Chemical Geology: Isotope Geoscience section最新文献

In order to estimate the isotope fractionation effect between coals and methane during coalification a maturity-related fractionation model has been developed for coals and reservoir gases of NW Germany which is based on empirical data. Assuming that observed isotope shifts of the convertible carbon of coals of different maturities are related to a loss of methane during coalification and that this shift can be described by a Rayleigh distillation process, functions with preselected fractionation factors were fitted to measured isotope data of the convertible carbon of coals. The best approximation of theoretical and measured data was achieved with a low fractionation factor (α_c=1.003). Using this model theoretical methane carbon isotope data were determined and compared to the isotopic composition of reservoir methanes of NW Germany. Although the methane isotope data of reservoir gases and the related maturity of the coals show a slight scatter, the theoretical data plot within the same range and follow the increase of the ¹³C concentration of reservoir gases with increasing maturity of the coals.

Carbon and oxygen isotope data are presented for carbonatites and associated alkali silicate rocks from the Tertiary (49 Ma) Dicker Willem complex in southern Namibia. Carbonatites are grouped into: (1) coarse-grained sövites and associated silicate-oxide-phosphate-rich cumulates; (2) finer-grained alvikite intrusions, showing porphyritic and spinifex textures, comb layering and gravity-settled layers; (3) late-stage dykes, pipe breccias, veins and druses. The early sövites carry many inclusions of silicate rocks (ijolites, syenites). The most primitive carbon and oxygen isotope compositions are found in phenocrysts from calcite-phyric microsövite, bulk s6vites and interstitial carbonate in the ijolites, with δ¹³C (−5‰ vs. PDB) and δ¹⁸O ( +7 to +9‰ vs. SMOW ). Oxygen isotope fractionation between cumulus pyroxene, magnetite and biotite in the sövites yields near magmatic temperatures of 600–900°C. Carbonates in some cumulates yield magmatic temperatures, but commonly show evidence of secondary alteration. Phenocrysts in dolomite-phyric alvikite are slightly enriched in ¹³C (average $\text{δ}{}^{13}\text{C}\text{=−3.6‰}$) and ¹⁸0 (average $\text{δ}{}^{18}\text{O}\text{=+9.9‰}$) relative to primitive ratios, but taken together with data for phyric calcite define a linear trend of increasing δ¹³C with δ¹⁸O and can be modelled as being the product of combined carbonate-silicate-oxide-phosphate fractionation of a parent sövite. Groundmass carbonate in the porphyritic alvikites, as well as the bulk alvikites, all show variable degrees of ¹⁸O enrichment relative to the phenocrysts, and reflect partial recrystallization of carbonate in the presence of low-temperature hydrous fluids.

SmNd (IDM) model ages for gneisses from the coastal region between 66° and 68°N in southern East Greenland range from 3.02 to 2.79 Ga and indicate that these basement rocks were formed in a major late Archaean episode of sialic crust formation between 3.0 and 2.8 Ga ago. Very low concentrations of U have resulted in unradiogenic Pb-isotopic compositions, so that most samples do not yield chronologically useful $\text{Pb}\text{Pb}$ isochrons. The data have been used to calculate $\text{Pb}\text{Pb}$ model ages, and in the northern and southern parts of the study area these are in good agreement with the SmNd model ages. In the central part of the area, however, $\text{Pb}\text{Pb}$ model ages are much lower than SmNd ages, locally as low as ∼ 1.80 Ga. This discrepancy is attributed to loss of U long after the crust formation event, and probably related to a phase of Proterozoic granulite-facies metamorphism that has not previously been recognised. Rb-Sr isotope data support this interpretation.

Light hydrocarbon and isotope compositions of methane were analyzed in well steam samples from the Matsukawa vapour-dominated type geothermal system. Alkanes (C₁-C₄) and alkene (C₂) were detected in all samples. Light hydrocarbon contents of CO₂-type steam are slightly higher than those of C0₂-H₂S-type steam. The isotope composition of methane and the relationship between methane/ethane ratio and δ¹³C-value of methane suggest that these light hydrocarbon gases are mixtures of thermogenic and abiogenic components. The abiogenic hydrocarbon may be attributed to magmatic hydrocarbon gases equilibrated with carbon dioxide at f_O2 defined by the fayalite-magnetite-quartz buffer (FMQ).

The UThPb system of minerals and whole rocks from Hearst and Kapuskasing mafic dykes of the eastern Superior Province, Canada, were investigated to determine their suitability as chronometers for primary and secondary events. Minerals from the dykes all register secondary events ranging from 1.9 to 2.2 Ga coinciding with large-scale crustal deformation associated with uplift of the Kapuskasing Structural Zone (KSZ). Plagioclase phenocrysts and megacrysts from both sets of dykes indicate a later alteration event at 1.72 ± 0.04 Ga.

Dyke whole rocks have a restricted range of μ-values from 4.4 to 21 and present-day isotopic compositions that scatter on a Pb isochron plot. For the Hearst dykes, the scatter is primarily the result of resetting effects which gave rise to multistage evolution of the samples' isotopic systems. Two- and three-stage models of the UThPb systems of the disturbed dykes suggest that the $\text{U}\text{Pb}$ and $\text{Th}\text{Pb}$ ratios of the rocks were reduced during these events. There is little evidence fo large variations among the isotopic composition of these dykes at the time of their crystallization.

Pb isotope data for undisturbed whole rocks from Hearst dykes from both upper-crustal segments (Hornepayne) mid mid- to lower-crustal segments (Wawa Domal Gneiss Terrain) plot on a 2.48 ± 0.02-Ga isochron. We interpret this to indicate that these rocks were derived from a homogeneous (mantle) source and that significant in situ country-rock contamination did not occur during emplacement. The initial Pb isotope ratios (${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 14.02,}{}^{207}\text{Pb}{}^{204}\text{Pb}\text{=14.88,}{}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 33.9}$), and the ratio ${}^{232}\text{Th}{}^{238}\text{U}$ (3.6) for this source suggest an undepleted (or, more contaminated) source relative to the mantle which produced later-formed lamprophyric dykes and carbonatites of the KSZ. The Pb isotopic signatures of the lamprophyres and carbonatites indicate that neither of their sources were as depleted as that of widespread komatiitic and basaltic volcanism in the Wawa and Abitibi belts at 2.7 Ga.

The isotopic composition of calcite and the deuterium content of its fluid inclusions were determined in a Tertiary vein system that developed in a tectonically active region. The vein system is composed of three sets (I, II, III) with different trends and their relative age has been established by cross-cutting relations. Each set has its characteristic calcite δ¹⁸O- and δ¹³C-values: − 7.9 and + 1.59‰ for set 1; − 15.3 and + 0.54‰ for set II; and −15.9 and + 1.73‰ for set III, respectively, while fluid-inclusion δD of the oldest set (I, −23.99‰) resembles that of Israeli coastal plain rainwater and groundwater δD-values of the youngest set (III, −49.69‰) resemble rain- and groundwaters of Mount Hermon. We attribute the δD differences to the isotopic “altitude effect” and conclude that the major change in the hydrologic system was related to the mountain formation and the elevation of the Hermon region. Formation temperatures for set I (35°C) and set III (64°C) are calculated using the relationship between δ¹⁸O and δD of meteoric water and suggest that vein formation occurred at shallow depths (1.7 km) in this region. The calcite δ¹³C indicates that the country rock was the source of vein calcite and that δ¹³C was modified by ∼ 1‰ due to pressure release. A more general model for vein formation, based on the pressure dependence of calcite solubility and cycles of increasing pore pressure that cause hydrofracturing and pressure drop, is discussed.

The anomalous natural radioactivity of Niue Island is due to large excesses of both ²³⁰Th and ²³¹Pa compared with their parent radionuclides. It is suggested that material in Niue Island soils, consisting mainly of goethite and gibbsite partially trapped within porous calcite, was exposed to seawater a few hundred thousand years B.P. and the U content of the seawater adsorbed onto the soil minerals. The island then rose and remained essentially above sea-level until present. The daughter products of the U isotopes grew into radioactive equilibrium with the U parents, but then the U parents were leached away some time in the span 100-70 ka B.P., leaving large amounts of orphan daughters in the soils without supporting porents.

Isotopic and elemental ratios have been measured in three natural gas samples from Gujarat, India. The ³He/⁴He ratio is radiogenic for Kalol-166 and Kalol-183, while a mantle component is clearly indicated for Motwan-2. Although Ne is atmospheric in these Kalol gases, for Motwan-2, a clear excess ²¹Ne due to ¹⁸O(α,n)²¹Ne is present. All three gases have radiogenic ⁴⁰Ar and air values for ³⁶Ar/³⁶Ar. Krypton is atmospheric in composition for all three gases. Xenon in Kalol-183 is purely atmospheric, but for Kalol-166 a pure fission Xe from ²³⁸U is also present. Xenon from Motwan-2, on the other hand, shows excess ¹²⁹Xe, ¹³¹Xe, ¹³²Xe, ¹³⁴Xe and ¹³⁶Xe which do not match either ²³⁸U or ²⁴⁴Pu fission yields. The higher yields of ¹³¹Xe and ¹³²Xe are most likely a result of preferential migration or leaching of their radioactive precursors from their host rock. But the ¹²⁹Xe excess could not be due to such preferential effects, and represents a mantle component. Both ³He and ¹²⁹Xe excesses can be accounted for by the presence of ∼20% mantle component.

The δ¹⁵N-values for the three gases Kalol-166, Kalol-183, and Motwan-2 are (in ‰) +9.27, −12.56 and −11.48, respectively. They indicate a sedimentary source combined with effects due to migration.

Measurements of cosmogenic ³²Si and the U-decay series' nuclides ²¹⁰Ph and ²²⁶Ra in waters and sediments of lake Pavin are reported. Both ²¹⁰Pb and ²²⁶Ra are enriched in the anoxic deep waters compared to the oxic surface waters, respectively by a factor of 4 and 10, whereas ³²Si is depleted by a factor of ∼ 2. Redox conditions in the lake appear to have no marked effect on the ³²Si. Using a steady-state box model it is shown that the deep-water ³²Si concentration is controlled by the underground lacustrine springs. The residence times of ²¹⁰Pb, ³²Si and ²²⁶Ra are ∼ 1, ∼ 10 and ∼ 80 a, respectively. In the case of ³²Si where more data are available, the assessed inventory data from the overhead atmospheric fallout and that measured in the sediments agree very well as expected. The ²¹⁰Pb, and ³²Si-based deposition rates during the past ∼ 100 a ranged from 0.8 to 1.9 mm a⁻¹, earlier these were a factor of ∼ 3–5 faster. The geochemistry of ³²Si and ²¹⁰Pb, in lake Pavin in many ways resembles that in the ocean, only the time scales of the processes involved are faster.