PDB Values Research Articles

Stable isotope geochemistry of waters and gases (CO 2, CH 4) from the overpressured Morganza and Moore-Sams fields, Louisiana Gulf Coast

Oxygen isotope analyses of water, and carbon isotope analyses of dissolved inorganic carbon (DIC), CO 2(g), and CH 4(g) have been performed on samples from normal and overpressured horizons in the Louisiana Gulf Coast. δ 18O SMOW values of the waters range from −2.2 to +8.7%. Consideration of the δ 18O values, the chemical composition of the waters, and the water and gas production values indicate that mixing with a low salinity, isotopically light water has taken place. Mixing lines suggest that the isotopically light component may be water vapor condensed during gas production, with a Rayleigh fractionation process accompanying vapor condensation, rather than shallow groundwater. Mixing lines suggest that all formation waters have δ 18O≈ + 8‰ , which is normal for deep, basinal waters, although Cl − concentrations indicate there are 2 water populations (with approximately 35,000 mg/l and 15,000 to 20,000 mg/l). There appears to be no systematic difference between the overpressured and normally pressured samples, or between samples from different fields. δ 13C PDB values of DIC range from −10.6 to −3.2‰. δ 13C PDB values of CH 4 range from −43.9 to −40.8‰, and δ 13C PDB values of CO 2 range from −8.8 to −6.3‰, indicating that these gases have a thermogenic origin. Significant isotopic differences are observed between the Moore-Sams and Morganza samples which suggest that the Morganza gases were produced at higher temperatures and/or by the decomposition of more mature source material. The CO 2 and DIC are not in isotopic equilibrium with calcite and dolomite cements immediately above and below the seal, suggesting that precipitation of calcite and dolomite took place in an earlier event(s). The DIC of most samples is at, or near, isotopic equilibrium with CO 2, although it appears that some samples have derived a significant component of DIC from isotopically heavier samples located both higher and lower in the section. The lack of isotopic equilibrium between DIC and CO 2 for some samples suggest either that some water samples have little or no contact with coexisting gas samples, or that a dynamic process is taking place that prevents equilibrium being achieved.

GC-IRMS in the Authenticity Control of the Essential Oil of Coriandrum sativum L.

The essential oils of 10 authentic coriander samples of different origin were analyzed. Capillary gas chromatography, on-line coupled with isotope ratio mass spectrometry (cGC-IRMS) using Stabilwax as the stationary phase, was applied. Alternatively, heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin in PS 268 was used as the chiral stationary phase. The enantiomeric ratio and the δ 13 C PDB values of some characteristic compounds were compared with those of commercially available spices and essential oils. The influence of isotopic effects on the 6 values during the primary CO 2 fixation was eliminated by definition of a suitable internal isotopic standard (i-IST). The isotopic effects among genuine monoterpenes are, therefore, exclusively determined by the influence of enzymatic reactions during secondary biogenetic pathways.

Epithermal gold mineralization in a fossil hot spring system, Red Butte, Oregon

The Red Butte gold prospect is a well-preserved, sediment-hosted, fossil hot spring system located within the Lake Owyhee volcanic field of eastern Oregon, 10 km west of the Owyhee Reservoir. Its features include siliceous hot spring sinter, a blanket of intense argillic alteration produced by surficial steam-heated waters, and a 50-m-wide hydrothermal eruption crater filled with bedded breccia deposits. Mineralization is hosted by north- and northwest-striking faults in volcaniclastic sediments of the Miocene Deer Butte Formation. Red Butte is capped by 10 m of syngenetic fluvial sedimentary rocks consisting of a basal silicified mudstone overlain by arkosic sandstones and conglomerates interbedded with siliceous hot spring sinter. The sediments at this paleosurface horizon have been cemented by hydrothermal quartz and adularia and form the resistant cap of the butte. A blanketlike zone of pervasive argillic alteration occurs at the base of the fluvial sediments. Numerous hydrothermal veins and breccias located within the lacustrine sedimentary rocks beneath the argillic blanket contain a telescoped mineral assemblage related spatially to the paleosurface. Quartz and adularia are the most abundant hydrothermal minerals in this zone along with minor pyrite, marcasite, electrum, bladed calcite, stellerite (Ca 4 (Al 8 Si 28 O 72 )28H 2 O), illite-smectite, and smectite clays. The veins and argillic blanket are cut by a 50-m-wide hydrothermal eruption crater filled with bedded breccia deposits. The delta 18 O SMOW values for hydrothermal quartz and adularia range from 5.3 to 8.3 and 0.1 to 2.4 per mil, respectively. The delta 18 O SMOW and delta 13 C PDB values for calcite range from -3.6 to 8.9 and -8.9 to -5.2 per mil, respectively. These values are consistent with those of minerals from meteoric hydrothermal systems. A plot of delta 18 O versus delta 13 C values for calcite shows that HCO (super -) 3 was the dominant aqueous carbon species. INAA analyses of altered sedimentary rocks and hydrothermal vein and breccia samples reveal the presence of anomalous concentrations of Au, Ag, As, Sb, Hg, and Mo. Q-mode factor analysis indicates that much of the variation in the geochemical data set can be explained in terms of five end-member components (factors) which reflect the composition and evolution of the hydrothermal fluid and the initial composition of the host rock. Elemental associations for these factors are (1) Au, K 2 O, Ag, FeO, Hg; (2) CaO, Mo, Hg, As; (3) FeO, Na 2 O, Hg, Sb; (4) K 2 O, Zn, Na 2 O, As; and (5) As, Mo, Sb, Ag. The abundance of adularia and stability of stellerite at Red Butte suggests that fluids reaching the surface had experienced a high degree of vapor separation due to boiling. Au mineralization occurred at the peak of hydrothermal activity in response both to boiling and mixing of the boiling hydrothermal fluid with surficial steam-heated waters.

Variations in the chemical and stable isotope composition of carbon and sulfur species during organic-rich sediment alteration: An experimental and theoretical study of hydrothermal activity at guaymas basin, gulf of california

Organic-rich diatomaceous ooze was reacted with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity at 325–400°C, 400–500 bars, and fluid/sediment mass ratios of 1.56–2.35 to constrain factors regulating the abundance and stable isotope composition of C and S species during hydrothermal alteration of sediment from Guaymas Basin, Gulf of California. Alteration of inorganic and organic sedimentary components resulted in extensive exchange reactions, the release of abundant H 2S, CO 2, CH 4, and C organic, to solution, and recrystallization of the sediment to an assemblage containing albitic plagioclase, quartz, pyrrhotite, and calcite. The δ 34S cdt values of dissolved H 2S varied from −10.9 to +4.3‰ during seawater-sediment interaction at 325 and 400°C and from −16.5 to −9.0‰ during Na-Ca-K-Cl fluid-sediment interaction at 325 and 375°C. In the absence of seawater SO 4, H 2S is derived from both the transformation of pyrite to pyrrhotite and S released during the degradation of organic matter. In the presence of seawater SO 4, reduction of SO 4 contributes directly to H 2S production. Sedimentary organic matter acts as the reducing agent during pyrite and SO 4 reduction. Requisite acidity for the reduction of SO 4 is provided by Mg fixation during early-stage sediment alteration and by albite and calcite formation in Mg-free solutions. Organically derived CH 4 was characterized by δ 13C pdb values ranging between −20.8 and −23.1‰, whereas δ 13C pdb values for dissolved C organic ranged between −14.8 and −17.7%. Mass balance calculations indicate that δ 13 C values for organically derived CO 2 were ≥ − 14.8%. Residual solid sedimentary organic C showed small (≤ 0.7‰) depletions in 13C relative to the starting sediment. The experimental results are consistent with the isotopic and chemical composition of natural hydrothermal fluids and minerals at Guaymas Basin and permit us to better constrain sources and sinks for C and S species in subseafloor hydrothermal systems at sediment-covered spreading centers. Our data show that the sulfur isotope composition of hydrothermal Sulfide minerals in Guaymas Basin can be explained by derivation of S from diagenetic sulfide and seawater sulfate. Basaltic S may also contribute to hydrothermal sulfide precipitates but is not required to explain their isotopic composition. Estimates of seawater/ sediment mass ratios based on sulfur isotopic composition of sulfide minerals and the abundance of dissolved NH 3 in vent fluids range from 3–29 during hydrothermal circulation. Sources of C in Guaymas Basin hydrothermal fluids include thermal degradation of organic matter, bacteriogenic methane production, and dissolution of diagenetic carbonate.

Palaeotemperatures indicated by Upper Jurassic (Kimmeridgian-Tithonian) fossils from Mallorca determined by oxygen isotope composition

The oxygen and carbon composition of calcite has been determined for belemnites, ammonite aptychi and the matrix surrounding these fossils, from the Kimmeridgian-Tithonian of Mallorca. Petrography, cathodoluminescence and trace element analyses indicate that both ammonite aptychi and matrix has been diagenetically altered whereas the belemnites have been undergone minimal or no modification. The belemnites give the heaviest oxygen isotope values of +0.04 to −0.99% PDB and lighter values of −0.71 to −3.98% PDB come from the ammonite aptychi and matrix. The carbon isotopic compositions obtained from all the fossils are positive and are similar to those derived from modern marine shallow-water skeletons. Palaeotemperatures calculated on the basis of the oxygen isotopic composition of the belemnites give a range of values from 13.6° to 16°C. These results agree with Late Jurassic palaeotemperatures from similar palaeolatitudes.

Conditions of formation and diagenetic evolution of Upper Proterozoic phosphate nodules from southern Sweden: evidence from petrology, mineral chemistry and isotopes

Dark-gray to black cryptocrystalline francolite occurs as nodules (∼ 2–8 cm in diameter) which are embedded in organic-matter-rich mudstones of the Visingsö Group (Upper Proterozoic, southern Sweden). The δ 13C PDB values (−18.4 to + 0.98‰) of structural-CO 2, complemented by studies of the petrographic character and strontium isotopic composition of francolite, indicate precipitation in the suboxic and sulfate-reduction zones. Francolite is intergrown with berthierine that has formed in the suboxic Fe-reduction zone. Calcite enriched in Mn and, to a lesser extent Fe, occurs as fracture- and void-fillings, and as replacement of francolite. Mn- and Mg-rich calcian siderites have precipitated as pseudomorphs after bacteria in the zone of microbial methanogenesis. The carbon ( δ 13 C PDB =−0.75 to +7.04‰ ) and oxygen ( δ 18 O PDB =−14.21 to −10.58‰ ) isotopes of calcite suggest derivation of carbonate ions from microbial methanogenesis and from thermal decarboxylation of organic matter at temperatures of 60° to 80°C. Some of the void- and fracture-filling calcites ( δ 13 C= −20.8 to −18.63‰ δ 18 O = −5.46 to −5.21‰ ) have, however, precipitated at lower temperatures (∼ 25–30°C) in the sulfate-reduction zone.

Pluvial conditions in the eastern Sahara following the penultimate deglaciation: implications for changes in atmospheric circulation patterns with global warming

During the transition from the penultimate glacial to the last interglacial period (Isotope Stages 6-5e or Terminal II), large freshwater lakes abruptly infilled sandy depressions in arid regions of the eastern Sahara. Geochemical and mineralogical studies of samples from lacustrine sequences in the Western Desert of southern Egypt indicate: (1) The sediments are chalk composed of authigenic chemical precipitates, predominantly low-Mg calcite, with carbonate contents averaging about 80%. The low dilution of the chemical precipitate by detrital input implies that physical erosion and transport of detrital grains were minimized by extensive vegetation cover in the catchment area. The fauna and flora associated with the lake deposits and Middle Paleolithic occupation sites on the paleo-shorelines attest to a more amenable environment. (2) Vastly different rainfall or atmospheric circulation patterns than at present must have existed based on consistently negative δ 18O PDB values for the chalk, as low as −9.5‰, which would require precipitation from lake waters significantly more depleted in 18O than the modern local spring water ( δ 18 O SMOW = −1.8‰ ). (3) The lakes were stable perennial water bodies with relatively long residence times sufficient to produce isotopic covariant signals in the chalks. Apparently, the lakes disappeared as abruptly as they appeared without going through a terminal phase of evaporative enrichment. Extremely arid conditions again prevailed. During the transition from the last glacial to the present interglacial (Terminal I), a similar change from arid to pluvial conditions occurred. Thus, it is proposed that, during transitions from glacial to interglacial climates, rapid global warming can lead to temporary alteration to atmospheric circulation with an intersification of the southwest monsoons, bringing large quantities of isotopically light moisture to the Eastern Sahara.

Groundwater dolocretes from the Upper Triassic of the Paris Basin, France: a case study of an arid, continental diagenetic facies

ABSTRACTThick dolomite‐cemented horizons (dolocretes) occur within a fluvial sandstone‐mudstone sequence of Late Triassic age in the western part of the Paris Basin, France. Two types of dolomites can be distinguished: (a) nodular dolomitic beds less than a few metres thick, which formed within mottled overbank siltstones and mudstones; and (b) massive dolomite up to 16 m thick, which occurs in coarse grained channel sandstones and conglomerates.The majority of the dolomite consists of a finely crystalline groundmass of dolomicrospar and, less commonly, dolomicrite. Glaebules, irregular spar‐filled cracks, spheroidal dolomite, silicification and vuggy porosity are locally abundant in the massive dolomite. In contrast, biologically induced micromorphological features such as rhizocretions and alveolar‐septal fabrics were observed in the thin, nodular dolomite beds.The dolomite is near stoichiometric, well ordered and non‐ferroan. 18O values range from −7·7 to −0·4%o PDB and 18O values range from −5·1 to + 1·8%0 PDB and no obvious difference in the stable isotopic composition between both types of dolomites was observed. Sr isotope ratios range from 0·7101 to 0·7126 and are invariably higher than the contemporary Triassic sea water.A vadose—pedogenic origin for the thin dolocrete layers is indicated by the occurrence of rhizocretions and other biological structures. Several features, however, argue against a pedogenic origin for the massive carbonates, most notably the absence of biologically induced structures, the occurrence in coarse grained channel (and not overbank) deposits, and the great thickness. These units are thus interpreted as groundwater in origin. Phreatic calcretes of Quaternary age, widespread in inland Australia, are regarded as a modern analogue for the Triassic Paris Basin dolocretes.Petrographic observations argue in favour of primary (proto)dolomite precipitation, although early diagenetic replacement of calcite by (proto)dolomite cannot be ruled out. Strontium and carbon isotope data of early diagenetic dolocrete cements and oxygen isotope data of early diagenetic silica indicate an entirely non‐marine, continental origin for the groundwaters. The poorly ordered and non‐stoichiometric protodolomite probably underwent stabilization upon further burial resulting in a near‐stoichiometric, well ordered dolomite that clearly lacks evidence for pervasive recrystallization.

In situ stratospheric measurements of CH4, 13CH4, N2O, and OC18O using the Bliss tunable diode laser spectrometer

Simultaneous in situ measurements of stratospheric CH4, 13CH4, N2O, OC18O, pressure, and temperature have been made from Palestine, Texas (32°N) in September 1988 with the JPL Balloon‐borne Laser In Situ Sensor (BLISS). Measurements of CH4 and N2O in the altitude range 30–35 km agree well with other measurements, except for an anomalously high value for the N2O at 31 km. Measurements of CH4 support earlier observations of a fold in the vertical profile. A ratio for stratospheric 13CH4/CH4 of 0.0105 ± 0.0010 implies an enrichment of δ13C = −45 ± 92 parts per thousand over the PDB value, in agreement with previous measurements in the troposphere. A large mixing ratio of 1.9 ± 0.2 ppmv for OC18O is measured, corresponding to an enrichment of δ18O = 280 ± 50 parts per thousand for the 18O isotopic species over the SMOW value.

A stable isotope study of the magnesite deposits associated with the alpine-type ultramafic rocks of Yugoslavia

Veins and stockworks of fine-grained (microcrystalline to cryptocrystalline) massive magnesite are common in the ultramafic masses of the ophiolite zone of the Inner Dinarides. Strata-bound deposits of the same kind of magnesite and/or dolomite in Miocene fresh-water basins lie above and beside the ultramafic masses. A carbon and oxygen isotope study of 12 magnesite deposits in Yugoslavia addresses their genesis and bearing on a possible relationship between epigenetic and syngenetic magnesites. All but one of the vein deposits have delta 13 C PDB values between -10 and -15 per mil (the exception, Oshve, has values averaging 3.1ppm), and each deposit has a characteristic value. All sediment-hosted deposits have delta 13 C values between -9 and +5 per mil, again with relatively restricted, characteristic fields for each. The two stockworks analyzed have delta 13 C values ranging from -10.6 to +4.3 per mil, but only one of these has values that coincide with the associated sediment-hosted magnesite deposits. With the exception of the Oshve vein deposit, delta 18 O SMOW values range from approximately 24 to 36 per mil and correlate positively with the delta (super /13) C values (r = 0.90, n = 42). Some of the sediment-hosted magnesite (Bela Stena) and dolomite (Braneshci, Cherenje) deposits were deposited in lakes (assumed temperature of 20 degrees C) with a calculated water delta 18 O value of -2 per mil, suggestive of meteoric water; lower lake temperatures would yield more negative values. Assuming that meteoric water with this delta 18 O value was the mineralizing solution, then a temperature of deposition of approximately 70 degrees C is indicated in those veins not intimately related to dikes.The delta 13 C values of or = 75 degrees C. The suggested low temperatures are consistent with the monomineralic purity of the vein magnesites. At these low temperatures gravity-driven circulating carbonated meteoric waters would react with harzburgite or serpentinite dissolving magnesium, which precipitates as magnesite on pressure drop as CO 2 escapes. The higher delta 13 C values in certain sediment-hosted and stockwork magnesites indicate the involvement of atmospheric CO 2 and possibly carbonate released to the hydrothermal system during contact metamorphism. This latter process is particularly attractive for the Oshve vein deposit with the unusual delta 18 O and delta 13 C values of 19 and 3.1 per mil, respectively, as this is the one vein which is intimately related to a dacite dike.

Diagenesis of carbonate cements in Permo-Triassic sandstones from the Iberian Range, Spain: evidence from chemical composition and stable isotopes

Petrographic, chemical and stable isotope investigations have been made on calcite, dolomite, and Fe-dolomite/ankerite from Permian-Triassic clastic sequences from the Iberian Range (Spain). Calcite and dolomite cements have precipitated before the Fe-dolomite/ankerite. Calcite, however, has also formed by calcitization of dolomite due to uplift. The negative δ 13C PDB values (−9.51 to −2.13%) in these carbonates suggest derivation of carbon from oxidation of organic matter in the early carbonates and from decarboxylation of organic matter in the late Fe-dolomite/ankerite. Early diagenetic dolomitic cements have heavier δ 13C(> −3.54%), higher Mn/(Mn + Fe) and Mn/(Mn + Mg) and lower Fe/(Fe + Mg) ratios than later diagenetic dolomitic cements. The latters are also characterized by lighter δ 18O PDB (> −10.64%) than the early cements due to increase in temperature and to pore water-mineral reactions.

Stable isotopic and fluid inclusion indications of large-scale hydrothermal paleoflow, boiling, and fluid mixing in the Keno Hill Ag-Pb-Zn district, Yukon Territory, Canada

The Keno Hill vein system of the central Yukon is restricted predominantly to the highly fractured, graphitic Keno Hill Quartzite unit of Mississippian age. Hydrothermal mineral zoning is related spatially to a Cretaceous granitic pluton which intrudes the quartzite. During mineralization, the quartzite acted as a district-scale aquifer. Subsequent erosion has exposed a 40-km long vein system, from its plutonic roots, outward to polymetallic Ag-Pb-Zn veins, and further to assemblages of epithermal character. The δ 18O quartz values from veins near the pluton increase outwards from +10.6 to +20.1%. as a result of cooling of the hydrothermal fluids and exchange with the quartzite. Contours of isotope values outline broad paths of fluid movement within the quartzite. Proceeding further from the pluton, δ 18O quartz values decrease to +10.5%. at the outer edge of the system. The presence of meteoric water is indicated here, where late stage quartz has δ 18O SMOW values as low as −7.l%. The outward decreasing trend appears to have been established by mixing of isotopically light meteoric water with exchanged fluids that were in isotopic equilibrium with the quartzite. Fluid inclusions from quartz in the orebodies demonstrate an evolving H 2O-CO 2-NaCl-CH 4 system. Loss of CO 2 and CH 4 during water vaporization coincides with increasing salinity and decreasing temperature resulting from high enthalpy steam loss. Depressurization during active faulting is the principal mechanism. Late stage fluids are represented by dilute aqueous inclusions with lower homogenization temperatures. Quartz from silver-rich veins has been shifted to higher δ 18O values, by up to 4%. relative to adjacent silver-poor veins, the result of a minimum 10–25% adiabatic boiling and fractionation dominated by water vaporization and associated cooling. Graphite initially buffered the hydrothermal fluids to a high CO 2 content, with variable CH 4. Involvement of organic carbon from the host rocks is indicated by the negative δ 13 C PDB values for the carbonates, from −4.0 to −12.9%. Variations in the carbon isotopes result from fluctuating CO 2 CH 4 ratios, reflecting the contrasting volatility of the gas pair. Siderite formed as a late-stage product of the boiling event, and its formation coincides with a decreasing 18O trend in the water created by the equilibration of graphite and water in replacing exsolved CO 2. The formation of CH 4 during this stage had a reducing effect on the fluid, resulting in an increase in δ 13C siderite values in association with the decreasing δ 18O siderite values. A closed-system boiling model, together with calculations of water consumption during post-boiling CO 2 and CH 4 formation, indicates that greater than 50% of the original water in the ore fluid was removed. Relatively saline mineralizing fluids resulted.

Petrology, chemistry and diagenesis of calcite concretions in Silurian shales from central Sweden

Calcite concretions in the lower Silurian Rastrites Shale from central Sweden were formed within 1 m depth of the seafloor. Micrite (1–4 μm) was formed initially but recrystallized into microspar (> 4–50 μm) and subsequently transformed into pseudospar (> 50–150 μm). Skeletal calcite in the concretions and in the host shale has been replaced by microspar which is chemically similar to that occurring in the concretion body. The δ 13C PDB values (mostly between −2‰ and 2‰) suggest that carbonates were mainly derived from seawater. Some of the carbon within calcite was derived from the oxidation of organic matter. Recrystallization has resulted in increasingly negative δ 18 O PDB values ( −7‰ to −4.3‰ ) whereas the δ 13C was not changed significantly.

Petrography and stable isotope composition of baroque dolomite from the Shuaiba formation (lower cretaceous), Abu Dhabi, United Arab Emirates

Integration of petrographic and stable isotopic analyses of dolomite from the reefal facies of the Lower Cretaceous Shuaiba Formation of Abu Dhabi, indicates two types of dolomites: host (very fine to medium crystalline dolomite formed in a meteoric-marine mixing zone) and baroque (white, coarsely crystalline dolomite). The baroque dolomite is characterized by coarsely crystalline mosaics of anhedral to subhedral crystals with undulose extinction, curved crystal faces and cleavage planes, and abundant inclusions. Baroque dolomite from this formation was precipitated in veins and narrow solution channels and is commonly composed of coarse bladed crystals, sometimes fan-shaped toward the center, precipitated as void filling cements that partially occlude porosity. Analysis of the formation of water chemistry and stable isotope for the dolomite of the Shuaiba Formation reveals that it probably formed by mixing seawater or meteoric water in the subsurface, with interstitial brines derived from halite-bearing evaporites. Also, oxygen isotope analyses (δ 18O PDB values range from −4.8 to −9.6% and δ 13C PDB values range from 2.4 to 4.9%) suggest that baroque dolomite formed at temperatures ranging from 67 to 112°C.

Stable isotope compositions of sedimentary organic carbon in Tampa Bay, Florida, U.S.A.: implications for evaluating oil contamination

δ 13C pdb values for sedimentary organic carbon in Tampa Bay, Florida and adjacent riverine systems range from −28% up the rivers, to −20% in the middle of the bay, to −10% in several contiguous small bays along the Gulf of mexico coastline. These values reflect the compositions of the plant precursors, that is, C 3-land plants, phytoplankton and sea grasses, respectively, which predominate in each of these areas. Organic carbon percentages range from 5.0 to less than 0.1 with a moderately good correlation relative to the<63gm sediment grain size fraction ( = 0.76for67samples). In addition to providing another interesting scenario of δ 13C signatures that are useful in understanding the contemporary deposition of organic carbon on the one hand and ancient depositional environments on the other, these data are also shown to be useful in assessing the contribution of oil residues to the sediments should an oil spill occur in Tampa Bay at some time in the future.

Carbon and its isotopes in mid-oceanic basaltic glasses

Three carbon components are evident in eleven analyzed mid-oceanic basalts: carbon on sample surfaces (resembling adsorbed gases, organic matter, or other non-magmatic carbon species acquired by the glasses subsequent to their eruption), mantle carbon dioxide in vesicles, and mantle carbon dissolved in the glasses. The combustion technique employed recovered only reduced sulfur, all of which appears to be indigenous to the glasses. The dissolved carbon concentration (measured in vesicle-free glass) increases with the eruption depth of the spreading ridge, and is consistent with earlier data which show that magma carbon solubility increases with pressure. The total glass carbon content (dissolved plus vesicular carbon) may be controlled by the depth of the shallowest ridge magma chamber. Carbon isotopic fractionation accompanies magma degassing; vesicle CO 2 is about 3.8‰ enriched in 13C, relative to dissolved carbon. Despite this fractionation, δ 13C PDB values for all spreading ridge glasses lie within the range −5.6 and −7.5, and the δ 13C PDB of mantle carbon likely lies between −5 and −7. The carbon abundances and δ 13C PDB values of Kilauea East Rift glasses apparently are influenced by the differentiation and movement of magma within that Hawaiian volcano. Using 3He and carbon data for submarine hydrothermal fluids, the present-day mid-oceanic ridge mantle carbon flux is estimated very roughly to be about 1.0 × 10 13 g C/yr. Such a flux requires 8 Gyr to accumulate the earth's present crustal carbon inventory.

Oxygen and Carbon Isotopes of Early Permian Cold-Water Carbonates, Tasmania, Australia

ABSTRACT The Permian carbonates of Tasmania formed at a paleolatitude near 80°S during the Gondwanan Ice Age, contain abundant dropstones, and are associated with glaciomarine sediments. A shallow-marine carbonate depositional model with floating icebergs, calved from an adjacent ice shelf grading into an ice sheet, has been proposed. As the sea level rose during relatively warmer periods, large volumes of melt waters mixed with marine waters retarding limestone deposition and causing deposition of shales/siltstones. These phases are preserved in the stratigraphic record as interbedded units ( Australian Permian brachiopods and molluscs are characterized by unusually light O18 PDB values and heavier C13 PDB values than those that occur in many modern, cool-temperate to subpolar cold-water carbonates. Tasmanian Permian whole-rock O18 PDB values fall at the edge of the Normal Marine Limestone field of Keith and Weber (1964) and range towards lighter values (down to -16.9 PDB). The O18 values of cements (-7.6 to -25.6 PDB) in the Tasmanian Early Permian limestones partly overlap with those O18 values obtained for fresh-water cements in the Early Permian continental tillites from Antarctica and South Africa (Gondwanaland), indicating that the Early Permian sea was diluted by isotopically light melt waters during warming phases. The O18 values of Permian fauna, if one assumes no diagenetic equilibration, give unrealistic paleotemperatures because of this variable melt-water dilution of the Permian sea. However, calculated O18 values, corresponding to marine C13 values of brachiopods and Eurydesma and extrapolated from a model based on the linear trend of C13-O18 in modern and last-glacial cold-water carbonates, give reasonable estimates of Australian Permian temperatures of up t 15° C with the coldest waters of less than 4° C around Tasmania. The sequential deviation lines of O18-C13 of both cements and the fauna associated with these cements indicate that the original O18 value of fauna was as high as +6 PDB. This calculated original O18 value indicates an average seawater temperature for Tasmania in the Early Permian of -1.8° C, similar to the present average -1.9° C water temperature near ice shelves around Antarctica. The O18 values of ements suggest that the Tasmanian limestones reacted with subfreezing melt waters during early diagenesis. The O18 composition of the Early Permian sea is inferred to have been about + 1.2 PDB, similar to that observed during the Pleistocene glaciations, and was diluted by melt water as light as O18 SMOW = -31 at 5° C (-26 PDB) in variable amounts. It is unlikely that the O18 composition of the wellmixed open Permian sea ever reached a value as light as O18 PDB = -6. It is suggested that the Permian sea C13 value was about +2 PDB, heavier than that of modern and Pleistocene seawater.

Carbon, oxygen and sulphur isotope variations in concretions from the Upper Lias of N.E. England

Carbon, oxygen and sulphur isotope data for transects across two pyrite-bearmg carbonate concretions, and their host sediments, from the Upper Lias of N.E. England show symmetrical zonation. δ 13C PDB values of the calcite cement (−12.9 to −15.4%.) indicate that most of it originated from organic matter by bacterial reduction of sulphate, augmented with marine and, to a lesser extent, fermentation derived carbonate. Organic carbon ( δ 13 C PDB = −26.1 to −37.0%.). reflects the admixture of allochtho-nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation. Two phases of pyrite are present in each concretion. The earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has δ 34S CD values of −22 to −26%. indicating formation by open system sulphate reduction. The later euhedral phase is more abundant and reaches values of − 2.5 to − 5.5%. at concretion margins. This phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system. The δ 13C and δ 34S data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface. The δ 18O values of the calcite in one concretion ( δ 18 O PDB = 2.3 to −4.8%.) indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater. The other concretion is isotopically much lighter ( δ 18 O PDB −8.9 to −9.9%.) and large δ 18O differences between concretions in closely-spaced horizons imply that local factors control the isotopic composition of pore waters.

Epigenetic Zoning in Surface and Near-Surface Rocks Resulting from Seepage-Induced Redox Gradients, Velma Oil Field, Oklahoma: ABSTRACT

ABSTRACT Surface and near-surface Permian sandstone has been drastically altered over the productive part of the structurally complex Velma oil field as a consequence of petroleum microseepage. Buried Permian sandstone along the northwest-southeast-trending anticline is cemented with abundant pyrite, ferroan calcite and ferroan dolomite. At the surface along the anticlinal crest, iron disulfide is scarce; isotopically anomalous carbonate-cemented sandstone is overlain by sandstone that is massively impregnated by hematite cement. Permian sandstone is normally reddish brown throughout southern Oklahoma, but along the anticlinal flank it has been bleached yellow and white owing to iron loss; some units contain abundant solid bitumen. The mineralogy in the vertical section over the anticline follows the calculated stability relations for iron sulfide, iron carbonate, and iron oxide along a gradient from strongly reducing conditions at depth to oxidizing conditions at the surface. Reducing conditions were provided by seeping hydrocarbons from subsurface reservoirs of this multizone giant field. Production depths range from 120 to 2,975 m. The principal evidence that these alterations are seepage-induced is provided by reports of oil seeps in the early literature, by zones of solid bitumen cements, and by C13 PDB values for carbonate cements that range from -7.8 to -36.7 ppt.

The geochemical evolution of the Pleistocene Lake Lisan-Dead Sea system

The geochemical history of Lake Lisan, the Pleistocene precursor of the Dead Sea, has been studied by geological, chemical and isotopic methods. Aragonite laminae from the Lisan Formation yielded (equivalent) Sr/Ca ratios in the range 0.5 × 10 −2−1 × 10 −2, Na/Ca ratios from 3.6 × 10 −3 to 9.2 × 10 −3, δ 18O PDB values between 1.5 and 7%. and δ 13C PDB from −7.7 to 3.4%.. The distribution coefficient of Na + between aragonite and aqueous solutions, λ A Na , is experimentally shown to be very sensitive to salinity and nearly temperature independent. Thus, Na/Ca in aragonite serves as a paleosalinity indicator. Sr/Ca ratios and δ 18O values in aragonite provide good long-term monitors of a lake's evolution. They show Lake Lisan to be well mixed, highly evaporated and saline. Except for a diluted surface layer, the salinity of the lake was half that of the present Dead Sea (15 vs 31%). Lake Lisan evolved from a small, yet deep, hypersaline Dead Sea-like, water body. This initial lake was rapidly filled-up to its highest stand by fresh waters and existed for about 40,000 yr before shrinking back to the present Dead Sea. The chemistry of Lake Lisan at its stable stand represented a material balance between a Jordan-like input, an original large mass of salts and a chemical removal of aragonite. The weighted average depth of Lake Lisan is calculated, on a geochemical basis, to have been at least 400, preferably 600 m. The oxygen isotopic composition of Lake Lisan water, which was higher by at least 3%. than that of the Dead Sea, was probably dictated by a higher rate of evaporation. Na/Ca ratios in aragonite, which correlate well with δ 13C values, but change frequently in time, reflect the existence of a short lived upper water layer of varying salinity in Lake Lisan.