Hydrothermal alteration and mineralisation of basalts from the spreading zone of the East Pacific Rise (7°S–23°S)

At the southern part of the East Pacific Rise (EPR), between 6°S and 30°S a survey on volcanic and hydrothermal activity was performed and samples were obtained by means of TV-controlled grab.This paper deals with altered and mineralized basalt sampled between 7°S and 23°S from five sites in a hydrothermal field.These basalts of tholeitic composition are vitreous to holocrystaline. They have suffered pervasive alteration during which rock-forming minerals (pyroxene, plagioclase) have been replaced by kaolinite, chlorite and smectite. As a consequence, strong depletion of Ca, Cr, Ni, Mg, Sr and Al took place, accompanied by an enrichment of Fe, Cu, Co, Mo, Zn, and Pb. The ore mineral assemblage is rather uniform and consists of pyrite, marcasite, wurtzite, sphalerite, chalcopyrite, covellite and goethite.The igneous and hydrothermal activities can be subdivided into different stages: lithification, high-temperature alteration (<400 C), medium to low temperature alteration and two substages of ore formation (250°C – 150°C and <150°C, respectively).This active present-day ore deposition is interpreted in terms of a peripheral zone of a volcanic-hosted ore mineralisation. It may be compared with fossil ophiolite-hosted massive sulfides that formed throughout the Alpine (e.g. Arabia, Cyprus) as well as Caledonian orogeny (e.g. Scandinavia).

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Zusammenfassung Im Südteil des East Pacific Rise (EPR) zwischen 6°S und 30°S wurde eine Übersichtsbeprobung in einem Gebiet starker vulkanischer und hydrothermaler Aktivität durchgeführt. Diese Studie befaßt sich mit alterierten und mineralisierten Basalten die in einem Hyrothermalfeld, an 5 Positionen zwischen 7°S und 23°S genommen wurden. Die tholeiitischen Basalte sind z.T. hyalin. z.T. holokristallin strukturiert. Im Verlauf der sehr starken Alteration wurden die Minerale Pyroxen und Plagioklas in Kaolinit, Chlorit und Smektit ungewandelt. Dieser Verdrängungsprozeß wurde von einer starken Abreicherung an Ca, Cr, Ni, Mg, Sr, Al und einer Anreicherung an Fe, Cu, Co, Mo, Zn und Pb begleitet. Die Vererzung zeigt eine einfache Mineralvergesellschaftung mit Pyrit, Markasit, Wurtzit, Sphalerit, Chalkopyrit, Covellin und Geothit.Die magmatische und hydrothermale Aktivität läßt sich in verschiedene Stadien untergliedern: Basaltentstehung, Hochtemperturalteration (< 400°), Mittelbis Tieftemperaturalteration und Vererzung mit 2 Substadien (250°C–150°C, < 150°C).Diese rezenten Vererzungen lassen sich interpretieren als die Randzone einer vulkanitgebundenen Erzmineralisation. Sie läßt sich mit fossilen Vertretern ophiolit-gebundener massiver Sulfiderze, wie sie im Verlauf den alpidischen (z.B. Arab.-Halbinsel, Zypern) und kaledonischen Orogenese (z.B. Norwegen) entstanden sind, vergleichen.

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Résumé Une étude des activités volcanique et hydrothermale a été effectuée dans la partie sud de l'East Pacific Rise, entre 6° et 30° de latitude sud. Des échantillons y ont été prélevés au moyen d'un engin dirigé par TV.La présente note se rapporte aux basaltes altérés et minéralisés récoltés en cinq points d'un champ hydrothermal, entre 7°S et 23°S.Ces basaltes, de composition tholéiitique sont tantôt vitreux, tantôt holocristallins. Ils ont été le siège d'une forte altération hydrothermale au cours de laquelle les minéraux de la roche (pyroxène, plagioclase) ont été remplacés par de la kaolinite, de la chlorite et de la smectite. Il en est résulté un appauvrissement marqué en Ca, Cr, Ni, Mg, Sr et Al, accompagné d'un enrichissement en Fe, Cu, Co, Mo, Zn et Pb. La minéralisation présente une association simple à pyrite, marcassite, wurtzite, sphalérite, covelline et goethite.On peut distinguer plusieurs stades d'activité magmatique et hydrothermale: formation du basalte, altération de haute température (<400°C), altération de moyenne à basse température avec deux stades de minéralisation (250-150°C; <150°C). Ces dépôts de la nature actuelle peuvent s'interpréter comme la zone périphérique d'une aire de minéralisation volcanogène. On peut les comparer à des gisements fossiles de sulfures massifs liés à des ophiolites, comme il s'en est formé au cours des orogenèses alpine (p. ex.: péninsule arabique, Chypre) et calédonienne (p. ex.: Norvège).

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Deep Fractionation of Ciinopyroxene in the East Pacific Rise 13°N: Evidence from High MgO MORB and Melt Inclusions

Mid-ocean ridge basalts (MORBs) from East Pacific Rise (EPR) 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO-Al2O3 and negative MgO-Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a "clinopyroxene paradox". The highest magnesium.bearing MORB sample E13-3B (MGO=9.52%) is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure >4±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber (at pressure～1 kbar). The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization (PEC). The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at >4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene+plagioclase+olivine crystallization at >4±1 kbar to mainly olivine+plagioclase crystallization at <1 kbar, which contributes to the explanation of the "clinopyroxene paradox".     

Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N:Evidence from High MgO MORB and Melt Inclusions

Mid-ocean ridge basalts(MORBs)from East Pacific Rise(EPR)13°N are analysed for major and trace elements,both of which show a continuous evolving trend.Positive MgO-Al_2O3 and negative MgO-Sc relationships manifest the cotectic crystallization of plagioclase and olivine,which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenocrysts.However,the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO.Thus,MORB samples are believe...     

Mineral Components, Texture, and Forming Conditions of Hydrothermal Chimney on the East Pacific Rise at 9°-10°N

To characterize the hydrothermal processes of East Pacific rise at 9o-10oN, sulfide mineral compositions, textural, and geochemical features of chimney ores were studied using ore microscope, scanning electron microscope, X-ray diffraction analysis, and electron microprobe techniques. Results show that there are three mineral assemblages for the hydrothermal chimney ores, namely: (i) anhydrite marcasite pyrite, (ii) pyrite sphalerite chalcopyrite, and (iii) chalcopyrite bornite digenite covellite. Mineral assemblages, zonational features, and geochemical characteristics of the ore minerals indicate that ore fluid temperature changed from low to high then to low with a maximum temperature up to 400 ℃. The chimney is a typical black smoker. The initial structure of the chimney was formed by the precipitation of anhydrites, and later the sulfides began to precipitate in the inner wall.     

The formation of high temperature clay minerals from basalt alteration during hydrothermal discharge on the East Pacific Rise axis at 21°N

Aluminous, high-temperature clay minerals form from alteration of tholeiitic basaltic glass and calcic plagioclase during hydrothermal venting on the crest of the East Pacific Rise at 21°N. The clay alteration assemblages are layered crusts (up to 1 mm thick) completely replacing glass and calcic plagioclase adjacent to surfaces exposed to hydrothermal fluids. The interiors of the affected basalt samples have unaltered appearances and oxygen isotopic compositions just slightly heavier than that of MORB. The surficial alteration crusts are mixtures of beidellitic smectite (aluminous, dioctahedral), randomly interstratified mixed-layer Al-rich chlorite/smectite, minor chlorite, an x-ray amorphous aluminosilicate material, and possible minor serpentine (amesite). A δ¹⁸O value of +4.1 ± 0.2%. (SMOW) is determined for the beidellitic smectite. Assuming that this smectite equilibrated with hydrothermal fluid having an oxygen isotope value between that of seawater (0%.) and 350°C hydrothermal fluid from EPR, 21°N vents (+1.6%.), an equilibration temperature between 290°C and 360°C is calculated for the beidellitic smectite. This is substantially higher than any previously reported temperature for an oceanic smectite. The mixed-layer Al-rich chlorite/smectite has a δ¹⁸O value of +3.5%., which corresponds to equilibration at 295°–360°C. The aluminous composition of the alteration assemblage is uncommon for clay minerals produced by submarine hydrothermal basalt alteration. We suggest that this assemblage is largely the product of high-temperature interaction between basalt glass + plagioclase and Mg-poor, acidic hydrothermal fluids, with possibly some contribution of Mg from bottom seawater, and that the aluminous clays either incorporate Al³⁺ remobilized from basalt by lowpH hydrothermal fluids, or are residual phases remaining after intense alteration of basaltic glass + plagioclase.     

Uptake and fractionation of rare earth elements on hydrothermal plume particles at 9°45′N,East Pacific Rise

Particulate samples (>0.45 μm) from a neutrally buoyant hydrothermal plume at 9°45′N on the northern East Pacific Rise were collected using large volume in situ filtration and analyzed for Fe, Al, Mn, Ni, and fourteen rare earth elements (REE). The Sm/Fe ratio (a proxy for overall REE/Fe) and Nd/Er (light/heavy REE fractionation) increased moderately with decreasing particulate Fe. Chemically, the sense of these relationships matched that documented previously in the TAG plume on the Mid-Atlantic Ridge (German et al., 1990), although particulate Fe was about 10 fold lower at 9°45′N. Spatial trends relative to the vent source, however, were opposite of expectation because slow Fe(II) oxidation and Fe(III) colloid aggregation over this interval led to increased particulate Fe (10–26 nM) with distance from source (Field and Sherrell, submitted). After subtraction of non-plume background particle composition, plume particles at 9°45′N and TAG had indistinguishable ranges of light REE-enriched fractionation relative to ambient seawater and had very similar Sm/Fe (therefore Kd for Fe oxyhydroxides), demonstrating that plume particles in both oceans reflect to a first degree the local seawater REE composition. Within-plume REE variations at 9°45′N were investigated using a simple mixing model which accounts for the bulk Fe-Al-Mn variations in the plume using two endmembers: fresh hydrothermal oxyhydroxide precipitates and ridge-crest background particles (composed largely of locally resuspended sediment). Sm/Fe and Nd/Er plot linearly with mixing ratio (R > 0.96), implying that the observed REE trends result from mixing of these two endmembers. Extrapolation to the composition of pure hydrothermal precipitates suggests that Nd/Er is fractionated relative to seawater by a factor of 1.8 during adsorption onto fresh Fe oxyhydroxide particles. The ridge-crest background particles are 5 fold higher in Sm/Fe and Nd/Er is 2.49 relative to seawater, partly a result of enriched terrigenous component in the resuspended matter. A reinterpretation of REE at TAG reveals that positive curvature in REE vs. Fe plots, argued previously to reflect continuous REE uptake (i.e., increasing Kd; German et al., 1990), may result from local depletion of the dissolved REE pool by partitioning onto Fe particles at Fe > 100 nM. Similar drawdown effects could contribute to the variable degrees of curvature observed for all seawater-source particle-reactive species in plumes that are sampled at high particulate Fe concentration. In sum, REE behavior in hydrothermal plumes is more consistent with equilibrium adsorption and mixing of distinct particle types, than with kinetic uptake control. Precise measurements of REEs in modern ridge-crest metalliferous sediments could be compared to the endmember composition calculated from the plume data to evaluate long-term changes in REE of the hydrothermal component.     

Accumulation history of the metalliferous and ore-bearing sediments of the Krasnov hydrothermal field (MAR 16°38′N) for last 80 Ka: Part II

This paper is dedicated to the geochemical studies of two bottom sediment cores that were taken during Cruise 28 of the R/V Professor Logachev in the Mid-Atlantic Ridge (MAR) 16dg38′N area in 2006. The chemical compositions of background metalliferous and ore (ore-bearing) carbonate sediments are presented and inter-element correlations are examined. Individual episodes are distinguished in the accumulation history of the ore-bearing and metalliferous sediments on the basis of element factor analysis.     

Accumulation history of the metalliferous and ore-bearing sediments of the Krasnov hydrothermal field (MAR 16°38′N) for the past 80 ka BP: Part I

The lithological-facies, biostratigraphic, and geochemical studies of ore-bearing and metalliferous sediments were carried out using original material from six cores taken in the MAR 16°38′N area during 28th Cruise of the R/V Professor Logachev.     

Similarities Between Granites on the East and West Coasts of the Pacific Ocean

The traditional idea proposes that granites on the east and west coasts of the Pacific Ocean are different. According to field investigations of the geology of the western United States coupled with the authors' long-term studies on granites in South China, granites both in eastern China and in the western United States are similar.     

The structure of the Precambrian and Lower Paleozoic basement of the Central Andes between 22° and 32°S. Lat.

Three major diastrophic cycles, defined by their structural style and their spatial distribution are recognized in the Andean Basement of this region. The oldest structures are related to the Panamerican Orogeny (500 to 700 m. a.) which produced in the Central Craton multiply deformed complexes of schists, gneisses and granites, that are covered discordantly by unmetamorphosed Cambrian and Ordovician beds. West of the Central Craton Ordovician sedimentary beds are folded with a simple structural style and intruded by granites. Both the sedimentry beds and the granites are covered discordantly by undeformed Devonian sequences. The folding of the Ordovician is attributed to the ocloyic phase of the Caledonian movements. West of the ocloyic belt is another foldbelt consisting of strongly folded Devonian beds attributed to the chanic phase (hercynian). The chanic belt is intruded by carboniferous and permian granites and covered discordantly by Carboniferous and Permian sequences.The features observed in the eastern slope of the Andes suggest that the Paleozoic foldbelts are intracratonic. Whether there are accreted terrains in the Pacific Coastal Cordillera is matter of controversy.

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Zusammenfassung Im Andenbasement dieser Region lassen sich drei diastrophische Hauptzyklen, die durch ihren strukturellen Baustil und ihre geographische Verbreitung definiert sind, unterscheiden. Die ältesten Strukturen sind verknüpft mit den Bewegungen der Panamerikanischen Orogenese (500–700 m. a.), welche im Zentralkraton einen polydeformierten Komplex von Schiefern, Gneissen und Graniten erzeugte. Diesem lagern diskordant kambrische und ordovizische Sequenzen ohne Metamorphose und präandiner Deformation auf. Nach Westen zu gibt es einen Gürtel stark deformierter ordovizischer Sedimente, die einen einfacheren Baustil als das präkambrische Basement aufweisen, und diskordant von undeformierten devonischen Abfolgen bedeckt sind. Die Faltung des Ordoviziums wird der Ocloyischen Phase der Kaledonischen Bewegung zugerechnet.Weiter westwärts tritt ein jüngerer Faltengürtel aus devonischen Sedimenten auf, die während der Chanischen Phase der Herzynischen Bewegung intensiv deformiert wurden, und von karbonischen und permischen Sequenzen diskordant überlagert sind.Der Ocloyische Gürtel ist von postordovizischen Graniten die von devonischen Schichten überlagert sind, und der Chanische Gürtel von karbonischen und permischen Graniten intrudiert. Im präkambrischen Basement des Zentralkratons treten einerseits präkambrische Granite auf, denen diskordant kambrische Schichten auflagern. Andererseits findet man Granite mit paläozoischen Isotopenaltern, letztere aber in Gebieten, wo wegen des Fehlens von Deckschichten eine stratigraphische Kontrolle unmöglich ist.Die gemachten Beobachtungen scheinen darauf hinzudeuten, daß die paläozoischen Faltengürtel intrakratonisch sind. Ob es allochtone »terrains« in der pazifischen Küstenkordillere gibt, ist Gegenstand der Kontroverse.

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Resumen En el basamento de los Andes de esta región se distinguen tres ciclos diastróficos mayores definidos por sus estilos estructurales y su distributión espacial. Las estructuras mas antiguas responden a movimientos que culminaron con la Orogénesis Panamericana (500–700 m. a.) que produjo en el Cratógeno Central un complejo de esquistos, gneises y granitos polideformados, al que se le superponen discordantemente secuencias cámbricas y ordovicicas sin metamorfismo ni déformatión preandina. Hacia el oeste hay un cinturón de sedimentos ordovícicos intensamente deformados con un estilo de plegamiento mas simple que el del basamento precámbrico y cubierto discordantemente por secuencias marinas devónicas sin deformatión. Los movimientos responsables de este plegamiento se atribuyen a la fase oclóyica de los movimientos caledónicos. Más hacia el oeste se manifiesta una franja de deformatión mas jóven atribuída a la Fase chánica de los movimientos hercínicos, en ella se encuentran sedimentitas devónicas intensamente plegadas cubiertas discordantemente por secuencias carbónicas y pérmicas.El cinturón oclóyico esta acompanãdo por granitos postordovícicos cubiertos por Devónico. Asimismo, el cinturón chánico (hercínico) esta intruido por granitos probablemente carbónicos. Con respecta a los granitos intruidos en el zócalo antiguo, hay algunos indudablemente precámbricos por estar cubiertos discordantemente por secuencias cámbricas sin metamorfismo, además hay granitos de edad dudosa que han dado valores isotópicos paleozoicos pero en lugares donde el control estratigráfico no es posible por ausencia de la cobertura paleozoica. De acuerdo a los hechos observados en la ladera oriental de los Andes surge la idea de que los cinturones de plegamiento paleozoicos son intracratónicos, aun es materia de controversia si en la Cordillera de la Costa junto al Pacifico hay terrenos alóctonos acrecionados.

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Widespread tectonic extension at the Central Indian Ridge between 8°S and 18°S

The middle part of the Central Indian Ridge (MCIR) between 8°S and 18°S is representative of mid-ocean ridges in the Indian Ocean but has not previously been systematically surveyed. Here we present results from the first high-resolution mapping survey over both on- and off-axis sections of the MCIR including multibeam bathymetry, magnetics, hydrocasting, and seabed sampling. The 700-km-long MCIR consists of six first-order segments that are offset by > 30 km along well-developed transform faults. Three of the first-order segments are further divided into seven second-order segments with < 30 km offset along non-transform discontinuities. We have recognized for the first time 11 prominent ocean core complexes (OCCs). These occur at nearly all segment ends, corresponding to an occurrence every 60 km of the surveyed ridge. Seafloor spreading model studies using magnetic reversals show that the MCIR is a slow-spreading ridge with average full opening rates ranging from 33.7 to 45.1 mm/yr, increasing from north to south. The highly curved and intermittent axial ridge geometry, rugged flank fabric, variation in the depth and width of the middle valley, and the frequent occurrences of ocean core complexes and non-transform discontinuities demonstrate that asymmetric accretionary processes are dominant along the ridge. The spreading rate symmetry combined with morphotectonic features, reveal that the MCIR segments developed mainly via tectonic extension with little magmatism. Segments with asymmetric accretion controlled by tectonic extension makes up ~ 96% of the MCIR, whereas symmetric accretion controlled by robust magmatism make up < 4%. Hydrothermal plumes with high methane concentrations occur frequently over the OCCs. This finding indicates that abundant OCCs exposed by detachment faults lead to extensive hydrothermal circulation at off-axis areas and that detachment faults are the primary fluid path for hydrothermal fluid circulation at off-axis regions. Serpentinization of mantle-derived rock at OCCs may be one of the major sources of heat and methane in off-axis areas.     

Neoarchean atmospheric chemistry and the preservation of S-MIF in sediments from the São Francisco Craton

Sulfur mass-independent fractionation (S-MIF) preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry. Small ranges in Δ³³S that expanded into larger fractionations leading up to the Great Oxygenation Event (GOE; 2.45–2.2 Ga) are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons, thus obscuring local for global sulfur cycling dynamics. By expanding the Δ³³S record to include the relatively underrepresented São Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE. In an early Neoarchean sequence (2763–2730 Ma) from the Rio das Velhas Greenstone Belt, we propose that low δ¹³C_org (<?30‰) and dampened Δ³³S (0.4‰ to ?0.7‰) in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation. The overlying black shale (TOC up to 7.8%) with higher δ¹³C_org (?33.4‰ to ?19.2‰) and expanded Δ³³S (2.3‰ ± 0.8‰), recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur. The sequence culminates in a metasandstone, where concomitant changes to more uniform δ¹³C_org (?30‰ to ?25‰), potentially associated with the RuBisCO I enzyme, and near-zero Δ³³S (?0.04‰ to 0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in the context of other sequences worldwide, the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes. Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.     

Geochemical survey of Portugal: Comparative analysis of data from soils and stream sediments

This study reports some results of a low density geochemical survey covering the entire country, taking both topsoils and active stream sediments as the sampling media. Standardized methods for sampling, sample preparation, analysis and analytical quality…     

Past sedimentation rates and environments of the Mendeleev Rise inferred from Sr isotope and δ18O chemostratigraphy of its Late Cenozoic sediments

Doklady Earth Sciences - Multiproxy investigation of sediment core AF-0731 from the Mendeleev Rise revealed several epochs of high bioproductivity corresponding to climate amelioration and surface...     

Geochemically induced shifts in catabolic energy yields explain past ecological changes of diffuse vents in the East Pacific Rise 9°50'N area

The East Pacific Rise (EPR) at 9°50'N hosts a hydrothermal vent field (Bio9) where the change in fluid chemistry is believed to have caused the demise of a tubeworm colony. We test this hypothesis and expand on it by providing a thermodynamic perspective in calculating free energies for a range of catabolic reactions from published compositional data. The energy calculations show that there was excess H₂S in the fluids and that oxygen was the limiting reactant from 1991 to 1997. Energy levels are generally high, although they declined in that time span. In 1997, sulfide availability decreased substantially and H₂S was the limiting reactant. Energy availability dropped by a factor of 10 to 20 from what it had been between 1991 and 1995. The perishing of the tubeworm colonies began in 1995 and coincided with the timing of energy decrease for sulfide oxidizers. In the same time interval, energy availability for iron oxidizers increased by a factor of 6 to 8, and, in 1997, there was 25 times more energy per transferred electron in iron oxidation than in sulfide oxidation. This change coincides with a massive spread of red staining (putative colonization by Fe-oxidizing bacteria) between 1995 and 1997. For a different cluster of vents from the EPR 9°50'N area (Tube Worm Pillar), thermodynamic modeling is used to examine changes in subseafloor catabolic metabolism between 1992 and 2000. These reactions are deduced from deviations in diffuse fluid compositions from conservative behavior of redox-sensitive species. We show that hydrogen is significantly reduced relative to values expected from conservative mixing. While H₂ concentrations of the hydrothermal endmember fluids were constant between 1992 and 1995, the affinities for hydrogenotrophic reactions in the diffuse fluids decreased by a factor of 15 and then remained constant between 1995 and 2000. Previously, these fluids have been shown to support subseafloor methanogenesis. Our calculation results corroborate these findings and indicate that the 1992-1995 period was one of active growth of hydrogenotrophic communities, while the system was more or less at steady state between 1995 and 2000.     

Mineral Components, Texture, and Forming Conditions of Hydrothermal Chimney on the East Pacific Rise at 9°-10°N

To characterize the hydrothermal processes of East Pacific rise at 9°-10°N, sulfide mineral compositions, textural, and geochemical features of chimney ores were studied using ore microscope,scanning electron microscope, X-ray diffraction analysis, and electron microprobe techniques. Results show that there are three mineral assemblages for the hydrothermal chimney ores, namely：（i） anhydrite ＋ marcasite ＋ pyrite, （ii） pyrite ＋ sphalerite ＋ chalcopyrite, and （iii） chalcopyrite ＋ bornite ＋ digenite ＋ covellite. Mineral assemblages, zonational features, and geochemical characteristics of the ore minerals indicate that ore fluid temperature changed from low to high then to low with a maximum temperature up to 400 ℃. The chimney is a typical black smoker. The initial structure of the chimney was formed by the precipitation of anhydrites, and later the sulfides began to precipitate in the inner wall.     

Standardisation of Rock–Eval pyrolysis for the analysis of recent sediments and soils

Rock–Eval 6 analysis, a well established screening tool for petroleum geochemistry, is being increasingly used to characterise the varying species of organic matter (OM) in the bulk samples of recent aquatic sediments. This is particularly important due to recent scientific attention on the role of OM in biogeochemical distribution of environmentally hazardous compounds (e.g., trace metals) in recent sediment archives. Rock–Eval’s automated use, low sample volume requirements and its high analytical accuracy and precision makes it an ideal tool for relatively rapid screening of OM in sediment cores. However, to date, there has been no broad scale standardisation to determine what may be contributing to each signal (e.g., S1, S2, S3, RC). We have selected a wide variety of representative, pure biochemicals (proteins, lipids, carbohydrates and lignins) and biological standards (phytoplankton, copepods, tree bark and conifer needles) to better understand the Rock–Eval 6’s measured organic matter parameters in the unconventional environmental samples. These data have been corroborated with organic petrographical and elemental (CHNS/O) data. Our results show that small organic molecules (<500 Da) are largely responsible for the S1 hydrocarbon peak while lipids and aquatic biological standards are contributing most in the S2 signal, and in particular the more labile “S2a” signal. Furthermore, carbohydrates, lignins and terrigenous plant standards are most responsible for the S3 signal. We also note that the S3 signals (CO/CO₂ ratios: OICO, OICO₂ and OIRE6) are the best discriminants for the source of OM. Finally, step wise pyrolysis of biological standards coupled with elemental analysis (CHNS/O) suggests that S2 and, to a lesser extent, S3 (S3CO and/or S3CO₂), would be most responsible for metal-binding elements such as S and N, with implications for element biogeochemical cycles.     

Metamorphic P-T conditions and variation of REE between two garnet generations from granulites in the Sør-Rondane mountains,East Antarctica

Mineralogy and Petrology - In this paper, we describe the metamorphic conditions of Fe-rich granulite and variations in rare earth elements (REE) between peak garnet porphyroblasts and secondary...