Juvenile CO2 in enderbites of Tromøy near Arendal, southern Norway: a fluid inclusion and stable isotope study

The enderbites from Tromøy in the central, granulite facies part of the Proterozoic Bamble sector of southern Norway contain dominantly CO₂ and N₂ fluid inclusions. CO₂ from fluid inclusions in quartz segregations in enderbites was extracted by mechanical (crushing) and thermal decrepitation and the δ¹³C measured. Measurement was also made on samples washed in 10% HCl, oxidized with CuO at high temperatures, and step-wise extracted with progressive heating. Results between the different techniques are systematic. The main results show δ¹³C of -4.5±1.5% for crushing and -7±2% for thermal decrepitation. δ¹³C is about constant for CO₂ extracted at different temperatures and points to a homogeneous isotopic composition. Due to the presence of carbonate particles and/or induced contaminations for the extraction by thermal decrepitation, the results for the crushing experiments are assumed the most reliable for fluid-inclusion CO₂. Very low values of δ¹³C have not been found in enderbite samples and δ¹³C combined with δ¹⁸O of the host quartzes (8-11%) indicates juvenile values. In addition, the fluid inclusions were examined by microthermometry and Raman analysis and host quartz by acoustic emission and cathodoluminescence. CO₂ fluid inclusions have varying densities with a frequency maximum of 0.92 g cm^-3 and generally do not concur with trapping densities at granulite conditions. Textures show that CO₂ must have been trapped in fluid inclusions in one early event, but transformed to different extents during late isothermal uplift without important fractionation of isotope compositions. The present data support a model of intrusion and crystallization of a CO₂-rich enderbitic magma at granuiite conditions.     

Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition

In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The δ¹³C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from -18 to +2‰ and from -7 to -3‰, respectively. The maximum δ¹³C value of graphite globules in the metasediment rises from -14 to -5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ¹³C values of graphite inclusions in garnet are ∼7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ¹³C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively ¹³C-depleted graphite indicates that the regional metamorphism increased the δ¹³C values of the Isua graphite. This is consistent with the regional trend of ¹³C-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused ¹³C-enrichment of Isua graphite. The δ¹³C values of graphite reported here (δ¹³C > -18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ¹³C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the ¹³C-enrichment of Isua graphite, previously reported large ¹³C-depletion of graphite, especially armored by apatite (Mojzsis et al., 1996) was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga).     

Oxygen isotope composition of syngenetic inclusions in diamond from the Finsch Mine,RSA

Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associated minerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotope analyses of syngenetic inclusions weighing 20–400 μg have been obtained by laser heating in the presence of ClF₃. ¹⁸O/¹⁶O ratios are determined on oxygen converted to CO₂ over hot graphite and, for samples weighing less than 750 μg (producing <12 μmoles O₂) enhanced CO production in the graphite reactor causes a systematic shift in both δ¹³C and δ¹⁸O that varies as a function of sample weight. A “pressure effect” correction procedure, based on the magnitude of δ¹³C (CO₂) depletion relative to δ¹³C (graphite), is used to obtain corrected δ¹⁸O values for inclusions with an accuracy estimated to be ±0.3‰ for samples weighing 40 μg.Syngenetic inclusions in host diamonds with similar δ¹³C values (−8.4‰ to −2.7‰) have oxygen isotope compositions that vary significantly, with a clear distinction between inclusions of peridotitic (+4.6‰ to +5.6‰) and eclogitic paragenesis (+5.7‰ to +8.0‰). The mean δ¹⁸O composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle (5.25 ± 0.22‰) whereas syngenetic purple garnet inclusions possess relatively low δ¹⁸O values (5.00 ± 0.33‰). Reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with the diamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range of δ¹⁸O values from +2.3‰ to +7.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values >4.7‰.     

Geochemical characterization of secondary microbial gas occurrence in the Songliao Basin, NE China

A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C₁/C₂₊ > 100), high C₂/C₃ and i-C₄/n-C₄ ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ¹³C_CH4 from −79.5‰ to −45.0‰, δ¹³C_C2H6 from −53.7‰ to −32.2‰, δ¹³C_C3H8 from −36.5‰ to −20.1‰, δ¹³C_nC4H10 from −32.7‰ to −24.5‰, and δ¹³C_CO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with ¹³C enriched methane and ¹³C depleted CO₂, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, ¹³C enriched methane and variable δ¹³C of CO₂, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with ¹³C depleted methane and ¹³C enriched CO₂, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain ¹³C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.     

Controls on dissolved inorganic carbon and δC in cave waters from DeSoto Caverns: Implications for speleothem δC assessments

Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ¹³C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ¹³C_DIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems.[DIC] and δ¹³C_DIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ¹³C_DIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ¹³C_DIC = −2.49[DIC] + 0.64, r² = 0.84). A shallow pool fed by multiple drips shows a bimodal δ¹³C_DIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO₂ fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ¹³C_DIC variability. Measured values of high [DIC] and low δ¹³C_DIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ¹³C_DIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of partitioned between degassed CO₂ and precipitated CaCO₃(1000lnα_{[(CO2-HCO3)+(CaCO3(AR)-HCO3)]/2}) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰).On the basis of ¹⁴C mass balance and paired ¹⁴C-U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from ¹⁴C-dead dolomite cap while the remainder of ∼77% is derived from ¹⁴C-live biomass. δ¹³C measurements of aragonite (n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ¹³C values calculated using the shallow pool summer/early fall data thus confirming the δ¹³C seasonality in both drips and coeval aragonite. δ¹³C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ (n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ¹³C records of stalagmites at DeSoto Caverns with resolving power >10⁻¹ year will likely carry the imprints of drip annual means that record climate-driven δ¹³C seasonal biases.     

The isotopic composition of respired carbon dioxide in scleractinian corals: Implications for cycling of organic carbon in corals

The origin of δ¹³C variations within the skeletons of zooxanthellate scleractinian corals is still a matter of considerable debate. In particular, the role respired CO₂ plays in controlling the eventual δ¹³C of the skeleton remains unclear. In this study, the temporal variability of the δ¹³C of respired CO₂ produced by Montastraea faveolata has been measured at approximately monthly intervals over a 1-year period. In these experiments, three corals maintained on a platform at 8 m depth near Molasses Reef in the Florida Keys were incubated in closed chambers for 24-h periods and samples of the incubation water analyzed for the δ¹³C of the dissolved inorganic carbon (ΣCO₂) at ∼3-h intervals. Throughout the incubation, the concentration of O₂ was measured continuously within the chamber. Our results show that during daylight, the δ¹³C of the ΣCO₂ in the incubation water becomes enriched in ¹³C as a result of fractionation during the fixation of C by photosynthesis, whereas at night the δ¹³C of the ΣCO₂ becomes more negative. The δ¹³C of the respiratory CO₂ ranges from −9‰ in the late spring to values as low as −17‰ in the autumn. The lighter values are significantly more negative than those reported by previous workers for coral tissue and zooxanthellae. An explanation for this discrepancy may be that the corals respire a significant proportion of isotopically negative substances, such as lipids, which are known to have values up to 10‰ lighter compared to the bulk δ¹³C of the tissue. The clear seasonal cycle in the δ¹³C of the respiratory CO₂ suggests that there is also seasonal variability in either the δ¹³C of the coral tissue or the type and/or amount of organic material being respired. A similar temporal pattern and magnitude of change was observed in the δ¹³C of the coral tissue samples collected from a nearby reef at monthly intervals between 1995 and 1997. These patterns are similar in timing to the δ¹³C measured in the coral skeletons. We have also calculated an annual mean value for the fractionation factor between dissolved CO₂⁻ in the external environment and photosynthate fixed by the zooxanthellae of 1.0121 (±0.003). This value is inversely correlated with the ratio of photosynthesis to respiration (P/R) of the entire organism and shows the highest values during the summer months.     

Stable isotopic evidence in support of active microbial methane cycling in low-temperature diffuse flow vents at 9°50′N East Pacific Rise

A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH₄ and CO₂ in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in ¹³C by ∼10‰ in values of δ¹³C of CH₄, and by ∼0.55‰ in values of δ¹³C of CO₂, relative to the values of the high-temperature source fluid (δ¹³C of CH₄ =−20.1 ± 1.2‰, δ¹³C of CO₂ =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in ¹³C of both CH₄ and CO₂ at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and ¹³C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH₄ and CO₂ concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable ¹³C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis.     

Goethite, calcite, and organic matter from Permian and Triassic soils: carbon isotopes and CO2 concentrations

Pedogenic goethites in each of two Early Permian paleosols appear to record mixing of two isotopically distinct CO₂ components—atmospheric CO₂ and CO₂ from in situ oxidation of organic matter. The δ¹³C values measured for the Fe(CO₃)OH component in solid solution in these Permian goethites are −13.5‰ for the Lower Leonardian (∼283 Ma BP) paleosol (MCGoeth) and −13.9‰ for the Upper Leonardian (∼270 Ma BP) paleosol (SAP). These goethites contain the most ¹³C-rich Fe(CO₃)OH measured to date for pedogenic goethites crystallized in soils exhibiting mixing of the two aforementioned CO₂ components. δ¹³C measured for 43 organic matter samples in the Lower Leonardian (Waggoner Ranch Fm.) has an average value of −20.3 ± 1.1‰ (1s). The average value yields a calculated Early Permian atmospheric Pco₂ value of about 1 × PAL, but the scatter in the measured δ¹³C values of organic matter permits a calculated maximum Pco₂ of 11 × PAL (PAL = present atmospheric level). Measured values of the mole fraction of Fe(CO₃)OH in MCGoeth and SAP correspond to soil CO₂ concentrations in the Early Permian paleosol profiles of 54,000 and 50,000 ppmV, respectively. Such high soil CO₂ concentrations are similar to modern soils in warm, wet environments.The average δ¹³C values of pedogenic calcite from 9 paleosol profiles stratigraphically associated with MCGoeth (Waggoner Ranch Fm.) range from −6.5‰ to −4.4‰, with a mean δ¹³C value for all profiles of −5.4‰. Thus, the value of Δ¹³C between the pedogenic calcite data set and MCGoeth is 8.1 (±0.9)‰, which is in reasonable accord with the value of 7.7‰ expected if atmospheric Pco₂ and organic matter δ¹³C values were the same for both paleosol types. Furthermore, the atmospheric Pco₂ calculated for the Early Permian from the average measured carbon isotopic compositions of the paleosol calcite and organic matter is also analytically indistinguishable from 1 × PAL, with a maximum calculated atmospheric Pco₂ (permitted by one standard deviation of the organic matter δ¹³C value) of ∼5 × PAL.If, however, measured average δ¹³C values of the plant organic matter are more positive than the original soil organic matter as a result of diagenetic loss of ¹³C-depleted, labile organic compounds, calculated Permian atmospheric Pco₂ using these ¹³C-enriched organic values would underestimate the actual atmospheric Pco₂ using either goethite or calcite. This is the first stratigraphically constrained, intrabasinal study to compare ancient atmospheric CO₂ concentrations calculated from pedogenic goethite and calcite. These results demonstrate that the two different proxies record the same information about atmospheric CO₂.The Fe(CO₃)OH component in pedogenic goethite from a Triassic paleosol in Utah is significantly enriched in ¹³C relative to Fe(CO₃)OH in goethites from soils in which there are mixtures of two isotopic CO₂ components. Field-relationships and the δ¹³C value (−1.9‰) of the Triassic goethite indicate that this ancient paleosol profile experienced mixing of three isotopically distinct CO₂ components at the time of goethite crystallization. The three components were probably atmospheric CO₂, CO₂ from in situ oxidation of organic matter and CO₂ from in situ dissolution of preexisting calcite. Although mixing of three isotopically distinct CO₂ components, as recorded by Fe(CO₃)OH in goethite, has been described in modern soil, this is the first example from a documented paleosol. Its preservation affirms the need for careful, case-by-case assessment of ancient paleosols to establish that goethite in any particular soil is likely to be a valid proxy of atmospheric Pco₂.     

Ancient graphite in the Eoarchean quartz-pyroxene rocks from Akilia in southern West Greenland II: Isotopic and chemical compositions and comparison with Paleoproterozoic banded iron formations

We present detailed petrographic surveys of apatite grains in association with carbonaceous material (CM) in two banded iron formations (BIFs) from the Paleoproterozoic of Uruguay and Michigan for comparison with similar mineral associations in the highly debated Akilia Quartz-pyroxene (Qp) rock. Petrographic and Raman spectroscopic surveys of these Paleoproterozoic BIFs show that apatite grains typically occur in bands parallel to bedding and are more often associated with CM when concentrations of organic matter are high. Carbonaceous material in the Vichadero BIF from Uruguay is generally well-crystallized graphite and occurs in concentrations around 0.01 wt% with an average δ¹³C_gra value of −28.6 ± 4.4‰ (1σ). In this BIF, only about 5% of apatite grains are associated with graphite. In comparison, CM in the Bijiki BIF from Michigan is also graphitic, but occurs in concentrations around 2.4 wt% with δ¹³C_gra values around −24.0 ± 0.3‰ (1σ). In the Bijiki BIF, more than 78% of apatite grains are associated with CM. Given the geologic context and high levels of CM in the Bijiki BIF, the significantly higher proportion of apatite grains associated with CM in this rock is interpreted to represent diagenetically altered biomass and shows that such diagenetic mineral associations can survive metamorphism up to the amphibolite facies.Isotope compositions of CM in muffled acidified whole-rock powders from the Akilia Qp rock have average δ¹³C_gra values of −17.5 ± 2.5‰ (1σ), while δ¹³C_carb values in whole-rock powders average −4.0 ± 1.0‰ (1σ). Carbon isotope compositions of graphite associated with apatite and other minerals in the Akilia Qp rock were also measured with the NanoSIMS to have similar ranges of δ¹³C_gra values averaging −13.8 ± 5.6‰ (1σ). The NanoSIMS was also used to semi-quantitatively map the distributions of H, N, O, P, and S in graphite from the Akilia Qp rock, and relative abundances were found to be similar for graphite associated with apatite or with hornblende, calcite, and sulfides. These analyses revealed generally lower abundances of trace elements in the Akilia graphite compared to graphite associated with apatite from Paleoproterozoic BIFs.Graphite associated with hornblende, calcite, and sulfides in the Akilia Qp rock was fluid-deposited at high-temperature from carbon-bearing fluids, and since this graphite has similar ranges of δ¹³C_gra values and of trace elements compared to graphite associated with apatite, we conclude that the Akilia graphite in different mineral associations formed from the same source(s) of CM. Collectively our results do not exclude a biogenic origin of the carbon in the Akilia graphite, but because some observations can not exclude graphitization of abiogenic carbon from CO₂- and CH₄-bearing mantle fluids, there remain ambiguities with respect to the exact origin of carbon in this ancient metasedimentary rock. Accordingly, there may have been several generations of graphite formation along with possibly varying mixtures of CO₂- and CH₄-bearing fluids that may have resulted in large ranges of δ¹³C_gra values. The possibility of fluid-deposited graphite associated with apatite should be a focus of future investigations as this may prove to be an alternative pathway of graphitization from phosphate-bearing fluids. Correlated micro-analytical approaches tested on terrestrial rocks in this work provide insights into the origin of carbon in ancient graphite and will pave the way for the search for life on other ancient planetary surfaces.     

Limitations on the climatic and ecological signals provided by the δC values of phytoliths from a C4 North American prairie grass

Silica phytoliths, which are deposits of opal-A that precipitate in the intra- and intercellular spaces of plant tissues during transpiration, commonly contain small amounts of occluded organic matter. In this paper, we investigate whether the δ¹³C values of phytoliths from a C₄ grass, Calamovilfa longifolia, vary in response to climatic variables that can affect the carbon-isotope composition of plant tissues. There is no significant correlation (r² < 0.3) between climate variables and the δ¹³C values of C. longifolia tissues (average δ¹³C_tissue = −13.1 ± 0.6 ‰; n = 70) across the North American prairies. However, plant tissue δ¹³C values are lower for grasses collected in populated areas where the δ¹³C value of atmospheric CO₂ is expected to be lower because of fossil fuel burning. Phytolith δ¹³C values are more variable (δ¹³C = −27.3 to −23.0‰; average = −25.1 ± 1.3‰; n = 34) and more sensitive to changes in aridity than whole tissue δ¹³C values. The strongest correlations are obtained between the δ¹³C values of stem or sheath phytoliths and humidity (r² = 0.3), latitude (r² = 0.4) and amount of precipitation (r² = 0.5). However, use of these relationships is limited by the wide spread in δ¹³C values of phytoliths from different plant tissues at the same location. We have been unable to infer any relationship between δ¹³C values of phytoliths and expected variations in the δ¹³C values of atmospheric CO₂. The C. longifolia phytoliths are depleted of ¹³C relative to tissue carbon by 10-14‰. This means that the phytoliths examined in this study have carbon isotopic compositions within the range reported previously for phytoliths from C₃ plants. This observation may further limit the usefulness of soil-phytolith assemblage δ¹³C values for identifying shifts in grassland C₃:C₄ ratios.     

Genesis of fumarolic emissions as inferred by isotope mass balances: CO2 and water at Vulcano Island, Italy

We have developed a quantitative model of CO₂ and H₂O isotopic mixing between magmatic and hydrothermal gases for the fumarolic emissions of the La Fossa crater (Vulcano Island, Italy). On the basis of isotope balance equations, the model takes into account the isotope equilibrium between H₂O and CO₂ and extends the recent model of chemical and energy two-end-member mixing by Nuccio et al. (1999). As a result, the H₂O and CO₂ content and the δD, δ¹⁸O, and δ¹³C isotope compositions for both magmatic and hydrothermal end-members have been assessed. Low contributions of meteoric steam, added at a shallow depth, have been also recognized and quantified in the fumaroles throughout the period from 1988 to 1998. Nonequilibrium oxygen isotope exchange also seems to be occurring between ascending gases and wall rocks along some fumarolic conduits.The δ¹³C_CO2 of the magmatic gases varies around −3 to 1‰ vs. Peedee belemnite (PDB), following a perfect synchronism with the variations of the CO₂ concentration in the magmatic gases. This suggests a process of isotope fractionation because of vapor exsolution caused by magma depressurization. The hydrogen isotopes in the magmatic gases (−1 to −‰ vs. standard mean ocean water [SMOW]), as well as the above δ¹³C_CO2 value, are coherent with a convergent tectonic setting of magma generation, where the local mantle is widely contaminated by fluids released from the subducted slab. Magma contamination in the crust probably amplifies this effect.The computed isotope composition of carbon and hydrogen in the hydrothermal vapors has been used to calculate the δD and δ¹³C of the entire hydrothermal system, including mixed H₂O-CO₂ vapor, liquid water, and dissolved carbon. We have computed values of about 10‰ vs. SMOW for water and −2 to −6.5‰ vs. PDB for CO₂. On these grounds, we think that Mediterranean marine water (δD_H2O ≈ 10‰) feeds the hydrothermal system. It infiltrates at depth throughout the local rocks, reaching oxygen isotope equilibrium at high temperatures. Interaction processes between magmatic gases and the evolving seawater also seem to occur, causing the dissolution of isotopically fractionated aqueous CO₂ and providing the source for hydrothermal carbon. These results have important implications concerning fluid circulation beneath Vulcano and address the more convenient routine of geochemical surveillance.     

Fluid Inclusions and Hydrogen,Oxygen, Sulfur Isotopes of Nuri Cu‐W‐Mo Deposit in the Southern Gangdese,Tibet

The Nuri Cu‐W‐Mo deposit is located in the southern subzone of the Cenozoic Gangdese Cu‐Mo metallogenic belt. The intrusive rocks exposed in the Nuri ore district consist of quartz diorite, granodiorite, monzogranite, granite porphyry, quartz diorite porphyrite and granodiorite porphyry, all of which intrude in the Cretaceous strata of the Bima Group. Owing to the intense metasomatism and hydrothermal alteration, carbonate rocks of the Bima Group form stratiform skarn and hornfels. The mineralization at the Nuri deposit is dominated by skarn, quartz vein and porphyry type. Ore minerals are chalcopyrite, pyrite, molybdenite, scheelite, bornite and tetrahedrite, etc. The oxidized orebodies contain malachite and covellite on the surface. The mineralization of the Nuri deposit is divided into skarn stage, retrograde stage, oxide stage, quartz‐polymetallic sulfide stage and quartz‐carbonate stage. Detailed petrographic observation on the fluid inclusions in garnet, scheelite and quartz from the different stages shows that there are four types of primary fluid inclusions: two‐phase aqueous inclusions, daughter mineral‐bearing multiphase inclusions, CO₂‐rich inclusions and single‐phase inclusions. The homogenization temperature of the fluid inclusions are 280°C–386°C (skarn stage), 200°C–340°C (oxide stage), 140°C–375°C (quartz‐polymetallic sulfide stage) and 160°C–280°C (quartz‐carbonate stage), showing a temperature decreasing trend from the skarn stage to the quartz‐carbonate stage. The salinity of the corresponding stages are 2.9%–49.7 wt% (NaCl) equiv., 2.1%–7.2 wt% (NaCl) equiv._, 2.6%–55.8 wt% (NaCl) equiv. and 1.2%–15.3 wt% (NaCl) equiv., respectively. The analyses of CO₂‐rich inclusions suggest that the ore‐forming pressures are 22.1 M Pa–50.4 M Pa, corresponding to the depth of 0.9 km–2.2 km. The Laser Raman spectrum of the inclusions shows the fluid compositions are dominated in H₂O, with some CO₂ and very little CH₄, N₂, etc. δD values of garnet are between ?114.4‰ and ?108.7‰ and δ¹⁸O_H2O between 5.9‰ and 6.7‰; δD of scheelite range from ?103.2‰ to ?101.29‰ and δ¹⁸O_H2O values between 2.17‰ and 4.09‰; δD of quartz between ?110.2‰ and ?92.5‰ and δ¹⁸O_H2O between ?3.5‰ and 4.3‰. The results indicate that the fluid came from a deep magmatic hydrothermal system, and the proportion of meteoric water increased during the migration of original fluid. The δ³⁴S values of sulfides, concentrated in a rage between ?0.32‰ to 2.5‰, show that the sulfur has a homogeneous source with characteristics of magmatic sulfur. The characters of fluid inclusions, combined with hydrogen‐oxygen and sulfur isotopes data, show that the ore‐forming fluids of the Nuri deposit formed by a relatively high temperature, high salinity fluid originated from magma, which mixed with low temperature, low salinity meteoric water during the evolution. The fluid flow through wall carbonate rocks resulted in the formation of layered skarn and generated CO₂ or other gases. During the reaction, the ore‐forming fluid boiled and produced fractures when the pressure exceeded the overburden pressure. Themeteoric water mixed with the ore‐forming fluid along the fractures. The boiling changed the pressure and temperature, oxygen fugacity, physical and chemical conditions of the whole mineralization system. The escape of CO₂ from the fluid by boiling resulted in scheelite precipitation. The fluid mixing and boiling reduced the solubility of metal sulfides and led the precipitation of chalcopyrite, molybdenite, pyrite and other sulfide.     

Evidence for a nitrogen flux directly derived from the European subcontinental mantle in the Western Eger Rift, Central Europe

In the Czech-German border region of the Vogtland and NW Bohemia (western Eger rift, Central Europe), chemical and isotopic compositions (C, N, He, Ar) of free gas from a thermal water escape (fluorite mine, Schönbrunn), two mineral springs (“Eisenquelle,” Bad Brambach; “Sprudel III,” Bad Elster) and a mofette (Bublak) located along an ∼40-km long traverse are reported. The gases of Bublak and Bad Brambach are CO₂-rich (>99 vol.%) and have δ¹³C values of −1.95 and −4.29‰, respectively. With distance from the center of CO₂ degassing (Bublak) the δ¹³C values decrease, most likely due to physico-chemical fractionation of CO₂ between gaseous and aqueous phases rather than to admixture of organic/biogenic CO₂. The δ¹⁵N values range between −3.2 and −0.6‰, compared to an upper mantle value of −4.0 ± 1.0‰. The four locations are characterized by ³He/⁴He ratios decreasing from 5.9 R_a in the center (Bublak) to 0.8 R_a in the periphery (Schönbrunn) and give evidence for mixing of He from a deep-seated magmatic source with a crustal source. The location with the highest ³He/⁴He ratio (5.9 R_a) is accompanied by the highest ⁴⁰Ar/³⁶Ar (550). We argue that the nitrogen of the Bublak mofette gas is a mixture of predominantly atmospheric and mantle-derived components, whereas at the other three locations crustal nitrogen may also be present. The Bublak δ¹⁵N value of ≈−4.5 ± 1.0‰ represents the first free gas δ¹⁵N reference from the European subcontinental mantle (ESCM) and indicates that, in contrast to the ³He/⁴He ratios, the δ¹⁵N values are equal for ESCM and MORB, respectively.     

Incremental vacuum dehydration-decarbonation experiments on a natural gibbsite (α-Al(OH3)): CO2 abundance and δC values

Incremental vacuum dehydration-decarbonation experiments were performed at 190°C on chemically “cleaned” aliquots of a gibbsite-dominated, Eocene-age bauxite sample with evolution of CO₂ and H₂O. “Plateau” F (CO₂/H₂O ratios) and δ¹³C values of the CO₂ derived from gibbsite were attained over the dehydration interval, X_v(H₂) = 0.16 to 0.67 (i.e., 16 to 67% breakdown of gibbsite). The plateau value of F for gibbsite was 0.0043 ± 0.0003, while the corresponding δ¹³C value of evolved CO₂ was −16.0‰±0.4‰. Additional experiments on chemically cleaned aliquots included (1) treatment with a solution of 0.3M Na-Citrate + 0.1M Na-Dithionite and (2) an exchange experiment with 0.1 bar of ¹³C-depleted CO₂ (−46‰) at 105°C for 64.5 h. Neither of these additional treatments resulted in a measurable perturbation of plateau values of F or δ¹³C for CO₂ evolved from gibbsite during dehydroxylation. These results support published work on Holocene samples which suggested that CO₂ occluded in gibbsite may preserve information on δ¹³C values of CO₂ in ancient terrestrial systems. The plateau values of F observed in the Eocene gibbsite indicate that it may be possible to experimentally calibrate a relationship between the concentration of CO₂ occluded in gibbsite and CO₂ in the environment at the time of crystallization. Such a calibration would significantly enhance the value of gibbsite as a source of information on ancient oxidized carbon systems.     

Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δC-N study of eclogite-hosted diamonds from the Jericho kimberlite

Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ¹³C values. While diamonds from high-MgO eclogites have low nitrogen contents (5-82 ppm) and extremely low δ¹³C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and δ¹³C values from −3.5‰ to −5.3‰.Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ¹³C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ¹³C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ¹³C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ¹³C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0 Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction.SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ¹³C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ¹³C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ¹³C-N yielded a partition coefficient (K_N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ¹³C values of diamond cores (−4‰) dictate the growth medium had higher δ¹³C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates.     

Insights into C and H storage in the altered oceanic crust: Results from ODP/IODP Hole 1256D

Carbon and hydrogen concentrations and isotopic compositions were measured in 19 samples from altered oceanic crust cored in ODP/IODP Hole 1256D through lavas, dikes down to the gabbroic rocks. Bulk water content varies from 0.32 to 2.14 wt% with δD values from −64‰ to −25‰. All samples are enriched in water relative to fresh basalts. The δD values are interpreted in terms of mixing between magmatic water and another source that can be either secondary hydrous minerals and/or H contained in organic compounds such as hydrocarbons. Total CO₂, extracted by step-heating technique, ranges between 564 and 2823 ppm with δ¹³C values from −14.9‰ to −26.6‰. As for water, these altered samples are enriched in carbon relative to fresh basalts. The carbon isotope compositions are interpreted in terms of a mixing between two components: (1) a carbonate with δ¹³C = −4.5‰ and (2) an organic compound with δ¹³C = −26.6‰. A mixing model calculation indicates that, for most samples (17 of 19), more than 75% of the total C occurs as organic compounds while carbonates represent less than 25%. This result is also supported by independent estimates of carbonate content from CO₂ yield after H₃PO₄ attack. A comparison between the carbon concentration in our samples, seawater DIC (Dissolved Inorganic Carbon) and DOC (Dissolved Organic Carbon), and hydrothermal fluids suggests that CO₂ degassed from magmatic reservoirs is the main source of organic C addition to the crust during the alteration process. A reduction step of dissolved CO₂ is thus required, and can be either biologically mediated or not. Abiotic processes are necessary for the deeper part of the crust (>1000 mbsf) because alteration temperatures are greater than any hyperthermophilic living organism (i.e. T > 110 °C). Even if not required, we cannot rule out the contribution of microbial activity in the low-temperature alteration zones. We propose a two-step model for carbon cycling during crustal alteration: (1) when “fresh” oceanic crust forms at or close to ridge axis, alteration starts with hot hydrothermal fluids enriched in magmatic CO₂, leading to the formation of organic compounds during Fischer-Tropsch-type reactions; (2) when the crust moves away from the ridge axis, these interactions with hot hydrothermal fluids decrease and are replaced by seawater interactions with carbonate precipitation in fractures. Taking into account this organic carbon, we estimate C isotope composition of mean altered oceanic crust at ∼ −4.7‰, similar to the δ¹³C of the C degassed from the mantle at ridge axis, and discuss the global carbon budget. The total flux of C stored in the altered oceanic crust, as carbonate and organic compound, is 2.9 ± 0.4 × 10¹² molC/yr.     

Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile

We evaluate the impact of exceptionally sparse plant cover (0-20%) and rainfall (2-114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile. δ¹³C and δ¹⁸O values of carbonates from the Atacama are the highest of any desert in the world. δ¹³C (VPDB) values from soil carbonate range from −8.2‰ at the wettest sites to +7.9‰ at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett.71, 229-240], in which we assumed an exponential form of the soil CO₂ production function, and relatively shallow (20-30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ¹³C value of soil carbonate in the Atacama, whereas the fraction C₃ to C₄ biomass at individual sites has a subordinate influence. The high average δ¹³C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO₂.δ¹⁸O (VPDB) values from soil carbonate range from −5.9‰ at the wettest sites to +7.3‰ at the driest and show much less regular variation with elevation change than δ¹³C values. δ¹⁸O values for soil carbonate predicted from local temperature and δ¹⁸O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use of δ¹⁸O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.     

Geochemical features and origin of natural gas in heavy oil area of the Western Slope,Songliao Basin,China

The Western Slope of the Songliao Basin is rich in heavy oil resources (>70 × 10⁸ bbl), around which there are shallow gas reservoirs (∼1.0 × 10¹² m³). The gas is dominated by methane with a dryness over 0.99, and the non-hydrocarbon component being overwelmingly nitrogen. Carbon isotope composition of methane and its homologs is depleted in ¹³C, with δ¹³C₁ values being in the range of −55‰ to −75‰, δ¹³C₂ being in the range of −40‰ to −53‰ and δ¹³C₃ being in the range of −30‰ to −42‰, respectively. These values differ significantly from those solution gases source in the Daqing oilfield. This study concludes that heavy oils along the Western Slope were derived from mature source rocks in the Qijia-Gulong Depression, that were biodegraded. The low reservoir temperature (30–50 °C) and low salinity of formation water with neutral to alkaline pH (NaHCO₃) appeared ideal for microbial activity and thus biodegradation. Natural gas along the Western Slope appears mainly to have originated from biodegradation and the formation of heavy oil. This origin is suggested by the heavy δ¹³C of CO₂ (−18.78‰ to 0.95‰) which suggests that the methane was produced via fermentation as the terminal decomposition stage of the oil.     

Isotopic composition of carbon in diamonds of diamondites: record of mass fractionation in the upper mantle

Carbon isotope data have been obtained for diamonds in diamondites (framesites, polycrystalline diamond) of unknown origin (presumably from southern Africa), which contain minor amounts of garnet (plus rare clinopyroxene). The carbon isotope abundance ratios show wide inter- and intra-sample ranges. The δ¹³C variations of diamonds from diamondites with “peridotitic” or “eclogitic” garnets are indistinguishable from each other, with a range from −3.2 to −27.9‰, and a peak around −18‰, indicating a depletion in ¹³C relative to most silicate-bearing single crystal diamonds. This ¹³C depletion could have been produced by mass fractionation of C from an isotopically homogeneous mantle source in a C-O-H fluid system. Fluids traveling between regions of varying redox conditions in the mantle could have provided the necessary vehicle. The variations of δ¹³C can be explained by different degrees of oxidation during the fluid/melt ascent.     

Indoor and outdoor urban atmospheric CO2: Stable carbon isotope constraints on mixing and mass balance

From July to November 2009, concentrations of CO₂ in 78 samples of ambient air collected in 18 different interior spaces on a university campus in Dallas, Texas (USA) ranged from 386 to 1980 ppm. Corresponding δ¹³C values varied from −8.9‰ to −19.4‰. The CO₂ from 22 samples of outdoor air (also collected on campus) had a more limited range of concentrations from 385 to 447 ppm (avg. = 408 ppm), while δ¹³C values varied from −10.1‰ to −8.4‰ (avg.=-9.0‰). In contrast to ambient indoor and outdoor air, the concentrations of CO₂ exhaled by 38 different individuals ranged from 38,300 to 76,200 ppm (avg. = 55,100 ppm), while δ¹³C values ranged from −24.8‰ to −17.7‰ (avg. = −21.8‰). The residence times of the total air in the interior spaces of this study appear to have been on the order of 10 min with relatively rapid approaches (∼30 min) to steady-state concentrations of ambient CO₂ gas. Collectively, the δ¹³C values of the indoor CO₂ samples were linearly correlated with the reciprocal of CO₂ concentration, exhibiting an intercept of −21.8‰, with r² = 0.99 and p < 0.001 (n = 78). This high degree of linearity for CO₂ data representing 18 interior spaces (with varying numbers of occupants), and the coincidence of the intercept (−21.8‰) with the average δ¹³C value for human-exhaled CO₂ demonstrates simple mixing between two inputs: (1) outdoor CO₂ introduced to the interior spaces by ventilation systems, and (2) CO₂ exhaled by human occupants of those spaces. If such simple binary mixing is a common feature of interior spaces, it suggests that the intercept of a mixing line defined by two data points (CO₂ input from the local ventilation system and CO₂ in the ambient air of the room) could be a reasonable estimate of the average δ¹³C value of the CO₂ exhaled by the human occupants. Thus, such indoor spaces appear to constitute effective “sample vessels” for collection of CO₂ that can be used to determine the average proportions of C₃ and C₄-derived C in the diets of the occupants. For the various groups occupying the rooms sampled in this study, C₄-derived C appears to have constituted ∼40% of the average diet.