40Ar(40Ar＊＋40ArE)、39Ar释气特征与过剩氩的甄别及年代学意义

对具有不同地质历史背景的3类⁴⁰Ar/³⁹Ar法样品中的⁴⁰Ar和³⁹Ar释出特征进行对比,研究结果表明,⁴⁰Ar/³⁹Ar法样品中的⁴⁰Ar、³⁹Ar释气曲线主要表现为以下3种形式:完全重合型、过剩氩型和不规则型。当⁴⁰Ar与³⁹Ar释气曲线呈完全重合型时,⁴⁰Ar/³⁹Ar法全熔年龄代表了岩体的形成年龄;当⁴⁰Ar、³⁹Ar释气曲线是过剩氩型时,⁴⁰Ar/³⁹Ar法全熔年龄则大于岩体的形成年龄;当⁴⁰Ar与³⁹Ar释气曲线呈不规则型时,表明样品中的放射成固氩(⁴⁰Ar^*)发生了丢失,其全熔年龄一般较岩体的形成年龄小。对于⁴⁰Ar、³⁹Ar释气曲线呈过剩氩型的样品,⁴⁰Ar/³⁹Ar法年龄谱通常呈马鞍形,且马鞍形年龄谱的底部年龄一般都具有地质意义,代表了岩体的形成年龄。对于⁴⁰Ar、³⁹Ar释气曲线呈不规则型的样品,对其年龄谱的解释应持谨慎态度。     

天然气中稀有气体同位素的分析技术

在天然气稀有气体同位素分析技术方面,目前国内存在着主机辅助设备不足和检测项目较少等急需解决的问题,为此,本文建立了高真空、低漏率和低本底的天然气进样系统,成功地解决了实验室分析过程中空气Ar对天然气样品Ar的污染,获得了天然气中40 Ar/ 36Ar的准确值,新测比值较国内外已发表的40 Ar/ 36Ar值普遍高,二者差值的大小与天然气中Ar的含量密切相关。同时,应用自制的液氮调温冷阱实现了Ar与Kr+Xe的粗分离和天然气Kr与Xe同位素组成的检测.     

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.     

石英~(40)Ar-~(39)Ar阶段加热法定年的实验技术改进及意义

40 Ar 3 9Ar阶段加热法测定石英年龄始于 1986年。近年来通过样品清洗、提高真空度、提高质谱计灵敏度、减小样品用量、增加熔样阶段等实验技术改进 ,数据质量比过去有所提高。但关键问题是选择有代表性的石英样品 ,并预先对样品做镜下鉴定和电子探针分析 ,挑选钾含量较高的石英用于定年 ,以便减少失败 ,提高年龄数据测试的成功率。当钾含量高于 0 0 5 %时 ,一般都可在中温 (70 0～ 10 0 0℃左右 )阶段获得 3个以上视年龄构成的坪年龄和等时年龄 ,比过去马鞍型年龄谱中由一个最小视年龄作为成矿年龄要准确。     

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。     

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。     

K－Ar测年法在确定沉积岩成岩时代中的应用——以鄂尔多斯盆地为例

根据伊利石Ｋ－Ａｒ测年法研究，确定鄂尔多斯盆地存在两期与构造运动有关的成岩作用事件：即与早期燕山运动有关的侏罗纪（１７０—１６０Ｍａ）和与晚期加里东运动有关的志留－泥盆纪（４２０－３７０Ｍａ）成岩作用时代。为了揭示碎屑物质对Ｋ－Ａｒ年龄的影响和伊利石成岩作用的机制，本文提出了一个通过观察Ｋ－Ａｒ年龄随深度变化的趋势来进行判断的模式。在Ｋ－Ａｒ年龄小于地层时代的条件下，Ｋ－Ａｒ年龄与深度呈正相关，或者很相近的Ｋ－Ａｒ年龄与深度变化无关均标志着没有或很少有碎屑物质的影响；同时，前者指示逐渐埋藏条件下的成岩作用，而后者则反映短暂的热事件引起的成岩作用事件。     

Loss of 40Ar(rad) from leucite-bearing basanite at low temperature: implications on K/Ar dating.

The Bakony-Balaton Highland Volcanic Field (BBHVF) is located in the central part of Transdanubia, Pannonian Basin, with over 50 alkali basaltic volcanoes. The basanite plug of Hegyestu erupted in the first phase of volcanic activity. K/Ar and Ar/Ar ages were published for the BBHVF. K/Ar and Ar/Ar ages of the leucite-bearing basanite of Hegyestás were conflicting. This is caused by the special Ar retention feature of leucite in this basanite. K/Ar ages measured in the usual way were 25–45% younger, but after HCl treatment of the rock, or after reducing the baking temperature of the argon extraction line from 250°C to 150°C, they became similar to the Ar/Ar ages. All Ar/Ar determinations were performed after HF treatment. HCl treatment dissolved olivine, nepheline, leucite, magnetite and from 1-1 sample analcime or calcite. K dissolution studies from different locations of Hegyestü have shown that K content is mostly ≈2%, but it may decrease to ≈0.3%. HCl treatment dissolved 28.0–63.5% of the K content. The calculated K concentration for the dissolved part of samples with ~2%K was 4.02-6.42%: showing that leucite is responsible for the low temperature loss of ⁴⁰Ar(rad). Ar may release at low temperature from very finegrained mineral, or when the Ar release mechanism changes. A ⁴⁰Ar(rad) degassing spectrum has been recorded in the 55–295°C range of baking temperature and the data were plotted in the Arrhenius diagram. The diagram shows that a change of the structure in the 145–295°C range caused the loss of ⁴⁰Ar(rad). On fractions of HCl treated rock 7.56±0.17 Ma isochron K/Ar age has been determined. This is regarded as minimum age of eruption and it is similar to the Ar/Ar isochron age (7.78±0.07 Ma).     

沉积岩钾、氩分布与天然气氩同位素--以渤海湾、四川和鄂尔多斯盆地为例

本文对渤海湾盆地、四川盆地和鄂尔多斯盆地等天然气产区41个不同深度、不同时代和不同岩性的油气源岩和储集岩利用K-Ar年龄测定法进行K、Ar分析、表面年龄测定和粘土矿物分析,探讨了沉积岩中钾、氩的时代和区域分布特征及泥质岩钾含量与粘土矿物中伊利石含量的关系,并对泥质岩和碳酸盐岩的钾氩分布进行对比研究,指出了沉积岩钾丰度的时代演化旋回及其特点.并讨论了岩石中的钾氩来源及赋存状态以及天然气中氩的来源及其进入气藏的运移机理.     

东喜马拉雅构造结岩体冷却的~(40)Ar/~(39)Ar年代学研究

对采自东喜马拉雅构造结核心地段雅鲁藏布大峡谷地区的13件标本中的20件矿物样品进行了系统的常规^40Ar／^39Ar年代学研究。数据显示,样品的（^40Ar／^39Ar）i值均接近尼尔值（295．5±5）,且绝大部分样品的坪年龄与其反等时线年龄在误差范围内一致。从数据统计结果来看,所测样品的^40Ar／^39Ar年龄大都集中在1．3Ma和2．5Ma左右,表明南迦巴瓦地区在上新世中期和更新世早期均经历了快速冷却抬升事件。本次测试的样品采自不同的高程及不同的构造单元,且样品原岩的成因及岩性各异,但沿着大峡谷由北向南不同地段的样品的不同矿物（角闪石、黑云母、白云母、钾长石）的^40Ar／^39Ar年龄相近,而同一样品中不同矿物的^40Ar／^39Ar年龄大小又并非完全按照矿物对氩同位素体系的封闭温度高低来分布,表明该地区在上新世以来的岩体冷却速率很大,以致该地区的矿物对氩同位素体系的封闭过程与处于缓慢冷却环境中的封闭过程明显不同。以本文报道的数据估算,南迦巴瓦地区的岩体在最近3Ma以来的冷却速率达120～240℃／Ma,岩体抬升速率达3．4—6．9mm／a。     

福建紫金山矿田深部似斑状花岗闪长岩锆石SHRIMP U-Pb和单矿物~(40)Ar/~(39)Ar年龄及其地质意义

通过对福建紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩3组锆石SHRIMP U-Pb和2组角闪石、钾长石~(40)Ar/~(39)Ar测年,获得锆石~(206)Pb/~(238()U加权平均年龄为101.8±1.5 Ma(n=34,MSWD=1.0),代表紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩的成岩年龄;同时获得角闪石~(40)Ar/~(39)Ar冷却年龄为100±11 Ma、102.2 Ma,钾长石的~(40)Ar/~(39)Ar冷却年龄为96.3±1.7 Ma、98.5 Ma。依据矿物封闭温度理论,估算紫金山矿田深部与成矿作用有关的主期似斑状花岗闪长岩由锆石结晶至角闪石40Ar/39Ar体系封闭、再到钾长石~(40)Ar/~(39)Ar体系封闭的岩石冷却速率分别是40.7~67.1℃/Ma、116.9~216.3℃/Ma,显示岩石的冷却速率较大;由古地温梯度推算主期似斑状花岗闪长岩结晶(101.8±1.5 Ma)至钾长石~(40)Ar/~(39)Ar体系封闭(96.3±1.7 Ma)期间岩体隆升剥露了约3 km,暗示地壳在这一时期发生了快速隆升剥蚀作用。紫金山矿田深部似斑状花岗闪长岩锆石206Pb/238U年龄佐证了紫金山矿田深部存在一个大岩基,并约束了紫金山矿田斑岩型矿床的成矿时代,单矿物的~(40)Ar/~(39)Ar年龄为矿区的隆升剥露研究提供新资料。     

自生伊利石40Ar/39Ar法定年技术及气藏成藏期的确定

为了确定天然气藏的成藏期,进行了自生伊利石的^40Ar／^39Ar法同位素定年实验．实验重点解决了自生伊利石的^40Ar／^39Ar法定年的几个技术难题：第一,粘土矿物的提纯,避免伊利石以外的含K矿物混入;第二,自生伊利石与碎屑伊利石的分离;第三,克服核反冲造成的Ar原子丢失．利用冷冻一加热循环碎样技术获得高纯度的粘土矿物;通过阶段加热得到的年龄谱可以区分自生伊利石与碎屑伊利石;利用“显微包裹”技术有效克服了核照射反冲问题．对鄂尔多斯盆地北部苏里格气田研究发现。二叠系储层中的伊利石有2种年龄图谱：一种只有自生伊利石的坪年龄;另一种图谱既有自生伊利石的坪年龄,也有碎屑伊利石的年龄,形成二阶式的图谱．通过自生伊利石的形成时间推断,天然气的最早充注时间晚于169～189Ma．实验的结果表明,冷冻加热循环碎样技术可以有效地避免伊利石以外的含K矿物混入．是获得高纯度粘土矿物的关键技术;自生伊利石^40Ar／^39Ar法定年技术可以用于确定天然气藏的成藏期.     

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.     

Evaluation of 40Ar/39Ar Geochronology of Authigenic Illites in Determining Hydrocarbon Charge Timing: A Case Study from the Silurian Bituminous Sandstone Reservoirs,Tarim Basin,China

The Silurian bituminous sandstones(SBS) in the Tarim Basin, China are important basinwide reservoirs with an estimated area of approximately 249000 km2. We investigated the ages of authigenic illites in the SBS reservoirs and constrained their formation timing by using the ~(40)Ar/~(39)Ar step wise heating method. The age spectra, ~(39)Ar recoil loss and their controlling factors were investigated systematically. The ~(40)Ar/~(39)Ar ages were compared with the conventional K/Ar ages of identical clay fractions. The clay in the SBS reservoirs is dominated by orderly mixed-layer illite/smectite(I/S) with 5%–30% smectite layers. The I/S minerals morphology comprises primarily honeycomb, short filamentous and curved-lath particles, characteristic of authigenic illites. The unencapsulated ~(40)Ar/~(39)Ar total gas ages(UTGA) of the authigenic illites range from 188.56 ± 6.20 Ma to 491.86 ± 27.68 Ma, which are 7% to 103% older than the corresponding K/Ar ages of 124.87 ± 1.11 Ma to 383.45 ± 2.80 Ma, respectively. The K-Ar ages indicate multistage accumulations with distinct distribution patterns in the Tarim Basin: older(late Caledonian-early Hercynian) around the basin margin, younger(late Hercynian) in the basin centre, and the youngest(middle to late Yanshanian) in the Ha-6 well-block, central area of the North Uplift. The age difference is believed to have been caused by the ~(39)Ar recoil loss during the irradiation process. Compared with the K/Ar ages, the estimated ~(39)Ar recoil losses in this study are in the range from 7% to 51%. The ~(39)Ar recoil loss appears to increase not only with the decreasing particle sizes of the I/S, but also with increasing percentage of smectite layers(IR) of the I/S, and smectite layer content(SLC) of the samples. We conclude that due to significant ~(39)Ar recoil losses, UTGA may not offer any meaningful geological ages of the authigenic illite formation in the SBS and thus can not be used to represent the hydrocarbon charge timing. ~(39)Ar recoil losses during ~(40)Ar/~(39)Ar dating can not be neglected when dating fine authigenic illite, especially when the ordered mixed-layer I/S containing small amount of smectite layers(IR30%) in the reservoir formations. Compared with the unencapsulated Ar-Ar method, the conventional K-Ar method is less complicated, more accurate and reliable in dating authigenic illites in petroleum reservoirs.     

西准噶尔晚古生代花岗岩类侵入体(40)~Ar/(39)~Ar热年代学

西准噶尔成矿带夹持在天山断裂与额尔齐斯断裂之间,是中亚成矿域西部的核心区域之一,广泛发育晚古生代深成岩浆活动、走滑断裂构造和斑岩铜矿、造山型金矿成矿作用。本文在西准噶尔成矿带包古图岩体、康德岩体、加曼岩体、库鲁木苏岩体、别鲁阿嘎希岩体、哈图岩体、阿克巴斯套岩体、庙尔沟岩体、克拉玛依岩体及红山岩体采集12个样品,通过黑云母和钾长石(40)~Ar/(39)~Ar阶段升温测年,给出了该地区(40)~Ar/(39)~Ar冷却年龄。其中,黑云母(40)~Ar/(39)~Ar年龄处在326~302 Ma范围内,钾长石(40)~Ar/(39)~Ar年龄为297~264 Ma,反映了西准噶尔地区晚石炭世-中二叠世的区域中温冷却历史。结合前人报道的锆石U-Pb、角闪石(40)~Ar/(39)~Ar、辉钼矿Re-Os、磷灰石裂变径迹等年龄数据,构建了西准噶尔成矿带晚古生代岩浆侵入,成矿作用与构造抬升,以及晚中生代剥露过程的整个热历史;并与区域左行走滑断裂活动的时间进行了对比,讨论了(40)~Ar/(39)~Ar冷却年龄的构造意义。     

Lattice diffusion of Ar in quartz, with constraints on Ar solubility and evidence of nanopores

Diffusion and solubility of Ar in optically clear natural and synthetic quartz crystals were examined at ∼500 to 1200 °C by treating polished specimens in pressurized Ar (1-185 MPa) and characterizing the resulting diffusive-uptake (or subsequent diffusive-loss) profiles using Rutherford backscattering spectroscopy (RBS). Analytical uncertainty leads to significant scatter in the data, but the Ar diffusivity, D, is reasonably well constrained by the following Arrhenius relationship:D = 8.2−4.2+8.8 × 10⁻¹⁹ exp[(−6150 ± 750)/T(K)] m²/sNo effects of crystallographic orientation or quartz structural form (α or β) are discernible.Apparent solubilities typically fall between 1000 and 3000 ppm (by mass), with large uncertainties (±50-60% 2σ), but some lower values (∼700 ppm) are observed near the low end of the Ar pressure range investigated. Occasional high-concentration outlier values fall between 5000 ppm and 3.8 wt.% Ar. These do not correlate with Ar pressure, suggesting extrinsic (non-lattice) siting of Ar in some cases. Field-emission SEM images and numerical simulations of the diffusion process document isolated nanopores as the hosts for the occasional very high concentrations of Ar (observable pores range down to ∼10 nm in diameter; indirect evidence points to smaller ones as the more common sinks for Ar). The systematics of the data suggest an actual (lattice) solubility of ∼2000 ppm at 100- to 200-MPa Ar pressure, which is equivalent to a partition coefficient of ∼0.001cm³STP/g · atm. Using either organic clathrate or fullerene as the Ar sources, 1-GPa experiments in a piston-cylinder apparatus result in similar uptake of Ar into quartz, in this case through partitioning equilibrium with C-O-H fluid (clathrate source) or amorphous carbon (fullerene source).The ability of quartz, relative to other minerals, to incorporate significant amounts of Ar may allow this ubiquitous and abundant mineral to serve as a local sink for Ar in crustal rocks lacking a free fluid phase. The diffusion data permit open-system behavior of Ar in quartz below the closure temperature of biotite and other ⁴⁰Ar source minerals.     

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.     

Helium and Argon Isotopes of the Tertiary Volcanic Rocks, Shandong Peninsula：Significance for Crust-Mantle Circulation

Noble gases have successfully been used to con strain the mantle structure (Allegre et al., 1983),discuss mantle heterogeneity (Ma and Tao, 2002),determine different mantle source components and soon, because of their chemical inertia and isotopicfractionation during the evolution of the earth. Het erogeneities of the subcontinental mantle beneatheastern China have been confirmed by abundant stud ies of mantle magmas and xenoliths, in mineralogy,petrology and isotop…     

含水矿物在真空下的释Ar机制:Ar-Ar热年代学面临的新问题

地质体的实际情况、激光显微探针束研究、XRD和SEM观测以及真空加热下Ar的释放特征都表明体积扩散不是含水矿物在真空加热中释放Ar的唯一机制。在低温下 ,Ar的释放主要受由缺陷引起的短程扩散和体积扩散共同作用的多途径扩散机制制约 ;而在高温下 ,由于含水矿物在真空中不够稳定 ,Ar的释放受到脱羟基、脱氢、氧化反应、分层作用等造成的晶体结构改变的强烈影响。含水矿物在高温下的氧化分解会导致矿物中原始Ar浓度梯度的均一化 ,因而无法得到真实的Ar分布剖面 ,也无法据此计算矿物的封闭温度 ,并进而可能影响到ArAr年龄坪的地质意义。     

论河北丰宁牛圈银(金)矿床的成矿时限问题

对河北牛圈银(金)矿区细粒钾长花岗岩株和含矿隐爆角砾岩墙钾长石样品进行了详细的40Ar/39Ar同位素年龄测定。细粒钾长花岗岩株钾长石样品主要加热阶段所获得数据构成一条相关性很好的直线,其对应的同位素等时线年龄值为(216.6±3.2)Ma(2σ),40Ar/39Ar初始比值为297±28。相比之下,含矿隐爆角砾岩墙钾长石样品特定加热阶段(1200～1300℃和1360～1450℃)所获数据同样也构成两条直线,其对应的同位素等到时线年龄值分别为(227.9±4.8)Ma(2σ)和(216.7±6.1)Ma(2σ),40Ar/39Ar初始比值分别为307±41和269±39。考虑到所测样品均采自未受到明显后期构造-岩浆活动或其他热事件影响的岩(矿)体,因此,细粒钾长花岗岩株和含矿隐爆角砾岩墙的形成时代为216Ma,与印支期富钾岩浆作用具有密切成因联系。根据含矿隐爆角砾岩墙的野外地质证据,结合矿物共生组合关系和钾长石同位素年龄数值,可以推测牛圈银(金)矿区早期成矿作用发生在(227.9±4.8)Ma,而主要成矿期为(216.7±6.1)Ma,是印支期多阶段富碱质成矿流体活动的产物。牛圈银(金)矿区成矿作用时间的厘定对于提高该区成矿理论研究水平和寻找隐伏金属矿床均具重要意义。