Chemical Geology: Isotope Geoscience section最新文献

SmNd, RbSr and Pb isotopic data have been obtained for a variety of samples from within and around the Fraser Complex, a large (> 400 × 30 km) composite body of metagabbro which is a major component of the Proterozoic Albany-Fraser Orogen.

Granites to the north and east of the complex and a granite sheet near its centre have SmNd model ages (t_DM) of ∼2000 to ∼ 2300 Ma, similar to values recorded in the western portion of the orogen. SmNd data for Fraser Complex gabbros scatter significantly on an isochron plot, but well-preserved igneous minerals extracted from one sample give 1291 ± 21 Ma with initial ϵ_Nd= -1.3 ± 0.2. RbSr for the biotite-rock pair gives a cooling age of 1268 ± 20 Ma with initial ⁸⁷Sr/⁸⁶Sr≈0.706. Slices cut from a mixed gneiss/ amphibolite zone of ductile deformation and retrogression along the western margin of the complex give a ∼ 1300-Ma Pb/Pb date for final tectonic emplacement. Intrusion, granulite-facies metamorphism and upper-crustal tectonic emplacement of the complex were probably all achieved within ≤ 30 Ma.

SmNd and RbSr characteristics of the western deformation zone suggest flushing by large volumes of fluid derived from a moderately depleted mantle source (ϵ_Nd ≈ + 2.4; ϵ_Sr, ≈ − 5 at 1300 Ma). The precursors to the Fraser Complex could have been derived from the same (or a similar) source at ∼ 1900 Ma.

The Trinity ophiolite consists of a widespread mantle unit (the Trinity Peridotite) and a thin crustal sequence ( ∼2 km) outcropping discontinuously. The well-exposed massifs of layered and isotropic gabbros, diabasic dikes and pillow lavas define a complete oceanic crustal sequence. The basalts, dikes and gabbros are characterized by retrograde metamorphic associations. Previous work has shown that the extent of alteration is directly correlated to the amount of circulating seawater and decreases downwards for increasing temperatures.

Oxygen isotopic compositions were measured on whole-rock samples and a few mineral separates across the ophiolite section. Clearly, hydrothermal alteration in the presence of seawater has introduced large variations of '80/'60 ratios with respect to those of the starting rocks whose composition is assumed to be typical of mantle-derived materials. The evolution of δ ¹⁸O-values as a function of depth is similar to the Oman example but there are some significant differences: with respect to the mantle reference composition, the upper section (pillow lavas) is '80 enriched up to 10.1‰. Low δ ¹⁸O-values down to +4‰ are recorded by some sheeted dikes, isotropic and cumulate gabbros. However, the Trinity ophiolite strongly differs from the Oman ophiolite by the presence of ¹⁸O-enriched rocks at deep levels of the ophiolite section.

Microthermometry on fluid inclusions from a few quartz-bearing samples and water/rock ratios inferred from previous Sr isotope data allow us to calculate a pattern of alteration temperatures consistent with a complex regime of water-rock interactions: low-temperature ( ∼ 110–170°C) exchange occurred pervasively through the pillow lava section but also irregularly affected deeper parts of the crust (isotropic and layered gabbros) in which a previous episode of high-temperature ( ∼ 300–390°C) interaction took place. Deep channelized penetration of seawater at low temperature could be related to the wider and deeper faulting pattern which characterizes slow-spreading centers. Such a low-temperature, off- axis interaction (pervasive in the upper section and heterogeneous at deeper levels) could destroy the δ ¹⁸) balance possibly established during the axial hydrothermal activity.

The oxygen isotope composition of hydrothermally-altered rocks in the Trinity complex is consistent with a δ ¹⁸O-value of 0 ± 2‰ for Silurian seawater.

Zeta (ζ) constants were determined using zircon, apatite and sphene age standards for a systematic calibration of fission-track dating. Identical crystals were double-checked in two facilities having significantly different degrees of neutron flux thermalization: Irradiation Pit-6 (IP-6) of the Korea Atomic Energy Research Institute (KAERI) TRIGA III reactor and the Graphite Pneumatic Tube (GrPn) of the Kyoto University Research Reactor (KUR-1) which have Cd ratios of 12.5 and ∼200, respectively, for Au. The final single weighted mean ζ baselines are given with errors (2σ) as: 324.5 ± 7.9 for SRM612, 110.9±2.7 for CN1 and 118.8±2.9 for CN2 from KAERI IP-6; and 333.7±8.5 for SRM612, 109.0±2.8 for CN1 and 119.5 ± 3.1 for CN2 from KUR GrPn. The final results from both facilities show anexcellent consistency in ζs for CN1 and CN2 and a discrepancy ( ∼2.8%) in SRM612-ζ values despite of a rough agreement within the error (2σ). Lower ratios (∼2.5–4%) of ϱ_d^CN1/ϱ_d^SRM612, ϱ_d^CN2/ϱ_d^SRM612 and ϱ_i^mineral/ϱ_d^SRM612 in KAERI IP-6 strongly imply such a decrease of ζ_SRM6l2.

The consistency in ζ_CN1 and ζ_CN2 in both facilities implies that the large difference in ²³²Th/²³⁵5U ratio between the mineral and CN1 or CN2 may not significantly affect thermal neutron dosimetry, even for apatite which has the largest difference of the ratio against dosimeter glasses. The larger (n,f ) reaction cross-section of ²³⁸U than that of ²³²2Th in epithermal and fast neutron energy regions also suggests that the difference of ²³⁸U /²³⁵U ratio as seen in glass SRM612, which has depleted U, may affect the thermal neutron dosimetry more significantly than that of ²³²Th/²³⁵U ratio regardless of Th/U ratios of the three minerals. Therefore, the discrepancy in ζ_SRM612 results mainly from the larger contribution of ²³⁸U fission induced by non-thermal neutrons to the thermal neutron dosimetry in the less thermalized KAERI IP-6. In consequence, the final ζ_SRM6l1 from KAERI IP-6 is significantly less precise. The final ζ baselines for CN1 and CN2, obtained independently from both facilities, are to be recommended for accurate fission-track analyses in preference to the SRM612-ζ baseline, even in the well-thermalized facility, e.g. KUR GrPn.

We report a nine-point RbSr whole-rock isochron age of 70±3 Ma (MSWD 3.97) for Mid-Jurassic volcanic rocks. The same rocks have also been dated by the UThPb method on zircon, giving a crystallization age of 166 ± 11 Ma, over twice as old as the RbSr age. The data demonstrate that whole-rock RbSr ages of volcanic rocks, even lava flows with SiO₂ content as low as 57 wt.%, are susceptible to complete resetting.

The rocks range in composition from rhyodacite tuffs to andesite lavas. The complete breakdown of all major minerals that contain Rb and Sr resulted in an alteration mineral assemblage consisting of phengite, albite, secondary quartz, and minor amounts of chlorite and epidote. Phengite is the K-bearing product of the breakdown of biotite and K-feldspar. Pressure during low-grade metamorphism of the volcanic rocks, estimated from phengite composition to have been in the range of 4 to 6 kbar, points to thrust-related burial as the main cause of resetting. Consequently, such reset isochrons may date large-scale events such as regional thrusting and metamorphism. The coherent resetting of the RbSr isochron suggests large-scale pervasive fluid movement during thrust-related burial metamorphism.

A method is described for measuring the ²²²Rn content of soil gas using a conventional liquid scintillation counter. Gas samples, collected in wetted ground-glass syringes, are equilibrated with a scintillation cocktail which is then expelled into a scintillation vial and counted. The method is straightforward and relatively fast (5–6 min. of operator's time per sample), and yields results having 95% confidence limits of ∼ ± 10% for samples containing ≥ 2·10⁴ Bq m⁻³ of ²²²Rn. The method is applied to a watershed near Bickford Reservoir in Massachusetts, U.S.A., where soil-gas ²²Rn content is found to be reasonably constant horizontally, but strongly and systematically increasing with depth, to > 5·10⁴ Bq m⁻³ at soil depths of ∼ 1 m.

The concentrations of Ra isotopes (²²⁴Ra, ²²³Ra, ²²⁸Ra and ²²⁶Ra) and ²²²Rn have been determined in brines from Kharaghoda, western India. The Ra isotopes concentrations in these samples are orders of magnitude higher than that reported in potable groundwaters. The ²²⁴Ra activities lie between 28 to 277 dpm per kg; about 4% to 96% of that expected from its production, calculated using ²²²Rn as a recoil flux monitor. The low [²²⁴Ra/²²²Rn] activity ratios are typical of less saline brines. Retardation factors for Ra in these brines, derived from the ²²²Rn-²²⁴Ra-²²⁸Ra system and based on a reversible exchange model, are in the range of about 0.3–114. These values are 3–4 orders of magnitude lower than that in potable groundwaters. The residence time of Ra in Kharaghoda brines is approximately one day. Our data suggest that in briny aquifers Ra is considerably less ‘particle-reactive’ than in potable groundwaters. The retardation factor for Ra shows a strong negative correlation with salinity. This anticorrelation suggests that salinity would play a crucial role in determining the mobility of Ra and its geochemical homologues through groundwater systems.

Sulfur, carbon and oxygen stable isotope analyses of sulfides, barite, quartz, magnetite and carbonate and initial ⁸⁷Sr/⁸⁶Sr ratios (I_Sr) of barite have been undertaken on representative samples from barite horizons mainly in the Foss sector of the Late Proterozoic, Middle Dalradian stratiform mineralisation, near Aberfeldy in the central Scottish Highlands. The main objective of the study was to use new isotope studies to test a SEDEX depositional model which involved mixing of reduced hydrothermal solution with contemporaneous anoxic and oxygenic seawater.

The assemblages studied were: massive pyrrhotite+pyrite +sphalerite+galena (scarce, and probably formed sub-seafloor only); barite +pyrite (common); and barite +magnetite (relatively uncommon). The sulfur isotope results (gd¹⁸SCDT) for the Foss samples, mainly from the open pit area, are: pyrrhotite and associated sulfides ( + 21 to + 24‰); pyrite (+ 27‰) with barite (+40‰); and barite (+35 to +38‰) with magnetite. Oxygen isotope analyses (δ¹⁸0_VSMOW) are: about + 14‰ for barite with pyrite and about + 16‰ for barite with magnetite. The variation in I_Sr is small; I_Sr being about 0.715 for barite with pyrite and about 0.714 for barite with magnetite. δ¹⁸O of magnetite is around +7.5‰.

The observed assemblages and the analytical results are considered to be reasonably consistent with the mixing model but with oxygenic seawater playing a very minor and localised role. The massive sulfide has inherited a δ³⁴S value of about +22‰ from hydrothermal sulfide. The end-member isotopic compositions of the sulfate resulting from mixing are likely to have been: sulfate from hydrothermal sulfide oxidised using oxygen dissolved in seawater, $\text{δ}{}^{34}\text{S}\text{=+22‰}$ and $\text{δ}{}^{18}\text{O}\text{= +25±5‰}$ and seawater sulfate, $\text{δ}{}^{34}\text{S}\text{= +40‰}$ and $\text{δ}{}^{18}\text{O}\text{= +14‰}$. A plot of δ³⁴S against δ¹⁸O for barite sulfate gives a line with a correlation coefficient of 0.766 (n=10), which is significant above the 99% level although most points cluster close to the seawater end of the expected variation. The I_Sr values of the barite suggest that the dominant source for this Sr was from the hydrothermal solution. There is no evidence for a significant contribution to sulfide by bacterial reduction of sulfide and this lack of a geochemical barrier of bacteriogenic sulfide could account for the dispersal of hydrotherma