standard deviation; Mineral name; MULT; Multiple investigations; Nickel oxide; Nickel oxide, standard deviation; ORDINAL NUMBER; Potassium oxide; Potassium oxide, standard deviation; Sample amount; Sample code/label; Silicon dioxide; Silicon dioxide, standard deviation; Sodium oxide; Sodium oxide, standard deviation; South Africa; Titanium dioxide; Titanium dioxide, standard deviation
    Type: Dataset
    Format: text/tab-separated-values, 250 data points
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  • 10
    Publication Date: 2023-05-12
    Keywords: Belssbank; Event label; Kaalvallei; MULT; Multiple investigations; ORDINAL NUMBER; Sample code/label; Sample comment; South Africa; δ18O; δ18O, standard deviation
    Type: Dataset
    Format: text/tab-separated-values, 52 data points
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