Distinct brief major events in the Karoo large igneous province clarified by new Ar/Ar ages on the Lesotho basalts

Recent mineral separate ages obtained on the Karoo large igneous province (southern Africa) suggest that the province was built by several distinct magmatic pulses over a rather long period on the order of 5–6 Ma concerning the main erupted volume [Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., Le Gall., B., 2005. The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new ⁴⁰Ar/³⁹Ar age data, Geology 33, 745–748]. Although this apparently atypical province is dated in more detail compared to many other large igneous provinces, volumetrically important areas still lack sufficient high-quality data. The timing of the Karoo province is crucial as this event is correlated with the breakup activity of the Gondwana supercontinent. The Lesotho basalts represent a major lava sequence of the province, but have not yet been precisely dated by systematic analysis of mineral separates. We analyzed plagioclase separates from five lava flows encompassing the complete 1.4-km-thick Lesotho sequence from top to bottom using the ⁴⁰Ar/³⁹Ar method. We obtained five plateau and mini-plateau ages statistically indistinguishable and ranging from 182.3 ± 1.6 to 181.0 ± 2.0 Ma (2σ). We derived an apparent maximum duration for this event of  0.8 Ma by neglecting correlated errors embedded in the age uncertainties.

A critical review of previous ages obtained on the Lesotho sequence [Duncan R.A., Hooper, P.R., Rehacek, J., Marsh, J.S., Duncan, A.R., 1997. The timing and duration of the Karoo igneous event, southern Gondwana. Journal of Geophysical Research 102, 18127–18138] shows that groundmass analyses are unreliable for high-resolution geochronology, due to alteration and ³⁹Ar recoil effects. Discrepancy between our ages and a previous plagioclase age at  184 Ma obtained by the later workers is tentatively attributed to the heterogeneity of the monitor used and/or cryptic excess ⁴⁰Ar. The current age database suggests that at least three temporally and spatially distinct brief major events (the Lesotho and southern Botswana lava piles and the Okavango dyke swarm) are so far recognized in the Karoo province. Identification of brief and volumetrically important Karoo magmatic events allows detecting the migration of the Karoo magmatism and potentially the stress regime that affected the southern African lithosphere at this time. A filtered compilation of 60 ages obtained with homogeneous intercalibrated standards suggests a shorter duration for the main pulses of the magmatism between 3 and 4.5 Ma, compared to a whole province duration of  10 Ma, between  182 and  172 Ma.     

Ar–Ar ages of intrusions in East Greenland: Rift-to-drift transition over the Iceland hotspot

Sixteen ⁴⁰Ar–³⁹Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56–54 Ma, 50–47 Ma and 37–35 Ma. In the Kangerlussuaq Fjord, which coincides with a major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and within the continental flood basalt succession, alkaline intrusions of the north–south trending Wiedemann Fjord–Kronborg Gletscher lineament range from 52 to 36 Ma.

We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord–Kronborg Gletscher lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland–Iceland Rise and are explained by mantle melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50–47 Ma) and later by uplift associated with regional plate-tectonic reorganization (37–35 Ma). The Iceland mantle plume was instrumental in causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and asymmetry in the magmatic history of the conjugate margins of the central Northeast Atlantic.     

延边海沟金矿床石英流体包裹体激光探针40Ar/39Ar测年与成矿背景

海沟金矿为一大型贫硫化物石英脉型金矿床,长期以来其形成时代一直存在争议,致使矿床成因研究受到较大影响,成矿预测等方面研究进展缓慢。采用石英流体包裹体的⁴⁰Ar/³⁹Ar激光探针定年法对该金矿床进行了精细的年代学测定,获得主成矿阶段石英流体包裹体的⁴⁰Ar/³⁹Ar正、反等时线年龄分别为（170±20） Ma和（172±16） Ma,MSWD分别为0.31和0.35,⁴⁰Ar/³⁶Ar比值分别为298±4和299±4;进一步剔除接近大气的成分点和过剩氩高的成分点,计算获得等时线年龄为（170±38） Ma（⁴⁰Ar/³⁶Ar Int.=299±10）和（165±52） Ma（⁴⁰Ar/³⁶Ar Int.=303±23）。结合已有同位素年代学研究成果,进一步明确了海沟金矿床主成矿期发生在早侏罗世晚期,成矿作用与中生代库拉板块向亚洲板块俯冲、特别是由此引发的华北板块东缘与兴蒙造山带东段的强烈拼贴并诱发的岩石圈加厚、减薄作用密切相关。     

南岭中段主要锡矿床He、Ar同位素组成及其意义

对南岭中段主要锡矿床的黄铁矿等进行了流体包裹体的He、Ar同位素研究。结果表明,成矿流体的⁴⁰Ar/³⁶Ar≈288～371,³He/⁴He≈0.09～28.58Ra ,不同地区流体包裹体He、Ar浓度变化较大,而在同一地区⁴⁰Ar/³⁶Ar、³He/⁴He值比较一致;⁴⁰Ar/³⁶Ar值介于大气饱和水(包括大气降水和海水)的同位素组成（⁴⁰Ar/³⁶Ar≈295.5）和夏威夷热点（⁴⁰Ar/³⁶Ar=350～360）之间,³He/⁴He值大大高于大陆地壳特征值（³He/⁴He≈0.03Ra）,介于地壳特征值与原始地幔特征值（³He/⁴He≈8.57Ra）和夏威夷热点特征值（³He/⁴He≈27.14Ra）之间,表明成矿流体与地幔柱的活动有关,为地幔、地壳和大气水的混合产物,以地幔流体为主。这一结果为南岭地区锡多金属矿床为壳幔相互作用的产物及区域地幔柱的存在提供了新证据。     

祁漫塔格韧性剪切带中绢云母40Ar-39Ar定年及地质意义

祁漫塔格韧性剪切带是祁漫塔格蛇绿混杂岩带与北昆仑岩浆弧的区域主构造边界,对剪切带内花岗质糜棱岩中绢云母40Ar-39 Ar法年龄测定,获得了(271.1±2.2) Ma的坪年龄,相应的36Ar/40 Ar-39 Ar/40 Ar反等时线年龄为(270.5±3.7)Ma (MSWD=2.7),39Ar/36 Ar-40 Ar/35 Ar正等时线年龄为(270.9±4.7) Ma (MSWD=0.22);坪年龄(271.1±2.2) Ma接近于绢云母矿物的形成年龄,也代表了祁漫塔格韧性剪切带的形成年龄.通过韧性剪切变形带内运动学特征的研究,表明剪切带具有右旋斜冲的性质.祁漫塔格韧性剪切带的形成与古特提斯洋的俯冲作用有关,是古特提斯洋向北俯冲造山过程的远程效应.     

辽宁阜新排山楼金矿的40Ar/39Ar成矿年龄

排山楼大型金矿床产于辽西高级变质太古宙花岗岩-绿岩带中,受绿岩中、上部层位的基性-中酸性火山岩系和东西向韧性剪切带控制。容矿岩石主要是变质安山质-英安质火山岩类。为解决主矿体的形成时代,选取含金石英细脉浸染状矿石中的石英测定了⁴⁰Ar/³⁹Ar年龄为2105.2±10.4Ma。谱线特征为马鞍型,其底坪年龄为石英的结晶年龄。由此提出排山楼金矿的成因主要与吕梁期的东西向韧性剪切带有关。     

Very fast exhumation of high-pressure metamorphic rocks with excess Ar and inherited Sr, Betic Cordilleras, southern Spain

In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for ⁴⁰Ar/³⁹Ar and Rb–Sr dating.

A single phengite grain from an amphibolite yielded an ⁴⁰Ar/³⁹Ar laser step heating plateau age of 86.9±1.2 Ma (2σ; 70% ³⁹Ar released) and an inverse isochron age of 86.2±2.4 Ma with an ³⁶Ar/⁴⁰Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2±6.3 Ma (2σ; 95% ³⁹Ar released). These ages are diagnostic of excess argon (⁴⁰Ar_XS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. ⁴⁰Ar_XS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation.

Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1±3.2, 40.6±2.6 and 14.1±2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrín unit has a whole-rock–mica–albite age of 17.2±1.9 Ma, which is within error of an ⁴⁰Ar/³⁹Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped ⁴⁰Ar/³⁹Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene.

Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 °C/Ma.     

Strain partitioning and preservation of Ar/Ar ages during Variscan exhumation of a subducted crust (Malpica–Tui complex, NW Spain)

The Malpica–Tui complex (NW Iberian Massif) consists of a Lower Continental Unit of variably deformed and recrystallized granitoids, metasediments and sparse metabasites, overridden by an upper unit with rocks of oceanic affinities. Metamorphic minerals dated by the ⁴⁰Ar/³⁹Ar method record a coherent temporal history of progressive deformation during Variscan metamorphism and exhumation. The earliest stages of deformation (D1) under high-pressure conditions are recorded in phengitic white micas from eclogite-facies rocks at 365–370 Ma. Following this eclogite-facies peak-metamorphism, the continental slab became attached to the overriding plate at deep-crustal levels at ca. 340–350 Ma (D2). Exhumation was accompanied by pervasive deformation (D3) within the continental slab at ca. 330 Ma and major deformation (D4) in the underlying para-autochthon at 315–325 Ma. Final tectonothermal evolution included late folding, localized shearing and granitic intrusions at 280–310 Ma.

Dating of high-pressure rocks by the ⁴⁰Ar/³⁹Ar method yields ages that are synchronous with published Rb–Sr and Sm–Nd ages obtained for both the Malpica–Tui complex and its correlative, the Champtoceaux complex in the French Armorican Massif. The results indicate that phengitic white mica retains its radiogenic argon despite been subjected to relatively high temperatures (500–600 °C) for a period of 20–30 My corresponding to the time-span from the static, eclogite-facies M1 peak-metamorphism through D1-M2 eclogite-facies deformation to amphibolite-facies D2-M3. Our study provides additional evidence that under certain geological conditions (i.e., strain partitioning, fluid deficiency) argon isotope mobility is limited at high temperatures, and that ⁴⁰Ar/³⁹Ar geochronology can be a reliable method for dating high pressure metamorphism.     

K-Ar、40Ar/39定年中尼尔值和初始氩比值

K-Ar、⁴⁰Ar/³⁹Ar定年中, 用以计算囚禁⁴⁰Ar绝对量的⁴⁰Ar/³⁶Ar, 称为初始氩比值.现代大气中的初始氩比值叫尼尔值, 为295.5.K-Ar、⁴⁰Ar/³⁹Ar定年的表观年龄都是在假设初始氩比值与现代大气中的尼尔值一致的基础上得到的.事实证明, 地球样品的初始氩比值不总是尼尔值, 因此这种假定不可靠, 尤其在对年轻样品进行K-Ar、⁴⁰Ar/³⁹Ar定年时, 会带来错误的表观年龄结果.对于K-Ar、⁴⁰Ar/³⁹Ar精细年代学, 特别对于年轻样品和低钾含量样品, 只有初始氩比值才对定年研究有意义, 而初始氩比值只能通过⁴⁰Ar/³⁹Ar等时线法求得.所以严格地讲, 只有等时线年龄才是可靠的.结合实验流程, 本文分析了⁴⁰Ar/³⁹Ar定年中为什么大多数等时线获取的初始氩比值接近尼尔值, 一是因为常规⁴⁰Ar/³⁹Ar坪年龄的计算方法, 二是因为吸附氩的干扰.初始氩比值的一致性, 被用来证明样品同位素同源和封闭特性, 对年龄数据的精度及可靠性有非常大的影响, 尤其是年轻样品.因此, 在⁴⁰Ar/³⁹Ar定年中要选取氩同位素初始比值一致的同源样品, 并在实验流程中尽量克服样品吸附气体.本文对传统⁴⁰Ar/³⁹Ar方法中坪年龄、年龄坪的意义提出了质疑, 对提高⁴⁰Ar/³⁹Ar定年的精度具有重要意义, 并使之能够应用到极年轻的火山岩定年中.     

Ar–Ar age determinations of eclogitic clinopyroxene and garnet inclusions in diamonds from the Venetia and Orapa kimberlites

Ar–Ar age measurements are reported for selected eclogitic clinopyroxene and garnet inclusions in Orapa diamonds and clinopyroxene inclusions in Venetia diamonds. Laser drilling of encapsulated clinopyroxene inclusions within Venetia diamonds released a maximum of 3% of the total ⁴⁰Ar, indicating little diffusive transfer and storage of radiogenic ⁴⁰Ar at the diamond–inclusion boundary. Apparent ages obtained during stepped heating of three diamonds are consistent with diamond crystallisation occurring just prior to the kimberlite eruption 520 Ma ago. Stepped heating of three clinopyroxene-bearing Orapa diamonds gave ages of 906–1032 Ma, significantly above the eruption age, but consistent with previously determined isotopic ages. A few higher apparent ages hint at the presence an older generation of Orapa diamonds that formed >2500 Ma ago. Orapa garnets also contain measurable K contents, and record a range of ages between 1000 and 2500 Ma. The old apparent ages and lack of significant interface ⁴⁰Ar released by the laser probe, suggests that pre-eruption radiogenic ⁴⁰Ar and mantle-derived ⁴⁰Ar components are trapped in microinclusions within the pyroxene and garnet inclusions.     

Ar/Ar dating of central European K–Mn oxides — a chronological framework of supergene alteration processes during the Neogene

K-bearing Mn oxides may potentially constitute useful objects for isotopic dating of ore-forming events. A comprehensive ⁴⁰Ar/³⁹Ar study performed on supergene K–Mn oxides sampled from different sub-alpine mountain terrains in Germany and France has been undertaken. The objective of these investigations was to provide new insight into how and when these secondary Mn accumulations may have formed. Developed in supergene environments at the expense of Mn²⁺/Mn³⁺-bearing precursor minerals, the Mn⁴⁺ oxides occur either as pseudomorphic ores or as cavity-fillings and linings.

The isotopic ages range from 25 to 1 Ma, indicating intense chemical weathering, especially during the Miocene and Pliocene. It is yet too early to decide whether the age range represents a more or less continuous process or distinct weathering episodes. Formation of supergene Mn oxides may result from combined climatic and tectonic factors: local uplift, exhumation, and associated fracturing of rocks provided fresh mineral surfaces for percolating meteoric fluids that induced subsequent weathering under warm–temperate to subtropical conditions.     

激光Ar/Ar微区定年的应用及展望

40Ar/39Ar定年中,存在一些使用传统的大样逐步升温技术难以处理的难题,如变质及沉积岩样品常含有多期次矿物而难以获得理想结果,过剩Ar在样品中的分布以及来源问题一直难以有效解决,根据缓慢冷却钾长石的40Ar/39Ar年龄谱提出的热演化模型也因为Ar在矿物中扩散机制的不明确性而不断受到质疑。20世纪90年代将紫外激光（UV-laser）成功引入40Ar/39Ar年代学分析后,为解决这些难题提供了一种思路。借助激光微区     

山西梨园金矿黄铁矿微量元素及S-Pb-He-Ar同位素地球化学特征及其地质意义

山西梨园金矿是近年华北克拉通内部金矿找矿的新进展,其矿化样式多,具有较高的研究价值。梨园金矿赋存于太古宙阜平群片麻岩和混合岩化花岗岩中,受区内北北东向断裂控制,主要由爆破角砾岩型的1号矿体和石英脉型的2号矿体组成。矿石矿物主要为黄铁矿、方铅矿和闪锌矿等,脉石矿物主要为石英、绢云母、钾长石等,蚀变类型主要为硅化、钾长石化、绢云母化、绿泥石化等。黄铁矿、黄铜矿和闪锌矿δ³⁴S值为0.9‰~4.3‰,平均值为3.0‰,说明硫源主要来自岩浆;方铅矿δ³⁴S值为-12.8‰~1.4‰,平均值为-4.2‰,pH值升高可能是造成方铅矿高负δ³⁴S值的原因。金属硫化物Pb同位素变化范围较小,²⁰⁶Pb/²⁰⁴Pb为16.697~16.890,²⁰⁷Pb/²⁰⁴Pb为15.239~15.267,²⁰⁸Pb/²⁰⁴Pb为42.186~45.334,均落入下地壳演化线附近,表明成矿流体运移过程中可能淋滤了富Th的下地壳物质。黄铁矿的³He/⁴He为0.27~1.04 R_a,⁴⁰Ar/³⁶Ar为1 194.61~5 488.80,³⁸Ar/³⁶Ar为0.204~0.218,表明成矿流体主要来源于地壳,部分来源于地幔流体,具有壳幔混合的岩浆流体特点。微量元素特征显示黄铁矿具有岩浆热液型黄铁矿的特点,辉绿岩脉和石英斑岩的部分物质可能加入了成矿过程。文章认为梨园金矿为与浅成岩浆侵入作用相关的岩浆热液型金矿床,爆破角砾岩型和石英脉型矿体不同的沉淀机制和矿物结晶过程是导致二者REE含量、S-Pb-He-Ar同位素等方面存在差异的主要原因。梨园金矿深部具有寻找细脉浸染型矿体的可能,其成矿作用是太行山中部地区燕山期构造岩浆活动的又一具体表现。     

雅鲁藏布江缝合带造山型金矿床成矿流体特征:来自He-A r同位素的约束

雅鲁藏布江缝合带造山型金成矿带是近些年来在青藏高原新确立的碰撞造山型金成矿带,成矿流体是一套CO2-H2 O-NaCl-CH4-N2体系,以变质水为主.为探讨地幔组分对成矿的影响,本文选取雅鲁藏布江缝合带内邦布矿床和念扎矿床两个大型的造山型金矿为研究对象,对与成矿相关的黄铁矿及胶黄铁矿进行He-Ar同位素分析.结果显示...     

高压-超高压变质岩外来40Ar成因探讨

高压—超高压变质带是地球上重要的地质单元,特殊的形成环境使该类变质岩的矿物中常因含有外来⁴⁰Ar而⁴⁰Ar／³⁹Ar年龄偏老,制约了⁴⁰Ar／³⁹Ar年代学在此类岩石中的应用。本文探讨了氩在不同相中的溶解度与分配系数的差异,并着重论述了扩散参数与压力的关系及压力对岩体封闭性的影响,系统总结了高压-超高压变质矿物中外来⁴⁰Ar的形成机制。此外,随Fe/Mg、Si/Al、K等组分不同,氩在矿物中保存能力的某种规律性变化、离子孔隙度研究等也从理论上为研究氩的扩散和封闭行为提供了新的视角。在高压—超高压变质过程中多种因素共同作用,氩在这类矿物中的扩散更为复杂。对外来⁴⁰Ar形成机制的探讨对揭示高压—超高压变质结晶环境、岩体开放与否、不同矿物对氩保存能力等有重要的指示意义。     

黑龙江省嫩江地区科洛杂岩隆升时代

科洛杂岩出露于黑龙江省嫩江县大兴安岭北段,构造解析指示其为一大型伸展滑脱构造,整体反映杂岩体向南东伸展滑脱的特征。依据杂岩体显微构造特征,推测科洛地区伸展滑脱变形带变形温度约为300～450 ℃。滑脱带中变形花岗片麻岩样品中黑云母单矿物的 ⁴⁰Ar/³⁹Ar 年龄为118.39～117.37 Ma,指示伸展滑脱带形成时代为早白垩世晚期。结合大兴安岭地区中生代岩浆活动特征和南北变质核杂岩对比研究,认为大兴安岭整体经历了早白垩世中晚期的快速隆升,形成一系列变质核杂岩,发育大规模伸展变形作用,与松辽盆地坳陷期具有良好的耦合关系。     

据40Ar/39Ar快中子年龄新资料讨论震旦系底界年龄

作者对相当于晋宁运动岩浆活动产物的湖北峡东黄陵岩体和江西九岭岩体分别进行了⁴⁰Ar/³⁹Ar快中子定年。对采自宜昌莲沱地区的黑云母斜长花岗岩体和黄陵庙黑云母花岗闪长岩体的样品测得两条年龄谱,HH-01黑云母的高温坪年龄为893.7±6.7Ma,HH-05角闪石坪年龄为886.4±4.6Ma。对采自江西宜丰黄岗口的九岭岩体中黑云母测得的高温坪年龄为937.1±6.4Ma。从上述三个年龄值,并综合以前测定的晋宁期侵入体的年龄值以及不整合面上覆盖层和基底最上部沉积层的一些年龄值认为,以900Ma作为震旦系底界年龄是合适的。     

大庙斜长岩同位素地质年龄、稀土地球化学及其地质意义

大庙斜长岩的⁴⁰Ar/³⁹Ar年龄测定呈现出一条典型的马鞍型年龄谱,在中温阶段有二个明显的坪年龄1656±15 Ma和1029±7 Ma,结合其构造位置和全球斜长岩分布来看,它们分别代表了侵位年龄和后期热扰动的时代。密云奥长环斑花岗岩中角闪石的⁴⁰Ar/³⁹Ar坪年龄为1716±21 Ma。两者时空上密切相关,代表了裂谷作用初期非造山环境中双模式岩浆作用产物。斜长岩类和苏长岩之间稀土配分模式的相似性表明,它们明显为同一成因的岩浆分异系列的产物。     

一种新的 K-Ar定年方法：峰值比较法

K-Ar法定年技术经历了体积法和稀释法等不断趋于成熟，但对于年轻火山岩进行定年时，由于同位素质量分馏的影响，上述方法都具有不可克服的缺陷。随着质谱仪稳定度和精度的提高，直接建立氩同位素含量与峰强度之间的定量关系，从而不再使用稀释剂（ 38 Ar）成为可能，这就是 K-Ar定年的新方法：峰值比较法（或无稀释剂法）。该方法可以消除质量分馏作用带来的误差，对年轻火山岩定年具有重要意义。近代火山物质都含有初始氩，且有不同程度的质量分馏，其同位素比值落在质量分馏线上，这表明质量分馏作用影响 K-Ar定年的准确性，尤其是非常年轻的样品。通过精确测定样品的 38 Ar／ 36 Ar比值，可以准确扣除样品中的初始 40 Ar，从而得到放射成因 40 Ar含量。这种测量方法可以用于较老岩石、矿物样品的年龄测定，其结果与 K-Ar稀释法以及 40 Ar－ 39 Ar法相一致；更重要的是可以用于年轻样品的年龄测定，可以测 2～5ka的岩矿样品，对富钾矿物的测量误差小于 1ka；对老于 0．1Ma的样品，相对误差不大于 1．5％。介绍了峰值比较法的基本原理和测试过程，同时还讨论它的使用条件和定年范围。     

Post-collisional Variscan lamprophyres (Black Forest, Germany): Ar/Ar phlogopite dating, Nd, Pb, Sr isotope, and trace element characteristics

Subsequent to high-pressure and temperature metamorphism of the axial zone of the Variscan foldbelt in central Europe at ca. 340 to 330 Ma, formation of lamprophyre dikes during transtensional tectonics may be viewed as the beginning of the post-collisional stage of the orogeny. We report the results of ⁴⁰Ar/³⁹Ar mica dating, major and trace element data, and isotope compositions for lamprophyre and rhyodacite samples from the southern Black Forest. The chemical compositions of these rocks shed light on the upper mantle and crust at the end of the Variscan orogeny. ⁴⁰Ar/³⁹Ar plateau ages for four phlogopite–biotite separates from lamprophyres indicate emplacement at 332 to 314 Ma. This event coincides with melting of the crust as indicated by ⁴⁰Ar/³⁹Ar biotite plateau ages of ca. 332 Ma for rhyodacite dikes which are probably related to coeval undeformed granites. Incompatible trace element patterns of the lamprophyre samples reveal the characteristics of evolved continental crust and are interpreted as evidence for melting of sediment in a subduction-modified mantle. Nd, Sr, and Pb isotope compositions indicate an enriched mantle source with _Nd-values of −1.5 to −6.8 which is similar to Variscan crust. Significant contamination of the lamprophyric melts by Variscan crust can be ruled out as mantle-derived phlogopite phenocrysts have similar _Nd-values as in the whole-rock samples. We propose that the isotope compositions and incompatible trace element characteristics of the lamprophyres were predominantly inherited from melted sediment. The isotope compositions of Variscan lamprophyres from western Europe suggest that enriched upper mantle was only partly delaminated when ascending hot mantle triggered melting of the lower crust, as has been invoked for the origin of post-collisional granites. The isotope compositions of Tertiary basalts and mantle xenoliths indicate a depleted upper mantle under western Europe, implying that the enriched Variscan material was efficiently removed and mixed into the convecting mantle.