10910309
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.
130427236
Fluxes of fluid and heat from the oceanic crustal reservoir
Abstract Recent discoveries define a global scale fluid reservoir residing within the uppermost igneous oceanic crust, a region of seafloor that is both warm and may harbor a substantial biosphere. This hydrothermal fluid reservoir formed initially within volcanic rocks newly erupted at mid-ocean ridges, but extends to the vastly larger and older ridge flanks. Upper oceanic crust is porous and permeable due to the presence of lava drainbacks, fissuring, and inter-unit voids, and this porosity and permeability allows active fluid circulation to advect measurable quantities of lithospheric heat from the crust to an average age of 65 Myr. A compilation of crustal porosities shows that this fluid reservoir contains nearly 2% of the total volume of global seawater. Heat flow and sediment thickness data allow calculation of reservoir temperatures, predicting 40°C mean temperatures in Cretaceous crust. Utilizing these temperature estimates, heat flow measurements and models for the thermal structure and evolution of the oceanic lithosphere, we have computed mean hydrothermal fluxes into the deep ocean as a function of plate age. The total hydrothermal volume flux into the oceans approaches 20% of the total riverine input and may contribute to the global seawater mass balance.
99916
The Deep Subsurface Biosphere in Igneous Ocean Crust: Frontier Habitats for Microbiological Exploration
We discuss ridge flank environments in the ocean crust as habitats for subseafloor microbial life. Oceanic ridge flanks, areas far from the magmatic and tectonic influence of seafloor spreading, comprise one of the largest and least explored microbial habitats on the planet. We describe the nature of selected ridge flank crustal environments, and present a framework for delineating a continuum of conditions and processes that are likely to be important for defining subseafloor microbial "provinces." The basis for this framework is three governing conditions that help to determine the nature of subseafloor biomes: crustal age, extent of fluid flow, and thermal state. We present a brief overview of subseafloor conditions, within the context of these three characteristics, for five field sites where microbial studies have been done, are underway, or have been proposed. Technical challenges remain and likely will limit progress in studies of microbial ridge flank ecosystems, which is why it is vital to select and design future studies so as to leverage as much general understanding as possible from work focused at a small number of sites. A characterization framework such that as presented in this paper, perhaps including alternative or additional physical or chemical characteristics, is essential for achieving the greatest benefit from multidisciplinary microbial investigations of oceanic ridge flanks.
1154322
Microbial diversity within basement fluids of the sediment-buried Juan de Fuca Ridge flank
Despite its immense size, logistical and methodological constraints have largely limited microbiological investigations of the subseafloor basement biosphere. In this study, a unique sampling system was used to collect fluids from the subseafloor basaltic crust via a Circulation Obviation Retrofit Kit (CORK) observatory at Integrated Ocean Drilling Program borehole 1301A, located at a depth of 2667 m in the Pacific Ocean on the eastern flank of the Juan de Fuca Ridge. Here, a fluid delivery line directly accesses a 3.5 million years old basalt-hosted basement aquifer, overlaid by 262 m of sediment, which serves as a barrier to direct exchange with bottom seawater. At an average of 1.2 × 104 cells ml−1, microorganisms in borehole fluids were nearly an order of magnitude less abundant than in surrounding bottom seawater. Ribosomal RNA genes were characterized from basement fluids, providing the first snapshots of microbial community structure using a high-integrity fluid delivery line. Interestingly, microbial communities retrieved from different CORKs (1026B and 1301A) nearly a decade apart shared major community members, consistent with hydrogeological connectivity. However, over three sampling years, the dominant gene clone lineage changed from relatives of Candidatus Desulforudis audaxviator within the bacterial phylum Firmicutes in 2008 to the Miscellaneous Crenarchaeotic Group in 2009 and a lineage within the JTB35 group of Gammaproteobacteria in 2010, and statistically significant variation in microbial community structure was observed. The enumeration of different phylogenetic groups of cells within borehole 1301A fluids supported our observation that the deep subsurface microbial community was temporally dynamic.
28399754
Oxygen consumption rates in subseafloor basaltic crust derived from a reaction transport model.
Oceanic crust is the largest potential habitat for life on Earth and may contain a significant fraction of Earth's total microbial biomass; yet, empirical analysis of reaction rates in basaltic crust is lacking. Here we report the first assessment of oxygen consumption in young (~8 Ma) and cool (<25 °C) basaltic crust, which we calculate from modelling dissolved oxygen and strontium pore water gradients in basal sediments collected during Integrated Ocean Drilling Program Expedition 336 to 'North Pond' on the western flank of the Mid-Atlantic Ridge. Dissolved oxygen is completely consumed within the upper to middle section of the sediment column, with an increase in concentration towards the sediment-basement interface, indicating an upward supply from oxic fluids circulating within the crust. A parametric reaction transport model of oxygen behaviour in upper basement suggests oxygen consumption rates of 1 nmol  cm(-3)ROCK d(-1) or less in young and cool basaltic crust.
46040371
Alteration of oceanic volcanic glass: textural evidence of microbial activity
The subsurface biosphere may constitute as much as 50 percent of Earth's biomass. Direct and indirect evidence suggests that an extensive biosphere exists in the rocks below the sea floor. This survey of basalts of the Atlantic, Pacific, and Indian Oceans supports the hypothesis that bacteria have colonized much of the upper oceanic crust, which has a volume estimated at 10(18) cubic meters. Although this is the largest habitat on Earth, its low abundance of bacteria constitutes much less than 1 percent of Earth's biomass.
128632332
3.5 billion years of glass bioalteration: Volcanic rocks as a basis for microbial life?
Abstract Alteration textures in volcanic glass from the seafloor fall into two classes, one suggestive of abiotic/diffusive hydration and chemical exchange, and another likely to be caused by microbial, cavity-forming, congruent dissolution. Glass bioalteration is common in submarine lavas throughout the world's ocean, dominant in the upper 300 m of the oceanic crust, and found in all well-preserved ophiolites and greenstone belts dating back to 3.5 Ga. It may yield a significant fraction of the global biomass and geochemical fluxes and is relevant to the development of the earliest life on Earth. We present a critical review concerning these glass bioalteration textures and present new data on their microchemical environment. We explore arguments for their biogenicity and further develop the prevalent model for their formation by relating corrosion morphology to the mechanism of microbial dissolution. Biological alteration produces conspicuous micron-scale granular and tubular textures. Granular glass alteration is well explained by colonizing microbes that selectively dissolve the glass in their contact area, forming a sponge-like interconnected network of micron-sized cavities along glass surfaces. Tubular alteration meanwhile, is more likely to be caused by filamentous cell extensions in a process similar to fungal tunneling of soil feldspars and marine carbonates. While we see clear functional similarities to fungal dissolution behavior, we do not know whether fungal or prokaryotic organisms are involved. However, this functional constraint may eventually help to identify potential microbes responsible for these features, potentially including eukaryotic or prokaryotic organisms. Yet, we caution that these organisms may be difficult to identify and to study, because they are likely to be sparsely distributed, slow growing, and difficult to cultivate.
83944478
Brock Biology of Microorganisms
Samenvatting:The book for introductory microbiology, Brock's Biology of Microorganismscontinues its long tradition of impeccable scholarship, outstanding art,and accuracy. It balances the most current coverage with the majorclassical concepts essential for understanding the science. A six-partpresentation covers principles of microbiology; evolutionary microbiologyand microbial diversity; metabolic diversity and microbial ecology;immunology, pathogenicity, and host responses; microbial diseases; andmicroorganisms as tools for industry and research. For researchers, groupleaders, senior scientists in pharmaceuticals, chemicals and biochemicalbiotechnology companies, and public health laboratories.
53059674
Iron and sulfide oxidation within the basaltic ocean crust: implications for chemolithoautotrophic microbial biomass production
Microbial processes within the ocean crust are of potential importance in controlling rates of chemical reactions and thereby affecting chemical exchange between the oceans and lithosphere. We here assess the oxidation state of altered ocean crust and estimate the magnitude of microbial biomass production that might be supported by oxidative and nonoxidative alteration. Compilations of Fe2O3, FeO, and S concentrations from DSDP/ODP drill core samples representing upper basaltic ocean crust suggest that Fe3+/ΣFe increases from 0.15 ± 0.05 to 0.45 ± 0.15 within the first 10–20 Myr of crustal evolution. Within the same time frame 70 ± 25% of primary sulfides in basalt are oxidized. With an annual production of 4.0 ± 1.8 × 1015 g of upper (500 ± 200 m) crust and average initial concentrations of 8.0 ± 1.3 wt% Fe and 0.125 ± 0.020 wt% S, we estimate annual oxidation rates of 1.7 ± 1.2 × 1012 mol Fe and 1.1 ± 0.7 × 1011 mol S. We estimate that 50% of Fe oxidation may be attributed to hydrolysis, producing 4.5 ± 3.0 × 1011 mol H2/yr. 
 
Thermodynamic and bioenergetic calculations were used to estimate the potential chemolithoautotrophic microbial biomass production within ridge flanks. Combined, aerobic and anaerobic Fe and S oxidation may support production of up to 48 ± 21 × 1010 g cellular carbon (C). Hydrogen-consuming reactions may support production of a similar or larger microbial biomass if iron reduction, nitrate reduction, or hydrogen oxidation by O2(aq) are the prevailing metabolic reactions. If autotrophic sulfate reduction or methanogenesis prevail, the potential biomass production is 9 ± 7 × 1010 g C/yr and 3 ± 2 × 1010 g C/yr, respectively. Combined primary biomass production of up to ∼1 × 1012 g C/yr may be similar to that fueled by anaerobic oxidation of organic matter in deep-seated heterotrophic systems. These estimates suggest that water-rock reactions may support significant microbial life within ridge flank hydrothermal systems, These estimates suggest that water-rock reactions may support significant microbial life within ridge flank hydrothermal systems.
1055042
Distributions of Microbial Activities in Deep Subseafloor Sediments
Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere.
128793935
The deep subterranean biosphere
Abstract The main purpose with this review is to summarise present research on the microbiology of deep subterranean environments, deeper than 50–100 m. Included are mainly studies where drilling, excavation, core sampling and ground water sampling have been made for research. Studies done in environments penetrated for commercial purposes, such as water wells, mining, oil recovery etc., have been dismissed because of the obvious risk for contamination during the penetration. Different measures that can be applied to reduce the risk of microbial contamination of sampled specimens by the access operations are discussed. The requirement for reliable estimations of the present microbial biomass, its activity and diversity in subterranean ecosystems, is fundamental. An array of different methods to achieve this goal are presented. The depth limit for subterranean life is suggested to be set by temperature, provided there is energy available for microbial life. If so, it should be possible to enrich thermophilic bacteria from deep hot ground waters which also has been done. There are only a few sites where the subterranean microbiology has been studied in multidisiplinary programs including chemistry and geology. The two most extensively published sites are the sediments of the Atlantic coastal plain of South Carolina, USA, studied in a subsurface program, initiated and sponsored by the U.S. Department of Energy, and crystalline bed-rock in Sweden studied in a program concerning the safety of future underground repositories for nuclear waste. This review presents an array of independent reports suggesting that microbial life is widespread at depth in the crust of earth—the deep subterranean biosphere. The obvious consequences is that microbes may be involved in many subterranean geochemical processes, such as diagenesis, weathering, precipitation, and in oxidation or reduction reactions of metals, carbon, nitrogen and sulfur—just as they are in most terranean environments.
176418752
Lithoautotrophic Microbial Ecosystems in Deep Basalt Aquifers
Bacterial communities were detected in deep crystalline rock aquifers within the Columbia River Basalt Group (CRB). CRB ground waters contained up to 60 μM dissolved H2 and autotrophic microorganisms outnumbered heterotrophs. Stable carbon isotope measurements implied that autotrophic methanogenesis dominated this ecosystem and was coupled to the depletion of dissolved inorganic carbon. In laboratory experiments, H2, a potential energy source for bacteria, was produced by reactions between crushed basalt and anaerobic water. Microcosms containing only crushed basalt and ground water supported microbial growth. These results suggest that the CRB contains a lithoautotrophic microbial ecosystem that is independent of photosynthetic primary production.
4407013
An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30° N
Evidence is growing that hydrothermal venting occurs not only along mid-ocean ridges but also on old regions of the oceanic crust away from spreading centres. Here we report the discovery of an extensive hydrothermal field at 30° N near the eastern intersection of the Mid-Atlantic Ridge and the Atlantis fracture zone. The vent field—named ‘Lost City’—is distinctly different from all other known sea-floor hydrothermal fields in that it is located on 1.5-Myr-old crust, nearly 15 km from the spreading axis, and may be driven by the heat of exothermic serpentinization reactions between sea water and mantle rocks. It is located on a dome-like massif and is dominated by steep-sided carbonate chimneys, rather than the sulphide structures typical of ‘black smoker’ hydrothermal fields. We found that vent fluids are relatively cool (40–75 °C) and alkaline (pH 9.0–9.8), supporting dense microbial communities that include anaerobic thermophiles. Because the geological characteristics of the Atlantis massif are similar to numerous areas of old crust along the Mid-Atlantic, Indian and Arctic ridges, these results indicate that a much larger portion of the oceanic crust may support hydrothermal activity and microbial life than previously thought.
4327881
A hydrogen-based subsurface microbial community dominated by methanogens
The search for extraterrestrial life may be facilitated if ecosystems can be found on Earth that exist under conditions analogous to those present on other planets or moons. It has been proposed, on the basis of geochemical and thermodynamic considerations, that geologically derived hydrogen might support subsurface microbial communities on Mars and Europa in which methanogens form the base of the ecosystem. Here we describe a unique subsurface microbial community in which hydrogen-consuming, methane-producing Archaea far outnumber the Bacteria. More than 90% of the 16S ribosomal DNA sequences recovered from hydrothermal waters circulating through deeply buried igneous rocks in Idaho are related to hydrogen-using methanogenic microorganisms. Geochemical characterization indicates that geothermal hydrogen, not organic carbon, is the primary energy source for this methanogen-dominated microbial community. These results demonstrate that hydrogen-based methanogenic communities do occur in Earth's subsurface, providing an analogue for possible subsurface microbial ecosystems on other planets.
25229925
Geomicrobiology in oceanography: microbe-mineral interactions at and below the seafloor.
Oceanography is inherently interdisciplinary and, since its inception, has included the study of microbe-mineral interactions. From early studies of manganese nodules, to the discovery of hydrothermal vents, it has been recognized that microorganisms are involved at various levels in the transformation of rocks and minerals at and below the seafloor. Recent studies include mineral weathering at low temperatures and microbe-mineral interactions in the subseafloor "deep biosphere". A common characteristic of seafloor and subseafloor geomicrobiological processes that distinguishes them from terrestrial or near-surface processes is that they occur in the dark, one or more steps removed from the sunlight that fuels the near-surface biosphere on Earth. This review focuses on geomicrobiological studies and energy flow in dark, deep-ocean and subseafloor rock habitats.
206137597
Radiolytic hydrogen and microbial respiration in subsurface sediments.
Radiolysis of water may provide a continuous flux of an electron donor (molecular hydrogen) to subsurface microbial communities. We assessed the significance of this process in anoxic marine sediments by comparing calculated radiolytic H(2) production rates to estimates of net (organic-fueled) respiration at several Ocean Drilling Program (ODP) Leg 201 sites. Radiolytic H(2) yield calculations are based on abundances of radioactive elements (uranium, thorium, and potassium), porosity, grain density, and a model of water radiolysis. Net respiration estimates are based on fluxes of dissolved electron acceptors and their products. Comparison of radiolytic H(2) yields and respiration at multiple sites suggests that radiolysis gains importance as an electron donor source as net respiration and organic carbon content decrease. Our results suggest that radiolytic production of H(2) may fuel 10% of the metabolic respiration at the Leg 201 site where organic-fueled respiration is lowest (ODP Site 1231). In sediments with even lower rates of organic-fueled respiration, water radiolysis may be the principal source of electron donors. Marine sedimentary ecosystems may be useful models for non-photosynthetic ecosystems on early Earth and on other planets and moons, such as Mars and Europa.
14210658
Subseafloor sedimentary life in the South Pacific Gyre
The low-productivity South Pacific Gyre (SPG) is Earth's largest oceanic province. Its sediment accumulates extraordinarily slowly (0.1–1 m per million years). This sediment contains a living community that is characterized by very low biomass and very low metabolic activity. At every depth in cored SPG sediment, mean cell abundances are 3 to 4 orders of magnitude lower than at the same depths in all previously explored subseafloor communities. The net rate of respiration by the subseafloor sedimentary community at each SPG site is 1 to 3 orders of magnitude lower than the rates at previously explored sites. Because of the low respiration rates and the thinness of the sediment, interstitial waters are oxic throughout the sediment column in most of this region. Consequently, the sedimentary community of the SPG is predominantly aerobic, unlike previously explored subseafloor communities. Generation of H2 by radiolysis of water is a significant electron-donor source for this community. The per-cell respiration rates of this community are about 2 orders of magnitude higher (in oxidation/reduction equivalents) than in previously explored anaerobic subseafloor communities. Respiration rates and cell concentrations in subseafloor sediment throughout almost half of the world ocean may approach those in SPG sediment.
56307161
Presence of oxygen and aerobic communities from sea floor to basement in deep-sea sediments
The depth of oxygen penetration and microbial activity in marine sediments varies by region. Sediment cores from the South Pacific Gyre host oxygen and aerobic microbial communities to at least 75 metres below the sea floor.
1019388
Under the sea: microbial life in volcanic oceanic crust
Exploration of the microbiology in igneous, 'hard rock' oceanic crust represents a major scientific frontier. The igneous crust harbours the largest aquifer system on Earth, most of which is hydrologically active, resulting in a substantial exchange of fluids, chemicals and microorganisms between oceanic basins and crustal reservoirs. Study of the deep-subsurface biosphere in the igneous crust is technically challenging. However, technologies have improved over the past decade, providing exciting new opportunities for the study of deep-seated marine life, including in situ and cross-disciplinary experimentation in microbiology, geochemistry and hydrogeology. In this Progress article, we describe the recent advances, available technology and remaining challenges in the study of the marine intraterrestrial microbial life that is harboured in igneous oceanic crust.
4462468
The contribution of the Precambrian continental lithosphere to global H2 production
Microbial ecosystems can be sustained by hydrogen gas (H2)-producing water–rock interactions in the Earth’s subsurface and at deep ocean vents. Current estimates of global H2 production from the marine lithosphere by water–rock reactions (hydration) are in the range of 1011 moles per year. Recent explorations of saline fracture waters in the Precambrian continental subsurface have identified environments as rich in H2 as hydrothermal vents and seafloor-spreading centres and have suggested a link between dissolved H2 and the radiolytic dissociation of water. However, extrapolation of a regional H2 flux based on the deep gold mines of the Witwatersrand basin in South Africa yields a contribution of the Precambrian lithosphere to global H2 production that was thought to be negligible (0.009 × 1011 moles per year). Here we present a global compilation of published and new H2 concentration data obtained from Precambrian rocks and find that the H2 production potential of the Precambrian continental lithosphere has been underestimated. We suggest that this can be explained by a lack of consideration of additional H2-producing reactions, such as serpentinization, and the absence of appropriate scaling of H2 measurements from these environments to account for the fact that Precambrian crust represents over 70 per cent of global continental crust surface area. If H2 production via both radiolysis and hydration reactions is taken into account, our estimate of H2 production rates from the Precambrian continental lithosphere of 0.36–2.27 × 1011 moles per year is comparable to estimates from marine systems.
128600347
Water Radiolysis: Influence of Oxide Surfaces on H2 Production under Ionizing Radiation
The radiolysis of water due to ionizing radiation results in the production of electrons, H· atoms, ·OH radicals, H3O+ ions and molecules (dihydrogen H2 and hydrogen peroxide H2O2). A brief history of the development of the understanding of water radiolysis is presented, with a focus on the H2 production. This H2 production is strongly modified at oxide surfaces. Different parameters accounting for this behavior are presented.
96614761
Introduction to Radiation Chemistry
Radiation Sources, the Interaction of Radiation with Matter. Radiation Dosimetry. Ions, Excited Molecules, and Free Radicals. Kinetic and Experimental Approaches. Gases. Water and Inorganic Aqueous Systems. Aqueous Organic Systems. Organic Compounds. Radiation Effects in Solids. Applied Radiation Chemistry. Appendices. Index.
128664155
Exploring the deep biosphere
Over the past two decades, scientific drilling into sediments and rocks in the ocean and the continent has revealed the presence of physiologically and phylogenetically complex microbial life in the deep subsurface. Microorganisms, most of which have no cultured or known relatives in the surface biosphere, have been discovered in sediments and rock at depth as deep as 1 km below the seafloor and more than 3 km deep in the continent. The deep biosphere probably represents the most voluminous part of the global biosphere and may constitute as much as 50% of the Earth’s biomass. In this article, we review the current knowledge of and most recent advances in studying the deep biosphere and suggest directions in future research.
53405154
The mean composition of ocean ridge basalts
[1] The mean composition of mid-ocean ridge basalts (MORB) is determined using a global data set of major elements, trace elements, and isotopes compiled from new and previously published data. A global catalog of 771 ridge segments, including their mean depth, length, and spreading rate enables calculation of average compositions for each segment. Segment averages allow weighting by segment length and spreading rate and reduce the bias introduced by uneven sampling. A bootstrapping statistical technique provides rigorous error estimates. Based on the characteristics of the data, we suggest a revised nomenclature for MORB. “ALL MORB” is the total composition of the crust apart from back-arc basins, N-MORB the most likely basalt composition encountered along the ridge >500 km from hot spots, and D-MORB the depleted end-member. ALL MORB and N-MORB are substantially more enriched than early estimates of normal ridge basalts. The mean composition of back-arc spreading centers requires higher extents of melting and greater concentrations of fluid-mobile elements, reflecting the influence of water on back-arc petrogenesis. The average data permit a re-evaluation of several problems of global geochemistry. The K/U ratio reported here (12,340 ± 840) is in accord with previous estimates, much lower than the estimate of Arevalo et al. (2009). The low Sm/Nd and 143Nd/144Nd ratio of all morb and N-MORB provide constraints on the hypothesis that Earth has a non-chondritic primitive mantle. Either Earth is chondritic in Sm/Nd and the hypothesis is incorrect or MORB preferentially sample an enriched reservoir, requiring a large depleted reservoir in the deep mantle.
135192578
Cobalt-based age models of pelagic clay in the South Pacific Gyre
Dating pelagic clay can be a challenge due to its slow sedimentation rate, post-depositional alteration, and lack of biogenic deposition. Co-based dating techniques have the potential to create age models in pelagic clay under the assumption that the flux of non-detrital Co to the seafloor is spatially and temporally constant, resulting in the non-detrital Co concentrations being inversely proportional to sedimentation rate. We apply a Co-based method to the pelagic clay sequences from Sites U1365, U1366, U1369, and U1370 drilled during Integrated Ocean Drilling Program (IODP) Expedition 329 in the South Pacific Gyre. We distinguished non-detrital Co from detrital Co using multivariate statistical partitioning techniques. We found that the non-detrital flux of Co at Site U1370 is approximately twice as high than at the other sites, implying that the non-detrital Co flux is not regionally constant. This regional variation reflects the heterogeneous distribution of Co in the water column, as is observed in the present day. We present an improved approach to Co-based age modeling throughout the South Pacific Gyre and determine that the Co-based method can effectively date oxygenated pelagic clay deposited in the distal open-ocean, but is less reliable for deposition closer to continents. When extending the method to geologically old sediment, it is important to consider the paleolocation of a given site to ensure these conditions are met. This article is protected by copyright. All rights reserved.
13831841
Dust, volcanic ash, and the evolution of the South Pacific Gyre through the Cenozoic
We examine the 0–100 Ma paleoceanographic record retained in pelagic clay from the South Pacific Gyre (SPG) by analyzing 47 major, trace, and rare earth elements in bulk sediment in 206 samples from seven sites drilled during Integrated Ocean Drilling Program Expedition 329. We use multivariate statistical analyses (Q-mode factor analysis and multiple linear regression) of the geochemical data to construct a model of bulk pelagic clay composition and mass accumulation rates (MAR) of six end-members, (post-Archean average Australian shale, rhyolite, basalt, Fe-Mn-oxyhydroxides, apatite, and excess Si). Integrating the results with Co-based age models at Sites U1365, U1366, U1369, and U1370, we link changes in MAR of these components to global oceanographic, terrestrial, and climatic transformations through the Cenozoic. Our results track the spatial extent (thousands of kilometers) of dust deposition in the SPG during the aridification of Australia. Dispersed ash is a significant component of the pelagic clay, often comprising >50% by mass, and records episodes of Southern Hemisphere volcanism. Because both are transported by wind, the correlation of dust and ash MAR depends on the site's latitude and suggests meridional shifts in the position of atmospheric circulation cells. The hydrothermal MARs provide evidence for rapid deposition from the Osbourn Trough spreading ridge before it went extinct. Excess Si MARs show that the abrupt increase in siliceous productivity observed at Site U1371 also extended at least as far north as Sites U1369 and U1370, suggesting large-scale reorganizations of oceanic Si distributions ~10–8 Ma in the southern SPG.
94271212
GeoReM: A New Geochemical Database for Reference Materials and Isotopic Standards
We have developed a new database named GeoReM (http://georem.mpch-mainz.gwdg.de) for reference materials and isotopic standards of geochemical and mineralogical interest. Reference samples include rock powders originating from the USGS, GSJ, GIT-IWG, synthetic and natural reference glasses originating from NIST, USGS, MPI-DING, as well as mineral (e.g., 91500 zircon), isotopic (e.g., La Jolla, E&A, NIST SRM 981), river water and seawater reference materials. GeoReM is a relational database, which strongly follows the concept of the three EARTHCHEM databases. It contains published analytical and compilation values (major and trace element concentrations, radiogenic and stable isotope ratios), important metadata about the analytical values, such as uncertainty, uncertainty type, method and laboratory. Sample information and references are also included. Three different ways of interrogating the database are possible: (1) sample names or material types, (2) chemical criteria and (3) bibliography. Some typical applications are described. GeoReM currently (October 2005) contains more than 750 geological reference materials, 6000 individual sets of results and references to 650 publications.
8622206
Thorium-Uranium Fractionation by Garnet: Evidence for a Deep Source and Rapid Rise of Oceanic Basalts
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.
128472382
Subduction cycling of U, Th, and Pb
Many studies argue, based partly on Pb isotopic evidence, that recycled, subducted slabs reside in the mantle source of ocean island basalts (OIB) [1]; [2] ;  [3] [A.W. Hofmann, W.M. White, Mantle plumes from ancient oceanic crust. Earth Planet. Sci. Lett. 57 (1982) 421–436; B.L. Weaver, The origin of ocean island basalt end-member compositions: trace element and isotopic constraints. Earth Planet. Sci. Lett. 104 (1991) 381–397; J.C. Lassiter, E.H. Hauri, Osmium-isotope variations in Hawaiian lavas: evidence for recycled oceanic lithosphere in the Hawaiian plume, Earth Planet. Sci. Lett. 164 (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 [4] [S.R. Hart, H. Staudigel, Isotopic characterization and identification of recycled components, in: Crust/Mantle Recycling at Convergence Zones, Eds. S.R. Hart, L. Gulen, NATO ASI Series. Series C: Mathematical and Physical Sciences 258, pp. 15–28, D. Reidel Publishing Company, Dordrecht-Boston, 1989]. 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” [5] [T. Elliott, A. Zindler, B. Bourdon, Exploring the kappa conundrum: the role of recycling in the lead isotope evolution of the mantle. Earth Planet. Sci. Lett. 169 (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.
129278040
Composition of altered oceanic crust at ODP Sites 801 and 1149
[1] 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%).
10829862
Permeability within basaltic oceanic crust
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.
52379931
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.
94874864
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
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.
94786426
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
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.
129354358
Mantle Dynamics and Generation of a Geochemical Mantle Boundary along the East Pacific Rise -Pacific/Antarctic ridge
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 Published by Elsevier B.V. All rights reserved.
56414852
Seafloor basalt alteration and chemical change in the ultra thinly sedimented South Pacific
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.
55013126
Recent eruptive history and magma reservoir dynamics on the southern East Pacific Rise at 17°30′S
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.
201611419
ALTERATION OF UPPER OCEAN CRUST IN A RIDGE-FLANK HYDROTHERMAL UPFLOW ZONE : MINERAL , CHEMICAL , AND ISOTOPIC CONSTRAINTS FROM HOLE 896 A 1
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.
140707720
Hydrothermal alteration of upper oceanic crust formed at a fast-spreading ridge: mineral, chemical, and isotopic evidence from ODP Site 801
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.
128833138
Effect of hydrothermal ridge flank alteration on the in situ physical properties of uppermost oceanic crust
[1] 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.
59578056
22. LOW-TEMPERATURE HYDROTHERMAL ALTERATION OF JURASSIC OCEAN CRUST, SITE 801
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.
21501351
Palagonitization of Basalt Glass in the Flanks of Mid-Ocean Ridges: Implications for the Bioenergetics of Oceanic Intracrustal Ecosystems.
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.
131018806
Environmental and chemical controls on palagonitization
[1] 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.