Chemical Geology: Isotope Geoscience section最新文献

We have analysed the Sr and C isotopic composition of samples of white sparry dolomite (WSD) interbanded with sulfides, vug-filling dolomite and calcite, and host dolostones associated with lead-zinc mineralization at Nanisivik, N.W.T. Canada. Ore stage WSD and late stage carbonates are considerably more radiogenic in ⁸⁷Sr and depleted in ¹³C and ¹⁸0 than dolomite of the host rocks. Within single WSD bands there is a negative linear correlation between ⁸⁷Sr/⁸⁶Sr and δ ¹³C and a positive correlation between ⁸⁷Sr/⁸⁶Sr and δ ¹⁸0. The ore-forming fluids varied considerably in isotopic composition during sulfide precipitation as a result of mixing between exogenous radiogenic ⁸⁷Sr and Sr from the host-rock dolostones, as well as mixing between bicarbonate from in situ oxidation of hydrocarbon, with carbonate from the host rock. High ⁸⁷Sr/⁸⁶Sr ratios in the ore fluid was probably the result of the interaction and equilibration with underlying metalliferous Arctic Bay formation and possibly basement rocks during migration to the site of ore deposition. The post-ore carbonates show a weak positive correlation between ⁸⁷Sr/⁸⁶Sr and δ ¹³C and apparently formed by remobilization of gangue and host-rock carbonate.

Whole-rock RbSr dating of three suites of greywacke and argillite from the Torlesse accretionary prism terrane of New Zealand provides age constraints on the timing of deposition of the sediments in the absence of fossil ages. The three suites produced four linear arrays. Two arrays are interpreted as isochrons with age significance (135 ± 3 and 114 ± 4 Ma) reflecting the diachronous deformation and metamorphism of separate thrust packets in the accretionary prism. Two arrays appear to be too old (154 ± 9 and 144 ± 8 Ma) with respect to other age data which suggests these rocks are Early Cretaceous in age; these linear arrays are consequently interpreted as pseudo-isochrons. A fourth suite from an overlying trench slope basin deposit also yields a pseudo-isochron, the age of which (153 ± 18 Ma) is older than the known age of deposition (c. 98–119 Ma).

Recent RbSr dating of Late Triassic Torlesse rock suites and suites from other accretionary prism terranes, have shown that meaningful ages can be obtained from rocks which have undergone complex deformation and low-grade metamorphism as a consequence of subduction and accretion. By comparison, the mixed provenance (and possibly different burial conditions), of Late Jurassic-Early Cretaceous Torlesse rocks has hampered complete Sr isotopic homogenisation. This study shows that RbSr dating can be applied to suites from these terranes, but a clear knowledge of the provenance (from petrography and geochemistry), tectonic setting, and any additional age constraints, are required to interpret the data.

Plant stable-carbon isotope fractionation models indicate that δ¹³C of atmospheric CO₂, CO₂ concentration, light and moisture stress, among other factors, may potentially affect the δ¹³C of fixed carbon. Seasonal δ¹³C variations in tree rings may therefore represent a new tool for paleoenvironmental reconstruction. The seasonal δ¹³C patterns in growth rings exist in trees (conifer and hardwood) from tropical and temperate localities, and isotopic variation is even seen in trees which are lacking or have poorly-defined rings. The patterns in different rings from a single tree are usually similar, although differences in amplitude and timing of maxima and/or minima are common. Some of the differences may be attributable to radial variation of the δ¹³C patterns which was found to be substantial in a severely water-stressed tree. Correlations of δ¹³C patterns and corresponding seasonal environmental variation for one well-monitored tree showed greater response of the δ¹³C change to measured soil moisture and precipitation than to temperature, calculated soil moisture, solar radiation, or net photosynthesis (as estimated from CO₂ release).

The sources, fluxes and sinks of dissolved organic carbon (DOC) in a groundwater recharge environment and along a well defined flowpath were investigated in a shallow unconfined aquifer system in Central Ontario, Canada. The initial recharge flux of DOC in the upper vadose zone is estimated to be 49 kg ha⁻¹ yr⁻¹, however, biochemical recycling and retention processes deeper in the vadose zone result in a net DOC flux of about 9.8 kg ha⁻¹ yr⁻¹ to the groundwater. Within the aquifer sequential redox processes observed along the flowpath (OZ consumption, SO₄²⁻ reduction, fermentation) further consume or produce DOC. Characterization of the DOC suggest a predominance of high molecular weight (HMW) aquatic fulvic acids and intermediate molecular weight compounds. Radiocarbon analyses of HMW and low molecular weight DOC fractions in the groundwater indicate: (1) DOC fractions recharging the groundwater system are derived from organic carbon sources in the upper soil zone; (2) ¹⁴C reductions in DOC observed along the groundwater flowpath are a combined result of redox processes (DOC oxidation, production) and the gradual input of bomb ¹⁴C; and (3) DOC oxidation in the groundwater does not significantly impact the inorganic geochemistry or carbon isotopic composition of the dissolved inorganic carbon pool.

Projected length analysis of surface tracks enables the calculation of length dispersive spectra of apatite fission track ages determined by the grain-population technique. Different T-t paths result in characteristic apparent age-spectra which reveal the low temperature ( < 140°C) history of rocks. The concept is successfully tested against geologic constraints.

Application of the thermoluminescence (TL), fine-grain (4–11 μm) dating method to modern coastal deposits and to tectonically displaced sediments near the Mendocino Triple Junction in northern coastal California demonstrates the suitability and limitations of dune sand, buried soil, estuarine mud, and beach deposits for TL dating. TL measurements on modern estuarine mud, modern beach sediments, and radiocarbon-dated dune sand and soils show that these deposits are suitably zeroed for TL dating under some conditions. A weighted mean TL age of 16.4 ± 1.3 ka was obtained for four samples (two from a buried soil and two from the straddling terrace dune sand), in close agreement with a radiocarbon age of 16.84 ± 0.06 ka B.P. on charcoal from this paleosol. Application to older sediments and soil profiles of similar origin may therefore provide useful geological ages.

Deposits associated with deformed marine terraces yielded TL ages of 176 ± 33 ka for a shallow marine mud at the Mad River site and 119 ± 31 ka for nearshore deposits at the Southport Landing site. These age estimates are consistent with the stratigraphic positions of the TL samples relative to deposits having previously inferred ages based upon soil profile correlations to local age assignments, which are in turn based upon uplift rates relating to eustatic sea level stands. These single TL age results suggest that more detailed TL dating applications in this region, and in similar geological settings, could provide accurate estimates of geologic ages and rates of deformation. However, the polymineralic fine grains from two other nearshore deposits (at the Table Bluff and Centerville Beach sites) yielded ambiguous plateau plots and thus can not be considered datable without additional TL experiments. This difficulty may be due to a high quartz/feldspar ratio in sediments from this area.

Organic carbon isotopes and the geochemistry of organic matter have been studied in suspended material from the Congo River and some of its tributaries and in a core located at 2000-m depth, off the mouth of the river.

The organic carbon isotopic composition of suspended material from the Congo tributaries identifies the floral cover and the lithologic and pedologic characteristics of their watersheds. Suspended material from the Congo River in the Stanley Pool emphasizes the sole influence of organic matter derived from C₃-rich plants. Outside the Congo estuary, differences of isotopic composition between suspended material near the surface and that near the bottom emphasize the respective marine or continental origin of particulate organic carbon.

The carbon isotopic composition of the marine deposits records biozonal fluctuations of the recent Quaternary. Shifts in the isotopic composition of organic matter may be explained by an increase of terrigenous organic matter derived from C₃ type plants during the wet biozones Z and X whereas during the drier biozone Y the associated organic matter shows a greater influence of the productivity of the oceanic biomass. However, a possible contribution of C₄ plant-derived particulate organic matter can be assumed for the very top of the Y biozone, introducing a ¹³C-enriched signal, indicative of marine planktonic input.

Carbonate mineral growths on concrete structures are endemic throughout the world. The growths take the form of calcitic crusts and stalactites, the formation mechanism for which has not been well defined. We have applied carbon and oxygen isotope analyses to calcites collected from structures in the Midland Valley of Scotland. Carbonate δ¹³C_PDB-values are in the range of −28.9 to −18.8‰ with δ¹⁸O_SMOW in the range of +8.5 to + 16.5‰. These results indicate that the calcites are produced by the interaction of atmospheric carbon dioxide (δ¹³C_PDB = −7‰ δ¹⁸O_SMOW = + 41‰ with hyper-alkaline water, in an isotopically open system. The water attains a high pH as it percolates through and interacts with the concrete, dissolving portlandite. Atmospheric carbon dioxide is fixed almost instantaneously as calcite in the water. A large kinetic carbon isotope fractionation of −10‰ occurs as carbon dioxide molecules cross the gas-liquid interface. The final δ¹³C-value of the calcites produced in the concrete geochemical environment is also shifted by another kinetic fractionation mechanism that may be pH controlled.

Seven crude oils and ten possible source rocks from Kuwait have been investigated for the stable carbon and sulfur isotopic compositions of chromatographic oil and extract fractions, asphaltenes, kerogen, carbonates and S-bearing minerals. The crudes are isotopically very similar and suggest a single organic source. They also show characteristics intermolecular isotopic distribution patterns.

On the basis of the criteria: organic richness [extractable organic matter-total organic carbon (EOM-TOC) relationships], carbon and sulfur isotopic similarities of oils, extracts, pyrite and kerogen, samples from the Ratawi Formation (Raudhatain oil field) and the Minagish Formation (Burgan oil field) showed indications of source potential. The excellent isotopic correlations with the sample from the Mauddud Formation (Raudhatain oil field) are explained by impregnation of rock samples by reservoir oil. In the core samples, an increase in the organic matter content is correlated with¹³C enrichment in the carbonates.

The results demonstrate the potential of stable carbon and sulfur isotope correlation methods in geochemical hydrocarbon exploration.