Comparison of Conventional K–Ar and Ar/Ar Dating of Young Mafic Volcanic Rocks

K–Ar and ⁴⁰Ar/³⁹Ar 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 K₂O measurements by flame photometry. The ⁴⁰Ar/³⁹Ar 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 ⁴⁰Ar/³⁹Ar technique. The precision of an ⁴⁰Ar/³⁹Ar 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 ⁴⁰Ar/³⁶Ar 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 ⁴⁰Ar/³⁹Ar ages can be measured on volcanic rocks as young as the latest Pleistocene, and perhaps even the Holocene.     

激光显微探针~(40)Ar/~(39)Ar同位素定年

已有 5 0年历史的K Ar定年法 ,由于过剩Ar和Ar丢失的普遍发现 ,使其最广泛的应用面临着严重挑战。40 Ar/ 3 9Ar分步加热释氩法是常规K Ar定年法的发展 ,它克服了常规K Ar定年法的一些局限 ,又可以测定岩浆构造热事件。激光显微探针40 Ar/ 3 9Ar定年法是在 2 0世纪末把聚焦激光束应用在40 Ar/ 3 9Ar分步加热释氩法中而发展起来的一种定年方法。它既具有常规K Ar定年法和40Ar/ 3 9Ar分步加热释氩法的所有优越性 ,又把定年引入微观领域。特别是在 2 0世纪的最后几年 ,以激光显微探针40 Ar/ 3 9Ar定年方法的完善和精度的提高为标志 ,把K Ar年代学研究推向了一个新的里程碑。微区微量高精度高分辨定年 ,把定年时限扩展到人类历史范畴 ,精细的分析技术拓宽了年代学的应用范围 ,使之解决的地质问题更广泛和深入 ,并且开始冲击着地球科学中的某些热点和难点课题。     

~(40)Ar/~(39)Ar热年代学现状与问题

Ar同位素体系定年矿物的封闭温度范围广,并且可以获得650℃～150℃温度段的非线性冷却历史,因此~(40)Ar/~(39)Ar热年代学成为研究地质体热演化历史过程中最有效的工具之一。但是由于矿物中Ar同位素扩散机制还有一些问题没有清楚地被认识,因此在一定程度上制约了~(40)Ar/~(39)Ar热年代学的发展。本文介绍了目前应用最广泛的两种~(40)Ar/~(39)Ar热年代学模式:多重扩散域模式和多路径扩散模式,讨论了它们的发展现状、存在的问题、可能解决的方法以及今后的发展方向。     

铜陵地区中酸性侵入岩年代学研究

本文选择了铜陵地区主要岩石类型的代表性岩体中黑云母为测定对象，准确地测定了侵入岩的４０Ａｒ／３９Ａｒ同位素年龄。测定结果表明，区内侵入岩的年龄均小于１４０Ｍａ，属燕山晚期的产物，后期热事件为成矿时代，晚于岩浆侵入时代，在此基础上，分析了ＫＡｒ法、ＲｂＳｒ法同位素年龄产生偏差的原因     

海绿石 K-Ar、40Ar/39Ar 年代学研究

海绿石是与沉积岩同时形成的高钾层状硅酸盐自生矿物，其 K-Ar、40Ar/39Ar 年龄存在失真现象。笔者认为，引起年龄失真的主要原因是由于该矿物结构中含有可膨胀层。通过测量可膨胀层含量及中子活化过程中39Ar的丢失率，使可对海绿石常规 K-Ar、40Ar/39Ar 年龄进行校正，得到其年龄校正公式。     

40Ar/39Ar age spectra of some undisturbed terrestrial samples

⁴⁰Ar/³⁹Ar age spectra and ⁴⁰Ar/³⁶Ar vs ³⁹Ar/³⁶Ar isochrons were determined by incremental heating for 11 terrestrial rocks and minerals whose geology indicates that they represent essentially undisturbed systems. The samples include muscovite, biotite, hornblende, sanidine, plagioclase, dacite, diabase and basalt and range in age from 40 to 1700 m.y. For each sample, the ⁴⁰Ar/³⁹Ar ratios, corrected for atmospheric and neutron-generated argon isotopes, are the same for most of the gas fractions released and the age spectra, which show pronounced plateaus, thus are consistent with models previously proposed for undisturbed samples. Plateau ages and isochron ages calculated using plateau age fractions are concordant and appear to be meaningful estimates of the crystallization and cooling ages of these samples. Seemingly anomalous age spectrum points can be attributed entirely to small amounts of previously unrecognized argon loss and to gas fractions that contain too small (less than 2 per cent) a proportion of the ³⁹Ar released to be geologically significant. The use of a quantitative abscissa for age spectrum diagrams is recommended so that the size of each gas fraction is readily apparent. Increments containing less than about 4–5 per cent of the total ³⁹Ar released should be interpreted cautiously. Both the age spectrum and isochron methods of data reduction for incremental heating experiments are worthwhile, as each gives slightly different but complementary information about the sample from the same basic data. Use of a least-squares fit that allows for correlated errors is recommended for ⁴⁰Ar/³⁶Ar vs ³⁹Ar/³⁶Ar isochrons. The results indicate that the ⁴⁰Ar/³⁹Ar incremental heating technique can be used to distinguish disturbed from undisturbed rock and mineral systems and will be a valuable geochronological tool in geologically complex terranes.     

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.     

柴北缘地区天然气40Ar/36Ar特征及其地质意义

测试并分析了柴北缘各油气田的14个天然气样品的氦、氩稀有气体同位素比值,并进行了气源对比。研究发现,柴北缘天然气40Ar/36Ar值分布在951～1712,平均1098,多数样品40Ar/36Ar较通常认为源于侏罗系的天然气40Ar/36Ar明显偏高,研究认为本区不存在幔源高40Ar/36Ar流体的介入,储层年代效应也不可能造成天然气40Ar/36Ar明显偏高,因此柴北缘天然气40Ar/36Ar明显偏高,主要是由于源岩年代积累效应引起的。据估算,气源岩年龄分布范围为164.7～460.8Ma,平均为345.1Ma,可能多数来源于石炭系。这一认识得到了柴北缘广泛分布有石炭系源岩和已发现源于石炭系原油的支持。柴北缘石炭系天然气的发现预示了柴北缘石炭系是一个新的油气勘探层系,从而拓展了柴北缘天然气勘探领域。     

湖北省随州三里岗地区二长花岗岩Rb-Sr、40Ar/39Ar同位素年龄

三里岗二长花岗岩与花山蛇绿混杂岩中的基性火山岩呈侵入接触关系,其年龄的确定可解决花山蛇绿混杂岩中基性岩形成年龄的上限问题. 本文对三里岗二长花岗岩分别进行了Sm、Nd同位素研究及Rb-Sr、40Ar/39Ar同位素年龄测定.3个全岩样品的Nd模式年龄平均值为1 064±105 Ma;12个全岩样品的Rb-Sr等时线年龄为422±53(2σ)Ma;二长花岗岩中所含角闪石矿物的40Ar/39Ar坪年龄为141.4±0.3 Ma,等时线年龄为142±2 Ma.这些结果暗示了花山蛇绿混杂岩中基性火山岩的形成年龄不可能晚于422 Ma.     

Age calibration of the Fish Canyon sanidine Ar/Ar dating standard using primary K-Ar standards

The ⁴⁰Ar/³⁹Ar dating technique requires the use of neutron fluence monitors (standards). Precise calibrations of these standards are crucial to decrease the uncertainties associated with ⁴⁰Ar/³⁹Ar dates. Optimal calibration of ⁴⁰Ar/³⁹Ar standards should be based on K/Ar standards having independent isotope dilution measurements of ⁴⁰K and ⁴⁰Ar*, based on independent isotope tracers (spikes) because this offers the possibility to eliminate random interlaboratory errors. In this study, we calibrate the widely used Fish Canyon sanidine (FCs) standard based on four primary K/Ar standards (GA-1550, Hb3gr, NL-25, and GHC-305) on which K and Ar* concentrations have been determined in different labs with independently calibrated tracers. We obtained a mean age of 28.03 ± 0.08 Ma (1σ; neglecting uncertainties of the ⁴⁰K decay constants) for FCs, based on the decay constant recommended by Steiger and Jäger [Steiger R.H., Jäger. E. 1977. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett.36, 359-362.]. This age corresponds to a mean ⁴⁰Ar*/⁴⁰K value of (1.6407 ± 0.0047) × 10⁻³. We also discuss several criteria that prevent the use of previous calibrations of FCs based on other primary standards (LP-6, SB-3 and MMhb-1). The age of FCs obtained in this study is based on the ⁴⁰K decay constants of Steiger and Jäger (1977) but we anticipate the imminent need for revision of the value and precision of the ⁴⁰K decay constants (representing the main source of uncertainties in ⁴⁰Ar/³⁹Ar dating). The ⁴⁰Ar*/⁴⁰K result of FCs obtained in this study allows therefore a rapid calibration of the age of FCs with uncertainties at the 0.29% level but perhaps more importantly this value is independent of any particular value of the ⁴⁰K decay constants and may be used in the future in conjunction with revised decay constants.     

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.     

Instrumentation Development for In Situ 40Ar/39Ar Planetary Geochronology

The chronology of the Solar System, particularly the timing of formation of extra‐terrestrial bodies and their features, is an outstanding problem in planetary science. Although various chronological methods for in situ geochronology have been proposed (e.g., Rb‐Sr, K‐Ar), and even applied (K‐Ar), the reliability, accuracy, and applicability of the ⁴⁰Ar/³⁹Ar method makes it by far the most desirable chronometer for dating extra‐terrestrial bodies. The method however relies on the neutron irradiation of samples, and thus a neutron source. Herein, we discuss the challenges and feasibility of deploying a passive neutron source to planetary surfaces for the in situ application of the ⁴⁰Ar/³⁹Ar chronometer. Requirements in generating and shielding neutrons, as well as analysing samples are described, along with an exploration of limitations such as mass, power and cost. Two potential solutions for the in situ extra‐terrestrial deployment of the ⁴⁰Ar/³⁹Ar method are presented. Although this represents a challenging task, developing the technology to apply the ⁴⁰Ar/³⁹Ar method on planetary surfaces would represent a major advance towards constraining the timescale of solar system formation and evolution.     

Amphibole Ar/Ar Geochronology from the Okcheon Metamorphic Belt, South Korea and its Tectonic Implications

An understanding of the Okcheon Metamorphic Belt (OMB) in South Korea is central to unraveling the tectono-metamorphic evolution of East Asia. Amphibole-bearing rocks in the OMB occur as calcsilicate layers and lenses in psammitic rocks, in the psammitic rocks themselves, and in the mafic volcanic layers and intrusives. Most amphiboles fail to show ⁴⁰Ar/³⁹Ar plateau ages; those that do have ages ranging from 132 to 975 Ma. The disturbed age pattern and wide variation in ⁴⁰Ar/³⁹Ar ages can be related to metamorphic grade, retrograde chemical reactions, excess Ar and amphibole composition. The oldest age (975 Ma) can be interpreted either as an old igneous or metamorphic age predating sedimentation or a false age caused by excess Ar. The youngest age of 132 Ma and the disturbed age pattern found in amphiboles from rocks located close to Jurassic granitoids are the result of retrograde thermal metamorphic effects accompanying intrusion of the granitoids. Some medium- or coarse-grained amphiboles in the calcsilicates are aggregates of fine-grained crystals. As a result, they are heterogeneous and prove to be readily affected by excess Ar. A disturbed age pattern in amphiboles from the calcsilicates occurring in the high-grade metamorphic zone may also be the product of excess Ar. On the other hand, the disturbed pattern of amphiboles present in the calcsilicates from the low-grade metamorphic zone could arise from both excess Ar and mixed ages. However, amphiboles from psammitic rocks and some calcsilicates in the high-grade metamorphic zone and in intrusive metabasites display real plateau ages of 237 to 261 Ma. The temperature conditions in the high-grade metamorphic zone were higher than the argon closing temperature for amphibole, and the amphiboles in this zone give plateau ages only when they are homogeneous in composition, lack excess Ar, and have not been thermally affected by intrusion of the granitoids. The unmodified ⁴⁰Ar/³⁹Ar ages prove rather younger than the age of the Late Paleozoic metamorphic event of 280 to 300 Ma, but they are close to muscovite K-Ar ages of 263 to 277 Ma. These ⁴⁰Ar/³⁹Ar amphibole ages are interpreted as the time of cooling that followed the main regional, intermediate-P/T metamorphic climax. The results demonstrate that interpretation of ⁴⁰Ar/³⁹Ar amphibole ages in an area subjected to several metamorphic events can be accomplished only by undertaking a thorough tectono-metamorphic study, accompanied by detailed chemical analysis of the amphiboles.     

40Ar/39Ar ages and 40Ar* concentrations of fine-grained sediment fractions from North Atlantic Heinrich layers

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.     

40Ar/39Ar and Rb-Sr geochronology of the Qingyang batholith,Anhui Province,China

The geochronology and genesis of the Qingyang batholith were investigated using⁴⁰Ar/³⁹Ar and Rb-Sr isotopic techniques. The Qingyang is a composite batholith consisting of two major rock types granodiorite and granite in the Yangtze fold belt.⁴⁰Ar/³⁹Ar spectra for biotite and amphibole separates are internally concordant. The concordance of the minerals and spectra indicate no thermal disturbance of the ages, and rapid cooling of the rocks. The granodiorite has an age of 137.6±1.4 m.y. and the granite 122.7±1.2 m.y. Whole-rock Rb-Sr analysis yields ages consistent with the⁴⁰Ar/³⁹Ar dates. Thus, the Qingyang batholith was formed in two major stages in the late Jurassic and early Cretaceous. The batholith is not Triassic as was previously proposed. Special⁴⁰Ar/³⁹Ar analysis of two granodiorite samples has precisely documented a 1.0 m.y. apparent age difference between these samples. Several factors could account for this difference, but different emplacement times seem most convincible. The granodiorite and granite show little variation in initial⁸⁷Sr/⁸⁶Sr ratio (about 0.7085). The high initial Sr ratios suggest that the magmas were formed by anatexis of older crustal materials.     

Calibration of GA1550 biotite standard for K/Ar and 40Ar/39Ar dating

The mineral separate GA1550 biotite has become an international standard for K/Ar and ⁴⁰Ar/³⁹Ar dating studies, although it was prepared as an intralaboratory standard at ANU to monitor tracer depletion from a gas pipette. It is one of a small number of samples that has been calibrated against ³⁸Ar tracers, some of which had been mixed with known amounts of atmospheric argon, so that a so-called primary calibration has been performed. By measuring GA1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1.342 (± 0.007) × 10^? 9 mol/g, and together with the measured K content of 7.645 (± 0.050) weight percent, we derive a best estimate for the K/Ar age as 98.5 ± 0.5 Ma, where the error is derived from averaging the ages determined relative to the ³⁸Ar tracer.     

Phlogopite40Ar/39Ar geochronology of mantle xenoliths from the North China Craton: Constraints on the eruption ages of Cenozoic basalts

Mantle xenoliths provide direct information about lithospheric evolution and asthenosphere–lithosphere interaction, and therefore precise dating of the host basalts which carried the xenoliths is important. Here we report ⁴⁰Ar/³⁹Ar geochronology of phlogopite separates from five spinel lherzolite xenoliths collected from the North China Craton (Hannuoba of Hebei Province, Sanyitang of Inner Mongolia Autonomous Region and Hebi of Henan Province), as well as the groundmass of the host basalts. Argon extraction was performed by conventional step heating technique and ultra-violet laser ablation microprobe (UVLAMP) technique. ⁴⁰Ar/³⁹Ar incremental heating results on groundmass yielded geologically meaningless ages. However, conventional step heating on phlogopites produced Miocene cooling ages, identical to the eruption ages obtained from the K–Ar dating methods of the Hannuoba and Sanyitang basalts. Adopting procedures to exclude potential influence of excess radiogenic Ar from a deep fluid source on a phlogopite separate from lherzolite yielded results with a good agreement of ages suggesting that the argon isotopes are distributed homogenously in this mineral, with no influence of excess argon. Phlogopites from Hebi yield ages between 6.43 and 6.44 Ma which are slightly older than those obtained from K–Ar method on whole-rocks. The discrepancy in the K–Ar ages obtained from the altered whole-rock samples suggests partial loss of ⁴⁰Ar. As a consequence, phlogopite Ar–Ar ages are considered more accurate than that of the whole-rocks. These results suggest that ⁴⁰Ar/³⁹Ar chronology of phlogopite provides reliable and precise ⁴⁰Ar/³⁹Ar ages of host basalts.     

Ar and Ar recoil loss during neutron irradiation of sanidine and plagioclase

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.     

南秦岭铧厂沟金矿床铬云母~(40)Ar/~(39)Ar年代学及其大地构造意义

南秦岭铧厂沟金矿床位于勉略缝合带以南的逆冲推覆带内,矿体呈透镜体或脉状产于新元古界蚀变细碧岩及泥盆系灰岩中,受控于近EW向叠瓦状逆冲断层及韧脆性剪切带。为了准确厘定其成矿时代,对矿区蚀变细碧岩型矿石和含矿石英脉中的铬云母进行了年龄测定。2件铬云母样品的~(40)Ar/~(39)Ar坪年龄分别为209.4±2.3 Ma和211.5±2.5 Ma,相应的等时线年龄211.4±3.6 Ma和215.3±3.9 Ma,与坪年龄在误差范围内一致。因此,铧厂沟金矿床的成矿年龄为212~209 Ma。结合铧厂沟金矿床的大地构造位置、矿床地质特征及成因类型,推测铧厂沟金矿床形成于扬子板块与秦岭微板块的碰撞过程,其成矿年龄代表了两板块发生碰撞的下限。     

A redetermination of the isotopic abundances of atmospheric Ar

Atmospheric argon measured on a dynamically operated mass spectrometer with an ion source magnet, indicated systematically larger ⁴⁰Ar/³⁶Ar ratios compared to the generally accepted value of Nier [Nier A.O., 1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Phys. Rev. 77, 789-793], 295.5 ± 0.5, which has served as the standard for all isotopic measurements in geochemistry and cosmochemistry. Gravimetrically prepared mixtures of highly enriched ³⁶Ar and ⁴⁰Ar were utilized to redetermine the isotopic abundances of atmospheric Ar, using a dynamically operated isotope ratio mass spectrometer with minor modifications and special gas handling techniques to avoid fractionation. A new ratio ⁴⁰Ar/³⁶Ar = 298.56 ± 0.31 was obtained with a precision of 0.1%, approximately 1% higher than the previously accepted value. Combined with the ³⁸Ar/³⁶Ar (0.1885 ± 0.0003) measured with a VG5400 noble gas mass spectrometer in static operation, the percent abundances of ³⁶Ar, ³⁸Ar, and ⁴⁰Ar were determined to be 0.3336 ± 0.0004, 0.0629 ± 0.0001, and 99.6035 ± 0.0004, respectively. We calculate an atomic mass of Ar of 39.9478 ± 0.0002. Accurate Ar isotopic abundances are relevant in numerous applications, as the calibration of the mass spectrometer discrimination.