Chemical Geology: Isotope Geoscience section最新文献

Combined Pb, Sr and Nd isotope determinations on several Nigerian Mesozoic (∼ 170 Ma) ring complexes, studied previously by van Breemen and co-workers, indicate a multistage petrogenetic process. Mantle-derived differentiated magmas assimilated crustal basement of average Early Proterozoic age. After crystallisation, some plutons were subjected to a second stage of crustal contamination by circulating hydrothermal fluids.

Crustal compositions were constrained by isotopic analysis of the Proterozoic basement of the Jos Plateau. Sm/Nd analysis of six gneisses yielded an average crustal residence age of 2 Ga, corresponding to the Burkinian event recognised elsewhere in western Africa. However, one sample yields a model age of 3 Ga, suggesting the presence of Archean crustal remnants. Pan-African granitoids yield a similar range of Nd model ages to the gneisses, suggesting that they were largely generated by crustal melting.

The Zaranda anorogenic complex has relatively radiogenic initial Nd and Pb isotope compositions and unradiogenic Sr (∼ 0.5126, ∼ 18.4 and ∼ 0.705, respectively), attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascent through the crust. Other ring complexes trend toward less radiogenic Nd and Pb isotope ratios and more radiogenic Sr, indicative of an increasing crustal contribution. Initial Pb isotope compositions yield a well-defined Pb/Pb isotope array with a slope age of ∼ 1.8 Ga which is consistent with the average Nd crustal residence ages of basement gneisses and granitoids.

The arfvedsonite albite apogranite from the Ririwai anorogenic complex has isotope ratios resembling Pan-African basement, probably resulting from hydrothermal overprinting with fluids equilibrated in the continental crust. Other Ririwai intrusions and one unit from the Shere Hills display evidence of hydrothermal overprinting of Sr and to some extent Nd isotope compositions, but only the Ririwai apogranite has been significantly overprinted by hydrothermal Pb.

The isotopic evidence supports a model for the Mesozoic anorogenic (“A-type”) granites of Nigeria in which mantle-derived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidus hydrothermal alteration in the continental crust.

δ ¹³C- and δ ¹⁸O-values of calcite, dolomite, rhodochrosite and siderite have been determined in situ using a laser extraction system, in which a focused Nd: YAG laser beam excites a sample surface producing a high-energy plasma cloud which forms CO and CO₂. Laser ionization of calcite, dolomite, rhodochrosite and siderite produces different yields of CO₂ and CO and different CO₂/CO ratios for each mineral. Isotopic analyses of combined CO and CO₂ are reproducible to ± 1–2‰ for δ ¹⁸O and ± 2–3‰ for δ ¹³C and are distinctly different from the isotopic compositions determined using standard acid dissolution techniques. CO₂ produced from laser ionization is consistently enriched in ¹³C and ¹⁸O compared to CO. A two-stage model is proposed to explain variations observed in the experimental results. In the first stage, the energy is absorbed by the mineral but the quantity and isotopic composition of gases depends on the chemical composition of the sample, especially the quantity of transition metals which have electronic transitions commensurate with the wavelength of the Nd: YAG. In the second stage, the relative isotopic compositions of CO and CO₂ are affected by reactions in the cooling plasma. This model should be applicable to all laser ionization systems.

Solutions of 1–5 m NaCl, KCl, CaCl₂ and MgCl₂, and synthetic brines were analyzed for hydrogen and oxygen isotopic compositions by vacuum distillation-microequilibration techniques. No effect on the hydrogen isotopic composition was observed for any of the solutions within the precision of the technique of ±5‰ determined from analyses of distilled water. Oxygen isotopic compositions of solutions measured by vacuum distillation-microequilibration indicate that oxygen isotopes are not fractionated during analysis in NaCl, KCl and CaCl₂ solutions. Vacuum distillation-microequilibration analyses of MgCl₂ solutions show depletions in ¹⁸O relative to pure water that can be related to the concentration of Mg²⁺ in the solution. The greatest depletion in ¹⁸O occurs in the 4 m MgCl₂ solutions which have δ¹⁸O-values 6‰ lower than that measured for pure water. Corrections based on the measurement of the δ¹⁸O-values of the pure MgCl₂ solutions can also be applied to mixed chloride brines. Both melting and crushing of halite grown in solutions of known isotopic composition followed by microequilibration to determine the stable isotopic composition of fluid inclusions give results within the precision determined from analyses of distilled water.

Two ferromanganese encrustations of hydrogenetic origin from water depths well below the CCD (carbonate compensation depth) from a seamount zone in the Central Indian basin were studied for growth rates using UTh series isotopes and transition-metal fluxes. The ²³⁰Th_excess inventories and transition-metal fluxes in depth profiles separated by a few cm on one crust (F-380) show insignificant variations. In another crust (SS-663X) this variation is within a factor of 2 as compared to variations by a factor of 5 in the Pacific crusts. The minor variations in growth rates, ²³⁰Th and transition-metal fluxes within a single crust as well as two adjacent crusts suggest that they are due to the contact of the accreting crust surface with a dynamic veneer of sediment in space and time.

Nearly uniform Mn/Fe ratios (1–1.6), δ-Mn0₂ as the main Mn mineral phase and a smooth exponential decay pattern of ²³⁰Th_excess and ²³⁰Th_excess/²³²Th activities with depth indicate that these crusts have not recorded any palaeoceanographic events of the past ∼ 0.4 Ma. The interpolated age of these crusts is between 10 and 13 Ma which is comparable to the period of increased carbonate dissolution due to enhanced activity of AABW (Antarctic Bottom Water) currents during the middle Miocene. Probably the middle Miocene oceanographic conditions, which were associated with increased carbonate dissolution, might have initiated the formation of crusts in the Central Indian basin.

A method is described for the preparation of sulphide precipitates for sulphur isotope analysis from sub-ppm (< 1 mg 1⁻ levels of sulphide in natural waters. Hydrogen sulphide is degassed from acidified water samples into a nitrogen stream and subsequently re-precipitated in a silver nitrate trap. Control experiments on model waters of known sulphur isotope composition show that the average fractionation of sulphide is small (0.069‰) but an uncertainty of ±0.6‰ is appropriate for this method.

30 ng Nd samples loaded with silica gel and phosphoric acid on single rhenium filaments reproducibly provide ¹⁴⁴NdO⁺ ion beams of ∼ 1.5-2 · 10⁻¹¹ A for several hours, without need of a poor source vacuum, or need to introduce oxygen into the mass spectrometer source. The loading technique discriminates strongly against CeO⁺, and accurate normalization for mass fractionation to ${}^{142}\text{Nd}{}^{144}\text{Nd}$ is possible. A 5-collector procedure for analysis and interference correction (¹⁸O, ¹⁷O overlaps, CeO⁺, PrO⁺, SmO⁺ ) is described which permits analysis of 30 ng Nd loads to internal precision of ±0.000005 (2se) in ∼2.5 hr. 17 loads (each 30 ng) of the laboratory Nd standard (analysed March–June 1990) yielded external precision of ±0.000007 (2sd) on ${}^{143}\text{Nd}{}^{144}\text{Nd}$, and ${}^{143}\text{Nd}{}^{144}\text{Nd}$ and ${}^{145}\text{Nd}{}^{144}\text{Nd}$ values within 2se of the value determined on 500 ng Nd⁺ runs, and a further 17, analysed October 1990–April 1991, give identical results within error. Eight sample analyses also agree within 2se between Nd⁺ and NdO+ analyses, the latter being loaded from rinses of the disposable pipette tips used to load the Nd⁺ runs.

Sulfur isotopes are fractionated during the reduction of sulfate by dextrose under hydrothermal conditions. The changes in the isotopic composition of sulfate during the reaction have been previously interpreted to reflect the kinetic isotope effect associated with the initial reduction reaction involving sulfate. By a separate pathway, isotope exchange between sulfate and newly formed sulfide may also affect the isotopic composition of sulfate. Computer simulations of the isotopic evolution of the experiments were pursued in this study to determine whether sulfate-sulfide isotopic exchange influenced the isotopic composition of sulfate and whether the reported kinetic isotope effects represent the kinetic isotope effects due solely to sulfate reduction. Numerical solutions of the rate laws governing the isotopic species show that a sufficiently fast isotope-exchange reaction influences the isotopic composition of sulfate. Comparison of the observed rates of sulfate reduction with the expected rates of sulfate-sulfide isotope exchange under the experimental conditions suggests that the exchange reaction largely determines the isotopic composition of sulfate in the experiments. Therefore, the kinetic isotope effects calculated previously are different from, and probably larger than, the true kinetic isotope effects associated with the reduction of sulfate by dextrose.

Different minerals of the Akouta sandstone-hosted uranium ore deposit in Niger have been investigated by Ub, SmNd and KAr isotopic systematics. The age of mineralization, derived from U-Pb and K-Ar results, is between ∼ 260 and ∼ 130 Ma, which contrasts with the Viséan age of the host Guézouman Formation (∼335 Ma) and consequently rules out a synsedimentary origin of the U ore. SmNd data on uranium oxide samples are widely scattered and do not provide valuable chronological information. However, Nd isotope data suggest that the regional peralkaline volcanic or plutonic rocks were a probable source of uranium for the Akouta ore deposit.

Pb isotope data for greenstone belt volcanic units, plutons and gneisses of the Archean Craton of Zimbabwe are presented together with SmNd isotopic analyses and model Nd mantle derivation (t_DM) ages for selected samples. Pb/Pb whole-rock isochrons yield well-determined dates for the Cardiff Hill rhyolite of the Shamva-Harare greenstone belt (2659₋₃₉⁺³⁸ Ma), the Somabula tonalite (2752₋₅₂⁺⁵⁰ Ma), the Gwenoro Dam migmatitic gneisses (2705_∂3⁺⁶⁰ Ma), and for various suites from the Chingezi tonalite (from 2874±32 to 2686₋₉₄⁺⁸⁸ Ma).

Comparison of geochronological results from this study with those of earlier work (mainly RbSr whole-rock dating) shows some significant discordances, and their possible causes are discussed with regard to time-integrated Th/U ratios and the geological settings of the relevant rock units.

The Chingezi and Sesombi plutons show good agreement between RbSr and Pb/Pb whole-rock isochron dates and also display a limited range of Th/U ratios, appropriate to a purely igneous differentiation history. Early to mid-Archean gneisses show large ranges of Th/U ratios, probably the results of U disturbances during metamorphism. In these rocks Pb/Pb dates may be older or younger than the corresponding RbSr dates, but t_DM model ages generally agree with the older of the isochron results.

The behaviour of the RbSr and UPb whole-rock systems during metamorphism may depend critically on the nature of the fluid phase evolved. CO₂-rich fluids appear to be implicated in U enrichment of early Archean gneisses at the Shabanie Mine. It is argued that CO₂-rich fluids may cause disturbance of the UPb system without resetting the RbSr system, while a H₂O-rich fluid phase could have the reverse effect.