Radiolytic Hydrogen Production in the Subseafloor Basaltic Aquifer

Hydrogen (H2) is produced in geological settings by dissociation of water due to radiation from radioactive decay of naturally occurring uranium (238U, 235U), thorium (232Th) and potassium (40K). To quantify the potential significance of radiolytic H2 as an electron donor for microbes within the South Pacific subseafloor basaltic aquifer, we use radionuclide concentrations of 43 basalt samples from IODP Expedition 329 to calculate radiolytic H2 production rates in basement fractures. The samples are from three sites with very different basement ages and a wide range of alteration types. U, Th, and K concentrations vary by up to an order of magnitude from sample to sample at each site. Comparison of our samples to each other and to the results of previous studies of unaltered East Pacific Rise basalt suggests that significant variations in radionuclide concentrations are due to differences in initial (unaltered basalt) concentrations (which can vary between eruptive events) and post-emplacement alteration. However, there is no clear relationship between alteration type and calculated radiolytic yields. Local maxima in U, Th, and K produce hotspots of H2 production, causing calculated radiolytic rates to differ by up to a factor of 80 from sample to sample. Fracture width also greatly influences H2 production, where microfractures are hotspots for radiolytic H2 production. For example, H2 production rates normalized to water volume are 190 times higher in 1 μm wide fractures than in fractures that are 10 cm wide. To assess the importance of water radiolysis for microbial communities in subseafloor basaltic aquifers, we compare electron transfer rates from radiolysis to rates from iron oxidation in subseafloor basalt. Radiolysis appears likely to be a more important electron donor source than iron oxidation in old (>10 Ma) basement basalt. Radiolytic H2 production in the volume of water adjacent to a square cm of the most radioactive SPG basalt may support as many as 1500 cells.

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title: Dust, volcanic ash, and the evolution of the South Pacific Gyre through the Cenozoic
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title: Thorium-Uranium Fractionation by Garnet: Evidence for a Deep Source and Rapid Rise of Oceanic Basalts
abstract: Mid-ocean ridge basalts (MORBs) and ocean island basalts (QIBs) are derived by partial melting of the upper mantle and are marked by systematic excesses of thorium-230 activity relative to the activity of its parent, uranium-238. Experimental measurements of the distribution of thorium and uranium between the melt and solid residue show that, of the major phases in the upper mantle, only garnet will retain uranium over thorium. This sense of fractionation, which is opposite to that caused by clinopyroxene-melt partitioning, is consistent with the thorium-230 excesses observed in young oceanic basalts. Thus, both MORBs and QIBs must begin partial melting in the garnet stability field or below about 70 kilometers. A calculation shows that the thorium-230—uranium-238 disequilibrium in MORBs can be attributed to dynamic partial melting beginning at 80 kilometers with a melt porosity of 0.2 percent or more. This result requires that melting beneath ridges occurs in a wide region and that the magma rises to the surface at a velocity of at least 0.9 meter per year.

title: Subduction cycling of U, Th, and Pb
abstract: Many studies argue, based partly on Pb isotopic evidence, that recycled, subducted slabs reside in the mantle source of ocean island basalts (OIB) ;  ;  [3] [A.W. Hofmann, W.M. White, Mantle plumes from ancient oceanic crust.  (1982) 421–436; B.L. Weaver, The origin of ocean island basalt end-member compositions: trace element and isotopic constraints.  (1991) 381–397; J.C. Lassiter, E.H. Hauri, Osmium-isotope variations in Hawaiian lavas: evidence for recycled oceanic lithosphere in the Hawaiian plume,  (1998) 483–496]. Such models, however, have remained largely untested against actual subduction zone inputs, due to the scarcity of comprehensive measurements of both radioactive parents (Th and U) and radiogenic daughter (Pb) in altered oceanic crust (AOC). Here, we discuss new, comprehensive measurements of U, Th, and Pb concentrations in the oldest AOC, ODP Site 801, and consider the effect of subducting this crust on the long-term Pb isotope evolution of the mantle. The upper 500 m of AOC at Site 801 shows > 4-fold enrichment in U over pristine glass during seafloor alteration, but no net change to Pb or Th. Without subduction zone processing, ancient AOC would evolve to low 208Pb/206Pb compositions unobserved in the modern mantle  [S.R. Hart, H. Staudigel, Isotopic characterization and identification of recycled components, in: Crust/Mantle Recycling at Convergence Zones, Eds.  Subduction, however, drives U–Th–Pb fractionation as AOC dehydrates in the earth's interior. Pacific arcs define mixing trends requiring 8-fold enrichment in Pb over U in AOC-derived fluid. A mass balance across the Mariana subduction zone shows that 44–75% of Pb but < 10% of U is lost from AOC to the arc, and a further 10–23% of Pb and 19–40% of U is lost to the back-arc. Pb is lost shallow and U deep from subducted AOC, which may be a consequence of the stability of phases binding these elements during seafloor alteration: U in carbonate and Pb in sulfides. The upper end of these recycling estimates, which reflect maximum arc and back-arc growth rates, remove enough Pb and U from the slab to enable it to evolve rapidly (≪ 0.5 Ga) to sources suitable to explain the 208Pb/206Pb isotopic array of OIB, although these conditions fail to simultaneously satisfy the 207Pb/206Pb system. Lower growth rates would require additional U loss (29%) at depths beyond the zones of arc and back-arc magmagenesis, which would decrease upper mantle κ (232Th/238U) over time, consistent with one solution to the “kappa conundrum”  [T. Elliott, A. Zindler, B. Bourdon, Exploring the kappa conundrum: the role of recycling in the lead isotope evolution of the mantle.  (1999) 129–145]. The net effects of alteration (doubling of μ [238U/204Pb]) and subduction (doubling of ω [232Th/204Pb]) are sufficient to create the Pb isotopic signatures of oceanic basalts.

title: Composition of altered oceanic crust at ODP Sites 801 and 1149
abstract: We present a comprehensive major and trace element dataset establishing ODP Site 801 as a geochemical reference for altered oceanic crust. The composition of old crustal sequences like those at Sites 801 and 1149 are critical to developing models of crustal aging and seawater chemistry evolution and to understanding the fate of crust consumed at subduction zones. Our estimate of the bulk composition of oceanic crust at Site 801 comprises ICP-AES and ICP-MS analyses of 117 discrete samples, 14 mixed composites and 5 glasses from the upper 500 m of Jurassic Pacific crust. Comparing the 801 “super-composite” with glass reveals enrichment of U (5x), Li (2x), K2O (4x), Rb (9x), and Cs (7x), similar to DSDP Sites 417/418, but little to no enrichment in Ba or Pb. The data also demonstrate good (∼10%) agreement between U measured on discrete samples and natural gamma logs, suggesting logging data is a reliable means of establishing bulk geochemical characteristics of oceanic crust. Data reported here serve to link other geochemical and mineralogical measurements on Site 801 and 1149 samples. We also document Boston University sample preparation procedures and instrument parameters for ICP-MS and ICP-AES analyses, and provide comparisons with other laboratories and techniques. We present new techniques for basaltic glass analyses using the Boston University 213 nm Nd-YAG LA-ICP-MS system, and show the data agree well with both solution-ICP-MS (5–10%) and ion probe measurements (∼10%).

title: Permeability within basaltic oceanic crust
abstract: Water-rock interactions within the seafloor are responsible for significant energy and solute fluxes between basaltic oceanic crust and the overlying ocean. Permeability is the primary hydrologic property controlling the form, intensity, and duration of seafloor fluid circulation, but after several decades of characterizing shallow oceanic basement, we are still learning how permeability is created and distributed and how it changes as the crust ages. Core-scale measurements of basaltic oceanic crust yield permeabilities that are quite low (generally 10−22 to 10−17 m²), while in situ measurements in boreholes suggest an overlapping range of values extending several orders of magnitude higher (10−18 to 10−13 m²). Additional indirect estimates include calculations made from borehole temperature and flow meter logs (10−16 to 10−11 m²), numerical models of coupled heat and fluid flow at the ridge crest and within ridge flanks (10−16 to 10−9 m²), and several other methods. Qualitative indications of permeability within the basaltic oceanic crust come from an improved understanding of crustal stratigraphy and patterns of alteration and tectonic modification seen in ophiolites, seafloor samples and boreholes. Difficulties in reconciling the wide range of estimated permeabilities arise from differences in experimental scale and critical assumptions regarding the nature and distribution of fluid flow. Many observations and experimental and modeling results are consistent with permeability varying with depth into basement and with primary basement lithology. Permeability also seems to be highly heterogeneous and anisotropic throughout much of the basaltic crust, as within crystalline rocks in general. A series of focused experiments is required to resolve permeability in shallow oceanic basement and to directly couple upper crustal hydrogeology to magmatic, tectonic, and geochemical crustal evolution.

title: The yield and isotopic composition of radiolytic H2, a potential energy source for the deep subsurface biosphere
abstract: The production rate and isotopic composition of H2 derived from radiolytic reactions in H2O were measured to assess the importance of radiolytic H2 in subsurface environments and to determine whether its isotopic signature can be used as a diagnostic tool. Saline and pure, aerobic and anaerobic water samples with pH values of 4, 7, and 10 were irradiated in sealed vials at room temperature with an artificial γ source, and the H2 abundance in the headspace and its isotopic composition were measured. The H2 concentrations were observed to increase linearly with dosage at a rate of 0.40 ± 0.04 molecules (100 eV)−1 within the dosage range of 900 to 3500 Gray (Gy; Gy = 1 J Kg−1) with no indication of a maximum limit on H2 concentration. At ∼2000 Gy, the H2 concentration varied only by 16% across the experimental range of pH, salinity, and O2. Based upon this measured yield and H2 yields for α and β particles, a radiolytic H2 production rate of 10−9 to 10−4 nM s−1 was estimated for the range of radioactive element concentrations and porosities typical of crustal rocks. The δD of H2 ( δ D = ( ( D / H ) sample / ( D / H ) standard − 1 ) × 1000 ) was independent of the dosage, pH (except for pH 4), salinity, and O2 and yielded an αDH2O-H2 of 2.05 ± 0.07 (αDH2O-H2 = (D/H)H2O to (D/H)H2), slightly less than predicted radiolytic models. Although this radiolytic fractionation value is significantly heavier than that of equilibrium isotopic exchange between H2 and H2O, the isotopic exchange rate between H2 and H2O will erase the heavy δD of radiolytic H2 if the age of the groundwater is greater than ∼103 to 104 yr. The millimolar concentrations of H2 observed in the groundwater of several Precambrian Shields are consistent with radiolysis of water that has resided in the subsurface for a few million years. These concentrations are well above those required to support H2-utilizing microorganisms and to inhibit H2-producing, fermentative microorganisms.

title: Self-radiolysis of tritiated water. 1. A comparison of the effects of 60Co γ-rays and tritium β-particles on water and aqueous solutions at room temperature
abstract: Monte Carlo simulations were used to investigate the chemistry of pure water and aqueous solutions after irradiation with different kinds of radiation: tritium β-rays and high-energy electrons or 60Co γ-rays. The objective of this work was to elucidate the mechanisms involved in the self-radiolysis of tritiated water, and to examine the importance of the effects of higher “linear energy transfer” (LET) by comparing 3H β-electrons (mean initial energy of ∼5.7 keV) with 60Co γ-rays (∼1-MeV electrons). We considered several chemical systems for which experimental data were available. These included pure water, aqueous solutions of sulfuric acid, and aqueous ferrous sulfate solutions in aerated 0.4 M H2SO4 (Fricke dosimeter). Simulations clearly showed quantitatively different yields of radical and molecular products produced by the radiolysis of water with tritium β− particles compared with corresponding yields from γ or energetic electron radiolysis. As a rule, lower radical and higher molecular yields were observed for 3H β-rays. These differences in yields are completely consistent with differences in the nonhomogeneous distribution of primary transient species (i.e., the structure of electron tracks) in the two cases. In the “short-track” (columnar) geometry of tritium β-electron radiolysis, radicals were formed in much closer initial proximity than in the “spur” (spherical) geometry of γ radiolysis. The “short-track” geometry favors radical-radical reactions in the diffusing tracks, which increases the proportion of molecular products at the expense of the radical products. The same trend in yields of radical and molecular products was also found under acidic conditions as well as in the aerated Fricke dosimeter. Unfortunately, comparison with experimental data was rather limited due to the paucity of experimental information for the radiolysis of water by 3H β-particles. Despite this deficiency, our simulations reproduced very well the significant increase observed in the yield of H2 at the microsecond time scale for 3H β-electrons (∼0.6 molecule/100 eV) compared to 60Co γ-rays (0.45 molecule/100 eV). Furthermore, our predicted yield of Fe3+ ions for tritium β-electron radiolysis of Fricke (acidic ferrous sulfate) solutions compared well with the literature values (∼11.9–12.9 molecules/100 eV). In particular, it was shown that the measured yield of the Fricke dosimeter was best reproduced if a single, “mean” or “equivalent” electron energy of ∼7.8 keV was used to mimic the energy deposition by the tritium β-particles (rather than the commonly used mean of ∼5.7 keV that mimics the tritium beta energy spectrum), in full accordance with a recommendation of ICRU Report 17. This decrease in G(Fe3+) compared to the value observed for 60Co γ-rays (15.5 ± 0.2 molecules/100 eV) was mostly due to the decrease in the yield of the escape radical products. Such results, even if fragmentary, corroborate very well with previous experimental and theoretical work, and support a model of tritium β radiolysis mainly driven by the chemical action of short tracks of high local LET.

title: Self-radiolysis of tritiated water. 3. The ˙OH scavenging effect of bromide ions on the yield of H2O2 in the radiolysis of water by 60Co γ-rays and tritium β-particles at room temperature
abstract: Monte Carlo track chemistry simulations were used to determine the yields (or G-values) of hydrogen peroxide in the radiolysis of neutral water and dilute aqueous bromide solutions by low linear energy transfer (LET ∼ 0.3 keV μm−1) radiation (e.g., γ-rays from 60Co, fast electrons or high-energy protons) and tritium β-particles (mean LET ∼ 6 keV μm−1) at 25 °C. We investigated the influence of Br− ions, as selective scavengers of ˙OH radical precursors of H2O2, on the inhibition of G(H2O2) for these two types of radiation. Studying this system under a wide range of Br− concentrations (5 × 10−7 to 0.2 M) and using a well-accepted mechanism for radiolysis in the presence or absence of air, we examined the chemical changes in the scavengeability of H2O2 produced by 300 MeV irradiating protons (used in this work to reproduce the effects of 60Co γ/fast electron radiolysis) and tritium β-electron radiolysis. We found that these changes could be related to differences in the initial spatial distributions of radiolytic species (i.e., the structure of the electron tracks, the low-energy β-electrons of tritium depositing their energy almost entirely as cylindrical “short tracks” and the energetic Compton electrons produced by γ-radiolysis forming mainly spherical “spurs”), in full agreement with previous experimental and theoretical work. Simulations showed that the short track geometry of higher LET tritium β-electrons in both water and aqueous bromide solutions favored a clear increase in G(H2O2) compared to 60Co γ-rays. Moreover, the presence of oxygen was seen to scavenge hydrated electrons (eaq−) and H˙ atoms on the ∼10−7 s time scale, thereby protecting H2O2 from further reactions with these species in the homogeneous stage of radiolysis. This protection against eaq− and H˙ atoms therefore led to an increase in the long time H2O2 yields, as seen experimentally. Finally, for both deaerated and aerated solutions, the H2O2 yield in tritium β-radiolysis was found to be more easily suppressed than in the case of cobalt γ-radiolysis, and interpreted by the quantitatively different chemistry between spurs and short tracks. These differences in the scavengeability of H2O2 precursors in passing from 300 MeV irradiating protons to tritium β-electron irradiation were in good agreement with experimental data, thereby lending strong support to the picture of tritium β-radiolysis in terms of short tracks of high local LET.

title: Mantle Dynamics and Generation of a Geochemical Mantle Boundary along the East Pacific Rise -Pacific/Antarctic ridge
abstract: A large-scale mantle compositional discontinuity was identified along the East Pacific Rise (EPR) and the Pacific-Antarctic Ridge (PAR) with an inferred transition located at the EPR 23 degrees S-32 degrees S. Because of the EPR-Easter hotspot interactions in this area, the nature of this geochemical discontinuity remains unclear. IODP Sites U1367 and 1J1368 drilled into the ocean crust that was accreted at similar to 33.5 Ma and similar to 13.5 Ma, respectively, between 28 degrees S and 30 degrees S on the EPR. We use lavas from Sites U1367 and U1368 to track this mantle discontinuity away from the EPR. The mantle sources for basalts at Sites U1367 and U1368 represent, respectively, northern and southern Pacific mantle sub-domains in terms of Sr-Nd-Pb-Hf isotopes. The significant isotopic differences between the two IODP sites are consistent with addition of ancient subduction-processed ocean crust to the south Pacific mantle sub-domain. Our modeling result shows that a trace element pattern similar to that of U1368 E-MORB can be formed by melting a subduction-processed typical N-MORB. The trace element and isotope compositions for Site U1368 MORBs can be formed by mixing a HIMU mantle end-member with Site U1367 MORBs. Comparison of our data with those from the EPR-PAR shows a geochemical mantle boundary near the Easter microplate that separates the Pacific upper mantle into northern and southern sub-domains. On the basis of reconstruction of initial locations of the ocean crust at the two sites, we find that the mantle boundary has moved northward to the Easter microplate since before 33.5 Ma. A model, in which along-axis asthenospheric flow to where asthenosphere consumption is strongest, explains the movement of the apparent mantle boundary. Crown Copyright (C) 2013

title: Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific
abstract: Determining the relationship between ocean floor basalt alteration and sedimentation is fundamental to understanding how oceanic crust evolves with time. Ocean floor basalts recovered at IODP Sites U1365 (similar to 100 Ma) and U1368 (similar to 13.5 Ma) in the South Pacific have been subjected to remarkably low sedimentation rates (0.71 and 1.1 m/Myr(-1), respectively). We report detailed petrographic and geochemical analysis of basalt cores from these sites in order to investigate what impact sediment insulation has on seafloor alteration beyond 10-15 Myr of ocean crust formation. Both sites exhibit low-temperature (< 150 degrees C) alteration (e. g., iron-hydroxides, carbonate, and quartz) within a predominantly oxidative regime, albeit with markedly different alteration styles and intensity. Alteration at Site U1365, which is predominantly composed of sheet flows, occurs mainly near sheet flow boundaries and fractures. In contrast, Site U1368 comprises interlayered pillows and thin sheet flows that have been subjected to relatively even levels of alteration. Variation of alteration style and intensity between Sites U1365 and U1368 appear closely tied to lithology and crustal structure. Although alteration-induced elemental changes at both sites are similar in, e. g., increasing K, Rb, U, Ba, and Fe3+ and decreasing Fe2+, Ca, and Ni, they show distinct differences in Th, which is significantly decreased at Site U1365 but relatively constant at Site U1368. At both sites enrichment of LREEs relative to HREEs is ascribed to alteration. The greater vein abundance and notably higher Fe3+/TiO2, K2O/TiO2, LOI/TiO2, and Rb/TiO2 ratios of representative samples at Site U1365 compared to Site U1368 are attributed to increased alteration intensity. This is mirrored by greater overall chemical change (Fe2O3, FeO, CaO, K2O, Li, Rb, Pb, and U) observed at Site U1365 than those of Site U1368 and other DSDP/ODP sites between 6 and 46 Ma. Since both Sites U1365 and U1368 endured only minimal sedimentation, we attribute the differences in overall chemical change across the two sites to duration of exposure to seawater.

title: Recent eruptive history and magma reservoir dynamics on the southern East Pacific Rise at 17°30′S
abstract: Submersible-based geologic observations and geochemical, magnetic paleointensity, and (210Pb/226Ra) radioactive disequilibria data indicate that at least five distinct lava sequences (three normal mid-ocean ridge basalt (N-MORB) and two transitional mid-ocean ridge basalt (T-MORB)) have been erupted within the last several hundred years along a 27-km-long portion of the fast spreading East Pacific Rise near 17°30′S. Isotopic and geochemical variations, both within and between eruptive units, indicate mixing of different primary magmas concurrently with differentiation in shallow-level subaxial magma reservoirs. Differentiation trends are linked to geographical variations in axial magma chamber (AMC) characteristics, with the lowest MgO samples erupted above the shallowest portion of the AMC, suggesting that pre-eruptive magma temperature is in part controlled by the depth-dependent efficacy of hydrothermal cooling. A third-order axial discontinuity at ∼17°29′S coincides with a narrowing of the subaxial melt lens and an increase in lava MgO to the south; we interpret the latter to reflect a sharp increase in the mixing proportion of recharge to low-MgO magma residing in the melt lens. Magmatic evolution of this area over the last few hundred years reflects continually evolving conditions in the subsurface and mantle melting processes that vary rapidly at rates that are at least as great as the eruption rate.

title: ALTERATION OF UPPER OCEAN CRUST IN A RIDGE-FLANK HYDROTHERMAL UPFLOW ZONE : MINERAL , CHEMICAL , AND ISOTOPIC CONSTRAINTS FROM HOLE 896 A 1
abstract: Hole 896A penetrates into the upper volcanic section of a ridge-flank hydrothermal upflow zone. Analyses of the secondary mineralogy and chemistry, whole-rock geochemistry, and oxygen, carbon, and strontium isotope ratios of whole rocks and secondary minerals were conducted to constrain the chemical and thermal evolution of hydrothermal alteration and its effects on the upper crust at Site 896. Celadonite ± Fe-oxyhydroxides are the earliest secondary minerals and formed at low temperatures. The crust was open to free circulation of seawater, but solutions derived from deeper in the crust may have provided some of the Fe, Si, and alkalies required for celadonite formation. Whole-rock chemical changes involved increased alkalies, and slight increases in H,O, Fe/ Fe(Totai)j §WQ and Sr/Sr. Subsequently, Fe-oxyhydroxides formed reddish alteration halos in the rocks in relatively young crust, where open circulation of large volumes of seawater maintained oxidizing conditions and low temperatures. Whole-rock chemical changes are characterized mainly by oxidation, but include increased H2O, alkalies, U, P, δ l 8 0, and Sr/Sr; local losses of S and possibly Tl; and possible minor losses of Ca and Mg. The next alteration stage was characterized by the pervasive formation of saponite in slightly older crust, where circulation of seawater was more restricted, conditions were less oxidizing, and temperatures were probably higher though less than 100°150°C. Whole-rock chemical changes include increased Mg, H2O, δ 1 8 θ, and Sr/Sr; slight alkali increases; and local gains of S and Tl. Significant uptake of Mg by the upper crust occurred through the formation of saponite in veins and breccias. Four saponites have Sr/Sr = 0.70842-0.70875 indicating that fluids were partly evolved seawater, but one fibrous saponite has Sr/Sr = 0.704363, requiring localized, rock-dominated fluid compositions. Calcium carbonates and zeolites were the last secondary phases to form. An early, lower temperature (26°-35°C) generation of carbonates, has low Mg, Fe, and Mn concentrations and high Sr contents. These carbonates formed from partly reacted seawater that had decreased Mg/Ca ratios and contained 2.5%-10% basaltic Sr (carbonate Sr/Sr = 0.708775 ± 0.000066 [2σ], N 11). A second generation of carbonates formed at higher temperatures (47°-67°C), from seawater-derived fluids with lowered Mg/Ca and Sr/Ca ratios and elevated Fe, and Mn concentrations. Trace-element chemistry of the high-temperature carbonates in general, and the lower Sr/Sr of rare high-temperature aragonites (0.7079-0.7084) suggest more restricted circulation of seawater and reducing conditions. The higher temperature carbonates formed at temperatures consistent with the present-day thermal regime at Site 896; a ridge-flank hydrothermal upflow zone with basement temperatures greater than 50°C. All rocks from Hole 896A have interacted with seawater at low temperatures, and samples commonly record the integrated legacy of superimposed alteration processes. The most intense chemical changes have occurred within hyaloclastite and fragmentation breccias that comprise at least 5% of the uppermost oceanic crust at Site 896. The sequence of alteration processes present in Hole 896A is broadly similar to that recorded in the upper crust (above 300 m sub-basement) of Hole 504B, which is located approximately 1 km to the northwest, in a zone of average regional heat flow. The main differences between the material from Holes 896A and 504B is the greater abundance of carbonates, and hyaloclastite and fragmentation breccias, and the common occurrence of thick ( l cm) saponite veins in the new hole.

title: Hydrothermal alteration of upper oceanic crust formed at a fast-spreading ridge: mineral, chemical, and isotopic evidence from ODP Site 801
abstract: ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10-20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5-100 [deg]C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (2O; local increases in FeT, Ba, Th, and U; and local losses of Mg and Ni.Secondary carbonate veins have 87Sr/86Sr=0.706337-0.707046, and a negative correlation with [delta]18O results from seawater-basalt interaction. Carbonates could have formed at any time since the formation of Site 801 crust. Variable [delta]13C values (-11.2[permil] to 2.9[permil]) reflect the incorporation of oxidized organic carbon from intercalated sediments and changes in the [delta]13C of seawater over time.Compared to other oceanic basements, a major difference at Site 801 is the presence of two hydrothermal silica-iron deposits that formed from low-temperature hydrothermal fluids at the spreading axis. Basalts associated with these horizons are intensely altered (60-100%) to phyllosilicates, calcite, K-feldspar, and titanite; and exhibit large increases in K, Rb, Cs, Ba, H2O, and CO2, and losses of FeT, Mn, Mg, Ca, Na, and Sr. These effects may be common in crust formed at fast-spreading rates, but are not ubiquitous. A second important difference is that the abundance of brown oxidation halos along fractures at Site 801 is an order of magnitude less than at some other sites (2% vs. 20-30%). Relatively smooth basement topography (<100 m) and high sedimentation rate (8 m/Ma) probably restricted the access of oxygenated seawater. Basement lithostratigraphy and early low-temperature hydrothermal alteration and mineral precipitation in fractures at the spreading axis controlled permeability and limited later flow of oxygenated seawater to restricted depth intervals.

title: Effect of hydrothermal ridge flank alteration on the in situ physical properties of uppermost oceanic crust
abstract: The circulation of cold water through the oceanic crust at mid-ocean ridge flanks causes changes in the composition and physical properties of the crust. This study focuses on the effect of hydrothermal ridge flank alteration on in situ physical properties. P wave velocity, electrical resistivity, and natural radioactivity are compared. They are obtained from downhole logging operations in seven boreholes drilled by the Deep Sea Drilling Project and Ocean Drilling Program. Basement age ranges from 5.9 to 167 m.y. The comparison shows that the average values of P wave velocity and electrical resistivity increase with the logarithm of basement age for lava flows and pillow basalts. Natural radioactivity and potassium content do not show a statistically significant relation to the basement age. No direct influence of spreading rate or thickness of the sediment cover on the in situ physical properties can be observed. The increase in P wave velocity with age reflects the reduction in pore space due to the precipitation of secondary minerals. Electrical resistivity is low in young crust where conductive clays are important alteration minerals. It increases in old crust because electrically insulating minerals, such as carbonates, are formed as late stage alteration minerals. Natural radioactivity is influenced by the composition of the magma but also rises in altered rocks due to the formation of K-rich clay minerals. The results of this study show that ridge flank hydrothermal alteration persists over longer-term intervals than indicated by seismic experiments.

title: 22. LOW-TEMPERATURE HYDROTHERMAL ALTERATION OF JURASSIC OCEAN CRUST, SITE 801
abstract: Low-temperature hydrothermal alteration of basement from Site 801 was studied through analyses of the mineralogy, chemistry, and oxygen isotopic compositions of the rocks. The more than 100-m section of 170-Ma basement consists of 60 m of tholeiitic basalt separated from the overlying 60 m of alkalic basalts by a >3-m-thick Fe-Si hydrothermal deposit. Four alteration types were distinguished in the basalts: (1) saponite-type (Mg-smectite) rocks are generally slightly altered, exhibiting small increases in H2O, δ 18 θ, and oxidation; (2) celadonite-type rocks are also slightly altered, but exhibit uptake of alkalis in addition to hydration and oxidation, reflecting somewhat greater seawater/rock ratios than the saponite type; (3) Al-saponite-type alteration resulted in oxidation, hydration, and alkali and 18 O uptake and losses of Ca and Na due to the breakdown of plagioclase and clinopyroxene; and (4) blue-green rocks exhibit the greatest chemical changes, including oxidation, hydration, alkali uptake, and loss of Ca, Na, and Mg due to the complete breakdown of plagioclase and olivine to K-feldspar and Phyllosilicates. Saponite- and celadonite-type alteration of the tholeiite section occurred at a normal mid-ocean ridge basalt spreading center at temperatures <20°C. Near- or off-axis intrusion of an alkali basalt magma at depth reinitiated hydrothermal circulation, and the Fe-Si hydrothermal deposit formed from cool (<60°C) distal hydrothermal fluids. Focusing of fluid flow in the rocks immediately underlying the deposit resulted in the extensive alteration of the blue-green rocks at similar temperatures. Al-saponite alteration of the subsequent alkali basalts overlying the deposit occurred at relatively high water/rock ratios as part of the same low-temperature circulation system that formed the hydrothermal deposit. Abundant calcite formed in the rocks during progressive "aging" of the crust during its long history away from the spreading center.

title: Palagonitization of Basalt Glass in the Flanks of Mid-Ocean Ridges: Implications for the Bioenergetics of Oceanic Intracrustal Ecosystems.
abstract: When basalt is exposed to oxygenated aqueous solutions, rims of palagonite form along fractures at the expense of glass. We employed electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of fresh glass and adjacent palagonite crusts to determine the geochemical changes involved in palagonite formation. Samples were retrieved from drill cores taken in the North Pond Area, located on the western flank of the Mid-Atlantic Ridge at 22°45'N and 46°05'W. We also analyzed whole rock powders to determine the overall crust-seawater exchange in a young ridge flank. Radioactive elements are enriched in palagonite relative to fresh glass, reaching concentrations where radiolytic production of molecular hydrogen (H2) may be a significant energy source. Based on these results, we hypothesize that microbial ecosystems in ridge flank habitats undergo a transition in the principal energy carrier, fueling carbon fixation from Fe oxidation in very young crust to H2 consumption in older crust. Unless the H2 is swept away by rapid fluid flow (i.e., in young flanks), it may easily accumulate to levels high enough to support chemolithoautotrophic life. In older flanks, crustal sealing and sediment accumulation have slowed down seawater circulation, and the significance of radiolytically produced H2 for catalytic energy supply is expected to increase greatly. Similar habitats on other planetary surfaces are theoretically possible, as accumulation of radiolytically produced hydrogen merely requires the presence of H2O molecules and a porous medium, from which the hydrogen is not lost.

title: Environmental and chemical controls on palagonitization
abstract: Palagonitized sideromelane from submarine volcaniclastic, seafloor volcanic, marine phreatomagmatic, lacustine phreatomagmatic, and subglacial volcanic settings was investigated using in situ microanalysis to test if palagonite composition and texture are related to depositional environment. Palagonitization extent varies linearly and inversely with original sample porosity, suggesting that porosity is a controlling factor of palagonitization. Water absorbance of reflected infrared light varies linearly with water content derived from electron microprobe totals. Palagonite water content has a linear, inverse relationship to palagonitization extent. REEs are immobile during palagonitization, so they can be used to construct isocon diagrams for estimating major-element concentration changes. Major-element and overall mass changes during palagonitization vary widely (particularly for FeO and TiO2) and indicate that palagonitization cannot be an isovolumetric process. These parameters depend strongly on original sideromelane composition, thus requiring composition to be taken into account when performing global oceanic cation flux calculations. Subalkaline sideromelane dissolves much more rapidly than alkaline sideromelane during palagonitization. Two styles of palagonitization, burial-diagenesis (relatively long-duration, low water/rock; passive fluid circulation) and hydrothermal (relatively short-duration, high water/rock; hydrothermal fluid circulation), are recognized. Observed palagonite REE concentration gradients indicate that sideromelane dissolution must continue in the zone behind the advancing palagonitization front. MgO was found to be highly mobile during palagonitization. Observed palagonite MgO gradients are not developed during sideromelane dissolution, but instead record initiation of syn- and/or post-palagonitization conversion of the gel-palagonite layer to a phyllosillicate layer, consistent with evolution of sideromelane alteration layers toward equilibrium with the solution.