Chemical Geology: Isotope Geoscience section最新文献

Stable and radiogenic isotopic compositions have been used to trace the origin and age of fluids associated with the formation and alteration of ore deposits such as the unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan, Canada. Since the formation of the U deposits at the unconformity between Aphebian basement rocks and the overlying Athabasca sandstones at ∼ 1200 Ma, the Athabasca Basin has expeienced episodic influxes of meteoric water that permeated large fault structures rooted in the basement graphitic metapelites and gneisses. Because these structures are associated with the initial formation of the U deposits, alteration due to late meteoric water influx is often prominant near the deposits. The development of late-stage kaolinite, lowering of δD-values in all clay minerals in fractures near the ore zones, loss of K₂O and decreasing KAr ages with increasing water contents in illite are indicative of retrograde alteration of clay minerals due to influx of late-stage meteoric water in the Athabasca Basin. Recent proposals that radiation damage and subsequent retrograde alteration of clay minerals near the U deposits have occurred are not consistent with either the data from the McArthur River area in Saskatchewan where kaolinite having low δD-values formed with small quantities of remobilized U at ∼ 400 Ma, or from data throughout the basin where clay minerals devoid of U mineralization also record this retrograde alterations. The results from McArthur River, in conjunction with data from a number of unconformity-type U deposits indicate that radiation does not promote retrograde alteration but do substantiate the simpler process of retrograde alteration of clay minerals in these deposits by influxes of late, low-temperature meteoric water along reactivated shear zones.

Stable isotope data from the Jurassic Qaqarssuk Carbonatite Complex show that carbonatites intruded during the main intrusive event have δ¹⁸O-values ranging from +7 to +8.1‰ SMOW and δ¹³C-values ranging from −3.5 to −3.1‰ PDB.

Late-stage søvite veins are enriched in light carbon relative to the main-stage carbonatites, with δ¹³C-values ranging from −5 to −4‰ PDB. This is interpreted as loss of heavy carbon to a gas phase.

Late-stage REE-carbonatites have δ³C-values in the same range as the main-stage carbonatites, but elevated δ³C-values relative to late-stage søvites. The REE-carbonatites have elevated δ¹⁸O-values (+7.4 to +9.2‰ SMOW) relative to both main-stage carbonatites and late-stage søvite.

Carbonates in metasomatically altered basement and contaminated carbonatite have elevated δ¹⁸O-values (+8.2 to +8.8‰ SMOW), probably caused by exchange of oxygen with the basement.

Oxygen isotope geothermometry give temperatures in the range 313–608°C, which is low relative to the expected igneous temperatures. These low temperatures are explained as caused by subsolidus reactions such as exsolution and recrystallization which can be observed in the carbonates. There is poor correspondence between oxygen and carbon isotope geothermometry as well as with solvus geothermometry, indicating that the calibration of the isotope geothermometer established in metamorphic carbonate rocks cannot be applied directly to carbonatites.

Seawater was evaporated, stepwise isothermally at 23.5°C, for 73 days, up to a degree of evaporation of 138 × by H₂O weight. At various stages of evaporation the precipitate was totally removed from the brine and the latter was allowed to evaporate further. The sulfur isotopic compositions of the precipitates and related brines show the following characteristics: The initial δ²⁴S of the original seawater is +20‰. The δ³⁴S of both precipitates and associated brines decrease gradually in the gypsum field up to the end of the halite field, where δ³⁴S_precipitate = + 19.09‰andδ³⁴S_brine = + 18.40‰. The precipitates are always enriched in ³⁴S relative to the associated brines in these fields, but the enrichment becomes smaller towards the end of the halite field. A crossover. where the δ³⁴S of the brines becomes higher than those of the precipitates, occurs at the beginning of the Mg-sulfate field. The δ³⁴S_precipitate increases from + 19.09‰ at the end of the halite field through +19.35‰ in the Mg-sulfate field to + 19.85‰ in the K-Mg-sulfate field, whereas the δ³⁴S_brine increased from +18.40‰, through +20.91‰ to +20.94‰, respectively. This evolution implies different values of fractionation factors (α) for the minerals precipitated at the late halite, Mg-sulfate and K-Mg-sulfate fields, other than that for gypsum (1.00165). The value of α_{precipitate-residual brine} would then be very slightly >1 in the late halite field and <1 in the two later fields.

The experimental pattern of evolution of the δ³⁴S-values of the precipitates is in good agreement with data for natural anhydrites interbedded in halites, where δ³⁴S-values are lower relative to basal gypsum (and secondary anhydrite), and of primary minerals of the Mg- and K-Mg-sulfate facies, reported in evaporitic sequences, such as those of the Delaware (U.S.A.) and of the Zechstein (Germany) basins. Thus, these results shed new light on observations of natural evaporitic sequences and suggest that the compositional trend of δ³⁴S under the present experimental conditions may simulate and explain natural evaporitic processes.

Noble gases in cherts of various origins were measured by means of the stepwise heating technique. The elemental abundance patterns of noble gases in the samples showed monotonic fractionation from ²⁰Ne to ¹³²Xe relative to the atmosphere; this pattern is designated as type-1 and is similar to that observed in water, natural gases and sedimentary rocks. The noble gas concentrations in cherts were low compared with those in other sedimentary rocks and related samples, and showed scarcely apparent correlations with chemical components. Crystallization from amorphous silica to quartz was responsible for the degassing loss of noble gases in cherts.

The probability distribution of the mean squared weighted deviation (MSWD) is derived and its dependence on degrees of freedom f is shown. The expectation (or mean) value of MSWD=1 and is not a function of f. However, the +1σ range of the expectation value of the MSWD decreases with increasing f. The standard deviation of the MSWD is σ = ±(2/f)^1/2. If MSWD 1+2(2/f)^1/2, there is only <5% probability that the data define an isochron. Use of MSWD as a criterion for accepting or rejecting the assumption of an isochron may be applied only if analytical errors σ_xi and σ_yi are well known.

107 rock samples from the Paraná plateau, southern Brazil (86 tholeiites and 21 various rock-types from the Precambrian crystalline basement) were measured for their oxygen isotopic composition. Both the crustally uncontaminated tholeiites from the Northern Paraná Province and the crustally contaminated basalts from the Southern Paraná Province exhibit quite positive δ¹⁸O-values ranging from +6.0 to +11.5% and from +7.0 to +12.8%, respectively. These values are considerably higher than those expected for rocks deriving from mantle partial fusion (+5.7±0.3%). A simple process of crustal contamination and fractional crystallization cannot explain the results obtained as the oxygen isotopic composition of the samples from the crystalline basement ( +4.8 to +12.7%, mean value +8.8%) cannot explain a δ¹⁸O increase of the basalt magma higher than ∼ 2%o. According to various considerations, we conclude that the positive oxygen isotope values exhibited by the Paraná basalts are the result of secondary post-eruptive hydrothermal exchange processes between rock and ¹⁸O-enriched water. The ¹⁸O-enriched water was substantially formed by isotopic exchange between normal (meteoric) groundwater and either the thick sedimentary sequence underlying the volcanic sequence in the Paraná basin or the crystalline basement rocks.

This paper presents and discusses volcano-sedimentary biotite radioisotopic ages, obtained from the Paleogene sequence of the northeastern Apennines using RbSr and KAr dating methods. In particular, we discuss criteria which enable us to select the most reliable ages. K contents of < 6.4%, or traces of vermiculite indicate dubious reliability. Ion microprobe analyses of individual biotite flakes from each dated sample allowed us to determine whether these volcano-sedimentary biotites are composed of one or more populations of minerals. Chemically heterogeneous samples suspected of yielding spurious radioisotopic ages were tested for geochronological homogeneity using ⁴⁰Ar³⁹Ar laser fusion probe dating on several groups of grains. ⁴⁰Ar³⁹Ar step-heating dating or biotite from two stratigraphic levels from one locality yielded age spectra with good plateaux. Two other samples representing the same stratigraphic levels in another locality yielded somewhat disturbed spectra. We selected K-rich biotites showing good geochemical homogeneity, and/or displaying a wide plateau. and/or giving reasonably consistent KAr and RbSr ages from at least five stratigraphically distinct layers; they draw a picture consistent with the sequence. The selected radioisotopic ages permit accurate calibration of well-known bio-, magneto- and chemostratigraphic events from 36.4 to 28.1 Ma including a tightly interpolated age value of 33.7 ± 0.4 Ma (2σ) for the much debated Eocene-Oligocene boundary.

We report a statistically significant negative correlation between the concentration (and the deposition rate) of the cosmogenic radionuclide ¹⁰Be and δ¹⁸O (of Globigerinoides ruber) in the sediments deposited in the anoxic Orca Basin (OB) of the Gulf of Mexico during the past ∼20 ka. Possible causes for this finding consecutively include: (1) enrichment of ¹⁰Be in the soils/easily erodable crustal material which were discharged into the OB during the Late Wisconsin Interglacial; (2) introduction of significant amounts of ¹⁰Be into the oceans due to the melting of continental ice sheets resulting in the enrichment of the radionuclide in the sediments deposited during that time; and (3) generation of a focussing effect resulting in the ¹⁰Be enrichment of the OB sediment.

After 10 days, 3-μmol CO₂ samples in containers having glass stopcocks with Viton^® or FETFE O-rings were enriched in ¹⁸O by 1.5% as a result of absorption by the elastomer; this amount of enrichment is ∼20 times greater than the precision of δ¹⁸O measurements of most laboratories. No change in ¹³C content was observed. Increasing the sample size to 100 μmol resulted in an ¹⁸O enrichment of 0.2% and did not affect the ¹³C content.

Caution needs to be exercised in selecting sample containers for CO₂ isotope-ratio samples of < 200 μmol. If stopcocks are used in construction of containers for such samples, the use of all-glass stopcocks with Apiezon N^® hydrocarbon-based grease will eliminate the fractionation of oxygen isotopes.

Thermal neutron fluence measurements have been performed with Au and Co standard monitors at the one side and Cu monitors (foils and wires) at the other. using three reactor channels with different neutron energy spectra and two calibrated Ge detectors. Care was taken to eliminate errors caused by epithermal activation and neutron self-shielding while special attention was also paid to the problem of incomplete annihilation of the ⁶⁴Cu β⁺ radiation. Our new measurements reveal consistent fluences between Cu and the Au and Co standard monitors, when using the recommended and/or recently evaluated nuclear parameters for the ⁶³Cu(n,γ)⁶⁴Cu reaction. However, we do not endorse the usage of Cu as a standard fluence monitor for fission-track dating because from a metrological standpoint too large uncertainties still exist on some of its relevant nuclear parameters such as the thermal neutron activation cross-section (σ₀) of ⁶³Cu.