电子探针化学测年在攀枝花大田晶质铀矿中的应用及其意义

晶质铀矿和沥青铀矿是热液铀矿床的主要工业铀矿物,在研究热液铀矿床成因及成矿规律方面具有重要的意义。攀枝花大田地区是我国混合岩型热液铀矿分布区,已发现粗粒特富铀矿滚石(铀含量10%)及较富基岩矿石(铀含量为0.1%~2%),主要铀矿物为晶质铀矿,对两种晶质铀矿成分及形成时代的研究对该区混合岩型热液铀矿成矿规律研究具有重要的价值。本文通过对大田地区滚石中的晶质铀矿和基岩矿石中的晶质铀矿进行矿物学及电子探针分析,研究了晶质铀矿的成分及形成时代。结果表明:(1)大田地区滚石和基岩矿石中的晶质铀矿除铅之外化学成分较为相似,两类矿石晶质铀矿中UO_2含量为77.36%~84.04%,ThO_2含量为0.98%~5.59%,PbO含量为1.79%~8.8%,其中滚石晶质铀矿中的铅含量低于基岩晶质铀矿,钍含量高于基岩晶质铀矿;(2)电子探针化学定年结果表明,基岩矿石晶质铀矿的形成时代为774.9~785.5 Ma,滚石晶质铀矿的形成时代为783.7 Ma,与传统同位素测年结果(775~777.6 Ma)非常一致,一方面说明滚石晶质铀矿和基岩晶质铀矿为同一时代的产物,另一方面说明电子探针原位测年方法是可靠的;(3)在后期的热液蚀变中,晶质铀矿先后发生了硅化、碳酸盐化及赤铁矿化,蚀变发生的时间分别为730.6Ma、699.8 Ma和664.0 Ma。此结论对研究攀枝花大田地区热液铀矿成矿时代及成矿作用过程提供了依据。     

电子探针测年方法应用于晶质铀矿的成因类型探讨

电子探针Th-U-Pb测年因其高分辨率与高精度的优势,在独居石、锆石等定年矿物中得到了推广,但在Th、U、Pb含量高的晶质铀矿、沥青铀矿等矿物中则应用较少。本文在铁矿床变质岩绿泥石、阳起石黑云母蚀变岩首次发现U含量高的晶质铀矿,基于此,结合该铁矿床地区的地质背景,利用偏光显微镜与电子探针等分析测试手段,将镜下蚀变现象、年龄计算与其他相关元素分析相结合,重点对晶质铀矿的成矿年龄及成矿规律进行探讨。研究发现:通过镜下观察判断,晶质铀矿的成因类型与澳大利亚著名的变质型铀矿相似,均为古老的变质型,且周围的脉石矿物均为绿泥石,绿泥石皆由黑云母退变质而成,铀矿的赋存位置显示其与黑云母、绿泥石之间有紧密联系,其成矿年龄与黑云母、绿泥石形成年龄息息相关。继而根据电子探针数据计算成矿年龄,判断成矿期次,得出主要成矿期在(1654±17)Ma~(1805±17)Ma,为中元古代中期,且主要成矿期与热液蚀变作用黑云母化有关,后期活化富集时期在(657±17)Ma~(807±17)Ma,为新元古代南华纪时期,此阶段是热液侵入、绿泥石化广泛发育的时期;选取较大颗粒对晶质铀矿的环带年龄进行计算,从年龄分布上证实后期有强烈的流体活动的发生,且主要与绿泥石化相关。另外,对比变质型与沉积型铀矿中Y2O3与UO2含量发现,两者之间存在负相关关系,此关系对判断铀矿成因即是否为变质型或沉积型可能有指示意义,但缺乏大量的数据佐证,需进一步研究。     

电子探针Th-U-Pb微区测年方法及其在铀矿地质研究中的应用前景

电子探针Th-U-Pb微区测年方法是新近发展起来的一项测年技术.该技术以放射性核素的衰变理论为基础,在一定条件下通过电子探针测量矿物中的Th、U、Pb含量,经过数据处理,最终计算出矿物的年龄.由于该项技术具有省时、费用低、不破坏样品等优点,现已广泛地应用于独居石、锆石的年龄研究中.在晶质铀矿年龄研究中的应用仅有零星报道,但由于该技术的高空间分辨率,将对不同产出状态的微小晶质铀矿颗粒的定年研究大有裨益.     

赣中紫云山花岗岩晶质铀矿的电子探针U-Th-Pb化学定年

紫云山岩体是赣中地区与钨铀成矿关系极为密切的过铝质花岗岩体,但目前该岩体的成岩时代尚不明确.通过偏光显微镜、扫描电镜、电子探针等手段,首次开展了紫云山花岗岩中赋存晶质铀矿的精细矿物学研究.结果表明:晶质铀矿主要赋存于黑云母之中,少数被黄铁矿包裹,部分晶质铀矿被不同程度溶蚀和交代,表明晶质铀矿是本区花岗岩型铀矿的主要铀源矿物之一.利用电子探针U-Th-Pb化学定年法测得蕉坑单元 (J₃J)5颗晶质铀矿年龄为154.5~168.9 Ma,加权平均年龄为161.8±2.4 Ma (MSWD=0.26,n=26),庙前单元 (J₃M) 三颗晶质铀矿年龄为152.8~164.7 Ma,加权平均年龄为159.7±3.2 Ma (MSWD=0.2,n=15).获得的年龄与南岭地区主要含钨花岗岩的侵入时间高度一致,对应华南中生代大规模岩浆活动的第二阶段.晶质铀矿年龄与华南含钨花岗岩锆石U-Pb年龄非常一致,验证了过铝质富铀花岗岩中晶质铀矿电子探针定年方法的可行性.      

砂岩型铀矿定年现状及测试方法研究

对砂岩型铀矿的大规模研究源于自上世纪九十年代开始的铀矿战略大转移,为保证后续分析成矿机制及物质来源等的准确性,对于砂岩型铀矿成矿时代的研究则是不可缺少的内容之一。目前,用以研究铀矿物年龄的最广泛的方法主要分为全岩-单矿物U-Pb等时线测试方法以及微区原位定年方法,而微区原位方法又分为电子探针化学测年和同位素原位微区精确定年。本文将通过对铀矿年代学的研究现状来总结对比,具体阐述和介绍目前定年的主要研究方法,从而为能够更为精确的分析构造演化历程提供具体依据。     

陕西华阳川铀-多金属矿床晶质铀矿电子探针U-Th-Pb化学定年及其地质意义

华阳川铀-多金属矿床位于小秦岭陆内造山带内,以铀铌铅为主,并伴生稀有、稀土元素矿化,含矿脉岩主要有伟晶岩和碳酸岩。文章利用电子探针U-Th-Pb化学定年技术对华阳川铀-多金属矿床中晶质铀矿开展了定年工作,并获得了2组U-Th-Pb年龄。第一组晶质铀矿加权平均年龄为(221.9±5.1)Ma(MSWD=0.36,n=18),该区印支期老牛山岩体及邻近的黄龙铺钼矿床、黄水庵钼矿床年龄均集中在220 Ma左右,说明晚三叠世是东秦岭地区一个重要的成岩成矿时代,华阳川铀-多金属矿床在220 Ma左右也发育一期铀-铌成矿作用。第二组晶质铀矿加权平均年龄为(137.1±2.0)Ma (MSWD=1.06,n=47),指示了华阳川铀-多金属矿床存在白垩纪的一期铀-铌活化-沉淀事件。测定的2组晶质铀矿化学年龄可能分别记录了该区自晚三叠世开始的后碰撞伸展环境和早白垩世的造山后伸展环境。     

光石沟晶质铀矿:一种潜在的铀矿物电子探针定量分析标准样品

电子探针定量分析是采用元素A在待测样品中的特征X射线强度与标准样品中元素A特征X射线强度相比较而进行的,要实现未知样品的元素定量分析必须要具有相应的标准样品,目前可用于铀元素分析的电子探针分析标准样品极少,且缺乏与天然矿物成分、结构近似的标准样品。国际和国内已经制定了电子探针标准物质研制的规范(GB/T 4930—2008/ISO14595:2003),按该规范规定的方法研究了产于陕西光石沟铀矿床的晶质铀矿,结果表明:这些晶质铀矿晶形发育好,颗粒大,具备良好的纯度、均匀性和稳定性。随机选择30个颗粒进行均匀性检测,UO₂和PbO在95%的置信区间的平均浓度不确定度分别为0.275%和0.060%,具备非常好的均匀性;该晶质铀矿在电子探针电子束长时间(如360s)轰击下和在自然条件下存放,均具有良好的稳定性;采用五家实验室化学分析定值方法确定了该晶质铀矿的化学成分,并计算了不确定度,主量元素UO₂为(86.80±0.36)%,PbO为(4.80±0.07)%,其他元素也给出了参考值。综合以上研究结果:产于光石沟铀矿床的晶质铀矿满足GB/T 4930—2008关于电子探针定量分析标准样品的各项判据,是一个潜在的适用于铀矿物化学成分电子探针定量分析使用的天然矿物标准样品。     

电子探针化学测年方法

介绍含Th U Pb矿物电子探针化学测年方法的基本原理 ,并利用应用实例说明该方法在研究岩石形成年龄、构造热事件年代等方面的重要作用 ,最后简要评价该方法的优、缺点 ,并指出它的应用前景 ,认为电子探针化学测年方法将有可能在深源岩石包体年龄研究、含放射性元素矿床形成年代研究等方面大有作为 ,并使地质年代学研究更加精细化 ,有助于深入探讨复杂地质事件及其演化历程。     

电子探针测年方法应用于粤北长江岩体的铀矿物年龄研究

晶质铀矿被认为是花岗岩型铀矿成矿的主要矿源提供者,在评价岩体的含矿性和确定成岩成矿年龄方面有重要意义。长江岩体属于诸广山复式岩体的一部分,是粤北地区重要的产铀花岗岩体,本文利用电子探针对该岩体中的铀矿物进行研究。结果表明:长江岩体中的铀矿物多以充填或被黄铁矿包围的形式存在,或者分布于石英、黑云母、绿泥石等矿物中;铀矿物类型主要有晶质铀矿、沥青铀矿、铀石、铀钍石四种。晶质铀矿/沥青油矿的化学年龄值可分为三组:~155 Ma、~106 Ma和~74 Ma。第一组年龄代表岩体的形成时代,后两组年龄代表铀矿的多期次成矿作用年龄。铀矿物从成岩后到~106 Ma,成分没有发生明显变化,直到~74 Ma后才发生明显的U元素活化、迁移。因此,可以推测长江岩体地区主要的铀矿成矿期应发生在~74 Ma及之后。     

独居石的电子探针钍—铀—铅化学测年

基于独居石富含Th,U,Pb,且具有非放射性铅的含量通常可忽略不计以及不易发生放射性铅的丢失的特点,为电子探针技术进行化学测年提供了重要的前提条件,通过对已知年龄的独居石标样以及新疆土屋－延东铜矿床的一个独居石样品进行电子探针测年分析,分析结果与标样的年龄标定值及样品年龄的推测值相吻合,表明独居Th-U-TPb电子探针化学测年技术是一种可靠有效的测年方法。     

钍石电子探针定年：记录小秦岭矿集区伟晶岩热液蚀变历史

电子探针化学定年法 (Chemical Th-U-total Pb Isochron Method,CHIME) 以其原位、极高的空间分辨率、样品制备简单方便、不破坏样品、测试速度快、成本低等优点,被成功用于独居石、褐帘石、锆石、磷钇矿、晶质铀矿等副矿物定年,但应用于钍石定年仍不成熟。本文尝试利用电子探针对小秦岭矿集区元古宙伟晶岩脉中的钍石进行定年。显微镜下观察发现,热液钍石颗粒细小（<30 μm）,多与独居石共生,包裹在褐帘石中。我们对未发生蚀变或者蜕晶化的小颗粒钍石进行了电子探针Th-U-Pb化学法定年,得到的加权平均年龄为120.9±2.1 Ma (95%置信区间)。这一年龄与枪马金矿的矿化年龄一致,代表了继该地区大规模花岗岩侵位活动之后,华北克拉通减薄过程中的又一次热液蚀变过程。     

内蒙古大青山逆冲推覆构造带中泥质板岩激光微区40Ar/39Ar年龄

大青山逆冲推覆构造下盘发育一套泥质板岩,它们是由二叠系泥岩、粉砂质泥岩和粉砂岩、细砂岩遭受逆冲推覆变形引起的动力变质作用改造形成的,发育一组透入性劈理构造.通过岩石薄片鉴定,沿劈理面形成绢云母、白云母和绿泥石等同构造新生矿物.对其中新生白云母矿物进行了激光微区40Ar/39Ar测年,获得了(121.6±1.6)Ma的等时线年龄.野外地质证据表明了大青山逆冲断层发生过两期逆冲推覆变形作用,早期发生在印支期,晚期发生在燕山期.(121.6±1.6)Ma年龄代表燕山期逆冲推覆事件发生时间.     

龙泉关韧性剪切带同变形花岗岩的构造特征及其独居石定年

龙泉关韧性剪切带强变形部位发育一系列平行剪切带走向展布的花岗岩脉体，构造变形特征表明它们可能是同变形花岗岩，是剪切作用下围岩部分熔融的产物。通过独居石电子探针定年，测得龙泉关韧性剪切带的年龄分别为：主变形幕发生于1877～1846Ma，第2个变形幕发生于1812-1782Ma，晚期1725Ma左右又经历了流体活动。龙泉关韧性剪切带的发生、发展与华北克拉通中部带古元古代晚期的变质事件同期，是东、西陆块碰撞的一个主要剪切带。     

电子探针化学测年技术及其在地学中的应用

电子探针主要应用于固体无机材料的微区化学成分定量测定和表面形貌研究。近十年来电子探针化学测年技术发展迅速，并取得了丰富的研究成果。本文在简单介绍该技术发展历程后，进一步详细介绍该技术的基本原理、样品制备与分析、年龄计算方法等内容，最后讨论了该技术的优缺点，同时指出电子探针测年技术在地学中的具有广泛的应用前景。     

Quantification of electron microprobe compositional maps of rock thin sections: an optimized method and examples

Quantification of discrete pressure–temperature domains in deformed chlorite + white mica‐bearing metapelites was undertaken on mineral compositions derived by two‐dimensional microprobe compositional mapping of selected areas of rock thin sections. In order to achieve compositional information at sufficient analytical precision, spatial resolution and sample coverage within a typical analysis time of 1 day, an optimization of measurement methods was necessary. The method presented here allows collection of raw counts for eight different element concentrations at an analytical precision of ～1–2 wt%. X‐ray intensity multiplane maps (one map per measured chemical element) are translated into concentration multiplane maps, utilizing selected conventionally measured spot analyses combined with the Castaing approximation for each mineral. As this step requires identification of the different minerals present in the mapped area, a statistical clustering technique to identify different groups of composition was developed, guided by simple petrographic inspection of the thin section, to delineate the important minerals in the mapped area. Finally, the compositions of each pixel are translated into a mineral structural formula thus yielding a new kind of image with a high content of petrological information. The reliability of the mineral composition images was emphasized by carrying out precision tests on the analytical data. The possible use of chemical maps to infer the P–T–deformation history of metamorphic rocks is illustrated with two samples from the Spitzbergen and the Sambagawa blueschist facies belts. In both samples, a strong correlation between structures and chemistry is observed. Qualitative estimates of P–T conditions from the Si‐content of mica and chlorite are in good agreement with their location in microstructures that formed at different times. Therefore, the combination of chemical maps with microstructural observations is a very powerful approach to understand both the evolution of complex metamorphic rocks and the control by deformation of mineral reactivity.     

Fold dating: A new Ar/Ar illite dating application to constrain the age of deformation in shallow crustal rocks

We propose a deformation dating method that combines XRD quantification and Ar chronology of submicroscopic illite to determine the absolute ages of folds that contain clay-bearing layers. Two folds in the frontal segment of the Mexican Fold-Thrust Belt (MFTB), which was deformed from Late Cretaceous to Eocene, are used to illustrate the method and its future potential.Variations in mineral composition, illite-polytype, crystallite-size (CS) and Ar total gas ages were analyzed in the limbs and hinge of two mesoscopic folds. This analysis examines potential effects of strain variation on illitization and the Ar isotopic system along folded layers, versus possible regional thermal overprints. The Ar total-gas ages for 9 samples in Fold 1 vary between 48.4 and 43.9 Ma. The % of 2M₁ (detrital) illite vs. Ar total-gas ages tightly constrains the age of folding at 43.5 ± 0.3 Ma. Nine ages from three samples in Fold 2 range from 76.2 to 62.7 Ma, which results in a folding age of 63.9 ± 2.2 Ma. Both ages are in excellent agreement with more broadly constrained stratigraphic timing. The method offers a novel approach to radiometric dating of clay-bearing folds formed at very low-grade metamorphic conditions, and has the potential to constrain dates and rates of regional and local deformation along and across foreland orogenic belts.     

Estimation of Fe3+ from electron microprobe analyses: observations on calcic amphibole and chlorite

ABSTRACT Metabasites and metagreywackes from the Pelona and Rand Schists of southern California were analysed using three different electron microprobes. For all three instruments, the estimated Fe³⁺ contents of calcic amphibole, chlorite and epidote are positively correlated. For some samples, there is an additional correlation between high estimated Fe³⁺ and the presence of magnetite. These results imply that microprobe analyses can be used to discern relative differences in Fe³⁺. However, microprobe data and calculations on the sensitivity of the correction procedures to systematic analytical errors indicate that estimated values of Fe³⁺ are not significant in an absolute sense. Thus, estimates of Fe³⁺ are meaningful when comparing samples analysed with a single microprobe, but must be used with caution when comparing analyses obtained on more than one probe.     

Electron microprobe dating of monazite substantiates ages of major geological events in the southern Brazilian shield

To obtain the chemical Th^*–Pb isochron ages and surface maps of monazite crystals in igneous and metamorphic rocks from the southern Brazilian Shield, we employ Th–U-total Pb dating by an electron probe microanalyzer. The ages of two Trans-Amazonian metamorphic events are given by a felsic, garnet-bearing granulite from the Santa Maria Chico granulitic complex. The age of the first event, at approximately 2.35 Ga, was obtained by surface mapping in a grain included in garnet. The dating of the second event, 1899±43 Ma, is in agreement with previous data obtained in zircon crystals with sensitive high-resolution ion microprobe. Other determinations belong to the Brasiliano cycle. In the São Gabriel block, an age of 643±129 Ma was obtained on monazite from a staurolite-garnet schist of the Cambaizinho Formation, whereas a staurolite-bearing schist from the Passo Feio complex yielded a 510±68 Ma age. Several units in the Dom Feliciano belt were dated, including the biotite-sillimanite gneisses of the Várzea do Capivarita complex (552±90 Ma), the sillimanite-garnet gneisses of Camboriú complex (565±77 Ma), the Três Figueiras granite (558±57 Ma), and the Plaza Itapema granite (545±55 Ma). The ages presented in this study, obtained through monazite chemical dating, are confirmed through comparison with previous data regarding zircon crystals from the same geological units.     

Chemical composition of macerals in bituminous coals of the Gunnedah Basin, Australia, using electron microprobe analysis techniques

The chemical composition of the organic matter in the principal macerals of high-volatile bituminous coals from the Gunnedah Basin, New South Wales (Rv_max of telocollinite between 0.6 and 1.1%) has been evaluated from polished section specimens using an electron microprobe technique. Highest proportions of carbon occur in the inertinite macerals, especially fusinite and secretinite (formerly resino-sclerotinite), as well as in sporinite; lowest proportions of carbon occur in the different macerals of the vitrinite group. Oxygen shows the reverse trend, being most abundant in vitrinite and least abundant in the inertinite components, whereas sulphur is lowest in the inertinites and highest in the liptinite (mainly sporinite) present. Evaluations of maceral composition, using the carbon content of telocollinite as a rank indicator, show that carbon is more abundant in both sporinite and semifusinite, relative to vitrinite, in low-rank high-volatile bituminous coals. The difference decreases with increasing rank, and the proportion of carbon in telocollinite becomes essentially the same as that in sporinite and semifusinite at carbon contents of about 89 and 91%, respectively. The carbon content of fusinite and secretinite, on the other hand, does not seem to vary appreciably with rank advance. No significant difference in composition occurs in the rank range studied between the three vitrinite varieties present, desmocollinite, telocollinite and a more highly reflecting telocollinite resembling pseudovitrinite. No evidence was found to indicate a higher hydrogen content, relative to telocollinite, for the vitrinite matrix of desmocollinite.