小秦岭文峪和娘娘山花岗岩体锆石SHRIMP U Pb年龄及其意义

对小秦岭文峪和娘娘山花岗岩体进行的锆石SHRIMP U Pb定年和岩石地球化学分析结果表明,文峪和娘娘山黑云母二长花岗岩体属于Ⅰ型花岗岩,是太古宙太华群高级变质岩系经部分熔融的产物。它们的成岩年龄分别为1384±25 Ma和1417±25 Ma,略早于本区拆离伸展活动的时限,暗示了中下地壳部分熔融形成的岩浆热穹窿和花岗质岩浆在上地壳的大面积侵位导致了小秦岭变质核杂岩的发育。花岗岩体的成岩年龄都早于本区金矿床的主期成矿时代,从时间上表明了花岗质岩浆活动与金矿成矿作用无直接关系。     

周口店洞穴堆积物及大同火山烘烤层的热发光年龄

本文是对采自北京周口店洞穴及山西大同火山地区的一批样品进行热发光年龄测定的结果。 1.在周口店第一地点,自上而下采集了4-4、4-5、10-6三个灰烬样。测定结果表明,天然热发光强度随取样的层位变低而增强。4-4、10-6测样热发光相对强度4.5和9.5,为一底部较宽的不完全对称的200℃左右的中温峰。     

冀东迁西-迁安地区早太古代变质岩系原岩恢复及其地质意义

冀东早太古代变质岩系的主体是一套呈层分布的,各种成分麻粒岩及其某些退变质产物——斜长角闪岩类。在其上部发育变质含铁建造,而在其下部常夹有变质的镁铁质和超镁铁质岩(如变闪辉岩、变辉石岩和蛇纹石化橄榄岩等)薄层或透镜体。 麻粒岩的矿物组合是斜方辉石+单斜辉石+斜长石(常常为反条纹长石)+石英+磁铁矿±角闪石±黑云母。在变质含铁建造中局部出现包括铝硅酸盐的矿物组合(蓝晶石-石榴石-黑云母-斜长石-石英)。变质铁硅质岩可以分为两类:辉石磁铁石英岩和英榴易熔岩(张儒瑗,从柏林,1981)。     

西藏南部同位素地质年龄的测定与喜马拉雅运动的分期

作者对1973-1975年中国科学院青藏高原综合科学考察队和1975年中国登山队科考分队地质组,在西藏南部和珠穆朗玛峰（以下简称珠峰）地区采集的部分同位素地质年龄样品,经镜下鉴定,选择适宜者用钾氩法做了同位素地质年龄的测定,从而对本区广泛出露的岩浆岩和变质岩的时空分布特征加深了认识。在工作中钾是用火焰光度法测定的。放射成因氩的含量大部分是用体积法测定,小部分用以Ar³⁸为稀释剂的稀释法测定。     

测定地质年代的钾-氩稀释法

随着地质科学工作的发展,要求应用钾-氩稀释法测定年青的新生代岩石矿物年龄越来越迫切。我们的钾-氩稀释法工作,开始于1975年,我们走了一条有别于国外稀释法的路子,从实验结果看,收到了较好的效果,有些地方也还需要进一步完善。     

沉积物的光释光测年和对沉积过程的指示意义

应用光释光测年技术测量了位于陕西、河南、湖北和安徽等地9个剖面的49个沉积物样品,这些样品的光释光信号都是以快组分为主,等效剂量预热坪区和剂量复原实验结果也表明这些样品都适合应用单片再生剂量法测量其等效剂量。根据测量的光释光年龄数据和样品的埋深,建立了各剖面的年龄-深度曲线和函数,通过它们推算出了各剖面的长期堆积速率,并据此指出了可能的沉积环境变化。在取样的深度范围内,所测剖面的堆积速率从0.02mm/a到2.58mm/a,并显示一般风成黄土的堆积速率要比水成堆积物的堆积速率要慢。所测样品的年龄从0.2±0.01ka到53.8±4.7ka。在9个剖面中,有两个自然剖面发生了长时期的沉积间断。在有文化层的剖面中,文化层下部地层的年代明显老于文化层和上部地层的年龄,文化层中沉积物的光释光年龄也可以用来指示文化层的堆积情况。另外,对两个剖面也进行了¹⁴C定年,结果显示沉积物全样的¹⁴C测年值得进一步研究。     

关于《二元混合体系的端元Sm-Nd模式年龄计算方法》几个问题探讨

关于《二元混合体系的端元Sm-Nd模式年龄计算方法》几个问题探讨杨晓松(中国地质大学地质系武汉430074)作者在《二元混合体系的端元Sm-Nd模式年龄计算方法》一文(下简称原文)中(杨晓松等,1993),提出了端元Sin-Nd模式年龄的概念及计算方法,其目的是解决模式年龄在解释时常遇到的困境(Arndt et al.,1987)。     

关于中国岩石绝对年龄的讨论

这个問題的研究是根据中国科学院地貭研究所和中国地貭部地貭研究所的倡議而进行的。参加这項工作的有中苏两国的科学工作者,其中包括中国科学院、苏联科学院、中国地貭部和苏联地貭及保矿部有关研究所的人員。由于有許多人参加这項工作,因而能在1957和1958年夏季收集了必需的岩石标本,并在1958-1959年室内工作期間对它們作了部分的研究。用以鑑定年龄的标本是为了編制中国第一个地貭建造年代簡表和进行华北前寒武系研究而采集的。它们包括了中国主要岩浆旋迴和变貭杂岩体。     

鉀-氩法測定岩石矿物絕对年龄数據的报导

本文提出了98个K-Ar法绝对年龄测定的新数据,样品是由中国科学院地质研究所和其他兄弟单位提供的。我们的试验条件大致与以往相同,但部分样品采用内加热法熔样。年龄计算常数仍采用:λ_K=5.57×10^-11,λ_β=4.72×10^-10年^-1,K⁴⁰（克/克样品）=1.22×10^-4K％。利用同位素铅法验证了集宁地区伟晶岩中云母以K-Ar 法测定的年龄值,并对同一岩石样品中所含云母、长石斑品及全岩分别进行了试验分析,得出其年龄相对偏差的范围。     

苏联科学院地质-地理学部地质建造绝对年龄测定委员会召开第八次会议

1959年5月18-22日以維尔納茨基命名的地球化学和分析化学研究所在莫斯科召开了地貭建造絕对年龄测定委員会第Ⅷ次常会。这次会議听取和討論了一系列的报告。其中右A.B.波尔勘諾夫和ЭK.格尔林克关于:“波罗的海地盾前寒武紀絕对年龄問題”的报告,А.П.維諾格拉托夫、А.И屠格林诺夫、К.Г.克洛林、E.B.比比柯娃、B.B.日洛址和С.И.吉可夫的关于“俄罗斯地台結晶基底、前寒武纪岩石的时代”的报告。     

Evaluation of KAr isochron methods

The KAr isochron method is an attempt to obtain the initial age or reset age and the extraneous argon isotopic ratio of a suite of cogenetic samples of different K-contents. Some samples contain excess argon-40 and others lose argon-40. The resultant ages on single samples are not significant geologically. We have discussed the principles of three types of isochrons commonly used, the ⁴⁰Ar vs ⁴⁰K isochron, the ⁴⁰Ar/³⁶Ar vs ⁴⁰K/³⁰Ar isochron, and the ⁴⁰Ar/³⁶Ar vs ³⁹Ar/³⁶Ar isochron, and evaluated the first two types for ten different hypothetical cases. If a straight line is obtained in the ⁴⁰Ar vs ⁴⁰K isochron, a positive intercept indicates an approximately constant amount of excess argon, whereas a negative intercept indicates argon loss. A curved line or scattering of points indicates that the basic assumptions are not valid for the set of samples under consideration. The ⁴⁰Ar/³⁶Ar vs ⁴⁰K/³⁶Ar isochron method is valid, rigorously, only when all samples of the system under consideration have the same non-radiogenic argon isotope composition. This requires that either no excess argon is present in the system, or else each analysis contains the same proportion of excess and atmospheric argon. If these conditions do not hold, approximately, invalid ages and invalid intercepts are obtained. Any KAr isochron needs to be used with caution.     

吉林陨石原始捕获的40Ar/36Ar比值及其地质意义

地球大气圈中⁴⁰Ar/³⁶Ar的现代比值被公认为295.5,在利用K-Ar法测定地球物质年龄时,它是校正大气氩混染量的重要参数。陨石中原始捕获(trapped)的(⁴⁰Ar/³⁶Ar)_t比值是否和地球大气圈中氩同位素丰度相一致,这个问题不但在地质年龄测定中具有现实意义,而且对探索太阳系的形成和演化也有重要理论价值。近十几年,随着质谱分析技术的提高以及登月计划的实施,国外出现了一批有关月岩中氩同位素原始组分的资料,但至今对陨石中原始氩同位素丰度的报道很少。笔者采用⁴⁰Ar/³⁹Ar快中子活化技术和约克方程^[3,4],对吉林陨石雨2号标本进行了原始捕获的(⁴⁰Ar/³⁶Ar)_t比值进行了测定,并探讨了它的地质意义。     

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.     

(极)年轻火山岩激光熔蚀40Ar/39Ar定年

对中国大量年轻或/和极年轻火山岩的定年实践研究表明,(极)年轻火山岩的激光熔蚀40Ar/39 Ar定年具有不同于第四纪以前喷发火山岩定年的显著特点.激光熔蚀40Ar/39Ar定年技术因为本底低、样品用量小以及与现代惰性气体同位素质谱设备在灵敏度、高精度方面的相一致,在年轻火山岩的定年中得到深入运用.借助激光在年轻或/和极年轻火山岩的40 Ar/39 Ar定年中,实践证明,样品形成时限越年轻(特别是相当于第四纪时期的样品),Nier值与样品中初始氩比值的偏离会引起K-Ar和40Ar/39 Ar表观年龄的偏差越大.对于小于0.2Ma的样品,Nier值与样品中初始氩比值的偏离对K-Ar和40Ar/39Ar表观年龄的偏差影响呈指数增长;当样品年龄相对较老(老于第四纪)时,Nier值和初始氩比值的偏离对K-Ar和40Ar/39 Ar表观年龄的影响较小.以40Ar/ArAr定年为出发点,定量给出界定年轻与极年轻火山岩的年龄:2～0.2Ma的火山岩界定为年轻火山岩,0.2Ma以来的火山岩称为极年轻火山岩.实验结果还证实,测定(极)年轻火山岩基质年龄时要尽量剔除非同源分馏的斑晶,以便去除斑晶可能带来的过剩氩影响;年轻火山岩样品的测年,应根据岩石结构和粒度特征选取合适的粒度,通常情况下,推荐0.2mm颗粒直径(60～80目)为理想粒径;年轻火山岩样品在快中子辐照后冷却放置时间不宜过长,否则造成37 Ar测不准,影响数据结果,带来较大偏差;激光40Ar/39Ar精细定年对标准样品的均一性有很高的要求,通过标定常用的国内外监测标样发现,标样SB-778-Bi,Bem4M,BT-1均一性很好,适合用作激光熔蚀40Ar/39Ar定年监测;测试数据的处理中,火山岩喷发后冷却结晶中同时形成的斑晶和基质的等时线处理能够帮助获得客观真实和精细的年龄结果.在此基础上,北京大学惰性气体同位素实验室建成了专用于(极)年轻火山岩精细定年的激光熔蚀40Ar/39Ar定年实验流程.     

Open system alkaline magmatism in northern Kenya: evidence from U-series disequilibria and radiogenic isotopes

U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (²³⁰Th/²³²Th) (≥1.06) with low (²³⁸U/²³⁰Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (²³⁰Th/²³²Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (²³⁸U/²³⁰Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion ⁴⁰Ar/³⁹Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (²³⁰Th/²³²Th)₀ ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano. Received: 29 May 1996 / Accepted: 24 November 1997     

应用40Ar—39Ar定年技术研究某些火山岩及陨石样品的受热历史

常规的K-Ar法是基于⁴⁰K通过K-层电子捕获衰变成⁴⁰Ar^*这一机理,应用衰变定律而定年的。它具有测定对象广、测定年龄范围大等优点,是同位素地质定年的主要方法之一。但是,由于⁴⁰Ar^*是气体,当岩石、矿物形成以后受到搅动时(如岩浆的侵入、构造活动、宇宙物质的冲击等),⁴⁰Ar^*容易丢失,使年龄值偏低。1962年由Sigurgeirsson提出的,后经Merrihue、Turner等人逐步完善的⁴⁰Ar-³⁹Ar快中子活化定年技术,很好地克服了K-Ar法的局限性。⁴⁰Ar-³⁹Ar定年分为两种:一是全熔融法(total fusion),样品被快中子照射后一次加热熔融,然后计算年龄,此值与常规K-Ar法结果相当;另一种是阶段加热法(step-heating),被照射的样品从低温到高温被逐步加热,分别计算各温度阶段的⁴⁰Ar/³⁹Ar视年龄,并进而得到一条年龄谱和一个坪年龄(plateau age)。后一种方法对研究地质体是否受过热的挠动、岩石矿物的早期结晶年代、后期热挠动次数、热挠动年代、岩石矿物对氢的保存性、过剩氩的存在状态等具有独特作用,它开辟了同位素地质年代学的一个新领域。本文将着重介绍应用⁴⁰Ar-³⁹Ar阶段加热技术研究地球物质及陨石受热历史的某些成果。     

Eruption of the Continental Flood Basalts at -259 Ma in the Emeishan Large Igneous Province, SW China： Evidence from Laser Microprobe ^40Ar/^39Ar Dating

A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province (LIP), SW China was investigated for laser microprobe 40Ar/39Ar dating. There are two 40Ar/39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger 40Ar/39Ar ages for some basalts with low-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The 40Ar/39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration (about 3 Ma) of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.     

Argon isotope fractionation induced by stepwise heating

Noble gas isotopes are widely used to elucidate the history of the rocks in which they have been trapped, either from distinct reservoirs or by accumulation following radioactive decay. To extract noble gases from their host rocks, stepwise heating is the most commonly used technique to deconvolve isotopically different components, e.g., atmospheric, in situ radiogenic, or excess radiogenic from mantle or crustal reservoirs. The accurate determination of the isotopic composition of these different components is of crucial importance, e.g., for ages obtained by ⁴⁰Ar-³⁹Ar stepheating plateaus. However, diffusion theory-based model calculations predict that the stepwise thermal extraction process from mineral phases induces isotope fractionation and, hence, adulterates the original composition. Such effects are largely unconsidered, as they are small and a compelling experimental observation is lacking. We report the first unequivocal evidence for significant mass fractionation of argon isotopes during thermal extraction, observed on shungite, a carbon-rich Precambrian sedimentary rock. The degree of fractionation, as monitored by ³⁸Ar/³⁶Ar and ⁴⁰Ar/³⁶Ar ratios, very well agrees with theoretical predictions assuming an inverse square root dependence of diffusion coefficient and atomic mass, resulting in easier extraction of lighter isotopes. Hence, subatmospheric ⁴⁰Ar/³⁶Ar ratios obtained for argon extracted at low temperatures may not represent paleoatmospheric argon. Shungite argon resembles modern atmospheric composition, but constraints on the timing of trapping appear difficult to obtain, as shungites are multicomponent systems.In ⁴⁰Ar-³⁹Ar stepwise heating, the isotope fractionation effect could cause systematic underestimations of plateau ages, between 0.15 and 0.4% depending on age, or considerably higher if samples contain appreciable atmospheric Ar. The magnitude of this effect is similar to the presently achieved uncertainties of this increasingly precise dating technique. Our results also indicate the importance of thermally activated diffusion as a possible fractionation mechanism, e.g., for hydrothermal gas exhalations, or for carbonaceous carrier phases such as “Q” in meteorites that have been suggested as carriers of highly fractionated noble gas residues from the early solar nebula.     

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.     

西昆仑库地韧性剪切带的40Ar/39Ar年龄

西昆仑库地以南有一套变质变形较强的岩系,前人依照区域对比关系将其划为前寒武的古老基底。对西昆仑早期构造演化的论述均基于该观点,但没有提供确凿的同位素年代学证据。笔者通过野外观察、室内研究,确认库地以南的变质变形岩系是大型韧性推覆剪切作用的产物。通过对新生变质矿物角闪石和黑云母单矿物的⁴⁰Ar/³⁹Ar年龄分析,确定剪切变质年龄为426-451Ma,说明库地的变质变形岩系是形成于早古生代晚期的一条大型韧性剪切带,这对于解释西昆仑的早期构造演化具有重要意义。