How to cut a sediment core for <Superscript>210</Superscript>Pb geochronology: a supplement

To obtain the profile of excess ²¹⁰Pb in sediment column, it is necessary to section the collected core to date. Sectioning thickness is generally not explicit. An approach when sedimentation rate is constant has been suggested by Lu and Matsumoto (Environ Geol 47:804–810, 2005). This paper addresses the case when sedimentation rate is variable. Actually, it is possible to assume that sedimentation rate is constant within the sectioning thickness and based on this concept, a tentative approach for determining reasonable sectioning thickness is proposed.     

Dating recent colluvial sequences with Pb and Cs along an active fault scarp, the Eliki Fault, Gulf of Corinth, Greece

Reliable dating is an essential element of palaeoseismological studies, yet whilst a suite of geochronological methods can now provide late Quaternary age control it remains very difficult to date modern events (i.e., those occurring within the last 150 years). This is significant because the starting point for many palaeoseismological investigations is a modern surface-rupturing event, whose geological effects need to be disentangled in trench stratigraphies from palaeoseismic ruptures. Two dating methods which, in combination, can provide robust dating control in recently deposited sediments are the ²¹⁰Pb and ¹³⁷Cs dating methods. Here, we test the applicability of using ²¹⁰Pb and ¹³⁷Cs to date colluvial sediments exposed in three trenches excavated across an earthquake fault—the Eliki fault, Gulf of Corinth, Greece—which ruptured in an earthquake in 1861. The ²¹⁰Pb and ¹³⁷Cs profiles observed in these colluvial sequences are relatively erratic due to the mixed nature of the sediments, i.e., their deposition in an environment where the supply of slope sediments is driven by seasonal rainfall, causing non-uniform sediment accretion and sediment reworking. In one trench, however, ²¹⁰Pb dating, corroborated by ¹³⁷Cs dating, indicates that a proposed post-1861 surface colluvial unit has been deposited over the period 1950 AD–present (at a rate of ca. 9 mm/year), and overlies a significantly older unit (>120 years old). The dating control provided here by ²¹⁰Pb and ¹³⁷Cs dating corroborates the published interpretation of the trench stratigraphy, and refines the ¹⁴C-based estimated dates for the upper unit. At two other trenches ²¹⁰Pb and ¹³⁷Cs dating only provided minimum ages (based on the presence or absence of ²¹⁰Pb_excess and ¹³⁷Cs). Such approximate ages, however, may still useful in corroborating interpretations made using the trench stratigraphy, or, at sites which have long earthquake recurrence intervals, determining which earthquake event was responsible for a particular bed offset.     

放射性同位素定年技术在崇明岛潮滩沉积速率上的应用研究

选取崇明岛北侧由东至西4个潮滩沉积柱,分析137Cs、226Ra和210Pb放射性比活度,应用210Pb和137Cs放射性同位素定年方法计算崇明岛潮滩沉积物沉积速率。对于东西两端,210Pb方法测得速率分别为3.08cm/a和2.34 cm/a,而137Cs测得速率分别为6.19 cm/a和2.06 cm/a。结果表明,137Cs定年方法计算出的潮滩沉积速率普遍大于210Pb方法结果;但两者反映了相同的速率规律。崇明岛主力生长方向为东和东北,西侧沉积作用相对较弱,表现出"东快西慢"的特点。沉积纵向上,1954年以来,自下而上沉积速率逐渐减缓。     

东北四海龙湾玛珥湖沉积物纹层计年与137Cs、210Pb测年

对东北四海龙湾玛珥湖SHLF6孔纹层沉积物的¹³⁷Cs放射性测量表明:¹³⁷Cs比活度的最大值出现在55cm处,对应于1963年世界原子弹试爆高峰期。纹层计年表明0～6cm共有35个纹层层偶。从75cm到65cm,¹³⁷Cs比活度从256±009dpm/g急剧增加到1868±017dpm/g,可能65cm对应于1954年。通过测量²²⁶Ra子核²¹⁴Pb和²¹⁴Bi（能量为295keV,352keV和609keV）放射的光子数获得²²⁶Ra比活度数据,然后求得过剩²¹⁰Pb_{比活度（²¹⁰Pbuns）。²¹⁰Pbuns比活度随深度增加而呈指数衰减,其异常波动可能与人类活动以及沉积速率变化有关,例如55cm处²¹⁰Pb比活度较高,与¹³⁷Cs的峰值对应,这可能与1963年前后人工核实验的高峰有关,因为核试验不仅产生¹³⁷Cs,而且可以产生208Pb和²¹⁰Pb;45cm处²¹⁰Pb比活度较低,而²²⁶Ra较高,可能与人类活动加剧,导致沉积速率增加有关。根据²¹⁰PbunsCRS模式,SHLF6孔0～19cm的平均沉积速率为20mg/cm²·a,或约为011cm/a。²¹⁰Pb测年数据与¹³⁷Cs时标及纹层计年均有很好的一致性。四海龙湾玛珥湖发育的纹层为年纹层,可以建立高分辨率时间序列。}     

年轻火山岩的定年:进展与问题

年轻火山岩定年对人类的起源与进化、地质年表建立、古环境演化、火山灾害、岩浆演化及地球动力学等研究非常重要。由于年轻样品放射性成因子体积累较少、大气氩含量高,微量的过剩氩极难识别,使得年轻火山岩及其沉积地层高精度定年一直是地质年代学中最具挑战性的课题之一。过去几年,年轻火山岩定年在技术上的进展主要体现在定年精度和准确度不断提升,尤其是利用新型多接收稀有气体质谱对单颗粒样品进行逐步升温~(40)Ar/~(39)Ar定年,是方法学上最重要的进展之一。本文总结了年轻火山岩~(40)Ar/~(39)Ar定年的现状、影响因素和相关定年技术及应用,以引起大家的关注和思考,共同提高我国年轻火山岩定年的技术水平。利用新型多接收质谱和创新的实验方法,通过对全球重要火山喷发事件和年轻国际标准样的研究,广泛开展实验室间的对比是提高国内~(40)Ar/~(39)Ar年代学水平并在年轻火山岩定年中取得突破的重要途径,也是当前~(40)Ar/~(39)Ar年代学研究的重要方向。     

210Pb and 137Cs measurements in the Circum Bohai Sea coastal region: sedimentation rates and implications

The representative ²¹⁰Pb and ¹³⁷Cs age-depth profiles were selected from more than 100 sites in the Circum Bohai Sea (CBS) coastal region to examine the sedimentation rates and associated environmental changes. There are three unique ²¹⁰Pb_exc distribution patterns: 1) ideal-decaying type, representing relatively constant sediment supply and quieter environmental setting; 2) wiggling type, but with approximately equivalent amplitude, representing coarser sediments and associated stronger flow current environment; and 3) episodic-eventinfluencing type, each representing typhoon storm-influenced depositional environment. Our results also show that there is a lower sedimentation rate (ca. 0.1 cm/a) in the coastal lowlands, a higher sedimentation rate (ca. 0.5–3 cm/a) in the intertidal flat and an intermediate rate of ca. 0.58 cm/a in the subtidal zone near the Huanghua Harbor. The interbedded silt layer occurring in the core sediment reveals lower ²¹⁰Pb activities, indicating a discontinuous sedimentation mostly due to typhoon events which dwarfs against ²¹⁰Pb and ¹³⁷Cs applicability. Overall, the ²¹⁰Pb and ¹³⁷Cs measurements of the present study provide physical insight into the evaluation of the coastal-marine ecological environment and associated management.     

The use of ‘bomb spike’ calibration and high-precision AMS C analyses to date salt-marsh sediments deposited during the past three centuries

A combination of ‘bomb spike’ calibration and conventional calibration of AMS ¹⁴C dating has been used to determine a detailed age-depth model for a 1-m sediment section collected from a salt marsh in Poole Harbour, southern England. These data were compared with the chronology obtained from ²¹⁰Pb analysis and ¹³⁷Cs age markers. We report post bomb values of over 1.46 F¹⁴C (> 146% modern ¹⁴C), and both the rising and falling limbs of the atmospheric ‘bomb spike’ are identified. Five pre-bomb samples were analysed using multi-target high-precision 2‰ AMS analysis, and after the replicates were combined the one-sigma uncertainty was as low as ± 9 ¹⁴C yr on some ages. These data, and an additional three normal-precision pre-bomb ¹⁴C samples, were calibrated using CALIB 5.0 and the chronology constrained using the ‘prior knowledge’ of independent age markers obtained from the analysis of pollen and spheroidal carbonaceous particle (SCPs). No agreement was found between the ¹⁴C ‘bomb spike’ dates and the CRS ²¹⁰Pb chronology modelled for this sequence. In addition, poor agreement was found between the signal of the 1960s weapons test fallout indicated by the ¹⁴C ‘bomb spike’ dates and the timing suggested by the ¹³⁷Cs data. This disagreement is attributed to the influence of the local discharge of ¹³⁷Cs from the former UKAEA site at Winfrith. We use our new chronology to confirm the existence of an acceleration in sedimentation rates in Poole Harbour during the last 100 yr previously reported for this site by Long et al. (Long, A.J., Scaife, R.G., Edwards, R.J. 1999. Pine Pollen in intertidal sediments from Poole Harbour, UK; implications for late-Holocene sediment accretion rates and sea-level rise. Quaternary International, 55, 3–16.), and conclude that ‘bomb spike’ ¹⁴C calibration dating may offer a more robust alternative to the use of ²¹⁰Pb chronologies for dating sediment deposition in salt-marsh environments. In addition, we demonstrate how the use of high-precision AMS analysis has the potential for reducing some of the uncertainties involved in the high-resolution dating of recent salt-marsh sediments.     

现代沉积的210Pb计年

²¹⁰Pb具有百年时间尺度沉积计年的重要价值。²¹⁰Pb_ex计年假设：沉积物是封闭系统；进入水体的²¹⁰Pb能有效地转移到沉积物中并不发生沉积后迁移；非过剩²¹⁰Pb与其母体²²⁶Ra保持平衡。²¹⁰Pb_ex计年可用稳定输入通量-稳定沉积物堆积速率模式、常量初始浓度模式或恒定补给速率模式。沉积物柱芯必须保持原态并以0．5～1cm间隔分截；用相应层节²²⁶Ra校正。沉积物表层混合作用及²²²Rn的丢失可能导致顶部²¹⁰Pb_ex异常。季节性缺氧湖泊沉积物顶部可能存在²¹⁰Pb及²¹⁰Po的再迁移。²¹⁰Pb与¹³⁷Cs两种计年方法原理上具有根本差别。²¹⁰Pb，¹³⁷Cs与沉积纹理方法对比是准确计年的重要保证。     

High-resolution210Pb dating of Lake Constance sediments: Stable lead in Lake Constance

Two sediment cores (BO90/13b and BO90/17b) from Lake Constance were investigated by-spectrometry for²¹⁰Pb,¹³⁴Cs,¹³⁷Cs,²⁴¹Am,²³⁴Th, and other members of the²³⁸U decay chain. The sediments were dated using the constant-flux model for²¹⁰Pb, and accumulation rates were determined. These range from 0.04 to 0.65g/cm ²/yr (BO90/13b) and 0.04 to 0.8g/cm ²/yr (BO90/17b), respectively. The mean accumulation rate amounts to 0.16g/cm ²/yr for both cores. The cores had already been dated by lamination counting and reconstruction of high-water events at the Limnological Institute at Constance, so that a very precise time scale was available. Both ages derived are in agreement within statistical error up to 1900, which means dating with the constant-flux model for²¹⁰Pb was confirmed up to that age. The position of the maxima of bomb cesium and americium confirm the stratigraphic and²¹⁰Pb datings. With²⁴¹Am a further radioactive isotope is available, which can, due to the half-life of²⁴¹Pu (t _1/2=14.4yr) be detected now by-spectrometry and can serve as an additional time indicator, the maximum being dated at 1963. By applying the various time scales, the depth profiles of stable lead and zinc of core BO90/13b were dated. Both heavy metals show a very significant maximum located beneath the layer of the maxima of bomb cesium and americium, showing that these maxima are older than those of the bomb isotopes. It is remarkable in this context that the maximum of zinc concentration occurs a little later than that of stable lead. Similar concentration profiles are observable in core BO90/17b and other, older sediment cores (CS6-CS10) on a transect across the lake. In contrast to a former assumption, the depth profile of stable lead in Lake Constance sediments does not reflect the anthropogenic gasoline lead emissions into the atmosphere for Germany, their maximum being dated at 1971.     

Historical record of heavy metal pollution deduced by lead isotope ratios in core sediments from the Osaka Bay,Japan

Three marine sediment cores from Osaka Bay were analyzed for ²¹⁰Pb geochronology, heavy metal concentrations (Zn, Cu, and Pb) and stable lead isotope ratios (²⁰⁶Pb/²⁰⁷Pb) in order to reconstruct high-resolution heavy metal pollution history from 1900–2006. Anthropogenic metal accumulation in sediments peaked in 1970 in agreement with the high economic growth period in Japan. The comparison of temporal patterns of ²⁰⁶Pb/²⁰⁷Pb ratio with other areas of Japan suggested that the heavy metals, imported from several different countries during the periods of economic growth (1955 to 1973), are the main pollution source for the country. For the period 1970–2006, the sediment data reflect the result of stricter environmental regulations applied after the late 1960s. However, heavy metal concentrations in the surface sediments are still elevated to levels several times higher than the levels at the bottoms of the cores. Additionally, the lead isotope ratio does not show significant change after the 1980s. Secondary heavy metal pollution through the mixing of deeper polluted sediment appears to be the likely reason for the deterioration of present time submarine sediment environments. In conclusion, this study has demonstrated that it is difficult to recover over a period of several years the benthic quality of a bay, once it is heavily polluted.     

福建龙海—漳浦沿海地区火山岩激光~(40)Ar/~(39)Ar年龄

福建东南沿海龙海—漳浦地区是新生代佛昙群玄武岩的分布区之一。对该区域火山岩而言,前人的研究主要集中在岩石地球化学特征、形成演化方面,对年代学研究较少,玄武岩喷发期次划分仍以20世纪80、90年代测定的K-Ar结果为依据,或通过下覆地层孢粉组合时代推断而来。为了更精确地测定该地区火山作用的时代及进一步确定其喷发期次,选取龙海—漳浦地区4个玄武岩样品,利用激光~(40)Ar/~(39)Ar测年方法进行精细定年。样品年龄为10.1～14.8Ma,明确了龙海-漳浦新生代玄武岩在中新世中晚期存在一次喷发期次。     

40Ar/39Ar年代学中几个重要问题的讨论

40Ar/39Ar年代学是同位素地质年代学中重要的两个"金钉子"手段之一(另一个为U-Pb法),广泛应用于重大地质事件、地质界线的精确定年,是确定地质年表的主要手段。40Ar/39Ar年代学测定的母体元素钾为常量元素,且实现分析时由于只需测定Ar同位素的比值,因而具有很高的分析精度,因此可以测定非常年轻(数千年)地质体的年龄。此外,由于在自然界中不易发生(物理、化学)反应的特性,Ar在矿物中的扩散可被准确地定量描述,因此40Ar/39Ar年代学也是热年代学的重要支柱,被广泛应用于地球深部物质上涌、折返、剥露、变质的冷却历史,率先提供了解析造山带、地壳作用过程等热历史的定量模型。这些特点使得40Ar/39Ar年代学成为地质年代学的三大支柱之一。那么,近年来该方法发展到了什么程度,其精确度和准确度达到了怎样的高度?为何年轻火山样品中极少发现过剩Ar?高压环境中的样品过剩Ar为何难以辨认?压力影响矿物的封闭温度吗?缓慢冷却K-长石的年龄谱是否可以真实地反映岩体所经历的热历史?多重扩散域模型(MDD)遇到了哪些挑战、该如何应用?40Ar/39Ar法和U-Pb法在构造热过程研究中有何不同的应用?本文对这些问题进行了思考和讨论,以期推动大家对40Ar/39Ar年代学的深入探索,推动其在我国地质研究中的应用。     

Chronology of a lacustrine core from Lake Linggo Co using a combination of OSL, 14C and 210Pb dating: implications for the dating of lacustrine sediments from the Tibetan Plateau

Vast palaeolakes once occupied the Qinghai‐Tibetan Plateau (TP). Analyses of the sedimentary records of these lakes could potentially provide an extensive dating archive. Many previously constructed age‐depth models simulating lacustrine cores have been principally based on radiocarbon (¹⁴C) dating. However, such dating could have been hampered by the so‐called ‘lake reservoir effect’ (LRE) and the reworking of lakebed sediments, resulting in inaccurate ¹⁴C age‐depth models and limiting interpretations of existing lacustrine palaeoclimatic records. Lake Linggo Co is located on the central TP, in one of the coldest and most arid regions of Tibet. We dated a 9.87‐m‐long lacustrine core extracted from the lakebed at a water depth of ~60 m using a combination of ²¹⁰Pb, ¹⁴C and optically stimulated luminescence (OSL) techniques. Some ¹⁴C ages showed significant age reversals; all the ¹⁴C ages were much older than the ²¹⁰Pb and OSL ages for the same depths. This could possibly be attributed to the presence of old, inherited carbon, causing the inherited ¹⁴C age to appear unstable during the deposition period. The ²¹⁰Pb and OSL ages were roughly concordant, and were also consistent with the stratigraphical succession. We therefore suggest that ¹⁴C dating may, on its own, be inadequate for accurate dating of lacustrine sediment sites on the TP, and that the OSL method should also be applied in order to evaluate the reliability of any ¹⁴C ages. With this approach, we constructed an age‐depth model, revealing sedimentation rates of 1.7, 0.8, 6.8 and 0.6 mm a⁻¹ between 0–1.9, 1.9–4.2, 4.2–4.4 and 4.4–9.4 ka, respectively.     

SHRIMP U–Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins

SHRIMP (Sensitive High‐Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable ²⁰⁷Pb/²⁰⁶Pb data are more readily obtained than ²⁰⁶Pb/²³⁸U, which also favours application to the Precambrian. However, it is demonstrated that ²⁰⁶Pb/²³⁸U dating of larger overgrowths (>10 μm) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of ± 7 Ma. In contrast, greenschist‐facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin.     

Determination of recent deposition rates in Lake Constance with radioisotopic methods

Sedimentation rates were determined with the ²¹⁰Pb method in eight sediment cores from Lake Constance. The rate of deposition in the main basin (Obersee) varies from about 0.06 g cm^?2 y^?1 in the central part to 0.13 g cm^?2 y^?1 in the eastern part of the lake and then increases rapidly towards the Rhine delta. In the central lake area the rate of deposition has been approximately constant since 1900, and dating with the ²¹⁰Pb method is in good agreement with sedimentological observations. In the Konstanzer Trichter area, the deposition rate has been increasing since about 1955 as a result of eutrophication and subsequent high carbonate production. Dating with ¹³⁷Cs is fairly accurate for sediments deposited at a high rate, but is questionable for slowly accumulating ones. A positive correlation of ²¹⁰Pb fluxes and sedimentation rates indicates that ²¹⁰Pb flux into sediments follows the distribution pattern of solids. ²¹⁰Pb profiles in four sediment cores interpreted in terms of a constant flux model display synchronous fluctuations of the sedimentation rate; however, their relation to long-range particulate input variations remains to be proved. Sedimentation rates determined with the ²¹⁰Pb method were used to calculate recent nutrient and heavy metal fluxes. Anthropogenic fluxes of Zn and Pb are in the same range of magnitude as in other polluted areas in Europe and America.     

Temporal dynamics of airborne lead-210 in Missouri (USA): implications for geochronological methods

 Lead-210 (²¹⁰Pb) deposited from the atmosphere is used in the dating of certain geological materials such as glacial ice and lacustrine sediments, but its long-term atmospheric behavior has been little studied. The present investigation reports measurements of airborne ²¹⁰Pb at 21 monitoring stations in Springfield, Missouri, during 1975–1995. Seasonal and diurnal patterns of atmospheric concentrations are established, and the mean annual concentrations of ²¹⁰Pb in ground level air during the 20-year period are examined. Although airborne ²¹⁰Pb concentrations are found to vary diurnally and seasonally, mean annual concentrations in southwest Missouri have remained relatively constant during this time period. This finding is important for geochronological methods that assume a constant ²¹⁰Pb flux from the atmosphere. Received: 8 September 1998 · Accepted: 12 October 1998     

Isotopes and Geochronology of the Meixian-type Pb-Zn-(Ag)Deposits,Central Fujian Rift,South China:Implications for Geological Events

Central Fujian Rift is another new and important volcanogenic massive sulfide Pb-Zn polymetallic metallogenetic belt. In order to find out the material genesis and mineralization period of Meixian-type Pb-Zn-Ag deposits, S and Pb isotope analysis and isotope geochronology of ores and wall rocks for five major deposits are discussed. It is concluded that the composition of sulfur isotope from sulfide ore vary slightly in different deposits and the mean value is close to zero with the 834S ranging from -3.5‰ to ＋5.6‰ averaging at ＋2.0‰, which indicates that the sulfur might originate from magma or possibly erupted directly from volcano or was leached from ore-hosted volcanic rock. The lead from ores in most deposits displays radioactive genesis character （206pb/204pb〉18.140, 207Pb/204pb〉15.584, 208pb/204pb〉38.569） and lead isotope values of ores are higher than those of wall rocks, which indicates that the lead was likely leached from the ore-hosted volcanic rocks. Based on isotope data, two significant Pb-Zn metallogenesis are delineated, which are Mid- and Late-Proterozoic sedimentary exhalative metailogenesis （The single zircon U-Pb, Sm-Nd isochronal and Ar-Ar dating ages of ore- hosted wall rocks are calculated to be among 933-1788 Ma.） and Yanshanian magmatic hydrothermal superimposed and alternated metallogenesis （intrusive SHRIMP zircon U-Pb and Rb-Sr isochronal ages between 127-154 Ma）.     

Pb chronology and trace metal geochemistry at Los Tuxtlas, Mexico, as evidenced by a sedimentary record from the Lago Verde crater lake

Lago Verde is a fresh-water maar found on the lower slopes of San Martin volcano, at the Sierra de Los Tuxtlas, Mexico, currently the northernmost remnant of the tropical rain forest in America. ²¹⁰Pb and ¹³⁷Cs analyzed in a sediment core were used to reconstruct the historical fluxes of Ag, Cd, Cu, Pb, Hg and Zn to the site during the last ∼ 150 yr. The ²¹⁰Pb_xs-derived sediment accumulation rates, the magnetic susceptibility, C/N ratios and δ¹³C data evidenced background conditions at the lake until 1960s, when enhanced erosion related to the clearing of large forested areas at Los Tuxtlas promoted higher accumulation rates of a heavier and more magnetic sedimentary material. Recent sediments from Lago Verde were found enriched by Pb (26-fold natural concentration level [NCLs]) and moderately enriched by Cd > Cu > Zn and Hg (6-, 5-, 4- and 4-fold corresponding NCLs, respectively). The fluxes of Cu, Hg, Pb and Zn have significantly increased since 1940s, with peak ratios of total modern to pre-industrial fluxes of 11, 11, 19 and 49, respectively. The lake occupies a relatively pristine, non-industrialized basin, and therefore the increased metal fluxes might be related to long-distance aeolian transport of trace metals.     

Geochemistry of heavy metals and sedimentation rate in a bay adjacent to the Caspian Sea

In the present investigation down core variations of heavy metals such as; Zn, Ni, Co, Mn, Fe and Al in a sediment core from Bay of Gorgan adjacent to the Caspian Sea was studied. Inter-relationship amongst various parameters was brought out through correlation coefficients and cluster analysis. The results of present study reveals that Zn, Ni, Co and Mn possess both lithogenous and non-lithogenous sources. The increasing trend of Al towards top of sediment core is indicative higher erosion in the recent years. Interestingly, concentration of all studied metals increases toward core top (except for Fe) that might be indicative of influence of man’s activities residing in catchment area of Gorgan Bay and also oil exploration in the Caspian Sea. Further, ²¹⁰Pb and ¹³⁷Cs techniques were used to find out rate of sedimentation. The result of dating is indicative of sedimentation rate between 1.4 to 2.45 mm/yr. based on ²¹⁰Pb and ¹³⁷Cs activities respectively.     

准噶尔盆地周边硅质岩激光40Ar/39Ar法定年

由于较低的钾元素含量以及过剩氩的存在,长期以来对硅质岩的40Ar/39Ar定年一直存在较大难度。近年来,由于仪器水平的不断提高,新实验技术和方法的应用,特别是激光全熔40Ar/39Ar定年技术的应用,40Ar/39Ar定年方法具有了足够高的测试精度和稳定的低本底水平,可以满足测试极低钾元素含量的硅质岩样品的要求。利用多组矿物颗粒测试数据计算等时线年龄的方法可以很好地去除过剩氩对硅质岩年龄的影响。本文利用激光全熔40Ar/39Ar定年方法对新疆准噶尔盆地边缘的两个硅质岩样品进行了定年研究。采自白碱滩地区的08BJT-3样品的年龄测试结果为294±14Ma,该年龄结果与硅质岩样品所处的晚石炭世地层沉积年代基本一致。采自卡拉麦里地区的KML-2样品的年龄测试结果为266±14Ma,该年龄结果与强烈变形改造硅质岩样品的卡拉麦里构造变形带活动年代十分一致,表明激光全熔40Ar/39Ar定年方法可以准确地对硅质岩进行定年。