Small-angle X-ray scattering measurement of the internal microstructure of natural zircon crystals

Zircon crystals change their physical properties significantly over time in response to the radiation damage (metamictization) induced in the lattice by the presence of radionuclides U and Th. Crystalline zircon has extremely low diffusion rates of the radiogenic daughter product, Pb. Lead diffusion is enhanced in metamict volumes, but the observed lack of correlation between Pb loss and metamictization in natural zircons requires that other mechanisms control the incidence of Pb migration. The proposition that self-induced stress and elasticity contrasts in zoned natural crystals create fast-track Pb migration pathways, in response to time-integrated radiation damage, requires a means of detecting the microstructures within zircons at the interatomic scale at which Pb migration takes place. Small-angle X-ray scattering (SAXS) is introduced as a means of detecting candidate microstructures including subgrain boundaries, defect networks and microfractures produced by differential metamictization. It is shown that a classical X-ray source yields measurable SAXS response from contrasting metamict and crystalline domains within a crystal, and these properties are quantified for a metamict zircon megacryst. Detection of the weaker SAXS response expected from microfractures and networked defects requires the more intense X-rays of synchrotron-source radiation.     

Metasomatic replacements and isotope relationships in zircon crystals and crystallogenetic models

Secondary textures formed in natural zircon crystals under the influence of hydrothermal solutions are interpreted as products of metasomatic substitution. Such textures are clearly seen in metamict zircons and zircons enriched in radioactive elements, which are often found in potassic granites, pegmatites, and other rocks with high U and Th contents. Specific secondary textures in zircons contained in microcline granites, metasedimentary and eclogite-like rocks of the Kola Peninsula are reviewed. Detailed mineralogical and geochemical studies of zircons (CL, BSE, microprobe analysis of incompatible elements) have been carried out with the subsequent U-Pb dating of different zircon zones. The mechanism of secondary texture formation is considered in terms of metasomatic isomorphic substitution. Analogy between natural structure of zircon and products of experimental isomorphic substitution makes it possible to obtain additional information on conditions of the natural zircon growth important for interpreting geochronological data. When metamict zircons are substituted without gaining new material and with a decrease in volume, the U-Pb age of initial zircon remains unchanged and the lower intercept of discordia and concordia corresponds to the time of fluid impact. Substitution with a gain in matter is accompanied by excess volume. In this case, the timing of zones of transitional composition is devoid of geological sense.     

锆石红外光谱在地质研究中的应用

以甘肃白银厂地区的7件锆石同位素年龄样品为例,根据其红外光谱中615cm-1吸收峰的高和半高宽度的比值,将锆石样品的变生程度量化,并将其分为晶质锆石、半变生锆石和变生锆石3类。参考相关资料,利用7件锆石样品的红外光谱特征对其铀钍含量和"红外年龄"做了估测,并将估测年龄与其U Th年龄和全岩Rb Sr年龄进行了比较和分析。     

赣南不同类型花岗岩体的锆石形态群特征及其意义

根据赣南不同时期的各类型花岗岩体的锆石形态群的研究,确定了赣南在前燕山期形成的隘地体、安西－龙州岩体都是造山花岗岩,寨背岩体,陂头岩体,柯树北岩体和安西－龙州岩体均可能是地幔物质混染地壳物质后形成的。     

变质岩中锆石U—Pb计时问题评述——兼论大别造山带锆石定年

锆石是U－Pb计时的首选对象，对于地质历史复杂的变质岩地区，如大别碰撞造山带的年代学研究，具有不可替代的重要性。变质岩中锆石经历了Pb的扩散丢失作用；晶格损伤导致的蜕晶化作用；增生－混合作用和重结晶作用。这些过程对锆石计时的准确性和有效性带来不同程度的影响。为了使测定的年龄有确定的地质意义，在进行锆石U－Pb定年前，必需对锆石进行成因矿物学和矿物内部结构研究，特别是阴极荧光和背散射电子成像研究，通过内部结构特征确定锆石成因过程。在化学U－Pb法定年时注意普通铅校正和^204Pb测定值对年龄的影响，尽量选择单一成因锆石。特别强调在大别造山带年代学中引入锆石微区离子探针定年技术。     

西昆仑上其木干花岗岩锆石饱和温度和Ti温度的地质意义

通过对上其木干花岗岩进行锆石微量元素和全岩主量元素分析,借助锆石饱和温度和Ti地质温度,反演岩浆源区的温度,进一步分析花岗岩的形成环境。锆石具有典型岩浆岩锆石向左倾斜的稀土元素配分模式,即轻稀土元素明显亏损、重稀土元素明显富集,具有显著的Ce正异常和Eu负异常。锆石饱和温度计算结果表明,上其木干花岗岩的锆石饱和温度介于772~829 ℃之间,平均值为800 ℃,属于高温花岗岩。锆石Ti地质温度的平均最低温度为614 ℃,平均最高温度为634 ℃,花岗岩中锆石形成的温度在614~634 ℃之间。结合锆石微量元素特征、锆石饱和温度和Ti温度与Th/U、10000/Hf的关系,可以判断上其木干花岗岩可能形成于西昆仑造山带增厚的岩石圈下部发生拆沉作用所导致的幔源岩浆底侵,岩浆结晶分异过程中发生了锆石的重结晶或存在多期岩浆(熔体-流体)注入事件。岩浆形成热源可能与羌塘地块沿着康西瓦大断裂和塔里木地块碰撞拼贴结束后,于后碰撞板内伸展体制下导致的软流圈上涌、幔源岩浆的底侵作用有关。     

Metamictization and U-PB isotopic discordance in single zircons: a combined Raman microprobe and SHRIMP ion probe study

Summary We report results of a combined SHRIMP ion microprobe and Raman microprobe study of the correlation between metamictization and U-Pb isotopic discordance of zircon. The spatial resolution of the SHRIMP and Raman probe used are about 200 m³ and 80 m³, respectively. This allows a direct comparison of U-Pb isotopic discordance and metamictization of small areas within zircon crystals. We show that the impact of the oxygen ion beam on the zircon during the SHRIMP analysis does not cause significant amorphization or recrystallization in the remaining zircon on a scale of 1 m. Consequently, it is possible to determine the initial degree of metamictization of zircon within and adjacent to a SHRIMP analytical spot by Raman microprobe measurements after performing SHRIMP analyses. A combination of the two microprobe techniques gives information on the concentration and distribution of radionuclides as well as the degree of metamictization and its heterogeneity and the lateral age distribution within the grain. We found that the degree of U-Pb isotopic discordance correlates closely with the degree of metamictization within single zircon grains, on a scale of 15 m, which is consistent with previous results on the scale of single and multiple zircon grains showing that metamictization enhances the relative potential for secondary loss of radiogenic lead in zircon.

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Metamiktisierung und U-Pb-Isotopendiskordanz in Einzelzirkonen: eine komplexe Untersuchung mittels Ramanmikrosonde und SHRIMP Ionensonde

Zusammenfassung Wir präsentieren die Ergebnisse einer Untersuchung des Zussammenhangs von Metamiktisierung und U-Pb Isotopendiskordanz in Zirkonen mittles kombinierter SHRIMP-Ionenmikrosonden- und Ramanmikrosondenanalysen. Die räumliche Auflösung der verwendeten Analysensysteme, experimentell mit 200 m³ (SHRIMP) und 80 m³ (Ramansonde) bestimmt, gestattet den direkten Vergleich der in Mikrobereichen innerhalb von Zirkon-Einkristallen gemessenen Isotopendiskordanzen und Metamiktisierungsgrade. Wir zeigen, daß der während der SHRIMP-Analyse auf den Zirkon einwirkende Sauerstoffionenstrahl weder signifikante Amorphisierung noch Rekristallisation des benachbarten Zirkonmaterials im 1 m-Bereich hervorruft. Es ist deshalb möglich, den Grad der Metamiktisierung eines Zirkon-Mikrobereiches auch in den SHRIMP-Analysengrübchen, d.h. erst nach dem Analysieren dieses Bereiches mit der Ionensonde, mittels hochauflösender Ramanmessungen unverfälscht zu bestimmen. Der Einsatz beider Mikromethoden liefert kombinierte Informationen zu den Gehalten und Verteilungen radioaktiver Elemente, zum Grad der Metamiktisierung und seiner Heterogenität und zur lateralen Altersverteilung innerhalb von Einzelkörnern. Wir weisen eine deutliche Korrelation des Grades der U-Pb-Isotopendiskordanz mit dem Grad der Metamiktisierung im Mikrobereich (15 m Auflösung) nach. Dies stimmt mit früheren, an Einzelzirkonen und Populationen getätigten Beobachtungen überein, daß die Metamiktisierung von Zirkon dessen Potential zu sekundären Bleiverlusten erhöht.

     

锆石的结构与化学稳定性 :核废料处置矿物类比物研究

锆石可作为核废料处置矿物类比物的重要矿物 ,近年来对此开展了大量的研究。在综合前人研究成果的基础上 ,文中对天然锆石遭受放射性照射所导致的晶格破坏、蜕晶质化以及锆石在各种热液条件下的稳定性、表面反应特性等几个方面进行了总结研究 ,特别是对各种热液条件下锆石的溶解动力学及元素传质进行全面讨论 ,进一步阐明了锆石作为核废料处置矿物类比物的优越性 ,指出今后一方面应着重对锆石的放射性破坏机制及蜕晶质化过程中的元素扩散、迁移和聚集进行系统的研究 ,另一方面着重研究不同变质程度锆石的淋滤机制和不同条件下淋滤溶液 (不同 pH值、无机溶液组分、有机溶液等 )对锆石中各种元素的影响。     

吐格尔明背斜核部花岗岩的年代学、地球化学与构造环境及其对塔里木地块北缘古生代伸展聚敛旋回的揭示

塔里木盆地北缘库车坳陷东部地区吐格尔明背斜核部花岗岩发育,该套花岗岩侵位于以云母石英片岩为主的元古界基底岩系中,上覆的三叠系以角度不整合与其接触。本区花岗岩的研究对揭示库车地区的基底性质与构造演化历史具有重要意义。对吐格尔明花岗岩锆石的U-Pb年代学和岩石地球化学的研究表明, 吐格尔明背斜核部的花岗岩锆石为典型岩浆成因锆石,其锆石SHRIMP U-Pb定年结果分别为626.4±5.2Ma~643.3±4.0Ma,表明它们的形成时代为新元古代晚期埃迪卡拉纪早期。该套花岗岩的SiO₂ (67.95%~78.59%)和Al₂O₃ (11.81%~16.21%) 含量均很高,A/CNK在1.2~1.6,为硅和铝过饱和类型,属典型的过铝质花岗岩;稀土元素总量为106×10^-6~207×10^-6,稀土元素配分曲线呈右倾型,具有明显的负Eu异常;高场强元素(Ta、Nb、Ti等)具有明显的负异常,大离子亲石元素(Rb、Ba、Sr等) 具有明显的正异常。吐格尔明花岗岩是由地壳物质在底侵热源或构造动力作用下引起的局部熔融形成的,属典型的壳源成因类型,形成于挤压环境向伸展环境转化阶段,推测塔里木北缘在震旦纪已开始伸展,代表了古亚洲洋早期的伸展构造环境。实验测得花岗岩钾长石⁴⁰Ar/³⁹Ar年龄平均为291.4~255.8Ma,代表了花岗岩的隆升剥蚀年龄,反映出吐格尔明花岗岩在二叠纪受到挤压发生剥露的过程。因而从年代学框架上更好地约束了塔里木地块北缘在古生代由伸展裂解到聚敛隆升的构造旋回。     

锆石变生特性及其类型划分

利用物理测试方法，如Ｘ衍射和红外光谱等方法，测定锆石结构的完整性，进而划分锆石类型。文中指出，随着锆石的结晶程度变差而衍射峰变弱，甚至消失。在红外分析中利用图谱上的６１０ｃｍ（－１）处的吸收峰的变化确定锆石的变生程度是既方便又经济的有效方法，根据该处的吸收峰参数计算出变生系数（Ｈ／Ｍ），从而引出锆石从全结晶形态到强变生状态的变化特征，以达到解决它们有关问题的目的。     

西藏过铝花岗岩锆石群型的成因信息

锆石是西藏过铝花岗岩最重要的副矿物,为了探讨过铝花岗岩的成因,对西藏过铝花岗岩体中的锆石含量、晶体形态和群型分布特征进行了研究,结果表明,可明显分出5种锆石晶体形态;锆石主要在过铝环境条件下,岩浆结晶温度较低时形成,结晶范围为600～900℃,其中冈底斯带结晶温度高于喜马拉雅带。不同构造-岩浆带锆石群型特征反映的成因信息不同,喜马拉雅带为壳源成因,而冈底斯带除了壳源成因之外,还有幔源成分的加入,反映了青藏高原岩石圈组成和演化的不均一性。     

Ra’s Abdah of the north Eastern Desert of Egypt: the role of granitic dykes in the formation of radioactive mineralization,evidenced by zircon morphology and chemistry

Syenogranitic dykes in the north of Egypt’s Eastern Desert are of geological and economic interest because of the presence of magmatic and supergene enrichment of radioactive mineralization. Zircon crystal morphology within the syenogranitic dykes allows precise definition of sub-alkaline series granites and crystallized at mean temperature of about 637 °C. The growth pattern of the zircons suggest magmatic and hydrothermal origins of radioactive mineralization. Hydrothermal processes are responsible for the formation of significant zircon overgrowth; high U-zircon margins might have occurred contemporaneously with the emplacement of syenogranitic dykes which show anomalous uranium (eU) and thorium (eTh) contents of up to 1386 and 7330 ppm, respectively. Zircon chemistry revealed a relative increase of Hf consistent with decreasing Zr content, suggesting the replacement of Zr by Hf during hydrothermal activity. Visible uranium mineralization is present and recognized by the presence of uranophane and autunite.     

Composition of coexisting zircon and xenotime in rare-metal granites from the Krušné Hory/Erzgebirge Mts. (Saxothuringian Zone,Bohemian Massif)

Zircon and xenotime, from two mineralogically and chemically contrasting granite suites occurring in the Kru?né Hory/Erzgebirge Mts., display extended compositional variability with respect to abundances of Zr, Hf, REE, Y, P, Th, Ca, Al, Fe and As. According to their geochemical signatures, P-rich (S-type) and P-poor (A-type) granites could be distinguished here. Both granite suites display high Ga/Al ratios (>2.6) and according to FeO_tot./(FeO_tot. + MgO) ratio can be classified as ferrous granites. Consequently, the both ratios cannot be used for discrimination S- and A-type granites. Both minerals are characterized by a variety of complex zircon-xenotime textures. They are usually strong hydrated and enriched in F. Zircon from P-rich granites displays a significant enrichment in P (up 0.24 apfu P), whereas zircon from P-poor granites has lower P and higher Y (up to 0.15 apfu Y). The xenotime-type substitution is the most important mechanism of isomorphic substitution in zircon in both granite suites. Zircon from both granite suites is typically enriched in Hf, especially unaltered zircon from P-rich granites (up to 8.2 wt. % HfO₂). However in altered zircons the Hf/Zr ratio is higher in the P-poor granites. The Hf-rich zircon from unaltered P-rich granite crystallised from low temperature granite melt, whereas altered zircons crystallised during post-magmatic hydrothermal alteration (greisenization). Xenotime from P-poor granites displays a considerable enrichment in HREE (up to 40 mol. % HREEPO₄) compared to xenotime from P-rich granites (up to 20 mol. % HREEPO₄). Xenotime compositions from P-rich granites are influenced by brabantite-type substitution, whereas for xenotime from P-poor granites the huttonite-type substitution is dominant. Unusual enrichments in HREE is significant for xenotime from P-poor granites, especially in Yb (up to 0.17 apfu Yb) and Dy (up to 0.11 apfu).     

Features of metamict zircons based on their micrographs and the data of electron microprobe analysis

A new quantitative approach on the basis of joint probability distribution (JPD) analysis of BSE- and CL-images is proposed for studies of the zoning of polygenic (polychronous) grains of zircon. It was shown for the example of zircon of the Mugodzhar metamorphic rocks (South Ural) that a series of zones might be distinguished in the grains of high chemical (structural) heterogeneity. Zones Ia and Ib belong to the fragments of crystalline and radiation-damaged zircon, respectively. Zone II corresponds to the primarily disordered areas (the “genetically metamict” zones) of high concentrations of the impurities of Y, P, REE, U, Hf, etc. The Ia-Ic trend is related to the process of autoradiation metamictization. Based on the data on oxygen content, the declinations of zircon composition from the stoichiometry and the hydration effects were considered.     

广东惠州石坝—黄沙洞地区岩石放射性生热率特征研究

岩石的放射性生热是地热资源的重要热源之一,为研究广东惠州石坝—黄沙洞地区岩石放射性生热率特征,本文系统采集了石坝—黄沙洞地区不同岩性的样品,测定其密度及产热元素含量,对花岗岩样品进行锆石LA-ICP-MS U-Pb同位素定年。结果表明：研究区内不同岩性中花岗岩的放射性生热率最高（均值5.81 µW/m³）,但是变化范围较大（2.83 µW/m³～9.07 µW/m³）;U、Th对生热率的贡献基本相等,K的贡献一般低于10%,部分沉积地层样品可达20%;花岗岩的生热率与时间关系密切,在～150 Ma具有明显峰值。初步结论认为：研究区内岩性对岩石生热率具有显著影响,花岗岩的生热率最高;花岗岩的生热率受岩体形成时的区域岩浆事件影响,在～150 Ma形成的花岗岩具有最高的生热率,有利于干热岩的勘查开发。     

冈底斯带巴嘎区二云母花岗岩SHRIMP锆石U-Pb定年

二云母花岗岩出露于冈底斯弧背断隆带上,沿色日荣-巴嘎东西向断层呈串珠状分布.岩石具有高SiO2、K2O、Al2O3,低CaO的特点,A/CNK>1.1,并且标准矿物计算的刚玉质量分数均大于2.1%,为过铝质花岗岩,具深熔花岗岩特点,稀土和微量元素显示该岩石形成于同碰撞构造环境.SHRIMP锆石U-Pb定年结果表明,花岗岩继承锆石给出了少量太古宙和大量1 120～1 489 Ma的年龄,代表了锆石主要源区的物质年龄,这与冈底斯带上念青唐古拉群原岩年龄基本一致,反映二者的亲缘性.有深熔特征的二云母花岗岩深熔时间为139 Ma.结合区域资料,巴嘎区二云母花岗岩是在晚侏罗世时期由于新特提斯洋的俯冲作用及其弧后洋盆的闭合碰撞使冈底斯带地壳增厚熔融的结果.     

大兴安岭东部花岗岩类锆石饱和温度及其地质意义

通过对花岗岩锆石饱和温度的计算,求得大兴安岭东部不同时代花岗岩浆的起源温度,为探讨花岗岩的形成条件和构造背景提供重要信息。计算结果表明,大兴安岭地区早古生代花岗岩、部分晚古生代I 型花岗岩( ＜ 300 Ma) 、晚古生代A 型花岗岩( 260 ～ 290 Ma) 属于高温花岗岩,高热的产生与造山后伸展构造背景导致的软流圈上涌/幔源岩浆的底侵作用有关; 部分晚古生代I 型花岗岩 ( ＞ 300 Ma) 及侏罗纪花岗岩属于低温花岗岩,可能反映了一种有流体参与的与俯冲有关的构造背景; 白垩纪I 型花岗岩的锆石饱和温度与侏罗纪花岗岩基本相同,属低温花岗岩,其形成可能与流体的加入有关,流体可能来自古太平洋俯冲板片残留体; 而白垩纪A 型花岗岩属高温花岗岩,高热的产生可能与白垩纪时期板内伸展构造体制下岩石圈减薄导致的地幔岩浆底侵有关。     

A zircon U-Pb study of metaluminous (I-type) granites of the Lachlan Fold Belt,southeastern Australia: implications for the high/low temperature classification and magma differentiation processes

Following seminal studies in the Lachlan Fold Belt (southeastern Australia), it has become almost axiomatic that metaluminous granites derive from infracrustal precursors, whereas strongly peraluminous plutons have metasedimentary or supracrustal sources, as reflected in the I- and S-type designation. Recently, zircon saturation thermometry has been used to further subdivide I-type granites into high- and low-temperature categories. That low-temperature I-type granites evolved by restite separation from magmas generated in the zircon stability field is implicit in this classification. To explore this hypothesis, we report an ion microprobe U-Pb (zircon) study into three hallmark ‘low-temperature’ Lachlan Fold Belt I-type suites. The combined patterns of zircon age inheritance and bulk rock Zr trends suggest that each suite formed from magmas that were initially zircon-undersaturated, and that fractional crystallisation, not restite unmixing, was the dominant differentiation process. The low temperature status presently applied to these rocks cannot therefore be justified. The inherited zircons in these I-type granites reflect melting and assimilation of metasedimentary rock, and testify to a supracrustal source component. Electronic Supplementary Material Supplementary material is available for this article at     

Mineralogy,M-type tetrad effect and radioactivity of altered granites at the G. Abu Garadi shear zone,central Eastern Desert,Egypt

The G. Abu Garadi area is covered mainly by metasediments, alkali feldspar granites and stream sediments. The alkali feldspar granite is traversed by a major strike-slip fault trending in an N-S direction as well as two subordinate sets of faults trending NW to WNW for the first one and NE for the second one. These faults represent the shear zones affected by magmatic (syngenetic) as well as hydrothermal (epigenetic) activities causing alteration of the granitic rocks. The most common alteration features are albitization, greisenization and koalinitization. The mass balance calculations of the studied altered samples show enrichments in Zr, Y, Ni, U, Th and Ga and depletions in Zn, Sr, Nb, Ba, Pb, Cu and V. Only the greisenized samples exhibit a significant enrichment in Nb, ∑REE budget and pronounced lanthanide tetrad effect (M-type), especially TE1,4, while weakly expressed tetrad effects are for the other albitized and koalinitized samples. Mineralogically, the common accessory minerals in the altered samples include samarskite-(Y), betafite, uranothorite, zircon, fluorite and cassiterite. The greisenized granites contain high eU and eTh than the other altered types, where they are characterized by an assemblage of the radioactive minerals; samarskite-(Y), betafite, uranothorite in addition to zircon. The inter-element relationships between U and Th and also their ratios illustrate that the radioelement distribution in these granites is mainly governed by magmatic processes, in addition to post-magmatic ones. The distribution of chemical elements and the fractionation of some isovalents within the shear zone are largely controlled by the newly formed mineral phases. With respect to uranium mobilization, uranium migrated from the host alkali feldspar granites of G. Abu Garadi, while the shear zones acted as traps for the migrated uranium. Moreover, U migrated in the shear zone during greisenization and albitization, and migrated out during koalinitization.     

东天山卡瓦布拉克地区片麻状花岗岩形成时代及地质意义

东天山卡瓦布拉克地区发育大量片麻状花岗岩,据野外特征分为含斑片麻状花岗岩和不含斑片麻状花岗岩,片麻理属后期韧性剪切叠加改造结果.本文获得含斑片麻状花岗岩LA-ICP-MS锆石U-Pb年龄为(942.1±7.2)Ma和(941.9±4.9)Ma;不含斑晶的片麻状花岗岩年龄为(424.2±3.4)Ma,这些年龄数据表明片麻状花岗岩分别形成于新元古代和中志留世.野外调查表明,片麻状花岗岩侵入于长城系星星峡群和卡瓦布拉克群中,被块状石炭纪花岗岩侵入.研究区西侧上覆泥盆纪地层未发生明显韧性变形.据此限定区内韧性剪切变形发生于晚志留世(约424 Ma)之后至泥盆纪之前.