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1.
The 40Ar/39Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). Recent investigations have improved the accuracy and precision of the ages of most of the Phanerozoic-aged standards (e.g. Fish Canyon Tuff sanidine (FCs), Alder Creek sanidine, GA1550 biotite and LP-6 biotite); however, no specific study has been undertaken on the older standards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmospheric contamination at the single grain level. The mean standard deviation of the 40Ar?/39ArK (F-value) derived from this study is 0.49%, comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards) between Hb3gr and FCs is RFCsHb3gr = 51.945 ± 0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is 1073.6 ± 5.3 Ma (1σ; internal error only) and ± 8.8 Ma (including all sources of error). This age is indistinguishable within uncertainty from the K/Ar age previously reported at 1072 ± 11 Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. 40Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19-35].The R-value determined in this study can also be used to intercalibrate FCs if we consider the K/Ar date of 1072 Ma as a reference age for Hb3gr. We derive an age of 27.95 ± 0.19 Ma (1σ; internal error only) for FCs which is in agreement with the previous determinations. Altogether, this shows that Hb3gr is a suitable standard for 40Ar/39Ar geochronology. 相似文献
2.
P. Alexandre 《Geochimica et cosmochimica acta》2011,75(15):4260-4272
In the case of volume diffusion, the closure temperature of a mineral is function of, among other factors, the characteristic diffusion dimension, which can be approximated by the grain size of the mineral analysed for grains smaller than or similar in size to the diffusion domains. The theoretical possibility of single mineral grain size thermochronology had been demonstrated empirically in earlier studies, mostly using biotite. In order to examine the potential of this method, it was tested alongside the widely used multi-mineral 40Ar/39Ar thermochronology. The sample comes from the granitic McLean pluton, in the south section of the Grenville orogeny. Seven grain size separates of biotite (ranging between 90 and 1000 μm), eight size fractions of amphibole (between 63 and 1000 μm), and three size fractions of K-feldspar (250-600 μm) were extracted and dated by the laser step-heating 40Ar/39Ar method. The total gas ages obtained behave as theoretically predicted, with increasing ages for increasing grain sizes, including for K-feldspar, but with the exception of the smallest and the largest grains for biotite and amphibole. The calculated cooling rates are ca. 0.7 °C/Ma for K-feldspar, ca. 2.5 °C/Ma for biotite, and ca. 11 °C/Ma for amphibole, corresponding very well to a monotonic cooling of the McLean pluton. A quick initial thermal re-equilibration with the cooler host-rocks is followed by a much slower cooling on a thermal path parallel to that of the Frontenac Terrain situated immediately to the southeast. The validity of the single mineral grain size thermochronology is demonstrated by comparison with the thermal evolution of the adjacent units and with the cooling history derived from a multi-mineral thermochronology, suggesting that it can be routinely used. The application of this method can be hampered by insufficiently low analytical uncertainties. 相似文献
3.
40Ar/39Ar plateau ages of biotite, plagioclase, K-feldspar and hornblende from the Biluoxueshan and Lincang plutons in the three-river area of Yunnan and the Quxu pluton in the Gandise belt of Xizang indicate that the three plutons were emplaced at 420, 234 and 43.4 Ma, respectively. Based on the study of closure temperatures and thermal histories, it is shown that the Lincang and Biluoxueshan plutons experienced the same thermodynamic event during 85–90 Ma, temporally representing the collision between the Sundaland and Eurasian plates, and that the Quxu pluton was uplifted at a much faster rate during 40–44 Ma(2.6 mm/a), marking the collision between the Indian and Eurasian plates. 相似文献
4.
西昆仑库地以南有一套变质变形较强的岩系,前人依照区域对比关系将其划为前寒武的古老基底。对西昆仑早期构造演化的论述均基于该观点,但没有提供确凿的同位素年代学证据。笔者通过野外观察、室内研究,确认库地以南的变质变形岩系是大型韧性推覆剪切作用的产物。通过对新生变质矿物角闪石和黑云母单矿物的40Ar/39Ar年龄分析,确定剪切变质年龄为426-451Ma,说明库地的变质变形岩系是形成于早古生代晚期的一条大型韧性剪切带,这对于解释西昆仑的早期构造演化具有重要意义。 相似文献
5.
The effects of deformation on radiogenic argon (40Ar∗) retentivity in mica are described from high pressure experiments performed on rock samples of peraluminous granite containing euhedral muscovite and biotite. Cylindrical cores, ∼15 mm in length and 6.25 mm in diameter, were drilled from granite collected from the South Armorican Massif in northwestern France, loaded into gold capsules, and weld-sealed in the presence of excess water. The samples were deformed at a pressure of 10 kb and a temperature of 600 °C over a period 29 of hours within a solid medium assembly in a Griggs-type triaxial hydraulic deformation apparatus. Overall shortening in the experiments was approximately 10%. Transmitted light and secondary and backscattered electron imaging of the deformed granite samples reveals evidence of induced defects and for significant physical grain size reduction by kinking, cracking, and grain segmentation of the micas.Infrared (IR) laser (CO2) heating of individual 1.5-2.5 mm diameter grains of muscovite and biotite separated from the undeformed granite yield well-defined 40Ar/39Ar plateau ages of 311 ± 2 Ma (2σ). Identical experiments on single grains separated from the experimentally deformed granite yield results indicating 40Ar∗ loss of 0-35% in muscovite and 2-3% 40Ar∗ loss in biotite. Intragrain in situ ultraviolet (UV) laser ablation 40Ar/39Ar ages (±4-10%, 1σ) of deformed muscovites range from 309 ± 13 to 264 ± 7 Ma, consistent with 0-16% 40Ar∗ loss relative to the undeformed muscovite. The in situ UV laser ablation 40Ar/39Ar ages of deformed biotite vary from 301 to 217 Ma, consistent with up to 32% 40Ar∗ loss. No spatial correlation is observed between in situ40Ar/39Ar age and position within individual grains. Using available argon diffusion data for muscovite the observed 40Ar∗ loss in the experimentally treated muscovite can be utilized to predict average 40Ar∗ diffusion dimensions. Maximum 40Ar/39Ar ages obtained by UV laser ablation overlap those of the undeformed muscovite, indicating argon loss of <1% and an average effective grain radius for 40Ar∗ diffusion ?700 μm. The UV laser ablation and IR laser incremental 40Ar/39Ar ages indicating 40Ar∗ loss of 16% and 35%, respectively, are consistent with an average diffusion radius ?100 μm. These results support a hypothesis of grain-scale 40Ar∗ diffusion distances in undeformed mica and a heterogeneous mechanical reduction in the intragrain effective diffusion length scale for 40Ar∗ in deformed mica. Reduction in the effective diffusion length scale in naturally deformed samples occurs most probably through production of mesoscopic and submicroscopic defects such as, e.g., stacking faults. A network of interconnected defects, continuously forming and annealing during dynamic deformation likely plays an important role in controlling both 40Ar∗ retention and intragrain distribution in deformed mica. Intragrain 40Ar/39Ar ages, when combined with estimates of diffusion kinetics and distances, may provide a means of establishing thermochronological histories from individual micas. 相似文献
6.
The 40Ar/39Ar stepwise crushing technique is applied for the first time to date garnet from ultra-high-pressure metamorphic (UHPM) eclogites. Three garnet samples from the Bixiling eclogites analyzed by 40Ar/39Ar stepwise crushing yield regular, predictable age spectra, and a clear separation between excess 40Ar and concordant plateau and isochron ages. All three age spectra begin with high apparent ages followed by step by step decreasing ages, and finally age plateaux with apparent ages in the range from 427 ± 20 to 444 ± 10 Ma. The data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 ± 34 to 459 ± 58 Ma, corresponding to initial 40Ar/36Ar ratios from 292.1 ± 4.5 to 294.5 ± 6.7, statistically indistinguishable from the modern air. The high initial ages are interpreted to derive from secondary fluid inclusions containing excess 40Ar, whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess 40Ar. The plateau ages are interpreted to approximate the time of garnet growth during initial UHPM metamorphism. Phengite analyzed by laser stepwise heating yielded a complicated two-saddle age spectrum with a scattered isochron corresponding to age of 463 ± 116 Ma and initial 40Ar/36Ar ratio of 1843 ± 1740 indicative of the presence of extraneous 40Ar within phengite. These concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that Dabie UHPM eclogites were first formed in the early Paleozoic, during the same event that caused the Qinling-Northern Qaidam Basin-Altyn Tagh eclogites. 相似文献
7.
Potassium-Ar and Rb-Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K-Ar and, in particular, 40Ar/39Ar dating of micas is still widely applied, Rb-Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb-Sr ages than the K-Ar or 40Ar/39Ar system. Moreover, a reduction of uncertainties typically reported for Rb-Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb-Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% (n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield 40Ar/39Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb-Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb-Sr and 40Ar/39Ar ages demonstrates the robustness of the Rb-Sr system in tri-octahedral micas and cautions against the sole use of 40Ar/39Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb-Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb-Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma.Combining the mica Rb-Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb-Sr system in 1-2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite. 相似文献
8.
William S. Cassata David L. Shuster Benjamin P. Weiss 《Geochimica et cosmochimica acta》2010,74(23):6900-6920
The thermal histories of Martian meteorite are important for the interpretation of petrologic, geochemical, geochronological, and paleomagnetic constraints that they provide on the evolution of Mars. In this paper, we quantify 40Ar/39Ar ages and Ar diffusion kinetics of Martian meteorites Allan Hills (ALH) 84001, Nakhla, and Miller Range (MIL) 03346. We constrain the thermal history of each meteorite and discuss the resulting implications for their petrology, paleomagnetism, and geochronology. Maskelynite in ALH 84001 yields a 40Ar/39Ar isochron age of 4163 ± 35 Ma, which is indistinguishable from recent Pb-Pb (Bouvier et al., 2009a) and Lu-Hf ages (Lapen et al., 2010). The high precision of this result arises from clear resolution of a reproducible trapped 40Ar/36Ar component in maskelynite in ALH 84001 (40Ar/36Ar = 632 ± 90). The maskelynite 40Ar/39Ar age predates the Late Heavy Bombardment and likely represents the time at which the original natural remanent magnetization (NRM) component observed in ALH 84001 was acquired. Nakhla and MIL 03346 yield 40Ar/39Ar isochron ages of 1332 ± 24 and 1339 ± 8 Ma, respectively, which we interpret to date crystallization. Multi-phase, multi-domain diffusion models constrained by the observed Ar diffusion kinetics and 40Ar/39Ar age spectra suggest that localized regions within both ALH 84001 and Nakhla were intensely heated for brief durations during shock events at 1158 ± 110 and 913 ± 9 Ma, respectively. These ages may date the marginal melting of pyroxene in each rock, mobilization of carbonates and maskelynite in ALH 84001, and NRM overprints observed in ALH 84001. The inferred peak temperatures of the shock heating events (>1400 °C) are sufficient to mobilize Ar, Sr, and Pb in constituent minerals, which may explain some of the dispersion observed in 40Ar/39Ar, Rb-Sr, and U-Th-Pb data toward ages younger than ∼4.1 Ga. The data also place conservative upper bounds on the long-duration residence temperatures of the ALH 84001 and Nakhla protolith to be °C and °C over the last ∼4.16 Ga and ∼1.35 Ga, respectively. MIL 03346 has apparently not experienced significant shock-heating since it crystallized, consistent with the fact that various chronometers yield concordant ages. 相似文献
9.
Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, 40Ar/39Ar ratios and K2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7-24 ppb 40Ar) than shock veins and matrix minerals (0.3-3 ppb 40Ar). The 40Ar/36Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic 36Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior (40Ar/36Ar: 480-490) in matrix minerals. Average apparent 40Ar/39Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These 40Ar/39Ar apparent ages are substantially older than the ∼170-474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic 36Ar and 38Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures. 相似文献
10.
Michael A. Cosca Eric J. Essene Michael J. Kunk John F. Sutter 《Contributions to Mineralogy and Petrology》1992,110(2-3):211-225
An 40Ar/39Ar thermochronological investigation of upper greenschist to granulite facies gneiss, amphibolite and marble was conducted in the Central Metasedimentary Belt (CMB), Ontario, to constrain its cooling history. Incremental 40Ar/39Ar release spectra indicate that substantial differential unroofing occurred in the CMB between 1000 and 600 Ma. A consistent pattern of significantly older hornblende and phlogopite 40Ar/3Ar cooling ages on the southeast sides of major northeast striking shear zones is interpreted to reflect late displacement due to extensional deformation. Variations in hornblende 40Ar/39Ar age plateaus exceeding 200 Ma occur over distances less than 50 km with major age discontinuities occurring across the Robertson Lake shear zone and the Sharbot Lake mylonite zone which separate the Sharbot Lake terrane from the Elzevir and Frontenac terranes. Extensional displacements of up to 14 km are inferred between the Frontenac and Elzevir terranes of the CMB. No evidence for significant post argon-closure vertical displacement is indicated in the vicinity of the Perth Road mylonite within the Frontenac terrane. Variations of nearly 100 Ma in phlogopite 40Ar/39Ar plateau ages occur in undeformed marble on either side of the Bancroft Shear Zone. Phlogopites from sheared and mylonitized marble within the shear zone yield 40Ar/39Ar diffusional loss profiles, but have older geologically meaningless ages thought to reflect incorporation of excess argon. By 900 Ma, southeast directed extension was occurring throughout the CMB, possibly initiated along previous zones of compressional shearing. An easterly migration of active zones of extension is inferred, possibly related to an earlier, overall easterly migration of active zones of regional thrusting and easterly migration of an ancient subduction zone. The duration of extensional shearing is not well constrained, but must have ceased before 600 Ma as required by the deposition of overlying undeformed Cambrian and/or Ordovician sedimentary rocks.Contribution No. 481 from the Mineralogical Laboratory, University of Michigan 相似文献
11.
Detailed zircon and apatite U-Pb dating and 40Ar/39Ar dating of actinolite have been carried out on the Carmen-Sierra Aspera Kiruna type magnetite-apatite and iron oxide Cu-Au (IOCG) district in the Coastal Cordillera of northern Chile (∼26°S). They define a precise succession of magmatic and hydrothermal events associated with early Cretaceous Andean magmatism. Apatite and magnetite from a magnetite-apatite tabular body with intergrowth texture in the Carmen deposit yield a total Pb-U isochron age of 131.0 ± 1.0 Ma. This result is the first direct dating of magnetite-apatite mineralization in an early Andean deposit, and the age coincides with zircon ages of a quartz diorite stock that partially hosts mineralization (130.6 ± 0.3 Ma). Magnetite from the studied tabular body contains only small amounts of radiogenic Pb and serves to constrain the initial common Pb isotopic composition. The high degree of correlation suggests that both minerals closed for Pb diffusion at essentially the same time and at a relatively high temperature (close to that of zircon), making the apatite-magnetite pair a reliable geochronometer for igneous or hydrothermal crystallization. Zircon from the Sierra Aspera composite pluton yields ages between 131.3 ± 0.3 Ma and 127.4 ± 0.1 Ma, clearly resolving the timing of intrusion of discrete intrusive phases. Actinolite 40Ar/39Ar ages partially overlap the ages of plutonic phases of the Sierra Aspera pluton, but are younger than the magnetite-apatite tabular body. The initial Pb isotopic composition of the melts and/or fluids from which the magnetite-apatite tabular bodies crystallized is very similar to the primitive Pb isotopic composition of granitic magmas associated with early Cretaceous plutons measured in K-feldspar. The Pb isotopic correspondence, combined with the temporal and spatial association between magnetite-apatite mineralization and the dioritic-quartz dioritic magmatism, strongly suggests a genetic relationship between early Cretaceous continental arc magmatism, massive magnetite-apatite deposits, and IOCG mineralization. 相似文献
12.
The Late Triassic Central Patagonian Batholith is a key element in paleogeographic models of West Gondwana just before to the break-up of the supercontinent. The preexisting classification of units of this batholith was mainly based on isotopic and geochemical data. Here we report the results of field mapping and petrography, backed up by three new 40Ar/39Ar biotite ages, which reveal previously unnoticed relationships of the rocks in the batholith. Based on the new information we present a reorganization of units where the batholith is primarily formed by the Gastre and the Lipetrén superunits. The Gastre Superunit is the oldest magmatic suite and is composed of I-type granites which display evidence of felsic and mafic magma interaction. It is formed by 4 second-order units: 1) equigranular hornblende–biotite granodiorites, 2) porphyritic biotite–hornblende monzogranites, 3) equigranular biotitic monzogranites and 4) hornblende quartz-diorites. Emplacement depth of the Gastre Superunit is bracketed between 6 and 11 km (1.8–3 kbar), and the maximum recorded temperatures of emplacement are comprised between 660 and 800 °C. The recalculated Rb/Sr age is 222 ± 3 Ma and the porphyritic biotite–hornblende monzogranites yielded a 40Ar/39Ar age in biotite of 213 ± 5 Ma. On the other hand, the Lipetrén Superunit is made up by fine-grained biotitic monzo- and syenogranites that postdate magma hybridization processes and intrude all the other units. The recalculated Rb/Sr age for this suite is identical to a 40Ar/39Ar age in biotite extracted from one of its monzogranites (206.4 ± 5.3 and 206 ± 4 Ma, respectively). This and the observed textural features suggest very fast cooling related to a subvolcanic emplacement. An independent unit, the “Horqueta Granodiorite”, which has previously been considered as the record of a Jurassic intrusive stage in the Central Patagonian Batholith, gave a 40Ar/39Ar age in biotite of 214 ± 2 Ma. This and the reexamination of available isotopic data allow propose that this granodiorite unit is part of the Late Paleozoic intrusives in the region. The Late Triassic Central Patagonian Batholith is overlain by 190–185 Ma volcano-sedimentary rocks, suggesting that it was exposed sometime between the latest Triassic and earliest Jurassic times, roughly coeval with a major accretionary episode in the southwestern margin of Gondwana. 相似文献
13.
Hornblende Ar/Ar geochronology across terrane boundaries in the Sveconorwegian Province of S. Norway
Bernard Bingen Ariel Boven Lea Punzalan Jan R. Wijbrans Daniel Demaiffe 《Precambrian Research》1998,90(3-4)
Hornblende incremental heating 40Ar/39Ar data were obtained from augen gneiss and amphibolite of the Sveconorwegian Province of S. Norway. In the Rogaland-Vest Agder and Telemark terranes, four pyroxene-rich samples, located close (≤ 10 km) to the anorthosite-charnockite Rogaland Igneous Complex, define an age group at 916 + 12/ − 14 Ma and six samples distributed in the two terranes yield another group at 871 + 8/ − 10 Ma. The first age group is close to the reported zircon U---Pb intrusion age of the igneous complex (931 ± 2 Ma) and the regional titanite U---Pb age (918 ± 2 Ma), whereas the second group overlaps reported regional mineral Rb---Sr ages (895-853 Ma) as well as biotite K---Ar ages (878-853 Ma). In the first group, the comparatively dry parageneses of low-P thermal metamorphism (M2) associated with the intrusion of the igneous complex are well developed, and hornblende 40Ar/39Ar ages probably record a drop in temperature shortly after this phase. In other hornblende + biotite-rich samples, with presumably a higher fluid content, the hornblende ages are probably a response to hornblende-fluid interaction during a late Sveconorwegian metamorphic or hydrothermal event. A ca 220 m.y. diachronism in hornblende 40Ar/39Ar ages is documented between S. Telemark (ca 870 Ma) and Bamble (ca 1090 Ma). Differential uplift between these terranes was mostly accommodated by shearing along the Kristiansand-Porsgrunn shear zone. The final stage of extension along this zone occurred after intrusion of the Herefoss post-kinematic granite at 926 ± 8 Ma. On the contrary, the southern part of the Rogaland-Vest Agder and Telemark terranes share a common cooling evolution as mineral ages are similar on both sides of the Mandal-Ustaoset Line the tectonic zone between them. The succession within 20 m.y. of a voluminous pulse of post-tectonic magmatism at 0.93 Ga, a phase of high-T-low-P metamorphism at 0.93-0.92 Ga, and fast cooling at a regional scale ca 0.92 Ga, suggests that the southern parts of Rogaland-Vest Agder and Telemark were affected by an event of post-thickening extension collapse at that time. This event is not recorded in Bamble. 相似文献
14.
Forty-four biotite samples collected about a lithologic contact between pelite and amphibolite were analyzed for 40Ar/39Ar and demonstrate the importance of bulk Ar diffusivity and system geometry—factors not usually considered in the interpretation and collection of 40Ar/39Ar age data. The resulting 40Ar/39Ar apparent ages range from 11.30 ± 0.05 Ma to 17.90 ± 0.10 Ma. The ages (and excess argon contents) are spatially and lithologically correlated. The pelite samples all yield ages clustering around ∼12 Ma, the age expected for cooling through biotite closure (∼360°C) in this region of the Alps. Ages in the amphibolite biotites are older, showing a smooth trend between 15 Ma at the contact with the pelite to 18 Ma, 34 cm from the contact. This data shows that characterization of the Ar closure age for biotite in a given system should not rest on a single sample, as otherwise irresolvable differences in age between samples within the same outcrop can exist. A generalized mechanistic model for excess argon is presented. The presence (or absence) of excess Ar depends on an intrinsic system parameter, τT, the transmissive timescale, which is the characteristic time for 40Ar to escape through the local intergranular transporting medium (ITM) to some sink for argon. To prevent buildup of geochronologically significant excess 40Ar, τT must be very short relative to the true closure age of the mineral. A FORTRAN code including radiogenic Ar production, diffusive loss of Ar from biotite, and bulk Ar diffusion through the ITM has been developed. Application of numerical modeling suggests that the time-averaged effective bulk diffusivity, DeffAr, in the biotite-amphibolite rock during early retrograde cooling is 2.2 ± 1.0 × 10−8 m2/yr (assuming steady state conditions) - the first such measurement available. Numerical modeling also provides information about the transmissivity and geologic history specific to the field site, including a drop in DeffAr at 15.5 ± 1.0 Ma. The timing of this drop is related to coincident rheological changes and the onset of rapid exhumation of the nappe stack. 相似文献
15.
Generally synmetamorphic granitic stocks intrude high-grade, Silurian-lowermost Devonian metasedimentary rocks near Augusta, Maine. Rb-Sr whole-rock isochrons (8 points each) define mutually overlapping crystallization ages of 394±8 m.y. (Togus quartz monzonite), 387±11 m.y. (Hallowell quartz monzonite), and 381±14 m.y. (Three Mile Pond biotite granodiorite), thereby providing a narrow chronologic bracket for Acadian tectonothermal activity in the area. Igneous hornblende, muscovite, and biotite display internally concordant 40Ar/39Ar age spectra with plateau dates of 350 m.y. (hornblende) and 300-265 m.y. (muscovite and biotite), with an overall southwestward younging trend. The mineral dates are similar to those recorded in adjacent portions of the regional metamorphic terrain and suggest a prolonged postmagmatic cooling which closely followed the diachronous northeast-southwest post-Acadian cooling of the country rocks. No evidence for a distinct Permian thermal overprint of older isotopic systems has been observed. 相似文献
16.
We report 39Ar-40Ar ages of whole rock (WR) and plagioclase and pyroxene mineral separates of nakhlites MIL 03346 and Y-000593, and of WR samples of nakhlites NWA 998 and Nakhla. All age spectra are complex and indicate variable degrees of 39Ar recoil and variable amounts of trapped 40Ar in the samples. Thus, we examine possible Ar-Ar ages in several ways. From consideration of both limited plateau ages and isochron ages, we prefer Ar-Ar ages of NWA 998 = 1334 ± 11 Ma, MIL 03346 = 1368 ± 83 Ma (mesostasis) and 1334 ± 54 Ma (pyroxene), Y-000593 = 1367 ± 7 Ma, and Nakhla = 1357 ± 11 Ma, (2σ errors). For NWA 998 and MIL 03346 the Ar-Ar ages are within uncertainties of preliminary Rb-Sr isochron ages reported in the literature. These Ar-Ar ages for Y-000593 and Nakhla are several Ma older than Sm-Nd ages reported in the literature. We conclude that the major factor in producing Ar-Ar ages slightly too old is the presence of small amounts of trapped martian or terrestrial 40Ar on weathered grain surfaces that was degassed along with the first several percent of 39Ar. A total K-40Ar isochron for WR and mineral data from five nakhlites analyzed by us, plus Lafayette data in the literature, gives an isochron age of 1325 ± 18 Ma (2σ). We emphasize the precision of this isochron over the value of the isochron age. Our Ar-Ar data are consistent with a common formation age for nakhlites. The cosmic-ray exposure (CRE) age for NWA 998 of ∼12 Ma is also similar to CRE ages for other nakhlites. 相似文献
17.
S.M. Aciego F. Jourdan D.J. DePaolo P.R. Renne 《Geochimica et cosmochimica acta》2010,74(5):1620-1635
Late Quaternary, post-shield lavas from the Mauna Kea and Kohala volcanoes on the Big Island of Hawaii have been dated using the 40Ar/39Ar and U-Th/He methods. The objective of the study is to compare the recently demonstrated U-Th/He age method, which uses basaltic olivine phenocrysts, with 40Ar/39Ar ages measured on groundmass from the same samples. As a corollary, the age data also increase the precision of the chronology of volcanism on the Big Island. For the U-Th/He ages, U, Th and He concentrations and isotopes were measured to account for U-series disequilibrium and initial He. Single analyses U-Th/He ages for Hamakua lavas from Mauna Kea are 87 ± 40 to 119 ± 23 ka (2σ uncertainties), which are in general equal to or younger than 40Ar/39Ar ages. Basalt from the Polulu sequence on Kohala gives a U-Th/He age of 354 ± 54 ka and a 40Ar/39Ar age of 450 ± 40 ka. All of the U-Th/He ages, and all but one spurious 40Ar/39Ar ages conform to the previously proposed stratigraphy and published 14C and K-Ar ages. The ages also compare favorably to U-Th whole rock-olivine ages calculated from 238U-230Th disequilibria. The U-Th/He and 40Ar/39Ar results agree best where there is a relatively large amount of radiogenic 40Ar (>10%), and where the 40Ar/36Ar intercept calculated from the Ar isochron diagram is close to the atmospheric value. In two cases, it is not clear why U-Th/He and 40Ar/39Ar ages do not agree within uncertainty. U-Th/He and 40Ar/39Ar results diverge the most on a low-K transitional tholeiitic basalt with abundant olivine. For the most alkalic basalts with negligible olivine phenocrysts, U-Th/He ages were unattainable while 40Ar/39Ar results provide good precision even on ages as low as 19 ± 4 ka. Hence, the strengths and weaknesses of the U-Th/He and 40Ar/39Ar methods are complimentary for basalts with ages of order 100-500 ka. 相似文献
18.
Fred Jourdan Jennifer P. Matzel Paul R. Renne 《Geochimica et cosmochimica acta》2007,71(11):2791-2808
The 40Ar/39Ar dating technique requires the activation of 39Ar via neutron irradiation. The energy produced by the reaction is transferred to the daughter atom as kinetic energy and triggers its displacement, known as the recoil effect. Significant amounts of 39Ar and 37Ar can be lost from minerals leading to spurious ages and biased age spectra. Through two experiments, we present direct measurement of the recoil-induced 39Ar and 37Ar losses on Fish Canyon sanidine and plagioclase. We use multi-grain populations with discrete sizes ranging from 210 to <5 μm. One population consists of a mixture between sanidine and plagioclase, and the other includes pure sanidine.We show that 39Ar loss (depletion factor) for sanidine is ∼3% for the smallest fraction. Age spectra of fractions smaller than ∼50 μm show slight departure from flat plateau-age spectrum usually observed for large sanidine. This departure is roughly proportional to the size of the grain but does not show typical 39Ar loss age spectra. The calculated thickness of the total depletion layer d0(sanidine) is 0.035 ± 0.012 (2σ). This is equivalent to a mean depth of the partial depletion layer (x0) of 0.070 ± 0.024 μm. The latter value is indistinguishable from previous values of ∼0.07-0.09 μm obtained by argon implantation experiments and simulation results.We show that it is possible to adequately correct ages from 39Ar ejection loss provided that the d0-value and the size range of the minerals are sufficiently constrained. As exemplified by similar calculations performed on results obtained in a similar study of GA1550 biotite [Paine J. H., Nomade S., and Renne P. R. (2006) Quantification of 39Ar recoil ejection from GA1550 biotite during neutron irradiation as a function of grain dimensions. Geochim. Cosmochim. Acta70, 1507-1517.], the d0(biotite) is 0.46 ± 0.06 μm. The significant difference between empirical results on biotite and sanidine, along with different simulation results, suggests that for biotite, crystal structures and lattice defects of the stopping medium and possibly subsequent thermal degassing (due to ∼150-200 °C temperature in the reactor or extraction line bake out) must play an important role in 39Ar loss.The second experiment suggests that 37Ar recoil can substantially affect the age via the interference corrections with results that suggest up to ∼98% of 37Ar can be ejected from the ∼5 μm grain dimension.Further investigation of silicates of various compositions and structures are required to better understand (and correct) the recoil and recoil-induced effects on both 39Ar and 37Ar and their influences on 40Ar/39Ar dating. 相似文献
19.
Herbert Kirsch Dipl.-Geol. Dr. Bernd Kober Prof. Dr. Hans Joachim Lippolt 《International Journal of Earth Sciences》1988,77(3):693-711
The Frankenstein gabbro complex in the northern Odenwald/FR Germany is one of the Hercynian plutonic bodies of the Saxothuringian zone in the Variscan belt.87Sr/86Sr isotopic investigations on pyroxene,40Ar/39Ar age determinations on hornblendes, plagioclases, biotites and pyroxenes and207Pb/206Pb dating on single zircon crystals have been carried out. The87Sr/86Sr initial ratio of the gabbro (0.70380) indicates uncontaminated derivation of the magma from the mantle. The results on hornblendes, plagioclases and zircons establish the isotopic age of intrusion and the mode of cooling. Pyroxene and biotite data indicate disturbed K-Ar systems of these minerals. The average ages of the hornblendes (363±7 Ma), of the plagioclases (359±3 Ma) and of the zircons (362 ±9 Ma) agree within the 1-sigma levels and constrain the early history of the plutonic intrusion at the turn from Devonian to Carboniferous times. These minerals must have been cooled below their closure temperatures within a time-interval shorter than the error margins of 10 Ma. The ages therefore are, unlike those of the neighbouring Bergsträßer Odenwald, not long-time cooling ages due to largescale and slow geological uplift of the crustal segment. Instead, they will closely estimate the time of intrusion of the Frankenstein pluton into a shallow crustal level. The hiatus of about 20 Ma compared to the average ages of the crystalline rocks of the Bergsträßer Odenwald in the south characterizes the mafic Frankenstein pluton as a separated unit of the Odenwald crystalline, and underlines the importance of the Carboniferous tectonic fault zone in-between. The present data set clearly demonstrates the importance of the applied combination of methods and techniques to constrain the history of intrusion and cooling of mafic plutonic rocks. 相似文献
20.
《Gondwana Research》2001,4(3):541-550
Since the late 1980s, it has been hypothesized that the wide range of apparent argon ages seen within single K-feldspar samples might be due to a distribution of diffusion domain sizes within the mineral. To test and apply this idea, an analytical technique that combines conventional laboratory degassing experiments (resistance heating) with numerical inversion procedures has been developed to extract cooling history information from feldspars. A key part of the method involves careful control of temperature in the laboratory to constrain the diffusion parameters of the feldspar samples. In our study, we have K-feldspar data from single crystals that mimic the types of data seen in classic resistance heater fusion experiments. Our step-heating data are based on using a continuous argon-ion laser with no direct control on temperature. However, with only a single added free parameter in the model, we show that it is possible to analyze this data in the multi-domain style, and make some simple inferences on the nature of the cooling history of the Carion pluton in central Madagascar. The Carion granitic pluton in central Madagascar was intruded into warm continental crust following orogenic events related to the final amalgamation of Gondwana. U-Pb SHRIMP dating of the pluton yields an emplacement age of 532.1 ± 5.2 Ma followed by relatively slow cooling as constrained by 40Ar/39Ar ages on hornblende, biotite and K-feldspar. Four hornblende samples yielded a mean 40Ar/39Ar age of 512.7 ± 2.6 Ma. A biotite sample yielded an age of 478.9 ± 1.0 Ma and modeled K-feldspar ages show cooling from 350° C at 466 Ma to 100° C by 410 Ma. Collectively, the data suggest that the pluton cooled from 850° C at 532.1 ± 5.2 (U-Pb zircon) Ma to 500° C at 512.7 ± 2.6 Ma (40Ar/39Ar hornblende), or approximately 18 °C/Ma slowing to ∼4 °C/Ma between 512 Ma and 478 Ma and finally to about 3°C/Ma between 478 and 410 Ma. 相似文献