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The evolution of illite to muscovite: mineralogical and isotopic data from the Glarus Alps,Switzerland 总被引：7，自引：0，他引：7
J. C. Hunziker M. Frey N. Clauer R. D. Dallmeyer H. Friedrichsen W. Flehmig K. Hochstrasser P. Roggwiler H. Schwander 《Contributions to Mineralogy and Petrology》1986,92(2):157-180
Thirty-five illite and muscovite concentrates were extracted from Triassic and Permian claystones, shales, slates and phyllites along a cross-section from the diagenetic Alpine foreland (Tabular Jura and borehole samples beneath the Molasse Basin) to the anchi- and epimetamorphic Helvetic Zone of the Central Alps. Concentrates and thin sections were investigated by microscopic, X-ray, infrared, Mössbauer, thermal (DTA and TG), wet chemical, electron microprobe, K-Ar, Rb-Sr, 40Ar/39Ar and stable isotope methods.With increasing metamorphic grade based on illite crystallinity data (XRD and IR) the following continuous changes are observed: (i) the 1Md2M1 polymorph transformation is completed in the higher grade anchizone; (ii) K2O increases from 6–8 wt. % (diagenetic zone) to 8.5–10% (anchizone) to 10–11.5% (epizone), reflecting an increase in the total negative layer charge from 1.2 to 2.0; (iii) a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone/ epizone boundary; iv) a shift of an endothermic peak in differential thermal curves from 500 to 750° C; (v) K-Ar and Rb-Sr apparent ages of the fraction <2 m decrease from the diagenetic zone to the epizone, K-Ar ages being generally lower than Rb-Sr ages. The critical temperature for total Ar resetting is estimated to be 260±30° C. K-Ar and Rb-Sr ages become concordant when the anchizone/ epizone boundary is approached. The stable isotope data, on the other hand, show no change with metamorphic grade but are dependent on stratigraphic age.These results suggest that the prograde evolution from 1 Md illite to 2M1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals, however this is not a recrystallization process. This restructuration is completed approximately at the anchizone/epizone boundary. The isotopic data indicate significant diffusive loss of 40Ar and 87Sr prior to any observable lattice reorganization. The restructuration progressively introduces a consistent repartition of Ar and K in the mineral lattices and is outlined by the 40Ar/39Ar age spectra.Concordant K-Ar and Rb-Sr ages of around 35-30 Ma. with concomitant concordant 40Ar/39Ar release spectra are representative for the main phase of Alpine metamorphism (Calanda phase) in the Glarus Alps. A second age group between 25 and 20 Ma. can probably be attributed to movements along the Glarus thrust (Ruchi phase), while values down to 9 Ma., in regions with higher metamorphic conditions, suggest thermal conditions persisting at least until the middle Tortonian. 相似文献
3.
We used the K–Ar ages of clay-sized mineral grains to investigate the timing of activity on the Minami-Awa Fault, which is a fossil seismogenic fault along a subducting plate interface separating the coherent strata of the Shimanto accretionary complex to the north from the tectonic mélange to the south. The K–Ar ages from the matrix shale of the mélange range from 85 to 48 Ma and decrease with decreasing amount of detrital mica, indicating that they record a mixture of authigenic illite related to burial diagenesis and detrital mica. In contrast, the K–Ar ages of an ultracataclasite within the fault core are significantly younger, ranging from 29 to 23 Ma, and are unrelated to grain size and amount of detrital mica. This indicates that s Ar diffused completely from the ultracataclasite between 29 and 23 Ma, which postdates the formation of authigenic illite by at least several million years. The diffusion of 40Ar in the ultracataclasite was probably caused by frictional heating or high-temperature fluid migration that occurred when the fault was reactivated. The results indicate that seismogenic faults that separate tectonic mélange from coherent strata in accretionary complexes may slip, not only during accretion, but also long after accretion. 相似文献
4.
40Ar/39Ar step-heating data were collected from micron to submicron grain-sizes of correlative illite- and muscovite-rich Cambrian pelitic rocks from the western United States that range in metamorphic grade from the shallow diagenetic zone (zeolite facies) to the epizone (greenschist facies). With increasing metamorphic grade, maximum ages from 40Ar/39Ar release spectra decrease, as do total gas ages and retention ages. Previous studies have explained similar results as arising dominantly or entirely from the dissolution of detrital muscovite and precipitation/recrystallization of neo-formed illite. While recognizing the importance of these processes in evaluating our results, we suggest that the inverse correlation between apparent age and metamorphic grade is controlled, primarily, by thermally activated volume diffusion, analogous to the decrease in apparent ages with depth observed for many thermochronometers in borehole experiments. Our results suggest that complete resetting of the illite/muscovite Ar thermochronometer occurs between the high anchizone and epizone, or at roughly 300 °C. This empirical result is in agreement with previous calculations based on muscovite diffusion parameters, which indicate that muscovite grains with radii of 0.05–2 μm should have closure temperatures between 250 and 350 °C. At high anchizone conditions, we observe a reversal in the age/grain-size relationship (the finest grain-size produces the oldest apparent age), which may mark the stage in prograde subgreenschist facies metamorphism of pelitic rocks at which neo-formed illite/muscovite crystallites typically surpass the size of detrital muscovite grains. It is also approximately the stage at which neo-formed illite/muscovite crystallites develop sufficient Ar retentivity to produce geologically meaningful 40Ar/39Ar ages. Results from our sampling transect of Cambrian strata establish a framework for interpreting illite/muscovite 40Ar/39Ar age spectra at different stages of low-grade metamorphism and also illuminate the transformation of illite to muscovite. At Frenchman Mtn., NV, where the Cambrian Bright Angel Formation is at zeolite facies conditions, illite/muscovite 40Ar/39Ar data suggest a detrital muscovite component with an apparent age ≥967 Ma. The correlative Carrara Fm. is at anchizone conditions in the Panamint and Resting Spring Ranges of eastern California, and in these locations, illite/muscovite 40Ar/39Ar data suggest an early Permian episode of subgreenschist facies metamorphism. The same type of data from equivalent strata at epizone conditions (greenschist facies) in the footwall of the Bullfrog/Fluorspar Canyon detachment in southern Nevada reveals a period of slow-to-moderate Late Cretaceous cooling. 相似文献
5.
LIU zaisheng Shi Hesheng Zhu Junzhang Qiu Huaning ZHANG Zhilin YUN Jianbing 《《地质学报》英文版》2012,86(2):383-392
The South China Sea began to outspread in the Oligocene. A great quantity of terraneous detritus was deposited in the northern continental shelf of the sea, mostly in Pearl River Mouth Basin, which constituted the main paleo-Pearl River Delta. The delta developed for a long geological time and formed a superimposed area. Almost all the oil and gas fields of detrital rock reservoir distribute in this delta. Thirty-three oil sandstone core samples in the Zhujiang Formation, lower Miocene (23–16 Ma), were collected from nine wells. The illite samples with detrital K feldspar (Kfs) separated from these sandstone cores in four sub-structural belts were analysed by the high-precision 40Ar/39Ar laser stepwise heating technique. All 33 illite 40Ar/39Ar data consistently yielded gradually rising age spectra at the low-temperature steps until reaching age plateaus at mid-high temperature steps. The youngest ages corresponding to the beginning steps were interpreted as the hydrocarbon accumulation ages and the plateau ages in mid-high temperature steps as the contributions of the detrital feldspar representing the ages of the granitic parent rocks in the provenances. The ages of the detrital feldspar from the Zhujiang Formation in the four sub-structural belts were different: (1) the late Cretaceous ages in the Lufeng 13 fault structural belt; (2) the late Cretaceous and early Cretaceous-Jurassic ages in the Huizhou 21 buried hill-fault belt; (3) the Jurassic and Triassic ages in the Xijiang 24 buried hill-fault belt; and (4) the early Cretaceous – late Jurassic ages in the Panyu 4 oil area. These detrital feldspar 40Ar/39Ar ages become younger and younger from west to east, corresponding to the age distribution of the granites in the adjacent Guangdong Province, Southern China. 相似文献
6.
The clay assemblages of the Late Jurassic-Early Cretaceous sediments of the Agadir Basin are affected by burial diagenesis. Apparent ages of these formations, from the K-Ar dating of fine grain size fractions (< 2 μm), decrease downward. This indicates an opening of the K-Ar isotopic system of detrital Hercynian minerals during burial. The extent of the isotopic rehomogenisation is more important in sandstones than in clayey marl facies. During burial diagenesis, a temperature of 165°C in clayey marl and 110°C in sandstone can be enough for isotopic rehomogenisation of fine-grained Hercynian illite. The migration of pore water seems to favour the diffusion of radiogenic 40Ar in sandstones. 相似文献
7.
40Ar/39Ar incremental-release analyses were carried out on whole-rock and constituent white mica (illite)-rich size fractions (0.63–1 to 6.3–20 m) within two very-low grade, penetratively cleaved metatuffs of contrasting anchizonal metamorphic grade (northeastern Rheinisches Schiefergebirge, Federal Republic of Germany). One sample from the upper anchizone displays internally concordant 40Ar/39Ar spectra with plateau ages ranging between ca. 316 and 325 Ma. These are similar to conventional K-Ar ages determined for the whole-rock and size fractions. Together the isotopic results suggest that cleavage formed at ca. 320 Ma during a concomitant very-low grade metamorphism. This is consistent with biostratigraphic controls which suggest that metamorphism and cleavage formation occurred during the Westphalian.A metatuff sample from the middle anchizone records more internally discordant 40Ar/39Ar age spectra with total-gas ages ranging from 366 to 372 Ma. These are ca. 35–45 Ma older than corresponding conventional K-Ar ages, indicating marked recoil-loss of 39Ar occurred during irradiation. Transmission electron microscopy reveals that white mica grains within size fractions from the upper anchizone sample have clearly defined, straight edges whereas those within the middle anchizone samples are embayed and diffuse. This results in an increase in surface/volume ratio and therefore greater susceptibility for recoil-loss of 39Ar in the middle anchizone sample. Grain-edge morphology appears to be a major factor in determining the extent of recoil-loss of 39Ar during 40Ar/39Ar analysis of fine-grained size fractions. 相似文献
8.
K.A. Foland 《Geochimica et cosmochimica acta》1979,43(6):793-801
Excess 40Ar in biotite from some relatively anhydrous charnockitic rocks in the Appalachian Piedmont indicates limited mobility of argon. Biotite from the Arden pluton of the granulite-facies Wilmington Complex apparently formed as a retrograde product at the expense of pyroxene and K-feldspar. Rb-Sr ages of biotite from all rocks are approximately 365 Myr. The same micas have apparent K-Ar ages which range from about 365–590 Myr, six of which clearly exceed the Sr isotope whole-rock date of 500 Myr. They contain variable amounts of excess40Ar incorporated during crystallization or recrystallization of biotite at about 365 Myr ago. None of the other minerals appears to contain significant amounts of excess argon. The K-Ar apparent ages show strong, positive correlation with whole-rock K concentrations. These relations yield a correlation between excess argon in the biotite phase and rock potassium. This suggests that excess 40Ar in biotite is of local derivation and is due to an imprint of the local argon activity. If the amount incorporated is roughly proportional to the prevailing argon partial pressure then substantial differences in pAr existed. Argon did not have a uniform chemical potential over large rock volumes. Analysis of closely spaced samples suggests different argon activity over the scale of less than 10m. This implies restricted transport of Ar and is probably due to very low effective permeability of the anhydrous assemblages. 相似文献
9.
Microtextures of authigenic Or-rich feldspar in the Upper Jurassic Humber Group, UK North Sea 总被引：4，自引：0，他引：4
ABSTRACT Detrital alkali feldspars currently at burial depths of 3·2–3·5 km in the Upper Jurassic Humber Group of the Fulmar oilfield, UK North Sea, are overgrown and have been partially replaced by authigenic Or‐rich feldspar. Intracrystal microtextures suggest several different provenances for the detrital grains. The overgrowths are uniformly non‐cathodoluminescent and have occasional celsian‐rich zones. Transmission electron microscopy shows that they are composed of a microporous mosaic of subµm‐ to µm‐sized sub‐grains associated with barite, illite and pyrite. The subgrains are somewhat rounded but have an approximate {110} Adularia habit and display a faint modulated microtexture on the nanometre scale. They have triclinic symmetry, but the lattice angles depart only slightly from monoclinic symmetry. These features are characteristic of K‐feldspar precipitated relatively rapidly and at low temperature. Authigenic Or‐rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains, often at a suboptical scale. Although, like diagenetic albitization, replacement by K‐feldspar is probably a very common diagenetic reaction, it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers. The permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for K/Ar and 40Ar/39Ar dating of diagenesis, and the existence of suboptical, replacive authigenic K‐feldspar within detrital grains may significantly modify the apparent Ar ages of detrital grains. Similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated. 相似文献
10.
22 hornblende K-Ar ages and 10 39Ar-40Ar spectra were obtained for hornblende garbenschists from the Western Tauern Window. The post-kinematic amphiboles were produced during the late Alpine prograde metamorphism (6–10 kb and 500–570° C). Two nearly potassiumfree cummingtonites rimming hornblende yield K-Ar ages of 120 Ma, while the 20 tschermakitic hornblendes scatter between 17 and 37 Ma. The reason for this scatter is excess Ar, possibly incorporated into amphiboles during healing of fractures, now traceable by trails of fluid inclusions. Excess Ar is semiquantitatively corrected for by combining cogenetic hornblende and cummingtonite with K-Ar isochrons. It can be quantified in 4 out of 10 hornblendes by 39Ar-40Ar stepwise heating experiments. Ages of 18–20 Ma result for corrected hornblendes. The retentivity of 40Ar, after correction for excess, shows no correlation with chemistry within the narrow compositional range observed; rather, it shows intriguing correlations with irregularities in Ca/K spectra, pointing to a microstructurally controlled mechanism for Ar loss. This observation leads to a critical evaluation of the closure temperature constant, which apparently depends on an incompletely known number of mineralogical and environmental parameters. In particular those 39Ar-40Ar release spectra which yield low temperature steps with younger ages than the plateaus are not interpretable in terms of a synchronous closure. This gives evidence that loss of radiogenic isotopes proceeds by a more complex mechanism than simple volume diffusion through isotropic media. 相似文献
11.
Six samples of a single carbonate-rich unit of the Swiss Préalpes, progressively metamorphosed from diagenesis to deep anchizone, yield 40Ar/39Ar spectra with variably developed staircase patterns, consistent with mixtures of detrital mica and neocrystallized mixed-layer illite/smectite. The lowest temperature heating steps for different size fractions (2–6?μm and 6–20?μm) converge to ～40?Ma providing an imprecise, maximum age of regional metamorphism. A method is described for distinguishing and quantifying the amount of pre-existing detrital mica versus neoformed illite layer in the illite/smectite formed during Tertiary Alpine metamorphism by comparison of X-ray diffraction patterns with Newmod© simulations. In the least metamorphosed samples the illite/smectite contains ～65% neoformed illite, and this illite accounts for approximately 17% of all dioctahedral phyllosilicate minerals in the rock (e.g., detrital mica and illite/smectite). In contrast, the illite/smectite from the more strongly metamorphosed samples contains >97% neoformed illite, which accounts for ～70% to >90% of all dioctahedral phyllosilicate minerals. Phyllosilicate morphologies viewed by scanning electron microscopy are consistent with these estimates. A process of dissolution/reprecipitation is inferred as a mechanism for the growth of the neoformed phyllosilicates. A plot of neoformed illite content versus 40Ar/39Ar total fusion age yields a near-linear curve with an extrapolated age of 27?Ma for 100% neoformed dioctahedral phyllosilicates. This age is interpreted as the time of incipient metamorphism and is consistent with independent biostratigraphic constraints. Model 40Ar/39Ar age spectra constructed with the XRD simulation results correspond well to the experimental data and illustrate the changes in degassing properties of progressively metamorphosed mixtures of detrital mica and neoformed illite. 相似文献
12.
Biotite is widely used for Rb-Sr and K-Ar isotopic dating and influences Sr isotope geochemistry of hydrological regimes. The isotopic system of biotite behaves diversely in response to surface weathering; i.e. the complete preservation of original Rb-Sr and K-Ar isotopic ages or dramatic reduction. In this study, we have explored the relation between the behavior of isotopic systems and complex weathering processes of biotites in the weathering profiles distributed on the Mesozoic granitoids in South Korea. In the lower parts of the profiles, biotite in the early stages of weathering was transformed into either oxidized biotite or hydrobiotite, with a mass release of 87Sr and 40Ar forced by the rapid oxidation of ferrous iron. During the transformation to oxidized biotite, 87Sr and 40Ar were preferentially released relative to Rb and K, respectively, via solid-state diffusion through the biotite lattice, resulting in a drastic reduction of original isotopic age. The reduction of Rb-Sr age was greater than that of K-Ar age because K was preferentially released over Rb whereas 87Sr and 40Ar were released proportionally to each other. However, during the transformation of biotite to hydrobiotite (i.e., to regularly interstratified biotite-vermiculite), 87Sr, Rb, 40Ar, and K were completely retained in the alternating biotite interlayer, and thus the original isotopic age can be preserved. In the upper parts of the profiles, where iron oxidation was almost completed, 87Sr, Rb, 40Ar, and K were gradually and proportionally released, with no further significant change in isotopic age during the gradual transformation of the early-formed oxidized biotite into hydrobiotite and vermiculite or during their final decomposition to kaolinite. The ratios and amounts of isotopes released from weathered biotites are dependent upon the degree of iron oxidation and the pathways of mineralogical transformation. Regional and local variations in isotopic systems affected by particular weathering processes should be considered when dating biotite or biotite-bearing rocks in weathering environments, modeling the transfer of Sr isotopes to hydrologic regimes, and tracking the provenance of sediments. 相似文献
13.
Four slate samples from subduction complex rocks exposed on the south coast of New South Wales, south of Batemans Bay, were analysed by K–Ar and 40Ar/39Ar step‐heating methods. One sample contains relatively abundant detrital muscovite flakes that are locally oblique to the regional cleavage in the rock, whereas the remaining samples appear to contain sparse detrital muscovite. Separates of detrital muscovite yielded plateau ages of 505 ± 3 Ma and 513 ± 3 Ma indicating that inheritance has not been eliminated by metamorphism and recrystallisation. Step‐heating analyses of whole‐rock chips from all four slate samples produced discordant apparent age spectra with ‘saddle shapes’ following young apparent ages at the lowest temperature increments. Elevated apparent ages associated with the highest temperature steps are attributed to the presence of variable quantities of detrital muscovite (<1–5%). Two whole‐rock slate samples yielded similar 40Ar/39Ar integrated ages of ca 455 Ma, which are some 15–30 million years older than K–Ar ages for the same samples. These discrepancies suggest that the slates have also been affected by recoil loss/redistribution of 39Ar, leading to anomalously old 40Ar/39Ar ages. Two other samples, from slaty tectonic mélange and intensely cleaved slate, yielded average 40Ar/39Ar integrated ages of ca 424 Ma, which are closer to associated mean K–Ar ages of 423 ± 4 Ma and 409 ± 16 Ma, respectively. Taking into account the potential influences of recoil loss/redistribution of 39Ar and inheritance, the results from the latter samples suggest a maximum age of ca 440 Ma for deformation/metamorphism. The current results indicate that recoil and inheritance problems may also have affected whole‐rock 40Ar/39Ar data reported from other regions of the Lachlan Fold Belt. Therefore, until these effects are adequately quantified, models for the evolution of the Lachlan Fold Belt, that are based on such whole‐rock 40Ar/39Ar data, should be treated with caution. 相似文献
14.
Minoru Ozima Kazuo Saito Masahiko Honda Sigeo Aramaki 《Geochimica et cosmochimica acta》1977,41(4):453-461
K-Ar dating and 40Ar-39Ar step heating experiments have been done for successive zones from the surface to the interior in two, altered dredged submarine basalts (16-2-6 and Aries V-23). The K contents show a systematic decrease (surface to the interior), whereas the K-Ar ages show a similar decrease in one sample (16-2-6) and almost no change in the other (Aries V-23).A simple diffusion model suggests that the K content decreases systematically from the surface to the interior, whereas the K-Ar age decreases or remains almost constant, depending on whether the times of solidification and of commencement of K-diffusion are close to each other or differ significantly. Comparison of the observed K content and K-Ar age variations within the rocks with the theoretical model then suggests that the solidification age of the sample Aries V-23 is much older than the commencement of the potassium-diffusion, the latter age perhaps being represented by the Eocene planktonic foraminifera on this sample. The 40Ar-39Ar isochron age obtained for the freshest specimen of Aries V-23 is 86.6 ± 3.7 m.y., supporting the above conclusion.The mathematical simulation indicates that an apparent concordance of the K-Ar age observed in some submarine basalts may be an artifact, only reflecting the significant gap between the solidification age and the time of the commencement of the K-diffusion in the rocks. Microprobe examination reveals that the potassium enrichment in the outer margin, is mainly due to K feldspar, which is an alteration product of plagioclase. 相似文献
15.
Late Pennsylvanian sedimentary rocks in the Narragansett basin were metamorphosed (lower anchizone to sillimanite grade) during late Paleozoic regional metamorphism at ca. 275–280 Ma. Twenty-five variably sized concentrates of detrital muscovite were prepared from samples collected within contrasting low-grade areas (diagenesis — lower greenschist facies). Microprobe analyses suggest that the constituent detrital grains are not chemically internally zoned; however, some grains within several concentrates display very narrow (<25 m), compositionally distinct, low-grade, epitaxial peripheral overgrowths. Detrital muscovite concentrates from the lower anchizone are characterized by internally concordant 40Ar/39Ar age spectra which define plateau ages of ca. 350–360 Ma. These are interpreted to date post-Devonian (Acadian) cooling within proximal source areas. Concentrates from lower grade sectors of the middle anchizone display slightly discordant spectra in which apparent ages systematically increase from ca. 250–275 Ma to define intermediate- and high-temperature plateaus of ca. 360–400 Ma. Detrital muscovite within samples from higher grade sectors of the middle anchizone and the upper anchizone are characterized by systematic low age discordance throughout both low-and intermediate-temperature increments. High-temperature ages only range up to ca. 330 Ma. Six size fractions of detrital muscovite from a sample collected within the lower greenschist facies have similarly discordant spectra, in which, apparent ages increase slightly throughout the analyses from ca. 250 Ma to 275 Ma. The detrital muscovite results are interpreted to reflect variable affects of late Paleozoic regional metamorphism. However, it is uncertain to what extent the systematic low age spectra discordance reflects intracrystalline gradients in the concentration of 40Ar and/or experimental evolution of gas from relatively non-retentive epitaxial overgrowths. However, low age discordance occurs regardless of the extent of epitaxial overgrowth. Intermediate-temperature increments evolved during 40Ar/39Ar whole-rock analyses of five slate/phyllite samples are characterized by internally consistent apparent K/Ca ratios. These are attributed to gas evolved from constituent, very fine-grained white mica. Samples from lower grade portions of the middle anchizone are characterized by intermediate-temperature apparent ages which systematically increase from ca. 275–300 Ma to ca. 360–375 Ma before evolution of a high-temperature contribution from detrital plagioclase feldspar. This age variation may reflect partial late Paleozoic rejuvenation of very fine-grained detrital material with a source age similar to that for the detrital muscovites. Slate/phyllite samples from upper sectors of the middle anchizone and from the upper anchizone were completely rejuvenated during late Paleozoic metamorphism and record intermediate-and high-temperature plateau ages of ca. 270–290 Ma. These data document that metamorphic conditions of the lower to middle biotite zone (ca. 325–350 °C) are required to completely rejuvenate intracrystalline argon systems of detrital muscovite. Therefore, the 40Ar/39Ar dating method may be useful in determination of detrital muscovite provenance and in resolution of the metamorphic evolution of low-grade terranes. 相似文献
16.
The Silurian bituminous sandstones(SBS) in the Tarim Basin, China are important basinwide reservoirs with an estimated area of approximately 249000 km2. We investigated the ages of authigenic illites in the SBS reservoirs and constrained their formation timing by using the ~(40)Ar/~(39)Ar step wise heating method. The age spectra, ~(39)Ar recoil loss and their controlling factors were investigated systematically. The ~(40)Ar/~(39)Ar ages were compared with the conventional K/Ar ages of identical clay fractions. The clay in the SBS reservoirs is dominated by orderly mixed-layer illite/smectite(I/S) with 5%–30% smectite layers. The I/S minerals morphology comprises primarily honeycomb, short filamentous and curved-lath particles, characteristic of authigenic illites. The unencapsulated ~(40)Ar/~(39)Ar total gas ages(UTGA) of the authigenic illites range from 188.56 ± 6.20 Ma to 491.86 ± 27.68 Ma, which are 7% to 103% older than the corresponding K/Ar ages of 124.87 ± 1.11 Ma to 383.45 ± 2.80 Ma, respectively. The K-Ar ages indicate multistage accumulations with distinct distribution patterns in the Tarim Basin: older(late Caledonian-early Hercynian) around the basin margin, younger(late Hercynian) in the basin centre, and the youngest(middle to late Yanshanian) in the Ha-6 well-block, central area of the North Uplift. The age difference is believed to have been caused by the ~(39)Ar recoil loss during the irradiation process. Compared with the K/Ar ages, the estimated ~(39)Ar recoil losses in this study are in the range from 7% to 51%. The ~(39)Ar recoil loss appears to increase not only with the decreasing particle sizes of the I/S, but also with increasing percentage of smectite layers(IR) of the I/S, and smectite layer content(SLC) of the samples. We conclude that due to significant ~(39)Ar recoil losses, UTGA may not offer any meaningful geological ages of the authigenic illite formation in the SBS and thus can not be used to represent the hydrocarbon charge timing. ~(39)Ar recoil losses during ~(40)Ar/~(39)Ar dating can not be neglected when dating fine authigenic illite, especially when the ordered mixed-layer I/S containing small amount of smectite layers(IR30%) in the reservoir formations. Compared with the unencapsulated Ar-Ar method, the conventional K-Ar method is less complicated, more accurate and reliable in dating authigenic illites in petroleum reservoirs. 相似文献
17.
雷州半岛第四纪火山岩激光40Ar/39Ar等时线定年研究 总被引：2，自引：1，他引：1
雷州半岛是我国新生代火山岩最重要的分布地区之一,火山活动主要集中在中晚更新世。前人对雷州火山岩的年代学研究以K-Ar法为主。研究表明,雷州火山岩测年结果大致分布在0.38～3.04Ma范围内。根据地层和火山岩层的叠置关系,雷州第四纪火山岩由于覆盖在被确定是1.87Ma和0.76Ma沉积的地层之上,故火山岩年龄应小于该地层年龄。K-Ar法定年结果与雷州地区地层叠置关系存在矛盾。本文通过对雷州半岛第四纪火山岩进行野外考察及采样,利用激光40Ar/39Ar年代学方法进行了精细定年。结果表明,雷州火山岩的喷发主要集中18万年前后。定年结果还表明,对于年轻样品,基于尼尔值计算的K-Ar年龄及40Ar/39Ar表观年龄偏老,等时线年龄相对较为可靠。对同一样品的斑晶、基质作斑晶-基质等时线计算,只有在斑晶基质满足同源条件时才有意义。本文首次提出,通过对比未照射样品的初始36Ar/38Ar值的均一性,以检验样品是否同源,确认斑晶-基质等时线年龄的可信度。据此,等时线的处理方法可以推广应用于特定区域内全部同源同时样品。 相似文献
18.
Five detrital white mica concentrates from very low-grade, metaclastic sequences within pre-Variscan basement and post-Variscan cover units of the Upper Austroalpine Nappe Complex (Eastern Alps) have been dated with 40Ar/39Ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas. Two samples from early Palaeozoic sandstone exposed within the same Alpine nappe record slightly discordant age spectra. The maximum age recorded in one is 562.2±0.7?Ma, whereas the other yielded a 40Ar/39Ar plateau age of 607.3±0.3?Ma. These results indicate a source area affected by Cadomian tectonothermal activity. Three detrital muscovite concentrates from post-Variscan, Late Carboniferous and Permian cover sequences exposed within three different Alpine nappes yielded 40Ar/39Ar plateau ages of 359.6?±?1.1?Ma, 310.5±1.2?Ma, and 303.3±0.2?Ma. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carboniferous molasse-type sequence and from a Permian Verrucano-type sequence record ages which indicate “late” Variscan (e.g. 330–300?Ma) metamorphic sources. By contrast, detrital white mica from another Permian Verrucano-type sequence suggests a source area affected by “early” Variscan (e.g. 400–360?Ma) metamorphism. These results help clarify palinspastic relationships and tectonic correlations between pre-Late Carboniferous metamorphic basement sequences and Carboniferous to Permian cover sequences. 相似文献
19.
Conventional K-Ar ages of tholeiitic basalts of the Ferrar Group in the central Transantarctic Mountains indicate significant loss of radiogenic 40Ar from this unit over much of its outcrop area. Argon loss varies inversely with amount of devitrified matrix in the basalts, which have not been thermally or tectonically disturbed since extrusion. 40Ar/19Ar age-spectra of these tholeiites are generally discordant and indicate significant inhomogeneity in the distribution of radiogenic 40Ar with respect to 39Ar, but are distinctly different from release patterns of thermally disturbed samples. Amounts of argon redistribution vary directly with amounts of devitrification and are reflected in progressive modification of the age spectra. A model of redistribution of radiogenic 40Ar by devitrification of originally glassy matrix is suggested that is consistent with disturbance of the conventional K-Ar systematics as well as the 40Ar/39Ar age-spectra. Samples with substantial redistribution but minor loss of radiogenic argon yield age spectra whose apparent ages decrease from low-temperature to high-temperature steps, similar to those reported for some lunar basalts, breccias, and soils. Modification of all the age spectra is attributed to redistribution of radiogenic 40Ar during progressive devitrification, although 39Ar-recoil effects suggested by Turner and Cadogan (1974) may be a factor in some cases. Where devitrification involves most potassium sites within the basalt, 40Ar/39Ar age-plateaux may be formed that have no geologic significance. 相似文献
20.
《Chemical Geology》2002,182(2-4):583-603
New K/Ar ages based on 40Ar/39Ar incremental heating of <2- and 2–20-μm size fractions of the well-characterized, carbonate-bearing Heinrich layers of core V28-82 in the eastern North Atlantic are 846–1049 Ma, overlapping with conventional K/Ar ages from the same Heinrich layers on the Dreizack seamounts of 844–1074 Ma. This agreement suggests the equivalence of the methods in fine-grained terrigenous sediments. Additionally, Heinrich layer H2 yielded a 40Ar/39Ar-based K/Ar age of 970±4 from Orphan Knoll in the southern Labrador Sea, within the range found in eastern North Atlantic Heinrich layers. Thus, the K/Ar data are robust in their indication of a dominant Labrador Sea ice-rafted source to even the finest sediment fraction in the eastern North Atlantic during the massive detrital carbonate-bearing Heinrich events of the last glacial cycle (H1, H2, H4, H5). Close correspondence of the radiogenic argon concentration (40Ar*) from the de-carbonated <63-μm fractions from V28-82 with the <2- and 2–16-μm fractions from the Driezack seamounts demonstrates that this measurement is a rapid and reliable method for correlating these layers within their belt of distribution.A 40Ar/39Ar-based K/Ar age of 433±5 million years for H11 in V28-82 is within the range of published data from background sediments in the eastern North Atlantic, and is consistent with published results across this interval in the Driezack seamounts. In contrast, the 40Ar/39Ar-based K/Ar age of H11 in the western Atlantic core EW9303-JPC37 is 614±5 million years. A brick red sample from approximately the interval of H3 of core EW9303-GGC40 yielded a 40Ar/39Ar-based K/Ar age of 567±1 million years, comparable to the published range of 523–543 Ma from the 2–16-μm fractions from that interval on the Dreizack seamounts. Both JPC37 and GGC40 are located in the path of the North Atlantic Drift. The older ages from western samples of H3 and H11 may result from dilution of a Hudson Strait source or an elevated age from southeastern Laurentide sources. 相似文献