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Halite possesses great potential for hosting and storing information vital to the reconstruction of Earth's ancient climate, seawater chemistry and evolving atmosphere. Here, we propose a screening protocol that not only distinguishes between primary and secondary halite, but also identifies fluid inclusions that carry original gas trapped during the primary crystallization process. An integrated multi-analytical protocol is presented for sample preparation, petrographic evaluation, analytical measurements and distinguishing original gases and contents from contaminants. The screening protocol starts with the visual inspection of halite for primary chevron and hopper features and/or milky appearance. Then, petrography is used to distinguish between primary and secondary (diagenetic) crystal fabrics and inclusions. The trace element and isotope geochemistry speak directly to the composition of the depositional and/or diagenetic fluids and to the formation in marine, non-marine or diagenetic settings. Furthermore, rare earth elements and redox sensitive elements may address the redox conditions of the salt pan/flat brines, whereas microthermometry helps characterize the depositional/diagenetic environment's temperature. Finally, gas extracted by quadrupole mass-spectrometer from gas bubbles in fluid inclusions is screened with concomitant CH4, CO2 and Ar contents to distinguish and quantify organic matter decomposition contributions and with 40Ar/36Ar to monitor modern atmospheric gas diffusion/leakage issues, and thus, contamination of the primary gas contents. Our test case shows that halite collected from cores is well-suited in maintaining its original mineralogical texture, chemistry and gas content. Our integrated screening protocol suggests that the 815 Myr old halite from the Tonian Browne Formation of the Officer Basin, Australia formed under atmospheric conditions of about 10.9% pO2 (back calculation) or 9.6% pO2 (mean calculation). The coeval Neoproterozoic gypsum collected in outcrop from Minto Inlet, Victoria Island, Canada underwent some mineralogical alteration. However, experiments suggest that depositional gypsum from outcrop/core may have potential of retaining vestiges of original gas in fluid inclusions.

The direct measurement of gases trapped in sulphide minerals shows that samples from gold-bearing deposits in Britain and Ireland are anomalously rich in methane. Chalcopyrite samples in de...

The Northwest quartz-pyrite-gold veins are situated 500 m east of the Lepanto fault in Mankayan, Luzon, Philippines. Most vein mineralization is hosted by the Lepanto metavolcanic basement rocks at an elevation between 700 and 1,050 m. The earliest stage, stage 1, is characterized by sphalerite + chalcopyrite + pyrite ± magnetite veins cutting the host rocks that were altered to chlorite + illite + epidote. Precious metal deposition started in stage 2 as electrum, native gold, and gold-silver tellurides deposited with pyrite, quartz, and carbonate. Deposition of gold and silver tellurides continued in stage 3a with abundant pyrite and tennantite-tetrahedrite solid solution intergrown with chalcopyrite, bornite, and minor sphalerite. The stage 3a veins and host-rock alteration are characterized by abundant muscovite and quartz, while the stage 3b veins and alteration consist of quartz, pyrophyllite, alunite, and dickite. Enargite and luzonite are the dominant sulfide minerals in stage 4, which are either disseminated in silicified host rock or within wide quartz veins. Lesser amounts of quartz and abundant pyrite with inclusions of enargite and luzonite were precipitated in stage 5. Microthermometry on fluid inclusions in quartz of stages 2, 3a, and 4 indicates boiling of the hydrothermal fluids. Bulk gas analysis on fluid inclusions in quartz shows that the stage 2 and 4 fluids had components derived from basaltic and andesitic magma, respectively. Fluids that formed stage 4 quartz were more diluted by meteoric water than the fluids that formed the stage 2 veins. Radiometric 40Ar/39Ar dating on alunite separated from the stage 3b advanced argillic alteration zone yielded 2.2 ± 0.1 Ma. Sulfur isotope compositions of the Northwest quartz-pyrite-gold deposit reveal a bulk δ34S of approximately 5‰, similar to the calculated value for the adjacent Far Southeast porphyry deposit. Calculated oxygen and hydrogen isotope ratios of the fluids of the Northwest quartz-pyrite-gold deposit stage 3b dickites are similar to those that formed the illite alteration in the Far Southeast porphyry deposit but are higher in δ18O when compared to the fluids that formed the kandites in the Lepanto enargite deposit. The northward cooling of mineralizing fluids previously reported in the Lepanto enargite deposit is not consistent with the mineralogic indications in the Northwest quartz-pyrite-gold deposit. These data indicate multiple mineralization events in the Mankayan district, which is one of the largest mineral districts in the western Pacific.

The Phoenix uranium deposit in the southeastern Athabasca Basin (Canada) is a typical unconformity contact-hosted uranium deposit, characterized by an association with reactivated basement faults, graphite-rich rocks in the underlying basement, and a pervasive clay alteration halo surrounding the mineralized zones. Petrographic results suggest that the mineralizing hydrothermal system was characterized by alternating desilicification and silicification events. Uraninite and clay-size minerals (tourmaline, kaolinite, illite and minor chlorite) mainly precipitated in the desilicification periods whereas hydrothermal quartz (mainly drusy quartz) formed during the silicification periods. Primary fluid inclusions in the hydrothermal quartz are inferred to represent the ore-forming fluids even though quartz did not co-precipitate with uraninite. The coexistence of multiple types of fluid inclusions (liquid-dominated biphase, vapor-dominated biphase, vapor-only and halite-bearing triphase) within individual fluid inclusion assemblages is interpreted to indicate fluid boiling and heterogeneous trapping. Bulk fluid inclusion volatile analysis by mass spectrometry indicates H2O as the dominant species, with less than 1 mol% non-aqueous volatiles that show a compositional trend typical of fluid boiling. Microthermometric and cryogenic Raman spectroscopic analyses indicate that the mineralizing fluids are of H2O-NaCl-CaCl2 ± MgCl2 composition, with salinities ranging mainly from 23.4 to 31.1 wt%. The liquid-dominated biphase inclusions (excluding those interpreted to have resulted from heterogeneous trapping) have homogenization temperatures from 90 to 157 °C. These data are generally consistent with the classical diagenetic-hydrothermal model in which basinal brines that extracted uranium from the basin or the basement were channeled along reactivated basement faults, and precipitated uraninite near the unconformity through reaction with reducing agents. However, the relatively low fluid inclusion homogenization temperatures documented in this paper, together with fluid boiling inferred from fluid inclusion assemblages, suggest that the deposit may have formed in a shallower environment (∼2.5 km) than assumed in the conventional diagenetic-hydrothermal model (>5 km). Abrupt fluid pressure drops during episodic faulting may have resulted in fluid boiling (flash vaporization) changing the fluid pH and promoting uraninite precipitation, with the alternating liquid-vapor conjugates of the boiling system resulting in alternating precipitation of quartz and uraninite respectively.

The Hishikari gold–silver deposits in southern Kyushu, Japan, are low-sulfidation vein-type epithermal deposits. Bulk composition analysis, fluid inclusion microthermometry and quantitative gas composition analysis on the basis of the crush-fast scan (CFS) method were conducted to elucidate characteristics and behavior of hydrothermal fluids responsible for the gold mineralization of the Hishikari deposits. Quartz vein samples were collected from widely scattered locations of ore veins: Zuisen-6, -3, -1 (Main deposit), Keisen-2 and Shosen-5 (Sanjin deposit), and Seisen-8 (Yamada deposit) between −70 and 64 ML (mine meter level). The samples are composed mainly of quartz and adularia, and ore minerals of electrum, pyrargyrite, pyrite and/or stibnite, having an average bulk Ag/Au weight ratio of 0.54. Fluid inclusions in quartz and adularia are two-phase liquid–vapor, and their trapping temperatures and salinities are ca. 190–220 °C and 1.5–2.7 wt.% NaCl eq., respectively. The salinity is highest with an average 2.4 wt.% NaCl eq. of the fluid inclusions from the deepest level (Zuisen-6 vein at −70 ML), while the fluid inclusion salinities from other samples are relatively low less than ca. 2.0 wt.% NaCl eq. Parallel- and lattice-bladed quartz occur in a number of veins, suggesting fluid boiling during the vein formation. Gases of fluid inclusions are composed mainly of H2O (93.4–99.8 mol%) and CO2 (0.15–6.2 mol%), N2 (0.007–0.96 mol%), Ar (0.0001–0.0047 mol%), CH4 (0.002–0.048 mol%), H2S (0.0004–0.0028 mol%), H2 (0–0.074 mol%) and He (0.0002–0.0011 mol%). CO2, N2 and Ar contents of the fluid inclusions are elevated in the samples collected from deep levels, while H2O, H2S and He contents are elevated in those from shallow levels. The Ar/He, CO2/CH4 and N2/Ar ratios of fluid inclusion gas indicate a systematic transition from magmatic to meteoric signatures in order from deep to shallow levels. The CO2/N2 ratio and volatiles contents are positively correlated suggesting condensation in relatively high Au-grade quartz vein samples (19.5–27.7 ppm Au), while negative correlations suggest boiling with volatile contents less than 1% are indicative of low Au-grade quartz vein samples (0.14 to 0.18 ppm Au). The elevated CO2 contents observed in the Zuisen veins are signatures of the deeply circulating hydrothermal fluid with a magmatic contribution. Due to intermittent boiling of hydrothermal fluid, CO2 and H2S were partitioned into the vapor phase in deeper portions and condensed into the steam-heated meteoric water in shallower portions. The signatures of both meteoric and magmatic fluids in fluid inclusion gas compositions were detected in the samples collected near the unconformity between Shimanto Supergroup (shale and sandstone) and Hishikari Lower Andesite. This suggests that a mixing of the deeply circulating hydrothermal fluids with the steam-heated meteoric water took place near the unconformity, and it was one of the mechanisms to have precipitated gold.

International audience; The evolution of Earth's atmosphere is of extreme importance to the lithosphere, hydrosphere and biosphere. This is particularly true for the role the biosphere played in the inceptual formation of atmospheric oxygen, and subsequently in its potential role in the evolution of higher life forms. Quantifying the change in atmospheric oxygen with geologic time is extremely challenging, with the redox sensitive isotope and element models subject to contradictory outcomes. Here, we present a new approach in determining past atmospheres, by a) directly measuring (DM) the amount of atmospheric oxygen trapped in fluid-gas inclusions of primary and unaltered halite, and by b) using a back-calculation method (BCM) applied to trapped halite fluid inclusion gas contents subjected to post-depositional biogeochemical reactions to calculate the amount of atmospheric oxygen. On average (±standard error), atmospheric oxygen content during the latest Ediacaran was 17.4 ± 2.1% compiled from DM of 17.4 ± 1.3% and from BCM of 17.3 ± 2.9%. For the earliest Cambrian average oxygen was 19.3 ± 1.4% compiled from DM of 18.8 ± 1.5% and from BCM of 19.8 ± 1.3%. The oxygen content during the mid and late Ordovician was relatively invariant with 15.6 ± 1.8% [15.4 ± 1.4% (DM), 15.8 ± 2.2% (BCM)], and 16.2 ± 1.2% [15.7 ± 1.7% (DM), 16.6 ± 0.7% (BCM)], respectively. The relatively stable atmospheric oxygen levels continued into the Silurian of 15.9 ± 1.1% (12.9 ± 0.4 to 16.5%, DM; 14.3 ± 1.7 to 19.8 ± 2.1% BCM) except for a peak to about present levels during the late Silurian of 23.2 ± 1.9% (21.6% DM, 24.7 ± 3.7% BCM). Early Carboniferous atmospheric oxygen returned to relatively lower and invariant levels at about 15.3 ± 0.7 to 15.7 ± 1.0% (15.0%, 15.5 ± 0.1% DM, 9.4 ± 3.9 to 15.8 ± 1.9% BCM). Similarly, the average atmospheric oxygen content in fluid inclusion of halites from the late Permian was similar with 15.7 ± 1.3% with 16.9% (DM) to 14.5 ± 2.6% (BCM). Indeed, our study suggests that atmospheric oxygen was relatively constant for most of the Paleozoic at about 16.5% (±0.6 SE, ±2.2 SD). This puts our proxy result at odds with the values and trends suggested by the COPSE, GEOCARBSULF AND GEOCARBSULFOR models for the early Paleozoic atmospheric oxygen, and continues the discord with the GEOCARBSULF and GEOCARBULFOR models during the late Paleozoic.

The Middle and Upper Cambrian Port au Port Group of western Newfoundland consists mainly of mixed carbonates and siliciclastics and is composed of the March Point, Petit Jardin, and Berry Head formations. Samples were collected from the core NF-02 that spans the uppermost Hawke Bay (40 m), the March Point (~83 m thick) and the lowermost Petit Jardin (~47 m). Petrographic examination of carbonates of the March Point and Petit Jardin carbonates reveals three generations of dolomite: (i) micritic to near-micritic (D1, 4 μm −35 μm) fabric-retentive dolomite, (ii) coarse crystalline sucrosic (D2, 60 μm−150 μm) fabric-destructive dolomite (sub-to euhedral zoned rhombs with clear rim and cloudy core), and (iii) coarse anhedral (D3, >0.5 mm) fracture-filling saddle dolomite. The microthermometric measurements of primary two-phased fluid inclusions retained in D2, and D3 samples yielded homogenization temperatures (Th) showing a general increase with burial. The mean values of Th and salinity are 89.1 ± 6.4 °C vs 21.2 ± 2.2 eq. wt% NaCl, and 110 ± 8.2 °C vs 21.1 ± 4.7 eq. wt% NaCl, respectively. The investigated dolomites show a decrease of Sr from D1 (102 ± 49 ppm) to D3 (47 ± 15 ppm) which reflects the influence of progressive burial. The mean δ18O values decrease from D1 (−6.3 ± −1.1‰ VPDB), to D3 (−9.7 ± 1.7‰ VPDB) where D2 (−7.6 ± 1.2‰ VPDB) is in the middle. The estimated δ18O values of the D1 parent fluid (−5 to −10‰ VSMOW) are bracketed by those suggested for the Middle Cambrian meteoric and seawaters. The fluid-inclusion gas analyses provided CO2/CH4 and N2/Ar ratios coherent with the conclusions from the petrographic examination and geochemical compositions.

The Beaverlodge district in northern Saskatchewan is known for “vein-type” uranium mineralization. Most of the uranium deposits are spatially related to major structures, and hosted by ca. 3.2–1.9 Ga granitic rocks (and albitite derived from them) and by ca. 2.33 Ga Murmac Bay Group amphibolite, all of which are unconformably overlain locally by deformed but unmetamorphosed redbeds of the ca. 1.82 Ga Martin Group, and by the flat-lying ca. 1.75–1.5 Ga Athabasca Group. The uranium mineralization is mainly hosted in fault rocks (breccias) and carbonate ± quartz ± albite veins, referred to as breccia-style and vein-style mineralization, respectively, with the latter being the focus of this study. Most of the mineralized veins occur in the basement rocks, although some crosscut the Martin Group. This study examines the field, petrographic, fluid inclusion and C-O isotope characteristics of mineralized and non-mineralized veins from 19 deposits/occurrences as well as from the Martin Group, with an aim to better understand the mineralizing environment and processes. The coexistence of liquid-dominated (L + V), vapour-dominated (V + L) and vapour-only (V) fluid inclusions within individual fluid inclusion assemblages (FIAs) in the veins suggests fluid immiscibility and heterogeneous trapping. The L + V inclusions with the lowest homogenization temperatures (Th) within individual FIAs are interpreted to represent homogeneous trapping of the liquid phase, which yield Th values from 78° to 330 °C (mainly 100° to 250 °C), and salinities from 0.2 to 30.8 wt.% NaCl equivalent. Mass spectrometric analysis of bulk fluid inclusions shows that the volatiles are dominated by H2O (average 97.2 mol%), with minor amounts of CO2, CH4, H2, O2, N2, Ar and He. Fluid pressures were estimated to be < 200 bars based on the inference of fluid immiscibility, fluid temperatures of 100° to 250 °C, and low concentrations of non-aqueous volatiles (< 3 mol%). The δ18OVPDB and δ13CVPDB of carbonate minerals associated with mineralization range from − 20.5 to − 8.9‰ and − 10.1 to − 0.9‰, respectively. The δ18OVSMOW values of the parent fluids calculated using the Th values range from − 9.6 to + 17.0‰, with the majority from 0 to + 5.0‰. O isotopes of paired equilibrium quartz and calcite, analyzed by secondary ion mass spectrometry (SIMS), yield temperatures from 161° to 248 °C, which are consistent with the fluid inclusion data. The new fluid inclusion and stable isotope data are inconsistent with a metamorphic or magmatic-hydrothermal model as proposed in some previous studies (for breccia-style and vein-style mineralization), but rather support a model in which the vein-type uranium mineralization took place at relatively low temperature (100° to 250 °C) and shallow (< 2 km) conditions, with fluid pressure fluctuating between hydrostatic and sub-hydrostatic regimes, possibly related to episodic faulting. The mineralizing fluids were mainly sourced from the Martin Lake Basin, and uraninite was precipitated as a result of mixing between this basin-derived fluid and fluids carrying reducing agents (Fe2 +, CH4) derived from the basement, although fluid-rock reactions and fluid immiscibility may have also played a role.

Strange Lake is a mid-Proterozoic peralkaline granite pluton (Quebec-Labrador, Canada) that underwent extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE). The HFSE mineralisation is confined to highly altered pegmatites and the most altered parts of the granites, implying a genetic association between hydrothermal fluids and HFSE enrichment. This study uses analyses of fluid inclusions to investigate the hydrothermal evolution of the Strange Lake pluton and the role of hydrothermal processes in concentrating the HFSE to potentially exploitable levels. Five groups of inclusions were distinguished. From earliest to latest, these groups are: primary aqueous inclusions (~ 25 wt% NaCl eq.) associated with melt inclusions (Group 1); primary aqueous inclusions (13–23 wt% NaCl eq.) associated with CH4 inclusions (Groups 2a and b); primary aqueous inclusions (~ 9 wt% NaCl eq.) associated with CO2 inclusions (Group 3); primary aqueous inclusions (~ 9 wt% NaCl eq.), which contain no carbonic component (Group 4); and finally, secondary aqueous inclusions (19 wt% NaCl eq.), including inclusions that outline mineral pseudomorphs, also with no detectable carbonic component (Group 5). Most of the inclusions (except those in Group 5) have conspicuous ‘implosion’ haloes, evident as numerous tiny voids. This indicates that the inclusions re-equilibrated, most likely during isobaric cooling. Fluid evolution commenced with the exsolution of a saline aqueous liquid (~ 25 wt% NaCl eq.) and an immiscible CH4 + H2 gas from the pegmatitic melt at temperatures of ~ 450–500 °C and a pressure of ~ 1100 bars. During isobaric cooling, the gas component of the fluid was gradually oxidised, evolving from being CH4-dominant to a CH4 fluid with a significant proportion of higher order hydrocarbons (due to oxidative coupling of methane induced by the consumption of O2 through the alteration of arfvedsonite to aegirine; ~ 325–360 °C), and finally to a CO2-dominated fluid at ~ 300 °C. The apparent salinity of the aqueous fluid decreased from ~ 25 to ~ 4.5 wt% NaCl eq. due to fluid-rock interaction. The latter also caused precipitation of nahcolite (as a result of the reaction of newly formed CO2 with sodium from decomposing minerals), formation of pseudomorphs after primary Na-zirconosilicates and Na-titanosilicates and replacement of primary REE-silicates by bastnasite-(Ce). Owing to this interaction, the carbonic component of the fluid was consumed which, together with the consumption of H2O to form Al-, K- and Fe-phyllosilicates, contributed to an increase in the fluid salinity (up to ~ 19 wt% NaCl eq.). Light rare earth elements (LREE) were remobilised over 10s to 100 s of metres by the high temperature high salinity fluid (preserved as Group 1 and 2a inclusions), whereas heavy rare earth elements (HREE) were remobilised on a much smaller scale by a late, low temperature high salinity fluid (trapped as inclusion Group 5). Fluid preserved as inclusion Group 3 may have been responsible for remobilisation of Zr and Ti.

The Lower Head Formation in the Parson’s Pond area (western Newfoundland, Canada) comprises siltstones with very fine grained to fine-grained sandstones. Petrography confirms that these sandstones are matrix rich, essentially wackes, with detrital minerals including quartz, feldspar, biotite, and numerous accessory minerals. Observed petrographic features suggest that the Lower Head sediments are the distal product of erosion, with sediment sourced from the Dashwoods microcontinent and Lushs Bight oceanic tract and thoroughly mixed in an earlier basin prior to final deposition in a trench slope basin. The Lower Head Formation sandstones have low porosity, with early diagenetic cements (C1) and later calcite in crosscutting calcite veinlets (C2). Petrographic, isotopic, and fluid inclusion analyses indicate that C1 cements formed during the early stages of diagenesis. Both δ13C and δ18O isotopes for the C1 calcite cements are isotopically heavier than the C2 calcite veins. Fluid inclusion homogenization temperatures for the later crosscutting C2 calcite generally range between 78 and 116 °C, with a mean of 100.7 °C (±9.75 °C) and fluid salinities of 5.41–15.98 equiv. wt.% NaCl. Fluid inclusion gas analysis from C2 calcite confirms that CO2/CH4 generally has an inverse correlation with N2/Ar. Petroleum-bearing fluid inclusions were also recorded in C2 calcite cements, indicating that these fractures were conduits for hydrocarbon migration prior and (or) during cementation. However, the early cementation and associated low porosity of the Lower Head Formation sandstones indicate that they offer restricted pathways to migrating fluids and volatiles, and any hydrocarbon migration must have been fracture controlled.

At Macmillan Pass (YT, Canada), the hydrothermal vent complexes beneath two shale-hosted massive sulphide (SHMS) deposits (Tom, Jason) are well preserved within Late Devonian strata. These deposits provide a unique opportunity to constrain key geochemical parameters (temperature, salinity, pH, f O 2 , ΣS) that are critical for metal transport and deposition in SHMS systems, and to evaluate the interaction between hydrothermal fluids and the mudstone host rock. This has been achieved using a combination of detailed petrography, isotopic techniques (δ 34 S, δ 13 C and δ 18 O values), carbonate rare earth element analysis (LA-ICP-MS), fluid inclusion analysis (microthermometry, gas analysis via incremental crush fast scan mass spectrometry), and thermodynamic modelling. Two main paragenetic stages are preserved in both vent complexes: Stage 1 comprises pervasive ankerite alteration of the organic-rich mudstone host rock and crosscutting stockwork ankerite veining (±pyrobitumen, pyrite and quartz) and; Stage 2 consists of main stage massive sulphide (galena-pyrrhotite-pyrite ± chalcopyrite-sphalerite) and siderite (±quartz and barytocalcite) mineralisation. Co-variation of δ 18 O and δ 13 C values in ankerite can be described by temperature dependent fractionation and fluid rock interaction. Together with fluid inclusion microthermometry, this provides evidence of a steep thermal gradient (from 300 to ∼100 °C) over approximately 15 m stratigraphic depth, temporally and spatially constrained within the paragenesis of both vent complexes and developed under shallow lithostatic ( 28), characteristic of diagenetic fluids, are coupled with positive europium anomalies and variable light REE depletion, which are more consistent with chloride complexation in hot (>250 °C) hydrothermal fluids. In this shallow sub-seafloor setting, thermal alteration of organic carbon in the immature, chemically reactive mudstones also had an important role in the evolution of fluid chemistry. Reduced sulphur generation via thermochemical reduction of Late Devonian seawater sulphate produced positive δ 34 S values in sulphide minerals (+7.5‰ to +19.5‰), coupled with a suite of volatile components (CO 2 , CH 4 , C 1 –C 4 hydrocarbons, N 2 ) trapped in Stage 2 quartz. Many of these geochemical features developed during the final stages of fluid ascent, in a system where the fluid cooled close to the site of mineralisation. Using this information, we have modelled the metal transporting capacity of the deep hydrothermal fluid, which even at modest salinities (6 wt.% NaCl) was high (≫100 ppm Pb, Zn), owing to the combined effects of high temperature and low pH (⩽4.5). Therefore in SHMS systems, enhanced geothermal gradients and rapid fluid ascent (with minimal fluid cooling) are considered to be the most important factors for transporting high concentrations of base metals to the site of mineralisation.

Speleothems frequently host “fossil” fluids that were trapped in small inclusions during growth. Such fluids may provide valuable clues to past microbial, geochemical, and climatic processes during their formation. However, one difficulty is to understand which gases represent background atmosphere and fluids within a given cave system at a particular time, and which may be the product of post-trapping residual microbial activity or abiotic chemical reactions? Do we have any hope of sorting out these differences? The success depends on a quantitative understanding of the gas composition trapped in the inclusions and an understanding of the interactions of cave mineralogy, air and water chemistry, and microbiological processes that may interfere with climatic or geochemical interpretations. Our proof-of-concept project uses time synchronous samples from several sites. We report here on this pilot investigation of speleothem inclusions using a methodology for quantitatively analyzing gases dissolved in inclusion fluids. We use incremental crushing of highly spatially resolved samples by mass spectrometry. Here, we report primarily on CH4, CO2, O2, and N2, but have included other detectable gases. The detection limit for He within aqueous fluid inclusions is ~0.2 ppm and gas ratios have ~5% precision using natural standards. We used chemically inert argon as a tracer gas to normalize results to air or air saturated water. This enables interpretation of gas data despite variability in hydrological and geological cave histories. Results are variable. For example, in one case oxygen was depleted while nitrogen was increased, which may be attributable to the breakdown of nitrate or nitrogen-containing biomolecules. In other cases, oxygen is enriched which may be attributed to several factors both geochemical and biological. We suggest potential interpretations between the competing hypotheses with larger future data sets. This first attempt tackles the complex and intertwined speleological questions using the inclusion gas method.

The oxidation of molecular hydrogen (H2) is thought to be a major source of metabolic energy for life in the deep subsurface on Earth, and it could likewise support any extant biosphere on Mars, where stable habitable environments are probably limited to the subsurface. Faulting and fracturing may stimulate the supply of H2 from several sources. We report the H2 content of fluids present in terrestrial rocks formed by brittle fracturing on fault planes (pseudotachylites and cataclasites), along with protolith control samples. The fluids are dominated by water and include H2 at abundances sufficient to support hydrogenotrophic microorganisms, with strong H2 enrichments in the pseudotachylites compared to the controls. Weaker and less consistent H2 enrichments are observed in the cataclasites, which represent less intense seismic friction than the pseudotachylites. The enrichments agree quantitatively with previous experimental measurements of frictionally driven H2 formation during rock fracturing. We find that conservative estimates of current martian global seismicity predict episodic H2 generation by Marsquakes in quantities useful to hydrogenotrophs over a range of scales and recurrence times. On both Earth and Mars, secondary release of H2 may also accompany the breakdown of ancient fault rocks, which are particularly abundant in the pervasively fractured martian crust. This study strengthens the case for the astrobiological investigation of ancient martian fracture systems.Deep biosphere-Faults-Fault rocks-Seismic activity-Hydrogen-Mars. Astrobiology 16, 690-702.

The End deposit is one of several uranium deposits in the Kiggavik area near the Proterozoic Thelon Basin, which is geologically similar to the Athabasca Basin known for its unconformity-related uranium deposits. The mineralization occurs as uraninite and coffinite in quartz veins and wall rocks (psammopelitic gneisses) in the sub-Thelon basement and is associated with clay- and hematite-altered fault zones. Fluid inclusions were studied in quartz cementing unmineralized breccias formed before mineralization (Q2), quartz veins that were formed before mineralization but spatially associated with uranite (Q4), and calcite veins that were formed after mineralization. Four types of fluid inclusions were recognized, namely liquid-dominated biphase (liquid + vapor), vapor-dominated biphase (vapor + liquid), monophase (vapor-only), and triphase (liquid + vapor + halite) inclusions. The first three types were found in Q2, whereas all four types were found in Q4 and calcite. The coexistence of these different types of inclusions within individual fluid inclusion assemblages is interpreted to indicate fluid immiscibility and heterogeneous trapping. Based on microthermometry, the fluids associated with Q2 are characterized by low salinities (0.4 to 6.6 wt%) and moderate temperatures from 148 to 261 °C, and the fluids associated with calcite show high salinities (26.8 to 29.3 wt%) and relatively low temperatures from 146 to 205 °C, whereas the fluids associated with Q4 have a wide range of salinities from 0.7 to 38.8 wt% and temperatures from 80 to 332 °C. Microthermometric and cryogenic Raman spectroscopic studies indicate that the high-salinity fluids in Q4 and calcite belong to the H2O-NaCl-CaCl2 ± MgCl2 system, with some dominated by NaCl and others by CaCl2. The fluid inclusions in Q2 are interpreted to be unrelated to mineralization, whereas those in Q4 and calcite reflect the mineralizing fluids. The fluid inclusion data are consistent with a genetic link of mineralization with basinal brines derived from the Thelon Basin. However, unlike the conventional deep-burial (>5 km) diagenetic-hydrothermal model proposed for the unconformity-related uranium deposits, the uranium mineralization in the End deposit is inferred to have formed in a shallow environment (probably

The Flemish Pass Basin is a deep-water basin located offshore on the continental passive margin of the Grand Banks, eastern Newfoundland, which is currently a hydrocarbon exploration target. The current study investigates the petrographic characteristics and origin of carbonate cements in the Ti-3 Member, a primary clastic reservoir interval of the Bodhran Formation (Upper Jurassic) in the Flemish Pass Basin. The Ti-3 sandstones with average Q 86.0 F 3.1 R 10.9 contain various diagenetic minerals, including calcite, pyrite, quartz overgrowth, dolomite and siderite. Based on the volume of calcite cement, the investigated sandstones can be classified into (1) calcite-cemented intervals (>20% calcite), and (2) poorly calcite-cemented intervals (porous). Petrographic analysis shows that the dominant cement is intergranular poikilotopic (300–500 μm) calcite, which stared to form extensively at early diagenesis. The precipitation of calcite occured after feldspar leaching and was followed by corrosion of quartz grains. Intergranular calcite cement hosts all-liquid inclusions mainly in the crystal core, but rare primary two-phase (liquid and vapor) fluid inclusions in the rims ((with mean homogenization temperature ( T h ) of 70.2 ± 4.9 °C and salinity estimates of 8.8 ± 1.2 eq. wt.% NaCl). The mean δ 18 O and δ 13 C isotopic compositions of the intergranular calcite are −8.3 ± 1.2‰, VPDB and −3.0 ± 1.3‰, VPDB, respectively; whereas, fracture-filling calcite has more depleted δ 18 O but similar δ 13 C values. The shale normalized rare earth element (REE SN ) patterns of calcite are generally parallel and exhibit slightly negative Ce anomalies and positive Eu anomalies. Fluid-inclusion gas ratios (CO 2 /CH 4 and N 2 /Ar) of calcite cement further confirms that diagenetic fluids originated from modified seawater. Combined evidence from petrographic, microthermometric and geochemical analyses suggest that (1) the intergranular calcite cement precipitated from diagenetic fluids of mixed marine and meteoric (riverine) waters in suboxic conditions; (2)the cement was sourced from the oxidation of organic matters and the dissolution of biogenic marine carbonates within sandstone beds or adjacent silty mudstones; and (3) the late phases of the intergranular and fracture-filling calcite cements were deposited from hot circulated basinal fluids. Calcite cementation acts as a main controlling factor on the reservoir quality in the Flemish Pass reservoir sandstones. Over 75% of initial porosity was lost due to the early calcite cementation. The development of secondary porosity (mostly enlarged, moldic pores) and throats by later calcite dissolution due to maturation of organic matters (e.g., hydrocarbon and coals), was the key process in improving the reservoir quality.

Oxygen concentration in the Precambrian atmosphere-ocean system is currently estimated from a series of geochemical proxies that depend on the concentration of redox-sensitive trace elements or their isotopic ratios measured in organic-bearing marine shales. This research has indicated that atmospheric oxygen content throughout the Precambrian was very low, Atmospheric O 2 % = 30 × P / 1 + P Where P = 10 0.89 × log Se/Co + 0.07 , r 2 ; r 2 = 0.85 The oxygen curve revealed by this relationship suggests a broad first-order trend of increasing oxygen of ~0.25 to 27% from 3500 to 1850 Ma, followed by a general decrease to around ~1.0% at ~1000 Ma, with a spike around 1400 Ma. Oxygen then rose gradually through the Ediacaran to reach a maximum of 20 to 29% in the early Cambrian. Detailed analysis of sedimentary pyrite in sixteen separate black shale formations, spaced throughout the Precambrian, demonstrates consistent results using the marine pyrite proxy, with standard deviations of the predicted oxygen for each formation of between 0.4 and 7%. In seven formations, we show that each formation demonstrates an increase in oxygen passing up stratigraphy from the base of the black shale facies toward the top.

δ2H were measured in water present in tiny primary fluid inclusions trapped in Mesoproterozoic (≈1.4 Ga) and Neoproterozoic (≈0.8 Ga) halite crystals. While the hydrogen concentrations range from 300 to 1500 ppm for the whole sample collection, δ2H values range from −74‰ to −54‰ (VSMOW) for the Mesoproterozoic halite sampled from the Sibley Group, Ontario, Canada, and from −89 to −38‰ for the Neoproterozoic halite from the Browne Formation, Officer Basin, Australia. The amount of evaporation required to precipitate halite is accompanied by a deuterium-enrichment of 30‰ to 90‰. It means that the original aqueous solutions had δ2H values comprised between −140‰ and −90‰. Considering the estimated tropical paleolatitudes for both depositional environments, surface waters were most likely significantly deuterium-depleted relative to modern ones. If such interpretation is partly at variance with the geological record, it could be reconciliated with some modelling outputs of the long-term water cycle. Indeed, several mechanisms produce a progressive deuterium-enrichment of the oceans. Those identified so far are the addition of deuterium-rich extraterrestrial water (cometary dust), the loss of H during water photodissociation by ultraviolet radiation in the upper atmosphere, and the isotopic fractionation that takes place during the hydroxylation of nominally-anhydrous mantle rock-forming minerals. We also propose that the major process able to shift through time the δ2Η of the hydrosphere is the reduction of H2O into H2 during the serpentinization of mantle rocks at mid-ocean ridges.

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International audience; The exposure of a carbonaceous siltstone sample to atmospheric entry, as part of the STONE 6 artificial meteorite experiment, has allowed a controlled investigation of the effect of heat shock during atmospheric entry on organic matter in carbonaceous meteorites and, potentially, sedimentary martian meteorites containing carbonaceous biomolecules. Thermal alteration is evident in an increase in structural order of the carbon (i.e. degree of graphitisation), preferential loss of thermally unstable compounds and substantial loss of extractable organic matter. There is a gradient of increasing alteration towards the outer, exposed margin of the rock, and also an increase in hydrocarbons that suggests outward migration following thermally-induced generation. The carbon has not been completely graphitised, and sufficient biomarker compounds survive to prove the biological origin of the organic matter. The experiment implies that meteorites of appropriate size could preserve evidence of biological activity on their parent body.

The Esino Limestone of the western Southern Alps represents a differentiated Ladinian-Lower Carnian (?) carbonate platform comprised of margin, slope and peritidal inner platform facies up to 1000 m thick. A major regional subaerial exposure event lead to coverage by another peritidal Lower Carnian carbonate platform (Breno Formation). Multiphase dolomitization affected the carbonate sediments. Petrographic examinations identified at least three main generations of dolomites (D1, D2, and D3) that occur as both replacement and fracture-filling cements. These phases have crystal-size ranges of 3–35 μm (dolomicrite D1), 40–600 μm (eu-to subhedral crystals D2), and 200 μm to 5 mm (cavity- and fracture-filling anhedral to subhedral saddle dolomite D3), respectively. The fabric retentive near-micritic grain size coupled with low mean Sr concentration (76 ± 37 ppm) and estimated δ18O of the parent dolomitizing fluids of D1 suggest formation in shallow burial setting at temperature ∼ 45–50 °C with possible contributions from volcanic-related fluids (basinal fluids circulated in volcaniclastics or related to volcanic activity), which is consistent with its abnormally high Fe (4438 ± 4393 ppm) and Mn (1219 ± 1418 ppm) contents. The larger crystal sizes, homogenization temperatures (D2, 108 ± 9 °C; D3, 111 ± 14 °C) of primary two-phase fluid inclusions, and calculated salinity estimates (D2, 23 ± 2 eq wt% NaCl; D3, 20 ± 4 eq wt% NaCl) of D2 and D3 suggest that they formed at later stages under mid-to deeper burial settings at higher temperatures from dolomitizing fluids of higher salinity, which is supported by higher estimated δ18O values of their parent dolomitizing fluids. This is also consistent with their high Fe (4462 ± 4888 ppm; and 1091 ± 1183 ppm, respectively) and Mn (556 ± 289 ppm and 1091 ± 1183 ppm) contents, and low Sr concentrations (53 ± 31 ppm and 57 ± 24 ppm, respectively). The similarity in shale-normalized (SN) REE patterns and Ce (Ce/Ce*)SN and La (Pr/Pr*)SN anomalies of the investigated carbonates support the genetic relationship between the dolomite generations and their calcite precursor. Positive Eu anomalies, coupled with fluid-inclusion gas ratios (N2/Ar, CO2/CH4, Ar/He), high F− concentration, high F/Cl and high Cl/Br molar ratios suggest an origin from diagenetic fluids circulated through volcanic rocks, which is consistent with the co-occurrence of volcaniclastic lenses in the investigated sequence.

International audience; We present a new and innovative way of determining the oxygen level of Earth's past atmosphere by directly measuring inclusion gases trapped in halite. After intensive screening using multiple depositional, textural/fabric, and geochemical parameters, we determined that tectonically undisturbed cumulate, chevron, and cornet halite inclusions may retain atmospheric gas during crystallization from shallow saline, lagoonal, and/or saltpan brine. These are the first measurements of inclusion gas for the Neoproterozoic obtained from 815 +/- 15-m.y.-old Browne Formation chevron halite of the Officer Basin, southwest Australia. The 31 gas measurements afford us a direct glimpse of the composition of the mid-to late Neoproterozoic atmosphere and register an average oxygen content of 10.9%. The measured pO(2) puts oxygenation of Earth's paleoatmosphere similar to 100-200 m.y. ahead of current models and proxy studies. It also puts oxygenation of the Neoproterozoic atmosphere in agreement with time of diversification of eukaryotes and in advance of the emergence of marine animal life.

A suite of Permian sylvite samples from Boulby potash mine, Yorkshire, UK, consistently yield traces of hydrogen upon analysis by a cold crush technique for liberating volatiles from entrapped fluid inclusions. In contrast, accompanying halite samples do not yield hydrogen. These data suggest the formation of hydrogen by radiolysis of water due to irradiation from potassium in the sylvite. The data indicate radiolysis as a mechanism for subsurface hydrogen generation, where it is available as an electron donor for a deep biosphere.

The origin of sediment-hosted Nevada gold deposits has been highly debated, especially regarding the relative contribution of multiple mineralizing events, particularly relating to the Cretaceous. We examined the Pipeline gold mine in north-central Nevada, focusing on data from the four vein sets in this atypical deposit where there is evidence for Cretaceous gold mineralization. Only the third, a quartz-sericite-pyrite-calcite vein set, has any link with the alteration styles and gold mineralization within the Pipeline deposit. Our geochemical results from fluid inclusion microthermometry and gas analysis show that the fluids from which quartz deposited were sourced from condensing magmatic volatiles and were trapped at ~300 °C and 2 kbar lithostatic pressure (~8 km). 40Ar/39Ar dating of sericite demonstrates that the quartz-sericite-pyrite veins formed at ~92 Ma, matching the dates of gold-associated epigenetic illite. Ore fluids enriched in CO2 and H2S caused decarbonation thereby releasing Fe2+ that reacted with H2S to form pyrite. Decreasing H2S destabilized gold bisulfide complexes and deposited gold. We conclude that this process can occur in a single Cretaceous event in advance of potential Tertiary mineralization.

New fluid inclusion analyses using a range of analytical techniques including quadropole mass spectrometric analyses coupled with gemmological investigations were conducted on rough and gemquality faceted emeralds from the Piteiras Mine, Minas Gerais, Brazil. These data complement those of Rondeau et al. (2003) who also presented analyses of the Piteiras emeralds. Emeralds are found typically as euhedral-to-anhedral crystals disseminated in biotite-phlogopite schist and range from 0.5 to 50 mm long. Emerald mineralization is associated closely with alkaline metasomatized pegmatite bodies, quartz boudin and veins, and talc-chlorite bands. Four types of fluid inclusions are recognized in the emeralds. These include aqueous brine and aqueous carbonic varieties containing one or two liquid phases, along with gas bubbles and/or solid crystals (e.g. carbonates). Primary fluid inclusions in emeralds record salinities of ~4–24 eq. wt.% NaCl and minimum trapping temperatures from ~350 to 480ºC. Combined microthermometry, Raman spectroscopy and crush-leach gas analyses indicate that the mineralizing fluid was an aqueous carbonic brine enriched in reduced volatile species such as CH4, N2, H2S and alkanes. With respect to their optical properties (RI ε = 1.573–1.580; RI ω = 1.580–1.588; birefringence = 0.006–0.008) and specific gravity (2.65–2.78), the Piteiras emeralds fall within the expected range for metasomatic, schist-hosted emeralds.

Background Hydrogen is known to occur in the groundwaters of some ancient cratons. Where associated gases have been dated, their age extends up to a billion years, and the hydrogen is assumed also to be very old. These observations are interpreted to represent the radiolysis of water and hydration reactions and migration of hydrogen into fracture systems. A hitherto untested implication is that the overwhelming bulk of the ancient low-permeability basement, which is not adjacent to cross-cutting fractures, constitutes a reservoir for hydrogen. Results New data obtained from cold crushing to liberate volatiles from fluid inclusions confirm that granites and gneiss of Archean and Palaeoproterozoic (>1600 Ma) age typically contain an order of magnitude greater hydrogen in their entrained fluid than very young (