40Ar/39Ar age constraints on deformation and mineralization, Rosebery Zn-Pb-Cu and Mount Lyell Cu deposits, Tasmania, Australia

The ⁴⁰Ar/³⁹Ar dating of alteration muscovite from the Rosebery Zn-Pb-Cu and Mount Lyell Cu deposits, Tasmania, Australia, has determined a succession of deformation events which occurred from 400-378 Ma, and comprises the Devonian Tabberabberan Orogeny. The dates from Rosebery range from 400-390 Ma, are a minimum age for mineralization, indicate the time of deformation, and provide a maximum age limit for granitoid emplacement in the vicinity of the deposit. The ages from the Mount Lyell field range from 400-378 Ma, are a minimum for mineralization, and date cleavage development. The North Lyell Cu mineralization, which was probably broadly coeval with deformation, may have formed at 400 Ma. All pre-Devonian alteration micas in the Rosebery and Mount Lyell areas have been recrystallized or reset. The Tabberabberan deformation in western Tasmania was broadly contemporaneous with widespread crustal shortening in southeastern Australia, as established from the dating of alteration minerals associated with deformation-related precious and base metal deposits.     

A K-Ar and 40Ar/39Ar study on white micas from the Brixen Quartzphyllite,Southern Alps

The Brixen Quartzphyllite, basement of the Southern Alps (Italy), consists of metasediments which had suffered progressive deformation and low grade metamorphism (p _max4 kbar, T _max375±25° C) during the Palaeozoic. It has been excavated by pre-Permian erosion, buried again beneath a pile of Permo-mesozoic to Cainozoic sediments (estimated T _max150° C), and is now exposed anew due to late Alpine uplift and erosion. The behavior of the K-Ar system of white micas is investigated, taking advantage of the narrow constraints on their thermal history imposed by the geological/stratigraphic reference systems.The six structurally and petrographically differing samples come from a single outcrop, whose position is roughly two kilometers beneath the Permian land-surface. White mica concentrates from five grain size fractions (<2 , 2–6 , 6–20 , 20–60 , 60–75 ) of each sample have been analyzed by the conventional K-Ar method, four selected concentrates additionally by the ⁴⁰Ar/³⁹Ar stepwise heating technique; furthermore, Ar content and isotopic composition of vein quartz were determined.The conventional ages of the natural grain size fractions (20–60 , 60–75) are in the range 316±8 Ma, which corresponds to the ⁴⁰Ar/³⁹Ar plateau age of 319.0±5.5 Ma within the error limits. The finer grain size fractions yield significantly lower ages, down to 233 Ma for fractions <2 . Likewise low apparent ages (down to 83 Ma) are obtained for the low temperature ⁴⁰Ar/³⁹Ar degassing steps.There is no correlation between microstructural generation of white mica prevailing in the sample and apparent age. This favours an interpretation of the 316±8 Ma values as cooling age; progressive deformation and metamorphism must be respectively older and their timing cannot be resolved by these methods. The data preclude any significant influence of a detrital mica component as well as of excess argon.The lower ages found for the fine grain-size fractions (respectively the low-T degassing steps) correspond to a near-surface period (p-T-minimum); the values are geologically meaningless. The effect is interpreted to result from partial Ar loss due to reheating during Mesozoic-Cainozoic reburial. A model based on diffusion parameters derived from the outgassing experiments and Dodson's (1979) equation yields a closure temperature of 284±40 °C for a cooling rate of 18° C/Ma. Furthermore, this model suggests that the observed argon loss of up to 5% may in fact have been induced by reheating to 150 °C for 50 Ma.     

The 40Ar/36Ar Isotope Ratio in Spontaneous Gases of Mineral Waters in the Caucasian Region

Regularities in the distribution of ⁴⁰Ar/³⁶Ar values in spontaneous gases were examined for the Caucasian region. It was revealed that the maximal values of this parameter are characteristic for methane water seepages located along the periphery of this mountain structure (Dagestan Wedge and southern and northwestern slopes of the Greater Caucasus). The excess ⁴⁰Ar here is probably genetically related to active tectonic processes (nappes, high seismicity, stresses, and others) rather than mantle source. By contrast, the carbon dioxide of recent volcanic areas is characterized by nearly atmospheric (296) and lower (up to 285) ⁴⁰Ar/³⁶Ar values. In such springs in the Elbrus region, an inverse correlation was revealed between the ⁴⁰Ar/³⁶Ar ratio in gases and concentrations of geothermometer elements (Li, K, Rb, and Cs) in water. This suggests that the Ar isotopic signature is governed by thermal conditions of the gas–water fluid circulation. The contribution of mantle gases with high ⁴⁰Ar/³⁶Ar values turns out to be undetectable against the background of these processes.     

Diffusion of 40Ar in metamorphic hornblende

Isothermal, hydrothermal experiments were performed on two compositionally contrasting hornblendes from amphibolites in order to examine Ar diffusion behavior in metamorphic hornblendes. Ten experiments on sample RF were performed at temperatures of 750°C, 800°C, and 850°C and pressures of 1 kbar using measured grain radii of 158, 101, and 34 m. Eight experiments on sample 118576 were performed under the same conditions using measured grain radii of 145, 77, and 25 m. Minor (<5%) alteration was observed in high temperature runs. Diffusion coefficients were calculated from measured radiogenic ⁴⁰Ar loss following treatment assuming a spherical geometry for the mineral aggregate. Diffusivities calculated for different grain sizes vary by up to an order of magnitude for a given temperature indicating that the effective diffusion radius was less than the measured grain radius. Diffusivities for RF and 118576 calculated for grain radii of 101 and 145 m, respectively, form a linear array on an Arrhenius diagram with slopes indicating activation energies of 60 kcal/mol. No correlation between Mg number (100 Mg/(Mg+Fe)) and activation energy was observed. Diffusivities calculated for these experiments are higher than previously reported results from similar experiments performed on hornblendes. A comparison of results for 34 m splits from these two studies indicates higher apparent diffusivities (by a factor of 5), which probably result from observed phyllosilicate inter-growths (chlorite) and/or exsolution lamellae that partition the metamorphic hornblendes into smaller subdomains. Diffusivities calculated for experiments performed on 65 m and 34 m splits of ⁴⁰Ar/³⁹Ar standard MMhb-1 at 800°C and 1 kbar are consistent with a previously reported activation energy of 65 kcal/mol. Arrhenius parameters which emerge from the empirical model of Fortier and Giletti (1989) agree with experimental results to within analytical uncertainty. Although results of these experiments support previously reported estimates of the activation energy of ⁴⁰Ar in hornblende (60 kcal/mol), phyllosilicate intergrowths and/or microstructures such as exsolution lamellae within the two metamorphic hornblendes result in extremely small diffusion domains, which may lead to lower Ar retentivities and lower closure temperatures. The effective diffusion dimension for ⁴⁰Ar in hornblende is not likely to be defined by dislocations but rather by some larger structure within the crystal. TEM and SEM studies may provide some insight into the effective diffusion dimension for ⁴⁰Ar in amphiboles, thereby enabling better estimates of closure temperatures and more precise temperature-time reconstructions.     

Comparison of 40Ar-39Ar and Rb-Sr Data on Phengites from the UHP Brossasco-Isasca Unit (Dora Maira Massif, Italy): Implications for Dating White Mica

Different lithologies (impure marble, eclogite and graniticorthogneiss) sampled from a restricted area of the coesite-bearingBrossasco–Isasca Unit (Dora Maira Massif) have been investigatedto examine the behaviour of ⁴⁰Ar–³⁹Ar and Rb–Srsystems in phengites developed under ultrahigh-pressure (UHP)metamorphism. Mineralogical and petrological data indicate thatzoned phengites record distinct segments of the P–T path:prograde, peak to early retrograde in the marble, peak to earlyretrograde in the eclogite, and late retrograde in the orthogneiss.Besides major element zoning, ion microprobe analysis of phengitein the marble also reveals a pronounced zoning of trace elements(including Rb and Sr). ⁴⁰Ar–³⁹Ar apparent ages (35–62Ma, marble; 89–170 Ma, eclogite; 35–52 Ma, orthogneiss),determined through Ar laserprobe data on phengites (step-heatingand in situ techniques), show wide intra-sample and inter-samplevariations closely linked to within-sample microchemical variations:apparent ages decrease with decreasing celadonite contents.These data confirm previous reports on excess Ar and, more significantly,highlight that phengite acted as a closed system in the differentlithologies and that chemical exchange, not volume diffusion,was the main factor controlling the rate of Ar transport. Conversely,a Rb–Sr internal isochron from the same eclogite yieldsan age of 36 Ma, overlapping with the time of the UHP metamorphicpeak determined through U–Pb data and thereby corroboratingthe previous conclusion that UHP metamorphism and early retrogressionoccurred in close succession. Different phengite fractions ofthe marble yield calcite–phengite isochron ages of 36to 60 Ma. Although this time interval matches Ar ages from thesame sample, Rb–Sr data from phengite are not entirelyconsistent with the whole dataset. According to trace elementvariations in phengite, only Rb–Sr data from two wet-groundphengite separates, yielding ages of 36 and 41 Ma, are internallyconsistent. The oldest age obtained from a millimetre-sizedgrain fraction enriched in prograde–peak phengites mayrepresent a minimum age estimate for the prograde phengite relics.Results highlight the potential of the in situ ⁴⁰Ar–³⁹Arlaser technique in resolving discrete P–T stages experiencedby eclogite-facies rocks (provided that excess Ar is demonstrablya negligible factor), and confirm the potential of Rb–Srinternal mineral isochrons in providing precise crystallizationages for eclogite-facies mineral assemblages. KEY WORDS: ⁴⁰Ar–³⁹Ar dating; Rb–Sr dating; phengite; SIMS; UHP metamorphism     

Thermal history of the Ecstall pluton from Ar/Ar geochronology and thermal modeling

New ⁴⁰Ar/³⁹Ar thermochronology results and thermal modeling support the hypothesis of Hollister et al. (2004), that reheating of the mid-Cretaceous Ecstall pluton by intrusion of the Coast Mountains Batholith (CMB) was responsible for spatially variable remagnetization of the Ecstall pluton. ⁴⁰Ar/³⁹Ar ages from hornblende and biotite from 12 locations along the Skeena River across the northern part of the Ecstall pluton decrease with proximity to the Quottoon plutonic complex, the nearest member of the CMB to the Ecstall pluton. The oldest ⁴⁰Ar/³⁹Ar ages are found farthest from the Quottoon plutonic complex, and are 90 ± 3 Ma for hornblende, and 77.9 ± 1.2 Ma for biotite. The youngest ⁴⁰Ar/³⁹Ar ages are found closest to the Quottoon plutonic complex, and are 51.6 ± 1.2 Ma for hornblende, and 45.3 ± 1.7 Ma for biotite. No obvious relationship between grain size and age is seen in the Ecstall pluton biotites. Spatial trends in ⁴⁰Ar/³⁹Ar ages are consistent with model results for reheating by a thermal wall at the location of the Quottoon plutonic complex. Although no unique solution is suggested, our results indicate that the most appropriate thermal history for the Ecstall pluton includes both reheating and northeast side up tilting of the Ecstall pluton associated with intrusion of the Quottoon plutonic complex. Estimates of northward translation from shallow paleomagnetic inclinations in the western part of the Ecstall pluton are reduced to ∼3000 km, consistent with the Baja-BC hypothesis, when northeast side up tilting is accounted for.     

Evidence for shock heating and constraints on Martian surface temperatures revealed by Ar/Ar thermochronometry of Martian meteorites

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.     

40Ar/39Ar ages of Precambrian manganese ore minerals from Sweden,India and Morocco

In order to test their chronometric potential, ⁴⁰Ar/³⁹Ar stepheating- (and ⁴He-) analyses have been carried out on five manganese ore minerals of the hollandite-cryptomelane series from three Precambrian manganese deposits (Ultevis/Sweden, Sitapar/India, Bachkoun/Morocco). Samples from the metamorphic occurrences Ultevis and Sitapar yielded Ar ages of 1.8 Ga and 0.95 Ga, interpreted as the age of postmetamorphic cooling (Hollandites/Ultevis) and of an early, K-introducing alteration process subsequent to amphibolite facies metamorphism (cryptomelanes/Sitapar). Both data are consistent with known chronologies of the Svecokarelian and Satpura orogenic cycles. A date of 670 Ma obtained for a hollandite from a volcanogenic vein deposit (Bachkoun), however, contrasts with published extrusion ages of 580–560 Ma for the volcanic host rocks (Ouarzazate Series), probably due to incorporation of excess argon. The use of the ⁴⁰Ar/³⁹Ar technique, together with multiple isotope systematics, made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 Ma. Measurement of fractional Ar losses during vacuum step heating (500–1600 °C), although indicating good Ar retentivities, failed to define model diffusion parameters because of non-linear Arrhenius arrays. Helium diffusion results (200–1200 °C) indicated retention of radiogenic ⁴He by the samples, corroborated by U/He mineral dates between 0.96 and 0.31 Ga. Potassium-bearing manganese oxides are therefore able to retain argon (possibly also ⁴He) through geological times and may thus provide ages of ore-forming processes (and perhaps later cooling and alteration stages).     

Sm−Nd dating of multiple garnet growth events in an arc-continent collision zone,northwestern U.S. Cordillera

Integrated petrologic and Sm–Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite,P–T studies indicate a single stage of metamorphism with final equilibration at 600°C and 8–9 kbar. The Sm–Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128±3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A⁴⁰Ar/³⁹Ar age for this hornblende indicates cooling through 525°C at 119±2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm–Nd garnet — whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent ages of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry, Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at 128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.     

1.1 Ga K-rich alkaline plutonism in the SW Grenville Province

U–Pb zircon and baddeleyite dating of six syenitic stocks establishes that the ultrapotassic, potassic alkaline and shoshonitic magmatism with island-arc affinities in the Central Metasedimentary Belt (CMB) of the southwestern Grenville Province, Canada took place between 1089 and 1076 Ma, along a 400-km-long, northeast-trending plutonic belt. These ages indicate that ultrapotassic rocks with arc affinities are not unique to the Phanerozoic. West to east emplacement ages along a northern and southern cross-section of this belt range from 1083±2 Ma (Kensington), through 1081±2 Ma (Lac Rouge) to 1076 _–1 ⁺³ Ma (Loranger) in the north, and from 1089 _–3 ⁺⁴ Ma (loon Lake) and 1088±2 Ma (Calabogie), to 1076±2 Ma (Westport) in the south. Although closely spaced in time, in detail these ages suggest a slight younging of this magmatic activity to the southeast. Integration of the geochronological data with the spatial extent and potassic character of the plutons shows that the K-rich alkaline suite is distinct from the nepheline-syenite belt of the Bancroft terrane and from the syenite-monzonite suite of the Frontenac terrane of the CMB, and it is considered to be a magmatic episode unique to the Elzevir terrane and its Gatineau segment. The timing and the postmetamorphic emplacement of these plutons indicate that the regional greenschist to granulite-facies metamorphism of the country rock (precise age unknown) is older than 1089 Ma throughout the entire Elzevir terrane. The potassic magmatism is interpreted as the initiation of the 1090–1050 Ma Ottawan Orogeny in the Elzevir terrane; thus, the regional metamorphism in this terrane, previously assigned to the Ottawan Orogeny, is an earlier event. The contemporaneous emplacement of this postmetamorphic plutonic belt with Keweenawan volcanism is at variance with current tectonic models which consider the Keweenawan rift to be formed at the same time as regional metamorphism in the CMB.     

40Ar/39Ar dating of cleavage formation in tuffs during anchizonal metamorphism

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.     

Cooling and uplift patterns in the Lepontine Alps South Central Switzerland and an age of vertical movement on the Insubric fault line

96 new fission track (FT) apatite and zircon, K/Ar and Rb/Sr biotite and muscovite ages are presented for 19 samples (mainly acid gneisses) from a 40 km traverse through the Lepontine Alps in the Maggia Valley, South Central Switzerland. Plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample. The entire profile shows a fairly uniform Late Neogene-Recent mean uplift rate of 0.5 mm/a, confirmed by a gradient of FT apatite age with elevation. Cooling from higher temperatures occurred earlier in the south, where uplift rates of 2.2 mm/a in the Steep Belt (root zone) indicate >9 km Early Miocene uplift of the northern Pennine block. This uplift started before 23 Ma and is interpreted as resulting from a major phase of backthrusting along the Insubric Line, and as dating the formation of the mylonite belt. Estimated cooling rates constrain the timing of Lepontine Mid-Tertiary metamorphism: 3 schematic models are proposed which also consider published Rb/Sr, K/Ar mica and hornblende and U/Pb monazite ages. Slow cooling, differential initial heating and subsequent cooling of different parts of the Central Alps and post-38 Ma cooling with syntectonic metamorphism at 27 Ma are postulated as alternative interpretations of isotopic data and geologic evidence. From extrapolation between K/Ar and Rb/Sr mica ages and apatite FT ages, 240±50° C is proposed as the closure temperature for the retention of fission tracks in zircon.     

Argon and halogen geochemistry of hydrothermal fluids in the Loch Ainort granite,Isle of Skye,Scotland

Highly turbid alkali feldspars from the Loch Ainort granite (59 Ma), Isle of Skye, have been analysed using the ⁴⁰Ar-³⁹Ar method to obtain chronological and chemical (K, Cl, Br, I) information concerning their origin and hydrothermal alteration. Three methods of gas extraction have been applied to neutron-irradiated samples: laser probe spot fusion of feldspars, in vacuo crushing of a feldspar/quartz separate, and laser stepped heating of the crushed residue. Apparent ages obtained by laser probe spot fusion are mostly similar to the 59 Ma intrusion age. Analyses of relatively pristine regions give some high apparent ages (>59 Ma) indicating the presence of small amounts of ⁴⁰Ar_E (excess ⁴⁰Ar). Crushing releases significant amounts of ⁴⁰Ar_E, but is dominated by an ⁴⁰Ar_A (atmospheric ⁴⁰Ar) component. ⁸⁴Kr/³⁶Ar values obtained by crushing are higher than air and are consistent with air equilibration with fresh water at low temperature 20°C). Therefore, ⁴⁰Ar_A was most probably introduced as palaeoatmospheric argon dissolved in the circulating hydrothermal fluids that interacted with the granite, thus supporting a meteoric origin for the fluids. Stepped heating gives a flat age spectrum and an age of 56±4 Ma. Crushing and stepped heating both released significant amounts of halogens with high Br/Cl and I/Cl ratios; excess Xe is also present as indicated by the high ¹³²Xe/³⁶Ar values. It seems likely that the halogen (and possibly Xe) enrichments resulted from interaction of the meteoric fluids with Jurassic sedimentary country rocks.     

Comment on ‘Paleozoic ages and excess Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from Ar/Ar dating by stepwise crushing by Hua-Ning Qiu and J.R. Wijbrans’

In a recent study, Qiu and Wijbrans (2006) [Qiu, H.-N. and Wijbrans, J. R. (2006). Paleozoic ages and excess ⁴⁰Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from ⁴⁰Ar/³⁹Ar dating by stepwise crushing. Geochimica et Cosmochimica Acta70, 2354-2370.] analyzed Ar-isotopes extracted by crushing garnets from the Dabie Shan Bixiling eclogite and claimed to have constrained a pre-Triassic (∼450 Ma) episode of UHP metamorphism from primary fluid inclusions.However, in the absence of careful sample characterization and stepped heating analyses, the reported ages are more easily explained as experimental artifacts related to Ar extracted from either mineral inclusions or the interface sites between mineral inclusions and the garnet matrix: Dabie Shan garnets commonly contain mineral impurities such as K-rich omphacite and/or K-feldspar.If Dabie Shan UHP metamorphism is of the generally accepted Triassic age (210-240 Ma), the apparent age of a phengite sample in equilibrium with the garnet can be explained by the presence of an extraneous ⁴⁰Ar^∗ component with mean ⁴⁰Ar/³⁶Ar value of ∼5000. This value is similar to the composition of extraneous ⁴⁰Ar^∗ in other eclogite facies terrane.     

Evidence from Ar/Ar ages of lunar impact glasses for an increase in the impact rate ∼800 Ma ago

Geochemical and ⁴⁰Ar/³⁹Ar data on nine impact glasses from the Apollo 14, 16, and 17 landing sites indicate at least seven distinct impact events with ages ∼800 Ma. Rock fragments analyzed by Barra et al. [Barra F., Swindle T. D., Korotev R. L., Jolliff B. L., Zeigler R. A., and Olsen E. (2006) ⁴⁰Ar-³⁹Ar dating of Apollo 12 regolith: implications for the age of Copernicus and the source of nonmare materials, Geochim. Cosmochim. Acta,70, 6016-6031] from the Apollo 12 landing site and some Apollo 12 spherules reported by Levine et al. [Levine J., Becker T. A., Muller R. A., Renne P. R. (2005) ⁴⁰Ar/³⁹Ar dating of Apollo 12 impact spherules, Geophys. Res. Let., 32, L15201, doi: 10.1029/2005GL022874.] show ∼800 Ma ages, close to the accepted age of the Copernicus event, 800 ± 15 Ma [Bogard D. D., Garrison D. H., Shih C. Y., and Nyquist L. E. (1994) ³⁹Ar-⁴⁰Ar dating of two lunar granites: The age of Copernicus, Geochim. Cosmochim. Acta, 58, 3093-3100]. These Apollo 12 samples are thought to have been affected by material from the Copernicus event since there is a Copernicus ray going through the Apollo 12 landing site. When all of these data are viewed collectively, including an Apollo 16 glass bomb [Borchardt R., Stöffler D., Spettel B., Palme H. and Wänke H. (1986) Composition, structure, and age of the Apollo 16 subregolith basement as deduced from the chemistry of post-Imbrium melt bombs. In Proceedings, 17th Lunar and Planetary Science Conference, pp. E43-E54], and in the context of diverse compositional range and sample location, there is a suggestion that there may have been a transient increase in the global lunar impact flux at ∼800 Ma. Therefore, the Copernicus impact event could have been one of many. If correct, there should be evidence for this increased impact flux around 800 Ma ago in the age statistics of terrestrial impact samples.     

Post-collision magmatism and tectonics in northwest Anatolia

A suite of biotite-hornblende granodiorite intrusions has been emplaced into blueschist-facies metasediments in northwest Anatolia, following collision between two continental margins, now represented by the Tavanli and Sakarya zones. The ⁴⁰Ar/³⁹Ar ages of phengites and glaucophanes from the blueschists, metamorphosed under unusually high P-low T conditions (P=20±2 kbar, T=430±30° C), suggest that metamorphism apparently occurred over a period spanning at least 20 Ma from 108 to 88 Ma. Post-tectonic granodiorites were emplaced during the Eocene (53 to 48 Ma) resulting in a cordierite and andalusite-bearing thermal aureole, indicative of pressures of 3 kbar. Trace-element systematics of the granodiorites are consistent with a derivation either from mantle-derived magmas by fractional crystallisation in shallow magma chambers, or from anatexis of crustal lithologies of internediate composition at pressures <10 kbar. The preservation of high P-low T assemblages in the blueschists together with the range of ages determined for blueschist-facies metamorphism are indicative of rapid exhumation of delaminated fragments from a subducted continental margin. However decompression melting of the crust is unlikely to have been a significant cause of magmatism, both because exhumation of the blueschists from deep crustal levels predated magmatism by at least 25 Ma, and because of the small melt fraction (<0.1) that may be generated in crustal lithologies by this process. Melting in the mantle wedge is required either to generate a primary melt for the derivation of magmas of intermediate composition or to provide an advective heat source for crustal melting. The cause of melt formation in the upper mantle may be related to the termination of subduction following collision during the Mid-Eocene.     

Shock implantation of Martian atmospheric argon in four basaltic shergottites: A laser probe Ar/Ar investigation

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.     

The tholeiite to alkalic basalt transition at Haleakala Volcano,Maui, Hawaii

Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of 10% to 15%, while the intercalated alkalic lavas reflect a range of 6.5% to 8% melting. However, within the uppermost 50 m of section. ⁸⁷Sr/⁸⁶Sr decreases from 0.70371 to 0.70328 as eruption age decreased from 0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at 0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.     

Direct dating of folding events by 40Ar/39Ar analysis of synkinematic muscovite from flexural-slip planes

Timing of folding is usually dated indirectly, with limited isotopic dating studies reported in the literature. The present study investigated the timing of intracontinental, multi-stage folding in Upper Proterozoic sandstone, limestone, and marble near Beijing, North China, and adjacent regions. Detailed field investigations with microstructural, backscattered electron (BSE) images and electron microprobe analyses indicate that authigenic muscovite and sericite crystallized parallel to stretching lineations/striations or along thin flexural-slip surfaces, both developed during the complex deformation history of the study area, involving repeated compressional, extensional and strike-slip episodes. Muscovite/sericite separates from interlayer-slip surfaces along the limbs and from dilatant sites in the hinges of folded sandstones yield muscovite ⁴⁰Ar/³⁹Ar plateau ages of ∼158–159 Ma, whereas those from folded marble and limestone samples yield ages of 156 ± 1 Ma. Muscovite from thin flexural-slip planes on fold limbs and hinges yields ages within analytical error of ∼155–165 Ma. Further muscovite samples collected from extensionally folded limestone and strike-slip drag folds yield younger ages of 128–125 Ma with well-defined plateaus. To assess the potential influence of the detrital mica component of the host rock on the age data, two additional muscovite samples were investigated, one from a folded upper Proterozoic–Cambrian sandstone outside the Western Hills of Beijing and one from a folded sandstone sampled 20 cm from folding-related slip planes. Muscovite separates from these samples yield significantly older ages of 575 ± 2 Ma and 587 ± 2 Ma, suggesting that the timing of folding can be directly determined using the ⁴⁰Ar/³⁹Ar method. This approach enables the identification and dating of distinct deformation events that occur during multi-stage regional folding. ⁴⁰Ar/³⁹Ar dating can be used to constrain the timing of muscovite and sericite growth at moderate to low temperatures (<400 °C) during folding, yielding well-defined plateau ages and thereby the age of deformation in the upper crust.     

Tectonothermal implications of laser40Ar/39Ar ages of sulphide-bearing veins and their host rocks in the Willyama Supergroup,South Australia

Summary ⁴⁰Ar/³⁹Ar age spectra of mica separates from sulphide-bearing veins and associated metasedimentary whole-rock samples in the Olary Block, South Australia, indicate widespread emplacement of epigenetic mineralisation in Willyama Supergroup metasediments between 450 and 480 Ma, during the Delamerian Orogeny. This age range complements petrographic, structural and geochemical studies, which indicate that periodic tectonic disturbances occurred in the Olary Block since the Mid-Proterozoic, triggering the intermittent development of hydrothermal fluid circulation and infiltration into reactivated faults and shear zones over time scales of more than 1000 Ma.Vein emplacement did not result in extensive outgassing of argon from Mid-Proterozoic potassic minerals, as long as alteration of the psammopelitic host rock was restricted to the immediate contact of the vein. The release patterns of whole-rock samples illustrate that loss of^40*Ar in these assemblages is a function of the degree of metasomatism. Resetting of the K-Ar system occurred only in host rock assemblages that experienced strong pervasive alteration. The good retention properties of the host rock assemblages and the lack of a pervasive thermal pulse allow us to see through the Delamerian events. A positive correlation between increasing apparent ages and distance from the vein contact in less altered samples strongly support the view that the epigenetic alterations assemblages record actual vein emplacement ages.

---

Tektonik und Temperaturgeschichte der Wllyama Supergroup (Süd-Australien)—Schlußfolgerungen anhand von Laser⁴⁰Ar/³⁹Ar Datierungen an sulfidführenden Gängen und assoziierten Nebengesteinen

Zusammenfassung ⁴⁰Ar/³⁹Ar Altersspektra von Glimmer-Separaten aus sulfidführenden Gängen und assoziiertem Nebengesteins-Proben im Proterozoischen Olary Block, Süd-Australien, weisen auf die weitverbreitete Entstehung von Gangvererzungen zwischen 450 und 480 Ma in den Metasedimenten der Willyama Supergroup hin. Diese Gangmineralisationen stehen somit im Zusammenhang mit der Delamerian Orogenese. Die Entstehungsalter unterstützen petrographische, strukturelle und geochemische Untersuchungen, wonach periodische tektonische Aktivität im Olary Block seit dem Mittleren Proterozoikum auftrat. Dies führte zu der wiederholten Bildung und Zirkulation von hydrothermalen Lösungen sowie deren Infiltration in re-aktivierte Störungen und Scherzonen über einen Zeitraum von mehr als 1000 Ma.Die Entstehung der Gangvererzungen bewirkte keinen signifikanten Verlust von Argon aus Mittel-Proterozoischen, Kalium-reichen Mineralen, solange die hydrothermale Alteration im Nebengestein auf den unmittelbaren Kontakt zur Gangvererzung beschränkt blieb. Die Gasspektren der Nebengesteins-Proben zeigen deutlich, daß Verlust von radiogenem Argon in diesen Gesteinen vom Grad der hydrothermalen Alteration abhängig ist. Neueinstellung des K-Ar Systems geschah nur im Fall von extremer, durchdringender hydrothermaler Alteration des Nebengesteins. Die⁴⁰Ar/³⁹Ar Analysen der Nebengesteins-Proben ermöglichen somit in Abwesenheit eines durchdringenden thermalen Pulses Alterbestimmungen über die Ereignisse der Delamerian Gebirgsbildung hinaus. Die positive Korrelation zwischen zunehmenden Ar/Ar Altern der Nebengesteine und deren Abstand vom Kontakt zu den Gangvererzungen in schwach alterierten Proben unterstützt die Ansicht, daß die epigenetischen Alterierungsparagenesen tatsächliche Vererzungsalter repräsentieren.

---

---

With 6 Figures