New high precision zircon ages from the Carboniferous of Scotland and their implications for the systematic bias between U-Pb and 40 Ar/ 39 Ar dating techniques
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Absolute dating
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Flood basalt
Large igneous province
Deccan Traps
Early Triassic
Radiometric dating
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The frequency statistics analysis is conducted for 207 pairs of difference values Δt between the hornblend-biotite40Ar-39 Ar ages(tAr) and zircon U-Pb ages(tZr) of granite plutons at home and abroad,the result indicates that theΔt(Δt=tZr-tAr) display a symmetrical normal distribution(skewness coefficient CSK=-0.193;kurtosis coefficient CKU=16.9) in the histogram,the Δt are both positive and negative with a mode of 0.40 Ma and a mean of 0.45 Ma.Using least squares regression procedure,a best liner regression equation(tAr=0.997 7×tZr+0.004 5) with high correlation coefficient(R=0.995) and regression coefficient(closed ito 1) is fitted for zircon U-Pb ages(tZr) and hornblend-biotite40Ar-39Ar ages(tAr) of granite plutons.These statistical analysis characteristics indicate that the hornblend-biotite40Ar-39Ar dating results are corresponding with zircon U-Pb dating results for granites,and there seams no a regularity of hornblende40Ar-39 Ar dating agesbiotite40Ar39Ar dating ages.
Kurtosis
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Abstract Zircon OD ‐3 from the P aleogene K awamoto G ranodiorite ( M ihara body) in J apan has been identified as a potential multi‐grain secondary standard for U–Pb dating. We have carried out an inter‐laboratory evaluation in order to evaluate possible heterogeneity amongst the OD ‐3 zircon grains. U–Pb ages were obtained using two analytical techniques (a sensitive high‐resolution ion microprobe and a laser ablation–inductively coupled plasma–mass spectrometry) in eight laboratories. All the 238 U– 206 Pb ages show good agreement, with an overall weighted average 238 U– 206 Pb age of 33.0 ± 0.1 Ma (2σ). The U–Pb age results revealed no significant variation or heterogeneity in the U–Pb ages of the OD ‐3 grains. Twelve fission‐track ( FT ) ages from three laboratories are also reported, and have a weighted average of 32.6 ± 0.6 Ma (2σ). Despite the different closure and annealing temperatures of the U–Pb and FT chronometers, respectively, the FT age is in good agreement with the U–Pb age. This suggests that the OD ‐3 zircon had a relatively fast cooling history and has not experienced later thermal annealing. The chronological dataset reported here clearly demonstrates that the OD ‐3 zircon could be a useful and reliable secondary standard for use during U–Pb dating studies of Cenozoic zircons.
Fission track dating
Closure temperature
Geochronology
Microprobe
Metamictization
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Recent papers (Black et al. 1997; Jagodzinski & Black 1999) reported that SL13‐referenced SHRIMP zircon ages are ca 1.3% younger than QGNG‐referenced ages. It is shown here that these authors overestimated their analytical errors and as a consequence failed to register the presence of large and variable fractions of Pb loss within both QGNG (median 40% of spots) and SL13 (25%). If only the oldest 206Pb/238U age groups within each of the same 16 analytical sessions are selected for comparison, the 206Pb/238U ages of QGNG relative to SL13 agree with the Concordia intersection age by isotope dilution to within 0.4% (standard deviation). This observation eliminates the possibility of systematic bias. Jagodzinski and Black (1999) also overlooked Pb loss in their ages for the Early Devonian Turondale and Merrions Formations, which are corrected here for time‐scale use.
Devonian
Late Devonian extinction
Isotope dilution
Geochronology
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In the past,it was difficult to get the accurate 40Ar/39Ar dating results of siliceous rocks with low potassium and high excess 40Ar content. In recent years,due to the improvement of instruments and the application of new techniques and methods,in particular,the application of laser 40Ar/39Ar dating method, 40Ar/39Ar dating method has both high enough precision and low enough background level to meet the requirements of getting precise age for siliceous rocks with low potassium. The method using data of multiple sets of mineral particles to calculate isochrone age can be adopted to eliminate the impact of excess 40Ar on the age of siliceous rocks. This paper use laser 40Ar/39Ar dating method to date two siliceous rocks collected from the edge of the Junggar Basin in north Xinjiang. The age of the sample 08BJT-3 collected from Baijiantan area is 294 ± 14Ma. It is basically consistent with the depositional age of the Late Carboniferous strata which the siliceous rock sample lies in. The age of the sample KML-2 collected from Karamaili area is 266 ± 14Ma. It is fully consistent with the activity age of the Karamaili fault zone which strongly transforms the siliceous rock sample. Both of these indicate that the laser 40Ar/39Ar dating method can accurately date the siliceous rocks.
K–Ar dating
Radiometric dating
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Abstract K–Ar and 40 Ar/ 39 Ar ages have been measured on nine mafic volcanic rocks younger than 1 myr from the Snake River Plain (Idaho), Mount Adams (Washington), and Crater Lake (Oregon). The K–Ar ages were calculated from Ar measurements made by isotope dilution and K 2 O measurements by flame photometry. The 40 Ar/ 39 Ar ages are incremental-heating experiments using a low-blank resistance-heated furnace. The results indicate that high-quality ages can be measured on young, mafic volcanic rocks using either the K–Ar or the 40 Ar/ 39 Ar technique. The precision of an 40 Ar/ 39 Ar plateau age generally is better than the precision of a K–Ar age because the plateau age is calculated by pooling the ages of several gas increments. The precision of a plateau age generally is better than the precision of an isotope correlation (isochron) age for the same sample. For one sample the intercept of the isochron yielded an 40 Ar/ 36 Ar value significantly different from the atmospheric value of 295.5. Recalculation of increment ages using the isochron intercept for the composition of nonradiogenic Ar in the sample resulted in much better agreement of ages for this sample. The results of this study also indicate that, given suitable material and modern equipment, precise K–Ar and 40 Ar/ 39 Ar ages can be measured on volcanic rocks as young as the latest Pleistocene, and perhaps even the Holocene.
Isochron dating
Isotopes of argon
Caldera
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Geochronology
Cenomanian
Radiometric dating
Aptian
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High-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40 Ar/ 39 Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U–Pb ages, despite the expected 1% theoretical bias between 40 Ar/ 39 Ar and U–Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40 Ar/ 39 Ar age uncertainties) from closure temperatures (T c ) of zircon (699–988 °C) to amphibole (500–600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40 Ar/ 39 Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole T c is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ 18 O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO 2 -rich carbonatite-derived fluids released during contact metamorphism.
Baddeleyite
Amphibole
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As a result of recent updating of decay constants and standard ages used for 40Ar/39Ar dating, it is necessary to recalibrate legacy ages obtained with older methods. These recalibrations bring legacy 40Ar/39Ar ages into better agreement with ages obtained using 238U/206Pb dating methods. We present nine recalibrated 40Ar/39Ar ages for the Upper Jurassic Morrison Formation of the Western Interior, U.S.A., along with the individual geographic and stratigraphic locations for each sample. These recalibrated ages will be useful for researchers looking to place better age constraints on the flora and fauna of the Morrison Formation, as well as for those working to understand stratigraphic relationships across the formation. The recalibrated ages also can now be used reliably for comparisons with newer 238U/206Pb ages obtained for the Morrison Formation.
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Overprinting
Isochron dating
Baltic Shield
Radiometric dating
Geochronology
Isotopes of argon
Impact structure
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