Chemical Geology: Isotope Geoscience section最新文献

Dating individual detrital muscovite micas in clastic sediments can be achieved using the⁴⁰Ar³⁹Ar laser microprobe technique. The results provide a powerful approach to understanding the relationship between intra-sample age variation and inter-sample age variation, something not possible using “bulk” analytical techniques. A key point in interpreting⁴⁰Ar³⁹Ar ages from detrital minerals is understanding their state of alteration which requires careful sampling and preparation procedures. The insights which this technique can provide, along with some of the pitfalls, are illustrated using data from turbidites in the Southern Uplands of Scotland. The ages of individual micas at the time of deposition, range from zero to 70Ma old. Distributions of ages vary from single populations with small standard deviations to widely distributed multi-mode populations. Intra-flow mica age variation is greater than inter-flow variation in Ordovician rocks, but higher in the sequence, in Silurian flows, the intra-flow variation is greater than inter-flow variation. This change reflects changes in the sedimentary environment with time.

Some sources of instrumental errors in the determination of¹³C/¹²C in organic compounds by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) have been investigated. For mass 44 ion beam intensities in the range 1·10⁻¹⁰ to 1·10⁻⁸ A, mass-spectrometric pressure effects do not significantly affect data accuracy, thus obviating the necessity of matching sample and reference ion beam intensities for each compound in a complex sample. Data quality is influenced by: (a) the quality of the furnace-tube packing; and (b) the performance of the cryogenic trap. On analysis of large (70 ng) samples, precisions (1σ) of⩽ ± 0.23‰ were obtained using 0.35–0.5-mm-grade CuO. However, there was a marked deterioration in precision (1σ⪯ ± 0.62) when coarse CuO (0.35–1 mm grade) was tested. Use of fine CuO also permits analysis of smaller samples. Inefficient trapping, and release of water during overnight defrosting leads to erroneously lowδ¹³C-values, accuracies of −1.1 to −0.25‰ resulting from analysis of 70-ng samples. Accuracy error increases to⩽1.86‰ on analysis of 10-ng samples. Efficient water trapping combined with the improvements in furnace performance results in precisions generally much better than±0.4‰ and accuracies better than±0.65‰ for high-molecular-weight compounds (268–338 amu), and very accurate and precise results (better than±0.2‰) for lower-molecular-weight compounds (142–173 amu).

Analysis of noble gases released from fluid inclusions by stepwise in vacuo crushing promises to provide for palaeocrustal fluids the wealth of information currently being obtained from the analysis of noble gases in contemporary ground-waters. In this paper, we report preliminary analyses and interpretation of noble-gas abundances and isotopes in: (a) minerals that precipitated in equilibrium with the atmosphere, in order to identify trapped air-saturated water; (b) quartz and fluorite from the North Pennines orrefield, to provide evidence on the source and interactions of the mineralising fluids; (c) Rhynie chert and associated vein quartz, an example of Devonian hot-spring activity; and (d) fluid inclusion-bearing mantle minerals, with MORB-type RbSr and SmNd isotope systematics.

Not surprisingly, these measurements indicate that many of the features of modern fluids are seen in palaeo-fluids. An important practical problem appears to be the presence of an additional component of unfractionated atmosphere in some samples.

The extent of boron isotope fractionation between tourmaline and aqueous vapor has been determined over theP−T range 50–200 MPa and 350–750°C. The results show that the extent of fractionation is inversely dependent on temperature. An unusual feature of the reaction is that the isotope fractionation is less of extensive at higher pressures for an equivalent temperature. Due to its refractory nature the experiments involved synthesis of tourmaline from constituents rather than equilibration of the preformed mineral with an isotopically distinct aqueous phase. For this reason we cannot, as yet, be certain whether the pressure effect is the result of equilibrium or kinetic processes.

Th isotope ratios have been measured on 3 standards using a VG54-30® mass spectrometer in static mode with a Faraday cup and ion counting Daly. The use of a 30-cm energy filter improves abundance sensitivity by an order of magnitude over a conventional VG Sector54 mass spectrometer. An abundance sensittivity of1·10⁻⁷ at mass 237 with respect to mass 238 and5·10⁻⁸ at mass 230 with respect to mass 232 can be achieved. Analysis of one standard on 4 different instruments over 18 months gives an accurate result compared with the alpha-counted value with a total external precision of±0.85% (1 standard deviation). Thus the instrument is capable of routine Th isotope ratio measurements.

The Axial Seamount site is located on the Juan de Fuca Ridge near the intersection of the Cobb-Eikenberg-Brown BEar seamount chain and the axial rift of the ridge. Several large (to 12 m) silica+sulfide+sulfate hydrothermal spires reflect high-temperature (to 300°C) hydrothermal activity. Laser microprobe analyses along traverses of five temporally distinct fluid conduits within an inactive high-temperature spire reveal significant temporal variation inδ³⁴S, both within individual conduits and between conduits. The maximum intra-conduitδ³⁴S variation (on wurtzite) is 5‰ and the maximum inter-conduit variation (on sphalerite) is 7.4‰ Inter-conduitδ³⁴S variation occurs primarily due to variable amounts of seawater mixing with hydrothermal fluids within the spire; generallyδ³⁴S_mineral decreases through time due to lesser amounts of seawater that invade more mature, heavily mineralized spires. Intra-conduitδ³⁴S variation has not been documented at this scale (5‰ within 1 mm), and the potential mechanisms responsible for this variation include: (1) within-spire seawater-hydrothermal fluid mixing, which also produces the inter-conduit variations, or (2) more deep-seated convection and redox processes in the sea-floor subsurface that alter theδ³⁴S_fluid.

Mixing of hydrothermal fluid with seawater cannot produce a range ofδ³⁴S_mineral-values of this magnitude (5‰), and deeper subsurface processes are required. Such deep-seated processes may involve the opening of new fluid conduits in the subsurface, exposing fresh basalt which will increase the reduction potential of the rock system. This in turn will promote increased reduction of seawater sulfate in the hydrothermal fluid and attendant increases ofδ³⁴S_fluid. The fluids will ulti precipitate³⁴S-enriched sulfide phases, although this excursion inδ³⁴S-values is ephemeral and will last only until the fluid has exhausted the reducing potential of the new conduit. At this point, sulfate reduction will be sharply reduced andδ³⁴S-values will correspondingly decrease. This process may explain the major (to +5‰) isotope excursions that occur within individual conduits on a very small (<1m) scale.

The Corsican Batholith provides an opportunity to test the single-zircon evaporation technique developed by B. Kober for Hercynian times. In this paper we attempt to clarify the method of calculating²⁰⁷Pb^*/²⁰⁶Pb^* ages and errors. The significance of the²⁰⁷Pb^*/²⁰⁶Pb^* ages are discussed with particular reference to the determination of the crystallization age of plutons. Zircon morphology is clearly an important criterion in selecting grains for analysis. Geological relations in the example that we are using are firmly established, enabling discussion of the²⁰⁷Pb^*/²⁰⁶Pb^* model age in the light of previous age determinations using other methods (RbSr,⁴⁰Ar/³⁹Ar and conventional UPb).

We interpret the well-defined322±12-Ma²⁰⁷Pb^*/²⁰⁶Pb^* age as the time of the crystallization of the U₁, MgK rocks. This is in good agreement with RbSr ages of the younger units U_2a and U_2b (312±9and290±6Ma, respectively).

Lead model ages on single-zircon crystals can be very precise and can give geologically more meaningful ages than those given by other methods, including the bulk-fraction conventional UPb method. The problem of episodic loss of radiogenic Pb can be avoided, making the grain concordant, and inherited grains or cores are easily detected.