Search

International audience; This study presents an inventory of possible chemical reactions affecting, or having affected, the Venus surface. Fluid rock reactions are simulated using experiments under conditions close to the present surface. Slabs or powder of several natural and synthetic silicate material (crystalline fresh basalt, altered basalt, obsidian, pumice and basalt glass) were reacted at 475°C in CO 2-H 2 O-H 2 S-SO 2-CO gas mixture. Most of the runs were carried out at roughly 90 bars with a duration of one week, some experiments having longer (one month) or shorter (one day) durations. The role of H 2 O content was explored through a wide range of water pressure: from dry gas for the current Venus conditions up to 590 bars (86%H 2 O) for early Venus (or other early terrestrial planets). The gas phase was sampled before the completion of the runs for chemical analysis of major gas components (CO, H 2 S, SO 2) as well as trace elements possibly released by the rocks. The altered samples were examined by a suite of mineralogical and chemical techniques (scanning and transmission electron microscopy, X-ray diffraction and spectroscopy). In dry atmosphere, the redox potential of the gas was close to the Ni/NiO buffer (−21.3 to −27.3 log fO 2), thus close to the current Venus conditions. The sample alteration is tenuous and limited to surface oxidation of glasses and coating of olivine by iron oxides, as well as the general deposition of (Ca,Na)SO 4 at the sample surface. The oxidation of glass is reflected in the formation of magnesioferrite under the surface and is accompanied by the release of Ca, Mg and Na into the gas phase or mineralized as sulfate at the surface. In wet atmosphere, obsidian recrystallizes into a mixture of plagioclase and amphibole while basaltic glass produced non-expandable clays minerals: chlorite-type (2:1:1) at the surface and likely celadonite (2:1) below the surface. Olivine is preserved. Using obsidian (the most alterable material) as a proxy of aluminosilicates, we discuss the surface reactions operating under supercritical conditions, and we used a shrinking-core equation for modeling the long-term reactions. These parametric exploration offer new insights into processes having affected the surface of Venus and contribute to the discussion of open questions such as the fate of water or the lifetime of vitreous dust or fine grain material if present in the current or past Venusian environment. Longer duration experiments will provide more kinetic parameters that can be extrapolated to the geologic history of Venus.

Terra verde, or Green Earth, a naturally-occurring mineral and green pigment consisting of celadonite or glauconite, is one of the most commonly-used pigments in works of art and their conservation. Its widespread use is owed primarily to its insensitivity to light, its neutral pH, and its high stability in air and moisture – attributes that collectively make it virtually unreactive with other components in an artefact. Notwithstanding this, the use of terra verde in the conservation of objects of cultural heritage is hindered due to its minimal tinting strength and hiding power, which makes the pigment effectively transparent when applied on the canvas with the most common binding media. This gives rise to problems in retouching during the restoration process. We report a robust and reliable method to overcome the problem of transparency through the application of crystal engineering techniques on the pigment Verona Green Earth, or Verona Terra Verde (VTV). The purchased pigment was co-crystallised with a selection of different co-formers and analysis was carried out on both starting compound and product via IR and XRF spectroscopy, hot-stage microscopy and powder X-ray diffraction. The results of the experiments conducted show an increase in the pigment's opacity without affecting its colour intensity.

Large amounts of stone wastes are accumulated alongside different exploiting mines of amethyst geodes in south Brazil, which are becoming an unsustainable environmental issue. For an adequate use of hydrothermal basalt in agriculture as soil remineralizer, it is fundamental to know the material's mineralogy and geochemical variability. This study aims to evaluate the mineralogical and geochemical variability of the basalt from five mines at two positions (near and far from the geodes) in different size fractions to discuss their potential use in agriculture. X-ray diffraction (XRD), X-ray fluorescence (XRF) and petrographic analyses of thin sections were used. Petrographic and mineralogical analyses showed similar compositions for the rocks, consisting of plagioclases, pyroxenes, and opaque minerals with apatite as an accessory mineral. The rock samples located near the amethyst geodes presented slightly higher clay mineral contents than the distant ones. The clay mineralogy was composed of smectite and celadonite. The subfractions preserved the characteristics of the original material. Small geochemical and mineralogical variations were observed in the hydrothermal basalts regardless of the positions and the fraction sizes studied. These characteristics confer this material a high potential for use in agriculture as a soil remineralizer. This approach contributes both to solving an environmental issue associated with geode exploitation and to create an alternative for soil fertilization.

Saint Demetrius of Stomion is a historical monastery placed in the geographical area of Larissa town, Greece, with a remarkable presence from its founding until nowadays. The monastery's present catholicon (main church) has been dated in the 16th century and its surviving wall paintings were constructed in 1758. In addition to the characterization of the materials applied, the purpose of this study is to determine the existence of a sole pictorial phase, that of the mid-18th century, or the occurrence of overpaintings. Additionally, the present study aims to begin the establishment of the documentation of materials and techniques of the late post-Byzantine iconographic guilds, which acted in Central and Northern Greece. The collected samples were analyzed by means of microscopic, spectroscopic and thermogravimetric methods. The use of fresco technique is implied by the major participation of calcium carbonate in all of the painting layers, its main contribution in the plaster layers and the microstratigraphic analysis of the samples' cross-sections, while the scarce and local presence of a proteinaceous material implies the limited use of egg-tempera technique for the highlights. The pigment analysis shows that mostly traditional pigments were used, including cinnabar, ferrous pigments (ochre, sienna, umber), minium, Naples yellow, massicot/litharge, green earth (celadonite), malachite, carbon and bone black, calcite, kaolinite and lead white. The absence of modern pigments, the consistency of all samples regarding microstratigraphy and applied materials, and the lack of the extensive use of an organic medium, strongly suggest the lack of overpaintings.

A range of phyllosilicate compositions have been detected spectroscopically on Mars, but the largest fraction by far corresponds to clay minerals rich in Fe and Mg. Given that most of our understanding of Martian clays comes from remote sensing data, it is critically important to explore the details of how compositional variation affects spectral features of phyllosilicates. The greatest efforts have focused so far on near-infrared (NIR) spectroscopy. Recently, ambiguities have been detected in the NIR spectra of 2:1 phyllosilicates with intermediate Fe Mg content that preclude mineral and chemical discrimination. Such ambiguities highlight the relevance of exploring the visible spectral range as a complementary tool to characterize Martian phyllosilicates precisely. This article reports the investigation of laboratory reflectance spectra (330–800 nm) from 34 Earth analogues of Martian phyllosilicates with a wide range of Mg Fe composition, including nontronite, celadonite and saponite end-members, as well as interstratified glauconite-nontronite, talc-nontronite, and talc-saponite. The spectra indicated the presence of Fe(III) by absorption modulations and a decrease in total reflectance, especially in samples with tetrahedral Fe(III). Absorption bands at 370 and 420 nm were diagnostic of octahedrically and tetrahedrally coordinated Fe(III), respectively. Band amplitudes in the second derivative of the Kubelka-Munk function correlated positively with Fe(III) content (R2 > 0.8). Standard color analyses of the visible reflectance spectra under the CIE illuminant D65 indicated that the CIELAB color parameter a*10 was positively correlated with tetrahedral Fe(III), b*10 was positively correlated with octahedral Fe(III), and L*10 was negatively correlated with Fe(III) in both structural sites. Because Fe(II) was in relatively low amount, it did not provide clear spectral evidence. Multiple regression models using the amplitude of the diagnostic absorption bands predicted well absolute Fe content in the phyllosilicates (R2 = 0.89) and the ratio Fe/(Fe + Mg + Al) (R2 = 0.84). CIELAB color parameters improved the prediction of total Fe (R2 = 0.92) and the ratio Fe/(Fe + Mg + Al) (R2 = 0.93). Application of these analyses to Martian data has challenges set by Fe oxide dust coating and spatial and spectral resolution. However, these results mark an avenue to develop testable tools using visible-wavelength spectral data from both satellite and lander probes to help establishing Fe content and mineral identification of Martian phyllosilicates.

Boles are markers of hiatus in successive eruptions of the Deccan flood volcanism (DFV) emplaced at the Cretaceous-Paleogene boundary that are interpreted to retain paleoenvironmental information from their subaerial exposure. To evaluate paleoenvironment during these hiatuses, mineralogy and geochemistry of three different colors of boles, i.e., red (dark = 10R-3/6 and light = 10R-4/8), brown (5YR-4/4) and green (5G-6/2) were examined. The plagioclase index of alteration (PIA) (loss of Ca and Na relative to Al) was between 76 and 81 for all the boles, indicating moderate chemical weathering that is probably due to their short subaerial exposures. Mass balance calculations show significant enrichments of K, Si, Mg and Rb in green and brown boles suggesting an allochthonous input that is interpreted as volcanic ash fallout. Mineralogically, the red bole is dominated by montmorillonite and hematite, the brown bole of ferruginous smectite and montmorillonite, and the green bole of celadonite and nontronite. Montmorillonite and hematite genesis in red boles indicates leaching in an oxidative environment at near neutral pH (~5.5 to 8) conditions. Further, heating of the altered basalt from emplacement of upper lava flows under subaerial-dry conditions could have also promoted the formation of hematite in red boles. The hydrothermal alteration of K and Mg enriched volcanic ash admixed with the altered lower flow basalt under slightly reducing oxidation potential led to the genesis of nontronite and celadonite in the green bole. The aluminous Fe-smectite in brown bole occurred from hydrothermal alteration of aluminous clays (e.g., montmorillonite). The different colored boles, therefore, suggest a range of weathering processes including chemical leaching to hydrothermal alteration in oxidative to reducing environmental conditions throughout DFV emplacement that could reflect changing surficial conditions associated with volcanically forced changes to the environment.

Agates from Ploczki Gorne hosted by Permian basaltic rocks are predominantly made of length-fast chalcedony, and subordinately megaquartz and quartzine. Moganite occurs in traces mainly in transparent, outer, darker regions of white-grey coloured agates. Silica matrix of agates comprises a wide variety of solid inclusions represented by celadonite, plagioclases, hematite, goethite, barite, calcite, heulandite-clinoptyloite, nontronite-saponite, and Mn-dioxides (ramsdellite). Mineral phases are locally accompanied by black aggregations of carbonaceous matter, which gives a Raman signature of disordered carbon. These organic components were probably deposited from a hydrothermal fluids at low-temperature conditions and originated from sedimentary rocks found in the surrounding area of Ploczki Gorne. The abundance of various SiO2 phases, mineral inclusions as well as various micro-textures (colloform, comb, feathery, and jigsaw-puzzle) in agates resulted from physicochemical fluctuations of SiO2-bearing mineralizing solutions at various stages of these gems formation. Agates from Ploczki Gorne formed during post-magmatic stage of basaltic host rocks evolution. Not only were the hydrothermal fluids enriched in silica, but also they contained other elements such as Na, Ca, Al, Mg, Mn, Fe, Ba, SO4, and CO2, which were mobilized from host rocks or surrounding area.

In this study, material characteristics of wall paintings executed in 11th Century Byzantine Church located in archaeological site of Anaia (Turkey) were investigated to provide historical information of the painting techniques. For this purpose, physical, chemical and mineralogical compositions of the paintings were determined by X-Ray Diffraction, Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy coupled with Energy Dispersive spectroscopy and Thermo-Gravimetric Analysis. Analysis results indicated that the paintings were executed using lime-secco technique. In this technique, pigments were mixed with lime and applied on a smooth plaster layer. Pigments used were mainly iron oxides for red, yellow and purple paintings, aluminosilicates such as celadonite for green paintings and lazurite for dark blue paintings.

Hydrothermally affected basaltic flows and volcaniclastic rocks in northwestern Parana (Brazil) display textural features associated with secondary minerals, reflecting the temperature intensity and evolution of the hydrothermal process. Determination of the secondary mineralogy and the geochemical processes associated with their precipitation are addressed in this paper. The lithotypes that comprise the bulk of the Parana Igneous Province have millimetric to centimetric vesicles filled by secondary minerals. Field data, petrographic descriptions, mineral characterization and mineral chemistry were used to analyze these secondary mineral phases. Matrix material in volcaniclastic rocks (e.g. shards, glass pisolites) precedes the precipitation of the secondary minerals. Coalescence and necking features are common in vesicles located on top of basaltic flows. Clay minerals identified as vesicle and/or fracture infillings include celadonite, illite/smectite (I/S) interestratifications, as well as montmorillonite and nontronite, with morphologies typical of authigenic volcanic processes such as cauliflower-like and honeycomb. The occurrence of different zeolites species is primarily controlled by CaO and Na2O activity in the hydrothermal fluid. Heulandite is the most pervasive zeolite, followed by stilbite and analcime. Calcite occurs in the center of vesicles, as the last crystallizing phase, or isolated in monomineralic vesicles. Vesicle formation occurred by magma degassing processes at temperatures of up to 1150 °C. However, infilling of these voids occurred later with the percolation of hydrothermal fluids external to the system. Although random from a geographical perspective, gradual decrease in temperature and fluid evolution were responsible for the systematic change in the mineral phases within individual vesicles. The hydrothermal fluid was marked by a Ca–Na signature, high cation/hydrogen ratio and low to intermediate temperatures, ranging between 50 and 200 °C. The controls for precipitation were mainly temperature and the chemical composition of the fluid, with high alkalinity and low H+ activity.

The aqueous chemistry, isotopic composition (δ18O and δ2H), and geothermometry of the Breitenbush Hot Springs area in the central Oregon Cascades were examined. Water samples were collected from springs (5), wells (8), and the Breitenbush River (2). Geothermometry was interpreted using the Reservoir Temperature Estimator (RTEst) software package and constrained by the reported mineralogy. Reservoir temperature was calculated assuming equilibration with chalcedony, celadonite, laumontite, heulandite, and epidote. Because of its relatively rapid reaction rate, calcite was presumed to remain equilibrated during cooling of the ascending reservoir waters. CO2(g) degassing from the springs was accounted for by adjusting the carbon content and temperature to be consistent with a cooling curve passing through the chemical composition of the deep well waters. The average estimated reservoir temperature of 137.1 ± 2 . 0 ∘ C is lower than the 174 to 180 °C reported in other multicomponent geothermometric studies. However, our new estimate is consistent with mineralogical, aqueous geochemical, and fluid inclusion data, and with geothermal borehole temperature measurements near the site. Stable oxygen and hydrogen isotope data indicate that the thermal waters at Breitenbush and Austin Hot Springs are a mixture of 4 to 8% "andesitic water" (Giggenbach, 1992) and local meteoric waters recharged at elevations of 1750 to 2200 m, along the crest of the Cascade Mountains. Br/Cl ratios, δ18O, and δD, and their correlation with Cl− concentrations from our study combined with data and analyses from other sources, suggests that Cl−, other halogens, and CO2 in the Breitenbush hot springs are primarily derived from degassing fluids rising from the serpentinized forearc mantle. The calculated reservoir temperature and average measured thermal water Cl− concentration were used with previously reported chloride-flux-based measurements by the USGS to estimate a hydrothermal discharge of 13.7 ± 1.6 L/s and an advective heat discharge of 7.1 ± 0.8 MW. This calculated hydrothermal heat loss is slightly less than Ingebritsen et al.'s (1992) previous estimate of 9 MW. However, it still represents a substantial transfer of heat from a relatively small, constant groundwater discharge.

X-ray Powder Diffraction (XRPD) is one of the non-destructive techniques habitually used for the structural analysis of crystalline solids. This technique can therefore be used to differentiate between crystalline and vitreous or amorphous solids such as Smalt Cobalt Blue or the binders used in paints. The “final” pigments used in the artist's palette are frequently composed of a complex mixture of inorganic and organic compounds, causing different interferences and matrix effects, such as filtering and signal overlapping and background noise increase. These effects can make difficult pigment identification and quantification contributing to mischaracterization. Nevertheless, when the experimental difficulties can be overcome, the results obtained are precise, definitive and useful. In the present research, the limitations were avoided or minimized by using an algorithm for data analysis like the one used by the XPowder™ software. In addition, most of the natural pigments used in the painting The Little Madonna of Foligno are identified and quantified. The preparation layers are composed of white lead, along with a homogeneous mix of several arsenic sulfides and small amounts of roselite. The color layer contains lapis lazuli and minor azurite for the blue colors, and cinnabar for the red ones. Other pigments found were: lead‑tin yellow, iron(III) oxy-hydroxides (red and yellow ochers), celadonite (green earth), and possibly olivine, diopside and other ferromagnesian minerals in the form of pyroxenes and amphiboles, whose diffraction lines also appear in the diffractograms, although with weak intensities and precise quantifications are not allowed. Crystalline Thenard's blue was not found.

Changes in the global Li-cycle, as recorded in the Li concentration and/or isotopic composition of seawater, have the potential to provide important insight into the controls on the long-term C-cycle. Understanding the magnitude and isotopic composition of the fluxes of Li into and out-of the ocean, and the controls on any variability in these, is necessary if we are to correctly interpret the paleo-record of the Li-cycle. Here the low-temperature (off-axis) hydrothermal sink is investigated using the volcanic section of the exceptionally preserved Troodos ophiolite. Using glass to define the protolith Li content, the uptake flux of Li is determined using bulk-rock analyses from four hydrologically distinct sections through the lava pile of the ophiolite. Differences in paleo-hydrological conditions in the crust appear to have played a significant role in controlling the uptake flux of Li with an ‘average’ uptake flux of equivalent to 14–21 × 10 9 mol yr −1 – this is considerably larger than generally assumed. Bulk-rock samples that contain a large seawater Li component have δ 7 Li of ∼10 ± 2‰. Celadonite separates have a δ 7 Li of ∼6 ± 1‰, considerably lighter than bulk-rock samples with the same Li content. Because celadonite is a significant repository for Li within the Troodos upper crust this means that another phase(s) must have markedly heavier δ 7 Li than the average bulk-rock; i.e. changes in the average mineralogy of altered crust will lead to changes in the bulk isotopic fractionation between the Li added to the upper oceanic crust and seawater (Δ SW-lava ). The shallowest samples in three of the four studied sections are isotopically lighter than deeper samples (but do not contain significant celadonite), again indicating that variations in alteration conditions and/or mineralogy can lead to variations in Δ SW-lava . Comparison with other studies of altered upper oceanic crust suggests that changes in alteration conditions lead to significant changes in Δ SW-lava . These changes likely reflect both a temperature dependence of the isotopic fractionation factor and a change in the fractionation factor due to changing mineral assemblage and/or mineral compositions and abundances. A significant portion of the increase in δ 7 Li of seawater over the past 50 Myr may be due to an increase in the bulk fractionation factor between seawater and Li added to the upper oceanic crust due to cooling bottom water.

The Late Triassic-Early Jurassic volcanic sequence of the Central Atlantic Magmatic Province (CAMP) of Morocco is classically subdivided into four stratigraphic units: the Lower, Middle, Upper and Recurrent Formations separated by intercalated sediments deposited during short hiatuses in volcanic activity. Although corresponding to a Large Igneous Province formed in continental environment, it contains subaqueous lava flows, including dominant pillowed flows but also occasional sheet flows. We present a study of the morphology, structure and morphometry of subaqueous lava flows from three sections located at the Marrakech High-Atlas (regions of Ait-Ourir, Jbel Imzar and Oued Lhar-Herissane), as well as an analysis of the sediments, in order to characterize them and to understand their environmental meaning. The analysis of clays by the diffraction method X-ray revealed the presence of illite, mica, phengite, celadonite, talc and small amounts of quartz, hematite, calcite and feldspar, as well as two pairs of interbedded irregular (chlorite Smectite/chlorite-Mica). Fibrous minerals such as sepiolite and palygorskite were not detected. The peperite of Herissane region (Central High Atlas) provided an excellent overview on the factors favoring the magma-sediment interaction. These are the products of a mixture of unconsolidated or poorly consolidated sediments, low permeability with a low viscosity magma. The attempt of dating palynology proved unfortunately without results.

To fully understand phyllosilicates on Mars, it is beneficial to study analog deposits on Earth. One attractive candidate for martian phyllosilicates is the intrabasaltic bole beds (palaeosols) from the Deccan Volcanic Province of India. Eleven samples from the upper-layer red Deccan bole beds and underlying yellow and green Deccan bole beds were analyzed by X-ray diffraction (XRD), near-infrared (1.0–2.5 µm) and mid-infrared (5–15 µm) reflectance spectroscopy. Analysis of the bole beds indicated that the red boles are composed of a mixture of montmorillonite and hematite, yellow boles contain vermiculite and minor montmorillonite and green boles are composed mainly of nontronite (smectite) and celadonite (mica). While the bole beds are all chemically similar to each other and to the underlying basalt from which they were weathered, they are mineralogically different. This suggests transformation from one mineral to the next without ion transfer or loss which could be indicative of a limited-water environment. In fact, celadonite can transform into smectites (such as montmorillonite), often with vermiculite as an intermediate step. This not only explains the stratigraphy and mineralogy of the Deccan bole beds but may also explain the layered phyllosilicates identified in various, global locations on Mars. The transition observed in the Deccan bole beds suggests an evolution of the alteration process from deuteric alteration to low-temperature weathering, likely due to changes in temperature, in a closed system (no significant ion transfer) as evidenced by the minerals’ similar chemistry. Thus, the Deccan bole beds are a good analog for the phyllosilicates layers on Mars and by studying the chemistry, mineralogy and spectral properties of the Deccan bole beds, we can link their formation and alteration processes to those of martian phyllosilicates. This will provide a clearer understanding of the environmental conditions on Mars at the time of the phyllosilicates’ formation and hence during Mars’ earliest history.

Celadonite is the most abundant K-rich hydrated mineral formed during basalt alteration. In convergent margins, celadonite from the altered oceanic crust sinks into the mantle and contributes to the mantle wedge metasomatism, recorded by incompatible element enrichment in lavas and mantle xenoliths samples from these settings. However, experimental data on the role of celadonite in subduction zones as a function of P-T are still scarce. Therefore, celadonite stability under “hot” and “cold” subduction settings is investigated experimentally in this study. Results show an early silica loss, with quartz/coesite replacing celadonite from 400 to 600 °C and 2.5–7.7 GPa. At 700 °C and 2.5 GPa, celadonite is partially converted to phlogopite-annite, and at 800 °C and 4.0 GPa celadonite is absent. At temperatures above 800 °C, enstatite and phlogopite are the stable silicate phases from 2.5 to 7.7 GPa. Mossbauer spectroscopy and X-ray diffraction analyses show that the reaction of celadonite to phlogopite-annite is progressive and controlled by the Fe3+ to Fe2+ reduction in the octahedral site. Coupled to this, there is a transition of pyrite to pyrrhotite and the release of reducing fluids enriched in H2S, SiO2, OH− and K+ in the mantle wedge. These fluids have the potential to mobilize chalcophile elements (e.g. Cu, Zn) and act as mineralizing agents in shallower and more oxidizing crustal environments. Data in this study demonstrate that celadonite-phlogopite-annite series minerals are stable down to at least 230 km in the mantle, contributing to the sources of mantle metasomatism, arc-related magma genesis and possibly chalcophile mineralization.

Celadonite (K[Mg2+,Fe2+][Fe3+,Al3+](Si4O10)(OH2)) is a common void-filling mineral in ocean floor and ophiolite lavas that forms during low-temperature, off-axis, hydrothermal alteration. Its occurrence provides important information about the sink of K from seawater into the upper oceanic crust. New age determinations for celadonitic samples from a ~20 km section of the lavas in the Troodos ophiolite, along with published ages, show that most celadonite formed within the first ~20 Myrs after crust accretion. However, significant regional differences in the duration of celadonite formation are observed. In comparison to a region of flat paleo-seafloor, in a paleo-seafloor topographic low celadonite started forming later and continued forming for ~20 Myrs longer. The paleo-seafloor low is associated with abundant hydrothermal sediments which are interpreted to have played a key role in controlling the chemical conditions required for celadonite precipitation. It is hypothesised that celadonite formed for longer largely because these hydrothermal sediments acted as a source for labile Fe for a longer time than in the region where the Fe required for celadonite formation was mainly leached from silicate phases. In the region of flat paleo-seafloor, sediment did not begin accumulating for ~20 Myrs after crustal accretion allowing prolonged oxidative alteration of the upper crust. This led to Fe released by silicate phase breakdown being incorporated in stable Fe3+ containing phases, such as Fe-oxyhydroxides, rather than being transported as aqueous Fe2+ to sites where celadonite could form. If this model is correct, the uptake of K from seawater into the oceanic crust may be highly spatially heterogeneous and intimately linked to the subsurface redox conditions and the distribution of hydrothermal sediments.

We report here a study of 30 fragments of green wall paintings from Roman times found in the Patio de Banderas excavation in Seville Alcazar. The sample characterisation was realised using optical microscopy, colourimetry, infrared and micro-Raman spectroscopy, X-ray diffraction, and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. The study of these pigments is important because it can help determine the source or the pictorial technique used. The samples studied in this work have been divided into two groups, according to the composition of their green pigments. In the first group, celadonite has been characterised as the primary component of the green colour; chlorite was also detected. Particles constituted by chromium accompanied by aluminium, iron and zinc were found in all studied samples of this group. Chlorite and chromium oxide could also be responsible for the green colour. The presence of chromium suggested the presence of green colour pigment from Verona. In the second group, a mixture of celadonite and glauconite was detected and could be responsible for the green colour observed. The addition of refracting material such as Egyptian blue was also used. A mixture of Egyptian green and Egyptian blue together with celadonite and glauconite was also found. Four classes of intonaco were recognised and classified based upon the composition of the aggregates.

The Central European Variscides and their epicontinental basins and grabens (Mesoeurope sensu Stille) subsided into the uplifted basement as well as the Alpidic Orogens, encompassing the Alpine Mts. Range, the Dinarides, the Northern Carpathians and the depressions in between (Neoeurope) are rife with a great variety of clay mineral assemblages. In many places the clay mineral deposits reach economic grade and several phyllosilicates can be used as an ore guide to non-clay mineral deposits. Time, climate and the geodynamic setting are the decisive parameters for the clay mineral accumulation. Time constitutes the x-axis for the plots illustrating the global climate change and the regional geodynamic crustal variation. It is also some kind of a yardstick to measure the preservation potential and the stability of phyllosilicates. The geological time scale is equivalent to a depth of a drill hole which penetrates different geological units characterized by various zones of post-depositional alteration. As such the geological age of formation is synonymous with an increase of the P-T regime where important boundaries concerning the micaceous phyllosilicates, kaolinite-, smectite-group minerals and the composition of glauconite are observed. The Mg-bearing phyllosilicates are marker for the Permo-Carboniferous to early Triassic and for the Middle to Late Jurassic extensional regimes in Central Europe. They indicate an ensialic geodynamic regime in Mesoeurope and an ensimatic regime in Neoeurope. This is also true for the presence and absence of Li-bearing phyllosilicates and Li-bearing tourmaline and spodumene which are a mirror image of the Permo-Carboniferous compressive regime in the Variscides and the extensional regimes in the Alpides and indicative of an ensialic and ensimatic regime, respectively. In the ensimatic geodynamic setting of the Neo- and Paratethys during the Miocene and Pliocene a felsic volcanic activity in an interarc-to back arc environment brought about an Fe-poor clay mineral assemblage in an ensimatic setting. In the ensialic Variscides only small-sized kaolinite deposits can be taken as an equivalent. Some ferroan saponite and chlorite represent the basic branch of the bimodal rift magmatic activity. Bentonitic clays and bentonites in the Alpine and Carpathian Foredeeps are marker beds for rift-related explosive volcanic activity sourced particularly in the North Atlantic Province and important mineral deposits. Fe-bearing phyllosilicates, e.g. chamosite, in the Early Ordovician and Jurassic ironstones were emplaced in a marginal and epicontinental basins, locally with restricted circulation. The Alpine extensional regimes saw the formation of celadonite in a calcareous depositional environment open to ocean. They act as facies indicators. Some phyllosilicates act as a natural multifunction display. Corrensite and mixed-layer chlorite-corrensite are not only in altered basic magmatic rocks marker for basement rocks but also affected by extraterrestrial impacts. The glauconite-kaolinite-illite-smectite-chlorite assemblages are efficient tools for the depositional environment and the post-depositional alteration, all in one. There exists an antagonism par excellence in terms of the sedimentary environment of formation between glauconite and kaolinite The clay mineral catena together with palygorskite and sudoite can be used as a proximity indicator from land to sea from the various subenvironments, e.g., from a coastal sabkha to the alluvial fan. Chromium muscovite, Cr smectite, and Cr chlorite formed at the brink from hypogene to supergene alteration and pave the way from the geodynamic/ lithofacial - to the climate-induced clay mineralization. They are the most recent clay mineral assemblages in Central Europe proved by radiometric age dating (Mio-Pliocene). The humid-tropical paleoclimate zone occurred from the Late Triassic to the Late Cretaceous, the Late Cretaceous to the Eocene, and during the Miocene. In Central Europe these paleoclimates and the resultant weathering conditions were most effective in the formation of residual clay deposits dominated by kaolin and Ni-Co laterites (garnierites) enriched in Al-, Fe- and Mn-duricrusts of economic grade. The climax of formation was reached around the Paleocene-Eocene Temperature Maximum (PETM). The tropical wet and dry paleoclimate zone in Central Europe was widespread during the Jurassic and Cretaceous leading to similar residual clay deposits impoverished in duricrusts. Residual clays in the tropical arid and semi-arid paleoclimate zones have palygorskite, corrensite and sudoite as typical phyllosilicates which denote the widespread occurrence of evaporates with the precipitation level reaching the K salt and even bittern stages. These clay minerals appeared episodically from the Late Permian through the Late Triassic. It is a paleoclimatic zone still under-explored as to Al-rich Li-Mg-bearing phyllosilicates. The high economic potential of clay minerals in Central Europe may be discovered among construction and non-construction raw materials from cement-making materials to structural clay products. Second to none are clay minerals in the construction sector used for all kinds of bricks and tiles. The majority of clay deposits is allocated to the Cenozoic, mainly Paleogene, followed by Mesozoic, Paleozoic and Precambrian rocks which is a manifesto for the expression “Clay & Time”.

International audience; Green earth is a common green pigment based on celadonite and glauconite, used since Antiquity by artists. Two geological minerals, eight commercial green earth pigments and a sample taken from a historical location in Monte Baldo were characterized. A set of different techniques including X-Ray diffraction (XRD), scanning electron microscopy coupled to energy dispersive spectroscopy (SEM-EDS) and numerous spectroscopies: spectrophotocolorimetry, near and mid infrared, Raman, Mössbauer were used to identify the structure and composition of the different earths. The results highlight complex composition with the presence of various phases, which can be due to the pigment sampling at a different location in the same deposit. Mobile and non-invasive analyses were carried out in order to suggest a protocol for the identification of green earth in artworks, and more specifically to distinguish celadonite and glauconite. With the available mobile non-invasive techniques, and the above analyses on the raw pigments, the green area in Nicolas Poussin’s painting, Bacchanales d’enfants (Galleria Nazionale d’Arte Antica (GNAA), Rome) was examined as a case study.

Recent work by Wallace (2016) and earlier work by Dunn (1982) recognized widespread, albeit relatively weak, phyllic alteration in the Copper Flat porphyry copper system of the Hillsboro district, Sierra County, New Mexico. Wereport shortwave infrared (SWIR) spectral absorption features of the phyllic alteration obtained using a probe mask in order to significantly reduce the area sampled by the spectrometer probe. This allows the detection of IR absorption anomalies on a much smaller spatial scale than previously possible. Analytical transects across individual samples of the early phyllic alteration that is widely distributed in the Copper Flat system generally show limited sample-specific wavelength variations for the main Al:OH bond absorption feature in the infrared spectrum. Detailed analyses utilizing the probe mask indicates that sericite in the altered mafic sites has slightly longer wavelength absorption by about 3-4 nanometers (nm) relative to sericite replacing adjacent plagioclase. This supports the conclusion of Wallace (2016) based on microprobe analysis that the sericite in this setting inherited a composition partly from its precursor silicatemineral, rather than being compositionally controlled by the fluid. On average, the early sericite has its main Al:OH absorption feature at around 2210nm, which is produced by mica of celadonitic composition. Analytical transects over samples showing multi-stage phyllic alteration have up to a 9nm wavelength variation in this absorption feature. Later stage phyllic alteration is characterized by well-developed sericite halos to thin quartz ±sulfide veinlets affecting all precursor minerals, but has limited representation in the system. The IR absorption features of this sericite have both longer (to 2214nm) and shorter wavelengths (2205nm) relative to the background sericite. The late sericite of longer-wavelength absorption tends to be a gray color and that of shorter wavelength is white. Based on the location of the Al:OH absorption feature, the late sericite ranges between muscovite and celadonite compositions (2205nm to 2214nm), and overlaps with the background sericite. No sericite of shorter wavelength has been found in the Copper Flat system. The Copper Flat hydrothermal system differs from other copper porphyry systems in a number of waysbut the characteristics of the phyllic alteration are particularly noteworthy. As in other porphyry systems, the widespread background carbonate-bearing phyllic alteration likely evolved from earlier potassic alteration fluids, butthese were much more restricted in volume, less oxidized and sulfur-poor at Copper Flat. Due to the fact that the hostrock rock strongly buffered the phyllic fluid, the resulting alteration was weak and targeted specific minerals with the general preservation of the texture of the rock and the dominance of celadonitic sericite compositions. In other porphyry systems, greater stability of shorter-wavelength sericite (i.e., < 2205nm), such as muscovite and paragonite,may result from various processes (absent at Copper Flat) that result in a reduction of the host rock’s capacity to buffer the hydrothermal fluid.

Synsedimentary and early diagenetic oxygen levels are estimated by evaluating celadonite–smectite formation in Jurassic marine black shale-hosted manganese carbonates. Celadonite formed under suboxic–dysaerobic conditions, Al-rich Fe-smectite formed under suboxic–anaerobic conditions, and nontronite formed under anoxic-anaerobic conditions during sedimentary burial. A genetic pathway by direct precipitation from solution is proposed for the enormous mass of celadonite based on mineral and textural evidence. Lamination of the manganese ore is independent of clay-mineral composition and was created by a series of mineralized Fe-rich biomats.

The Parana volcanic province is the world's largest producer of amethyst geodes, mostly from the Ametista do Sul mining district (20 by 30 km) in southern Brazil. The description of the lava flows in the Ametista do Sul mining district, and adjacent areas, combined with flow-by-flow geochemistry and scintillometry, defines the stratigraphy of the flows. Nine flows were identified in the Ametista do Sul region and four additional flows were identified in the Frederico Westphalen region, totalizing 13 flows in the stratigraphy of the Ametista do Sul mining district. Six of the flows belong to Pitanga magma type (more than 3 wt.% TiO 2 ); which occur at the base of the local stratigraphy and the other seven flows belong to Paranapanema magma type (2–3 wt.% TiO 2 ) at the top of the stratigraphy. The amethyst-bearing geodes occur in the uppermost three Pitanga flows, known as Veia Alta (main producer), Veia do Meio and Veia Baixa. These flows were highly altered (more than 60 vol.%) to clay minerals by a low temperature alteration process. The massive presence of clay minerals is an important component that controls the rheology of the rock, transforming the basalt into a very low grade smectite and celadonite metabasalt. Two Paranapanema flows, COOGAMAI and Linha Alta, from Ametista do Sul were identified in a lower position, at the base of the section in Frederico Westphalen which were correlated using scintillometry and flow-by-flow geochemistry. In addition, Veia Alta and Veia do Meio flows were also identified in lower positions, compared to Ametista do Sul. Thus, the identification of these flows in different places with different elevations is an evidence of the presence of downthrown blocks in a fault-block structure, lowering the stratigraphy more than 200 m to the west in four steps. The tectonic patterns that generate this structure are three NW-strike lineaments. The identification of the COOGAMAI flow at the base of the Frederico Westphalen region, suggests that the Veia Alta geode-hosting flow is below the ground or below the elevation of 250 m in this region. These evidences show that the fault-block structure is controlling vertically the deposit of amethyst-bearing geodes and is of major significance in the exploration for new amethyst geode deposits in the region.

Samples of glauconite, representing different stages of glauconitisation, as well as different formation environments, were analysed for rare earth elements (REE) and other trace elements using a combination of bulk sample and spatially-resolved in situ techniques. The results indicate that the high-sensitivity, spatially-resolved technique of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) produces values up to two orders of magnitude lower than the bulk sample analyses. This suggests that submicroscopic rare earth element-bearing phases are distributed within the glauconite aggregates comprising the bulk samples. Analytical scanning electron microscopy (ASEM) revealed the presence of micrometre-sized grains of apatite and pore filling precipitates of an unidentified (REE, Ca)-phosphate (approximate composition Ca 0.3–0.4 (Ce 0.4 La 0.1–0.2 Nd 0.1 )PO 4 ) in some glauconite grains. The inherent REE concentrations of the glauconite aggregates (i.e., glauconite crystallites without accidental mechanical inclusions or authigenic, not layer silicate mineral precipitates) was found to be relatively low (e.g., typically less than 100 ppm), and this value decreased with increasing glauconitisation (smectite–mica transformation through a series of recrystallisation processes). These results suggest that the REEs substitute for Ca in the interlayer space of the layer silicate structure and, therefore, the REE content decreases as Ca is progressively removed from the interlayer (smectite–mica transition). LA-ICP-MS, when combined with electron probe microanalysis (EPMA) or ASEM, offers an opportunity to exclude submicroscopic accessory minerals from glauconite trace element analyses, and so produces reliable trace element data for the respective minerals which host those elements. These results illustrate that accessory minerals are difficult to eliminate from clay samples, and that care needs to be taken in the interpretation of clay mineral REE distributions, irrespective of the aggregation state of the studied clay (i.e., whether finely dispersed within the sedimentary rock, or forming millimetre-sized aggregates). Model calculations showed that authigenic apatite associated with the studied green marine clays tends to have higher REE content than “bioapatites”, the total REE content being above 10 000 ppm.

Petrographic and geochemical analyses of basaltic rocks dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction have been completed in order to investigate water-rock interaction processes during mid-ocean ridge (MOR) hydrothermal alteration in the Indian Ocean. In the study area, we have successfully recovered a serial section of upper oceanic crust exposed along a steep rift valley wall which was uplifted and emplaced along a low angle normal fault. On the basis of microscopic observation, dredged samples are classified into three types: fresh lavas, low-temperature altered rocks, and high-temperature altered rocks. The fresh lavas have essentially the same chemical composition as typical N-MORB, although LILE and Nb are slightly enriched and depleted, respectively. Low temperature alteration brought about the enrichment of K2O, Rb, and U due to the presence of K-rich celadonite and U-adsorption onto Fe-oxyhydroxide and clay minerals. On the other hand, chloritization, albitization, and addition of base metals by high temperature hydrothermal alteration result in enrichments of MnO, MgO, Na2O, Cu, and Zn and depletions of CaO, K2O, Cr, Co, Ni, Rb, Sr, and Ba. In addition, U-enrichment is also observable in the high temperature altered rocks probably due to the decrease of uranite solubility in the reducing high-temperature hydrothermal solution. These petrological and geochemical features are comparable to those of the volcanic zone to transition zone rocks in the DSDP/ODP Hole 504B, indicating that our samples were recovered from the upper ∼1000 m section of the oceanic crust. Only the alteration minerals related to off-axis alteration are absent in our samples dredged from near the spreading axis. The similarity of alteration between our samples from the Indian Ocean and the Hole 504B rocks from the Pacific Ocean suggests that MOR hydrothermal systems are probably similar across all world oceans.

Two-mica granites from the Nkambe area (West Cameroon) comprise two-mica granite and magnetite-rich two-mica granite. They are composed of K-feldspar, phengite, biotite and quartz. Megacrysts of magnetite surrounded by pink-colored patches are present locally. They are peraluminous with a ferroan affinity and exhibit normative corundum. These two-mica granites were generated at the minimum temperature and pressure of crystallization of 640 °C/600 MPa and later on evolved in the late stage of differentiation to give magnetite-rich two-mica granite. During the differentiation, hydrothermal events took an important role and led to the genesis of megacrysts of magnetite and leaching of alkalis, calcium, iron and magnesium out of some minerals. However the substitution of Fe 3+ by Al vi has been more prominent in magnetite-rich two-mica granite than in two-mica granite leading the formation of muscovite adversely to celadonite during the hydrothermal process. This process lasted about 50 m.y and was associated to the cooling path that reached lower pressures with the formation of chlorite at temperatures below 400 °C.

The thermal effect induced by the Morron de Mateo volcanic dome (Cabo de Gata volcanic region, Spain) on the adjacent bentonitised tuffaceous beds has been studied as a natural analogue of the thermal behaviour of the bentonite-engineered barrier of a geological radwaste repository. These bentonites consist mainly of Fe-rich smectites and were formed in equilibrium with seawater at temperatures between 75 and 95 °C, according to the δ18O and δD values. In contrast, bentonites from other localities in the region consist mainly of Al-smectites, formed in equilibrium with meteoric water below 25 °C. This investigation is focussed on the detection of the chemical differences between smectites from proximal and distal zones to the dome, as well as to test whether the temperatures calculated based on the O and H isotopic values correspond to their formation or transformation. The initial hypothesis was that the chosen smectites could be formed under marine conditions, being later transformed and isotopically re-equilibrated as a result of the intrusion. To check this hypothesis, a detailed mineralogical, chemical, geochemical and isotopic study has been performed on the smectitised tuffaceous materials and the overlaying biocalcarenites outcropping near and far from the dome. The results show that distal smectites are dioctahedral Al-smectites, similar to those from other deposits in the region, while proximal smectites are Fe- and Mg-rich smectites, showing two evolutionary trends on a Fe–Mg–Al ternary diagram. Similar features are observed when their structural formulae are plotted on the muscovite–celadonite–pyrophylite diagram. Thus, they plot in the smectite domain with interlayer charge less than 1, which is mainly due to octahedral substitution for distal smectites, while for proximal ones it is caused by both octahedral and tetrahedral substitutions. In this ternary diagram, the domains of both proximal and distal smectites are partially overlapped. The coexistence of di- and trioctahedral smectites was only detected in one proximal sample. Further, proximal biocalcarenites are enriched in Fe-rich dolomite in relation to the distal ones. The 87Sr/86Sr and δ13C values in carbonates and δD in smectites indicate equilibrium with seawater. In contrast, δ18O values of carbonates and smectites indicate that they were transformed and re-equilibrated between 40 and 90 °C, and between 55 and 66 °C, respectively, independently of their location with respect to the dome. These data suggest that the transformation of calcite into Mg–Fe-carbonates and the occurrence of Fe- and Mg-rich smectites near to the dome resulted from a chemically induced process at similar temperatures. The compositional differences among samples suggests that Fe, Mg and minor Mn were supplied by a contaminant plume originated from the dome, migrating through the sediments and becoming more diluted away from the source. The absence of a well-defined thermal gradient in the system could be due to the small size, semi-closed and shallow character of the basin, as well as to its high underlying volcanic activity. Finally, the results are discussed in terms of analogue processes that can be expected in the bentonite barrier of a radwaste geological repository.

Many studies have focused on low-temperature alteration of the oceanic crust, nevertheless little is known about the initial processes of low-temperature alteration and their influence on the mineralogical and chemical composition of the oceanic crust once it leaves the spreading axis. The lack of such studies is primarily related to the difficulty of recovering samples representative of this initial alteration style. Such information is nevertheless important not only for crust–ocean mass-balances but also to characterize an important input to the subduction zones. We have studied dredged basalts from the eastern flank of the East Pacific Rise at 14°15′S, concentrating on the products of a single spreading segment in a corridor perpendicular to the spreading axis and covering a range of crustal ages from 0 to 9 Ma. Electron microprobe, X-ray fluorescence, X-ray diffraction and ICP-MS analyses have been carried out to examine the mineralogical and chemical changes in the basalts which make up the surface of the upper crust caused by low-temperature alteration. Fresh rocks were sampled at the ridge axis; off-axis basalts show features of progressive alteration. Celadonite is the main alteration component in 0.12–4.6 Ma old rocks, whereas phillipsite is more abundant in rocks older than 4.6 Ma. Changing compositions of secondary minerals, progressive sealing of fractures and the occurrence of more alteration rinds on older rocks show evidence for a slight change in redox conditions, from an oxidizing, water-dominated to a more reducing, rock-dominated environment with time. Iron oxyhydroxide and celadonite are the first alteration products, partly replaced or covered by saponite under more reducing conditions. The Fe necessary for the formation of these minerals is furnished by the dissolution of glass and the breakdown of olivine. Phillipsite is present in fractures and veins in rocks older than 1 Ma. Analyses also indicate an illite–smectite mixed layered mineral which is believed to be an intermediate between saponite and celadonite and small amounts of a chlorite/smectite mixed layered mineral. All samples are characterized by the lack of minerals formed by hydrothermal processes. We conclude that the alteration took place under seawater-dominated conditions at low temperature. A comparison of trace element analyses from altered whole rock samples and their appendant fresh glass chips provides a record of element mobility during alteration. Off-axis basalts show significant uptakes of Rb, Cs and Ba which are supplied by seawater and incorporated in or on secondary minerals. An enrichment of U is also apparent and appears to be especially strong when alteration conditions are oxidative. K2O is also gained in all altered off-axis basalts, believed to be linked to the formation of celadonite. However, the volume of rock being altered is so small relative to the volume of the oceans that this scavenging has no noticeable effect on the composition of the oceans. The composition of the oceanic crust, on the other hand, is affected significantly.

The Yellowstone geothermal system provides an ideal opportunity to test the ability of reactive transport models to accurately simulate water-rock interaction. Previous studies of the Yellowstone geothermal system have characterized water-rock interaction through analysis of rocks and fluids obtained from both surface and downhole samples. Fluid chemistry, rock mineralogy, permeability, porosity, and thermal data obtained from the Y-8 borehole in Upper Geyser Basin were used to constrain a series of reactive transport simulations of the Yellowstone geothermal system using TOUGHREACT. Three distinct stratigraphic units were encountered in the 153.4 m deep Y-8 drill core: volcaniclastic sandstone, perlitic rhyolitic lava, and nonwelded pumiceous tuff. The main alteration phases identified in the Y-8 core samples include clay minerals, zeolites, silica polymorphs, adularia, and calcite. Temperatures observed in the Y-8 borehole increase with depth from sub-boiling conditions at the surface to a maximum of 169.8°C at a depth of 104.1 m, with near-isothermal conditions persisting down to the well bottom. 1-D models of the Y-8 core hole were constructed to determine if TOUGHREACT could accurately predict the observed alteration mineral assemblage given the initial rock mineralogy and observed fluid chemistry and temperatures. Preliminary simulations involving the perlitic rhyolitic lava unit are consistent with the observed alteration of rhyolitic glass to form celadonite.

The purpose of this work is to study the mobility and budget of Fe isotopes in the oceanic crust and in particular during low-temperature interaction of seawater with oceanic basalt. We carried out this investigation using samples from Ocean Drilling Program (ODP) Site 801C drilled during Leg 129 and Leg 185 in Jurassic Pacific oceanic crust seaward of the Mariana Trench. The site comprises approximately 450 m of sediment overlying a section of 500 m of basalt, which includes intercalated pelagic and chemical sediments in the upper basaltic units and two low-temperature (10–30 °C) ocherous Si–Fe hydrothermal deposits. Fe was chemically separated from 70 selected samples, and 57Fe/54Fe ratios were measured by MC-ICP-MS Isoprobe. The isotopic ratios were measured relative to an internal standard solution and are reported relative to the international Fe-standard IRMM-14. Based on duplicate measurements of natural samples, an external precision of 0.2‰ (2σ) has been obtained. The results indicate that the deep-sea sediment section has a restricted range of δ57Fe, which is close to the igneous rock value. In contrast, large variations are observed in the basaltic section with positive δ57Fe values (up to 2.05‰) for highly altered basalts and negative values (down to −2.49‰) for the associated alteration products and hydrothermal deposits. Secondary Fe-minerals, such as Fe-oxyhydroxides or Fe-bearing clays (celadonite and saponite), have highly variable δ57Fe values that have been interpreted as resulting from the partial oxidation of Fe2+ leached during basalt alteration and precipitated as Fe3+-rich minerals. In contrast, altered basalts at Site 801C, which are depleted in Fe (up to 80%), display an increase in δ57Fe values relative to fresh values, which suggest a preferential leaching of light iron during alteration. The apparent fractionation factor between dissolved Fe2+ and Fe remaining in the mineral is from 0.5‰ to 1.3‰ and may be consistent with a kinetic isotope fractionation where light Fe is stripped from the minerals. Alternatively, the formation of secondary clays minerals, such as celadonite during basalt alteration may incorporate preferentially the heavy Fe isotopes, resulting in the loss of light Fe isotopes in the fluids. Because microbial processes within the oceanic crust are of potential importance in controlling rates of chemical reactions, Fe redox state and Fe-isotope fractionation, we evaluated the possible effect of this deep biosphere on Fe-isotope signatures. The Fe-isotope systematics presented in this study suggest that, even though iron behavior during seafloor weathering may be mediated by microbes, such as iron-oxidizers, δ57Fe variations of more than 4‰ may also be explained by abiotic processes. Further laboratory experiments are now required to distinguish between various processes of Fe-isotope fractionation during seafloor weathering.

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 (

Petrographic and mineralogical studies of a 45 m thick basaltic lava flow in southern Parana Basin (Rio Grande do Sul, Brazil) enables a detailed description of three structural levels: the lower vesicular zone (LVZ), inner massive zone (IMZ), and upper vesicular zone (UVZ). The three levels, inherited from cooling stages, are characterized by vertical zonation of the petrographic features and associated secondary mineral assemblages, such as clay minerals and zeolites. Zeolite crystallization is limited to the vesicle infilling and partial replacement of albitized plagioclases. The clay mineral sequence observed in vesicle infilling is a celadonite, saponite, chlorite/saponite mixed layer. The mesostasis of the three levels, which constitutes reduced sites of clay mineral crystallization from the peripheral levels (top and base) to the inner, massive, and vesicle-free part of the flow, presents a saponite to C/S mixed layer sequence. Petrographic and chemical observations support three steps for the alteration mechanisms. The earliest alteration stages are related to postmagmatic mechanisms. They are marked by earliest celadonite precipitation in the oxidative condition of the highly permeable UVZ, saponite with homogeneous compositions in reducing conditions, and C/S mixed layer conversion in the inner part of the flow, where temperature gradients have been preserved during the final stages of cooling. The albitization of plagioclase associated with zeolite crystallization and the compositional changes in clay mineralogy should be attributed to high water/basalt alterations during the low-grade burial metamorphic conditions in more permeable vesicular levels of the flow.

In this study, phlogopite synthesis experiments were carried out at 800 °C and at 1000 and 2000 MPa for low concentrations of Rb (XRb=Rb/(Rb+K)

In situ 40Ar/39Ar UV-laser ablation analyses of garnet, omphacite, epidote, phengite, barroisite and symplectite in a sample of UHP eclogite from Qinglongshan (Sulu terrain, China) reveal extremely heterogeneous argon isotopic ratios. Results from the 220–240 Ma eclogite indicate a significant and variable extraneous argon signature that is restricted to the early-formed phengite and epidote and later-formed barroisite. Individual, compositionally zoned, phengite grains record a large variation in 40Ar/39Ar dates (300–1100 Ma) that are positively correlated with celadonite content. These data are consistent with retrograde chemical exchange and argon isotopic repartitioning in response to decreasing partial pressures of argon during decompression. Consistent with the observation of atmospheric argon ratios in garnet, omphacite and symplectite these data support an interpretation that the extraneous Ar was incorporated early in the crystallization history of the eclogite, and not from a late infiltrating fluid. The ultimate source of the extraneous argon remains speculative, however on the basis of supporting stable oxygen and hydrogen isotope data from this locality, which support a closed isotopic system, it is inferred to have been inherited from the protolith of the UHP eclogite.

Celadonites from the Parana flood basalts have been dated by internal Rb/Sr isochrons, using the ammonium acetate leaching technique previously used by Hart and Staudigel (1986) for celadonites from DSDP holes. The residues after leaching have very high 87Rb/86Sr ratios (18.7–1900), and very radiogenic 87Sr/86Sr (0.73–2.96). Electron microprobe analyses show that potassium, and thus also presumably rubidium, became enriched in these minerals during the transformation of a proto-celadonite smectite phase into celadonite. Our leaching experiment suggests that radiogenic 17Sr and rubidium are very resistant to chemical fractionation during leaching and thus may have remained in closed-system for Rb/Sr chronology. Ages varying from 107 ± 1 to 81 ± 1 Ma have been obtained at two different sites, one on high-Ti basalts in the northern Parana basin and the other on low-Ti basalts in the southern basin. These ages are significantly younger than the eruption of the flood basalts (127–137 Ma), and in this respect are in good agreement with results obtained on celadonites from oceanic ridges. Initial 87Sr/86Sr ratios are distinctly higher in the celadonites than in the host basalts, probably due to contamination by radiogenic strontium transported by the hydrothermal fluids. Our results suggest that mineralogy and isotopic composition are different in celadonites from the northern and southern Parana basins, although this needs to be confirmed on a larger dataset.

Rb Sr dating of celadonites from several geographic/stratigraphic locations from the Troodos ophiolite, Cyprus, yield crystallization ages varying between 57.9 ± 3.9 and 89.0 ± 1.9 Ma. The oldest celadonite age (89.0 ± 1.9 Ma) agrees well with the crystallization age of the Troodos igneous complex suggesting that low-temperature chemical exchange with seawater started immediately after formation of the ophiolite. Drillcore samples suggest a duration of low-temperature alteration of ca. 10 Ma, whereas outcrop samples yield much longer durations of ≥ 30 Ma. The exact reason for this discrepancy is not fully understood, but may represent the difference in localized downwelling in different parts of the ophiolite, or may be related to the extensive weathering of the upper extrusive suite. Additionally, in nearly all cases Rb Sr age estimates are older than K Ar age data from the same samples. To evaluate the potential disturbances of Rb Sr isotope systematics from exchangeable interlayer and basal surface positions, a series of ion exchange experiments was carried out. These data show that significant amounts of Rb and Sr can be in exchangeable positions, and therefore a fraction of the parent and daughter inventory of these clays is vulnerable to late-stage chemical exchange. However, the overall agreement between the data of the whole mineral, and ion-exchanged clay separates suggests that the influence of this exchange on the Rb Sr isotopic system is minor. The post-crystallization chemical behavior of exchangeable K and Ar was not evaluated, but the loss of radiogenic argon from exchangeable sites could result in K Ar ages younger than Rb Sr ages of the same sample.

Microbial green marine clays in deep-sea sediments were recovered from the Izu-Bonin fore-arc basin at Leg 126 of the Ocean Drilling Program (ODP) along the Bonin-Mariana trench. Green spherical materials and remnants of dead microbial cells of filamentous bacteria are abundant in the green marine clays. Organic carbon and graphite were found in the volcanic sandstone. Transmission electron microscope observations clearly show processes of graphite mineralization from low-crystallinity to well-crystallized graphite. However, X-ray photoelectron spectrochemical analyses (micro-ESCA) indicate that the green marine clays contain high C (1s) of microbial origin carbon, and COO, CO, CC and graphite structures. Fourier transform infrared spectra (FT-IR spectra) of the green spherules show peaks between glauconite and celadonite. X-ray powder diffraction data show the presence of smectite and 1.03–1.22-nm mica-clay minerals with abundant zeolites, which are hydrothermal alteration products. The results suggest that fossil remnants of graphite mineralized organisms, in particular filamentous bacteria, might play an important role in the formation of green marine clays.

Direct phase analysis, based on the probabilistic estimate of triple and quartet structure invariant sums, was used to determine the crystal structures of three layer silicates, viz. celadonite, muscovite and phlogopite-biotite, from oblique texture electron diffraction intensity data published earlier by B.B. Zvyagin and his co-workers. In all cases, the resulting potential maps are sufficient to identify most of the atomic positions (including oxygen) found in the original determinations. In addition, most of the crystallographic phase values are identical to the ones found earlier. The major drawback for this technique is due to the paucity of measured intensities, requiring a multisolution approach to this analysis. However, atomic-resolution data enable the choice of the correct map to be made from chemical knowledge of a reasonable layer structure.

REE data for secondary minerals from low- and high-temperature alteration environments in the Troodos Ophiolite have been determined using a combined mass spectrometry-neutron activation technique. Smectite- and analcime-REE mimic fresh rock profiles and have concentrations ranging from < 1–5 times chondritic values. Celadonite is depleted in LREE and total REE relative to the rocks. Groundmass-replacing chlorite and epidote are similar to, or enriched in, REE relative to fresh rocks. In contrast, fault-related chlorite and void-filling epidote are LREE-enriched with large positive Eu anomalies and yield comparable REE profiles to those of hydrothermal fluids venting at mid-ocean ridges. The zeolites, clinoptilolite and laumontite, are also LREE-enriched with REE contents significantly lower than rock values. Carbonate is enriched in HREE relative to LREE and concentrations vary from 0.2 to 18 times chondritic values. REE data for these minerals indicate that seawater-REE contents are modified by seawater-rock interaction in volcanic sequences and sheeted dykes in oceanic crust during axial hydrothermal alternation and subsequent crustal aging. The absolute and relative abundances of REE in seawater-derived solutions evolve in response to the precipitation of secondary minerals and the prevailing water/rock ratios. Rocks altered at low temperatures will show no change or a decrease in REE content; the LREE/HREE ratio will reflect the proportion of smectite, celadonite and carbonate in the groundmass assemblage. In contrast, rocks altered at high temperatures will show no change or an increase in REE; the LREE/HREE ratio will reflect the proportion of void-filling and groundmass-replacing chlorite and epidote in the rock.

Oxygen and carbon isotope ratios were measured for hydrothermal minerals (silica, clay and calcite) from fractures and vugs in altered rhyolite, located between 28 and 129 m below surface (in situ temperatures ranging from 81 to 199°C) in Yellowstone drill holes. The purpose of this study was to investigate the mechanism of formation of these minerals. The δ18O values of the thirty-two analyzed silica samples (quartz, chalcedony, α-cristobalite, and β-cristobalite) range from −7.5 to +2.8‰ . About one third of the silica 7samples have δ18O values that are consistent with isotopic equilibrium with present thermal waters; most of the other silica samples appear to have precipitated from water enriched in 18O (up to 4.7‰) relative to present thermal water, assuming precipitation at present in situ temperatures. Available data on fluid-inclusion homogenization temperatures in hydrothermal quartz indicate that silica precipitation occurred mostly at temperatures above those measured during drilling and imply that 15O enrichments in water during silica precipitation were generally larger than those estimated from present conditions. Similarly, clay minerals (celadonite and smectite) have δ18O values higher (by 3.5 to 7.9‰) than equilibrium values under present conditions. In contrast, all eight analyzed calcite samples are close to isotopic equilibrium with present thermal waters. The frequent incidence of apparent 18O enrichment in thermal water from which the hydrothermal minerals precipitated may indicate that a higher proportion of strongly 18O-enriched deep hydrothermal fluid once circulated through shallow portions of the Yellowstone system, or that a recurring transient 18O-enrichment effect occurs at shallow depths and is caused either by sudden decompressional boiling or by isotopic exchange at low water/rock ratios in new fractures. The mineralogy and apparent 18O enrichments of hydrothermal fracture-filling minerals are consistent with deposition during transient boiling or rock-water exchange (fracturing) events.

Discrepancies between predicted and observed interstitial water profiles for sites 322 and 323, Leg 35, Bellingshausen Abyssal Plain, were used to identify sites of reaction for further mineralogical and chemical investigations. Two major reaction sites were identified at site 323: (1) In the silicification zone between 410 and 505 m depth, where dissolution of biogenic (opaline) silica, plagioclase and a few coccoliths and the formation of opal-CT, Mg-rich smectite and K-feldspar are responsible for the observed silica, Ca 2+ , Mg 2+ , and K + interstitial water gradients. Dissolution of biogenic silica provided most of the silica for the porcelanites. (2) In the basalt, weathering of pyroxene and plagioclase and the formation of celadonite, smectite, calcite, and goethite are probably responsible for the observed Ca 2+ , Mg 2+ , and K + interstitial water gradients below the silicification zone. The chemistry of authigenic smectites reflects the composition of their precursor(s).

Petrographic examination of amygdules and veins associated with moderately altered pillow basalts dredged from the Peru Trench has revealed that a consistent pattern of mineral crystallization has occurred. This sequence is: (1) green, weakly pleochroic clay (R.I. > 1.56); (2) dark yellowish brown, non-pleochroic clay (R.I. > 1.56); (3) light yellowish brown to colorless, fibrous, weakly pleochroic clay (R.I. < 1.56); and (4) calcite or celadonite. Chemical and X-ray diffraction analyses suggest that all clay mineral amygdule and vein fillings are dominated by intimate mixtures of an Fe-rich saponite and nontronite with very small admixtures of serpentine and illite. It is argued that sequential mineral fillings of fractures and vesicles may provide significant information about the chemistry of circulating interstitial fluids. For the pillow basalts studied the first-formed clays were enriched in nontronite, thereby suggesting Fe-rich fluids. These in turn were followed by saponite-rich clays and calcite. The change from Fe-and Mg-rich fluids to dominantly Ca-rich fluids is thought to correspond to a change from mafic mineral alteration to plagioclase alteration in the pillow basalts. An increase in the Fe3+/Fe2+ ratio of clays toward the centers of vesicles may indicate a change toward a more oxidizing environment of alteration.

Hydrothermal alteration of the 0.54 Ma Biscuit Basin rhyolite flow in Yellowstone drill cores Y-7 and Y-8 has involved significant changes in Ti-normalized whole rock concentrations of Li, Mg, Ca, Na, K, Mn, Rb, Sr, Sb, Cs, Ba, and U, whereas such changes have generally not occurred for Al, Fe, Sc, Co, Y, Zr, REE, Hf, Ta, and Th. The relatively mobile elements have been redistributed over distances from 2–5 cm to more than 10–100 m, and are associated mainly with zeolites and clays, in thoroughly altered samples; average whole rock changes in Ti-normalized mass range from about 0.5–17 g/Kg (9–49%) for major elements and 0.1–280 mg/Kg (19–4260%) for minor and trace elements. Compositional changes correlate with hydrothermal mineralogy. The relatively immobile elements have been redistributed over a distance of at least 10–100 μm but less than 2–5 cm, and are associated mainly with clays, in thoroughly altered samples. Addition of SiO2 ranges from 110–890 g/Kg in samples where porosity has been thoroughly sealed by silica deposition. Thermal water is not in oxygen isotopic equilibrium with any primary or secondary phases analyzed except calcite. Modest depletions of 18O (1–2%) due to exchange with thermal water apparently have occurred in Y-8 plagioclase and obsidian. Oxygen isotope ratios in obsidian-replacing smectite and in veinlet celadonite and β-cristobalite suggest that these phases precipitated from water locally enriched in 18O (up to ~8% heavier than present thermal water). The minimum integrated water/rock mass ratio in Y-7 and Y-8 is 103–104 (assuming advective transport).

A blue-green smectite (iron-rich saponite) and green mica (celadonite) are the dominant sheet silicates in veins within the 10.5 m of basalt cored during DSDP Leg 34, Site 321, in the Nazca plate. Oxygen isotopic analyses of these clays, and associated calcite, indicate a formation temperature of ≤25°C . Celadonite contains appreciable Fe 2 O 3 , K 2 O and SiO 2 , intermediate MgO, and very little Al 2 O 3 . Celadonite is commonly associated with goethite and hematite, which suggests that this phase formed by precipitation within a dominantly oxygenated environment of components leached from basalt and provided by seawater. A mass balance estimate indicates that celadonite formation can remove no more than 15% of the K annually transported to the oceans by rivers. In contrast, iron-rich saponite containing significant Al 2 O 3 appears to have precipitated from a non-oxidizing , distinctly alkaline fluid containing a high Na/K ratio relative to unmodified seawater. Seawater-basalt interaction at low temperatures, resulting in the formation of celadonite and smectite may explain chemical gradients observed in interstitial waters of sediments overlying basalts.

Chemical and mineralogical changes produced by very low-grade metamorphism in a 40 meter thick, K-rich, calc-alkaline andesite flow of the marine Jurassic Rio Grande Formation of southern coastal Peru are discussed. This metamorphism (=spilitization) was non-deformational and generated spilitic domains at (and near) both vesicular margins of the flow, whereas the massive central zone remained relatively unaltered. The metadomains are characterized by mineral associations of the zeolite facies. Primary minerals are Ca-plagioclase, augitic pyroxene, iron-titanium oxides, and (pseudomorphs after) olivine. Metamorphic minerals are: albite (three generations), K-feldspar, pumpellyite, chlorite, interlayered chlorite-celadonite, celadonite, various mixed-layer Si- and Fe-rich phyllosilicates, “iddingsite,” calcite, analcime, titanite, and white mica. The effect of the metamorphism on the rock chemistry is reflected in changes especially observed at the marginal zones of the flow which affect major, trace, and RE elements: 1) strong increase of the iron oxidation ratio (Fe2O3/FeO); 2) enrichment in Na2O accompanied by a concomitant depletion of CaO in non-amygdaloidal domains; 3) depletion of SiO2; 4) strong enrichment in H2O and CO2; 5) marked depletion of Sr and Rb; 6) enrichment in Cl and S; and 7) slight depletion in RE elements, notably in the top zone of the flow. Conversely, elements such as Ti, P, Nb, and Y were fairly immobile, whereas Zr and K were only slightly mobilized. The effect of the metamorphism on the mineral chemistry is expressed by the predominance of metastable equilibrium evidenced by the existence of wide compositional ranges in the phyllosilicates, the incomplete albitization of the Ca-plagioclase, and the Al-rich character of the pumpellyites. The metamorphism is considered to be of hydrothermal-burial type, which takes place at low temperature and pressure — probably about 125–230°C and less than 3 kb, and is produced mainly through permeability-controlled seawater/rock interaction.

Shallow marine lavas and pyroclastic rocks, several tens of metres thick, outcrop at the base of thrust slices in the Pindos nappe in the Lakmon Mountains of western continental Greece. These volcanic rocks are basalts and andesites formed during mid-Triassic subduction. Geochemically, on the basis of the potassium content, shoshonitic and calc-alkalic groups are distinguished. Principally the shoshonitic rocks contain primary K-feldspar. The calc-alkalic rocks contain complexly zoned feldspars and are depleted in intermediate REE: this depletion could reflect a small degree of partial melting of amphibole, or possibly phlogopite, during petrogenesis. These two characteristics are found in other examples of minor calc-alkalic rocks associated with shoshonites described in the literature. A petrogenetic model is developed involving deeper mantle enrichment in LILE and partial melting of this enriched mantle rock containing amphibole. Calc-alkalic rocks are the result of initial partial melting and may be mixed with small amounts of magma of deeper origin; further partial melting produces the more voluminous shoshonites. Celadonite developed as a secondary mineral during burial beneath about 1 km of Mesozoic to early Tertiary sediments. During nappe emplacement, vermiculite formed in the calc-alkalic rocks; in contrast, smectite developed in the shoshonites, because the potassium was sufficient to immobilize all available Fe in celadonite.

Magmatic muscovite of Hercynian S-type granite, muscovite of greisenized granite, muscovite of selvages at quartz vein-schist contacts and muscovite within the vein of the Portuguese W-Sn deposit from Panasqueira are distinguished mainly by trace-element contents and there is a tendency for log ( f H 2 O f HF ) to increase in this sequence. Several correlations were found between major elements of these muscovites and of muscovites of hypothermal quartz veins of the Sn deposit from Montezinho, the Sn-W deposit from Argozelo and the W deposit from Borralha (northern Portugal). F content of muscovite depends on unit-cell parameter b, Si, (Mg + Fe2+ + Fe3+), AlVI and OH. Sn content of muscovite seems to depend on its celadonite content and is carried by fluorine. Sn correlates positively with (Mg + Fe2+ + Fe3+) and F in muscovites from each deposit. But a positive correlation between Sn and Cs was found in hydrothermal muscovites from all deposits. The contents of both elements may be guides of Sn and W mineralizations. W is generally less abundant than Sn in muscovite and was probably transported by water as tungstic acid or WO2−4. The muscovite of quartz veins from different deposits can be distinguished by major- and trace-element contents. The muscovites of W quartz veins crystallized at greater log ( f H 2 O f HF ) than the muscovite of Sn, Sn-W and W-Sn quartz veins. Muscovite is also abundant in schists bordering quartz veins and has more distinct major- and trace-element contents than those of white mica of regional metamorphism and of hydrothermal muscovite of selvages along the vein-schist contact. It also crystallized at different log ( f H 2 O f HF ).

Cation exchange experiments (ammonium acetate and cation resin) on celadonite-smectite vein minerals from three DSDP holes demonstrate selective removal of common Sr relative to Rb and radiogenic Sr. This technique increases the Rb/Sr ratio by factors of 2.3 to 22 without significantly altering the age of the minerals, allowing easier and more precise dating of such vein minerals. The ages determined by this technique (site 261—121.4 ±1.6m.y.; site 462A—105.1 ±2.8m.y.; site 516F—69.9 ±2.4m.y.) are 34, 54 and 18 m.y. younger, respectively, than the age of crust formation at the site; in the case of site 462A, the young age is clearly related to off-ridge emplacement of a massive sill/flow complex. At the other sites, either the hydrothermal circulation systems persisted longer than for normal crust (10–15 m.y.), or were reactivated by off-ridge igneous activity. Celadonites show U and Pb contents and Pb isotopic compositions little changed from their basalt precursors, while Th contents are significantly lower. Celadonites thus have unusually high alkali/U,Th ratios and low Th/U ratios. If this celadonite alteration signature is significantly imprinted on oceanic crust as a whole, it will lead to very distinctive Pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust. Initial Sr isotope ratios of ocean crust vein minerals (smectite, celadonite, zeolite, calcite) are intermediate between primary basalt values and contemporary sea water values and indicate formation under seawaterdominated systems with effective water/rock ratios of 20–200.