Composition and chemical microprobe dating of U-Th-bearing minerals. Part I. Monazites from the Urals and Siberia

To develop chemical microprobe timing of U-Th-bearing minerals, monazite grains from several localities in the Ural and Siberia have been dated using upgraded measurement techniques and age calculation based on original software. The samples were taken from pegmatites of the Ilmeny Mountains and the Ilmeny-Vishnevy Mountains Complex in the South Urals; pegmatites from the Adui granitic pluton and its framework in the Central Urals; gneisses and granulites of the Taratash Complex in the South Urals; and felsic gneisses from the Transangara region of the Yenisei Ridge. Scrutiny of the composition, heterogeneity, and chemical substitution of U and Th ions is a necessary stage of chemical dating aimed at estimating the degree of closeness of the U-Th-Pb system and unbiased screening of analytical data. The results obtained have been compared with the known isotopic ages of the studied minerals; the compared data are satisfactorily consistent.     

High and low temperature alteration of uranium and thorium minerals, Um Ara granites, south Eastern Desert, Egypt

The Um Ara area, in the south Eastern Desert of Egypt contains a number of uranium occurrences related to granitic rocks. U-rich thorite, thorite and zircon are the main primary uranium- and thorium-bearing minerals found in mineralized zones of the Um Ara alkali-feldspar granites; uranophane is the most common secondary uranium mineral. U-rich thorite contains blebs of galena, has rims of uranophane and contains inclusions of Zr-rich thorite. Electron probe microanalysis (EPMA) provides an indication of a range of solid solution between thorite and zircon, in which intermediate phases, such as Th-rich zircon and Zr-rich thorite, were formed. These phases have higher sum of all cations per formula (2.05 to 2.06 apfu, for 4 oxygen atoms) than that of ideal thorite and zircon. This is attributed to the presence of substantial amount of interstitial cations such as Ca, U and Al in these phases. Some zircon grains are stoichiometric in composition, other altered grains display lower SiO₂ and ZrO₂ contents. Enrichment of Th and U in altered zircon preferentially involves coupled substitution (Ca²⁺ + (Th,U)⁴⁺ ↔ 2Zr⁴⁺ + 2Si⁴⁺), implying that significant U and Th may enter the Zr and Si position in zircon. Negative correlation of Zr vs. Hf and Al may indicate that Hf and Al have been introduced to the zircon during later fluid alteration rather than during the primary magmatic event. A two-stage metallogenetic model is proposed for the alteration processes and origin of U- and Th-bearing minerals in the Um Ara alkali-feldspar granite: 1) the first stage was dominated by hydrothermal alteration and accompanied by albitization, k-feldspathization, desilicification, chloritization, hematitization, silicification, argillization, fluoritization and corrosion of primary U-bearing minerals. Solid-solution between thorite and zircon occurred during this stage. The second stage occurred at the near-surface profile where circulating meteoric water played an important role in mobilizing the early formed primary U-bearing minerals. Uranium was likely transported as a calcium uranyl carbonate complexes. When these complexes lost their stabilities by precipitation of calcite, they decomposed in the presence of silica to form uranophane.     

CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data

This paper outlines the CHIME (chemical Th–U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a “pseudo-isochron” from which an age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca + Si)/(Th + U + Pb + S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, X-ray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO₄. The age-mapping procedure for young monazite and zircon includes acquiring PbMα (or PbMβ) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described.     

CHIME dating of monazite,xenotime, zircon and polycrase: Protocol,pitfalls and chemical criterion of possibly discordant age data

This paper outlines the CHIME (chemical Th–U-total Pb isochron method) dating method, which is based on precise electron microprobe analyses of Th, U and Pb in Th- and U-bearing accessory minerals such as monazite, xenotime, zircon and polycrase. The age-mapping technique that is applicable to young monazite and zircon is also described. CHIME dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation, and then these data are used to construct a “pseudo-isochron” from which an age can be obtained via regression. This method, when coupled with discrimination of possibly concordant age data by chemical criteria such as the (Ca + Si)/(Th + U + Pb + S) ratio for monazite and Ca and S contents for zircon, has the potential advantage of significant precision, and the ability to work with minerals that have a significant initial common Pb component. This technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis. Many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data. Critical factors include the roles of collimator slit, detector gas, background estimation, accelerating voltage, probe current, X-ray interferences and count rate in affecting the accuracy, and a way to apply the Th and U interference correction without pure Th- and U-oxides or synthesized pure ThSiO₄. The age-mapping procedure for young monazite and zircon includes acquiring PbMα (or PbMβ) intensity of individual pixels with multiple spectrometers, correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement, calibrating of intensity with standards and calculating of ages from the Th, U and Pb concentrations. This technique provides age maps that show differences in age domains on the order of 20 Ma with in monazite as young as 100 Ma. The effect of sample damage by irradiation of intense and prolonged probe measurement is also described.     

甘肃龙首山青井地区花岗质砾岩中钍、铀混合型矿化特征及成因探讨

青井是位于龙首山成矿带西段的一个钍、铀混合型异常点,通过钻孔查证在青井盆地的花岗质砾岩中发现了较好的矿化线索,矿石具有热液型矿化的特征,发育钾长石化、赤铁矿化、碳酸盐化和绿泥石化为主的钾交代蚀变组合。通过对含矿花岗质砾岩的岩矿鉴定、电子探针测试、地球化学分析和钍矿物U-Pb同位素年龄测定进行综合研究,认为花岗质砾岩中发育的是产于挤压-俯冲构造环境下受断裂构造和辉绿岩脉共同控制的热液成因钍、铀混合型矿化,含钍、含铀矿物主要为钍石、沥青铀矿和铀石,成矿期应为新生代中晚期。热液成矿过程中带来了大量的外来组分,矿石中的Al_2O_3、Fe_2O_3和K_2O等主量元素和轻、重稀土元素以及Rb、Nb、Nd、Zr、Hf、Ta、W、Sb等微量元素均随着Th、U含量的增加而增加。     

Disturbance versus preservation of U–Th–Pb ages in monazite during fluid–rock interaction: textural, chemical and isotopic in situ study in microgranites (Velay Dome, France)

Monazite is extensively used to date crustal processes and is usually considered to be resistant to diffusive Pb loss. Nevertheless, fluid-assisted recrystallisation is known to be capable of resetting the monazite chronometer. This study focuses on chemical and isotopic disturbances in monazite grains from two microgranite intrusions in the French Central Massif (Charron and Montasset). Petrologic data and oxygen isotopes suggest that both intrusions have interacted with alkali-bearing hydrothermal-magmatic fluids. In the Charron intrusion, regardless of their textural location, monazite grains are sub-euhedral and cover a large domain of compositions. U–Pb chronometers yield a lower intercept age of 297 ± 4 Ma. An inherited component at 320 Ma is responsible for the scattering of the U–Th–Pb ages. The Montasset intrusion was later affected by an additional F-rich crustal fluid with crystallisation of Ca-REE-fluorocarbonates, fluorite, calcite and chloritisation. Pristine monazite is chemically homogeneous and displays ²⁰⁸Pb/²³²Th and ²⁰⁶Pb/²³⁸U concordant ages at 307 ± 2 Ma. By contrast, groundmass monazite shows dissolution-recrystallisation features associated with apatite and thorite precipitation (Th-silicate) and strong chemical reequilibration. ²⁰⁸Pb/²³²Th ages are disturbed and range between 270 and 690 Ma showing that the Th/Pb ratio is highly fractionated during the interaction with fluids. Apparent U–Pb ages are older due to common Pb incorporation yielding a lower intercept age at 312 ± 10 Ma, the age of the pristine monazite. These results show that F-rich fluids are responsible for Th mobility and incorporation of excess Pb, which thus strongly disturbed the U–Th–Pb chronometers in the monazite.     

Discovery of a c.370 Ma granitic gneiss clast from the Hwanggangri pebble-bearing phyllite in the Okcheon metamorphic belt, Korea

The chemical Th–U total Pb isochron method (CHIME) of dating was carried out on accessory minerals in samples from the Okcheon metamorphic belt in Korea. Dated minerals include xenotime and monazite with overgrown mantles in a granitic gneiss clast from the Hwanggangri Formation, metamorphic allanite in garnet-bearing muscovite–chlorite schist of the Munjuri Formation, and polycrase and monazite in post-tectonic granite from the Hwanggangri area. Overgrowth of mantles took place at 369 ± 10 Ma on c. 1750 Ma cores of xenotime and monazite in the granitic gneiss. Allanite, occurring in textural equilibrium with peak metamorphic minerals, yields a CHIME age of 246 ± 15 Ma that is discriminably older than the polycrase (170 ± 6 Ma) and monazite (170 ± 3 Ma) ages of the post-tectonic granite. These chronological data suggest that some of the metasedimentary rocks in the belt formed through a single stage of metamorphism at c. 250 Ma from post-370 Ma sediments. Late Permian age signatures have also been reported from the Precambrian Gyeonggi and Yeongnam massifs that border the Okcheon metamorphic belt, and indicate that parts of the basement massifs and the metamorphic belt were affected by the same regional metamorphic event.     

A Raman spectroscopic study on the structural disorder of monazite–(Ce)

This study addresses whether Raman spectra can be used to estimate the degree of accumulated radiation damage in monazite-(Ce) samples whose chemical composition was previously determined. Our results indicate that the degree of disorder in monazite–(Ce), as observed from increasing Raman band broadening, generally depends on both the structural state (i.e., radiation damage) and the chemical composition (i.e., incorporation of non-formula elements). The chemical effects were studied on synthetic orthophosphates grown using the Li-Mo flux method, and non radiation-damaged analogues of the naturally radiation-damaged monazite–(Ce) samples, produced by dry annealing. We found that the “chemical” Raman-band broadening of natural monazite–(Ce) can be predicted by the empirical formula, $$ {\hbox{FWHM}} {\hbox{[c}}{{\hbox{m}}^{ - {1}}}{]} = {3}{.95} + {26}{.66} \times {\hbox{(Th}} + {\hbox{U}} + {\hbox{Ca}} + {\hbox{Pb)}} {\hbox{[apfu]}} $$ where, FWHM = full width at half maximum of the main Raman band of monazite–(Ce) (i.e., the symmetric PO₄ stretching near 970?cm^?1), and (Th+U+Ca+Pb) = sum of the four elements in apfu (atoms per formula unit). Provided the chemical composition of a natural monazite–(Ce) is known, this “chemical band broadening” can be used to estimate the degree of structural radiation damage from the observed FWHM of the ν₁(PO₄) band of that particular sample using Raman spectroscopy. Our annealing studies on a wide range of monazite–(Ce) reference materials and other monazite–(Ce) samples confirmed that this mineral virtually never becomes highly radiation damaged. Potential advantages and the practical use of the proposed method in the Earth sciences are discussed.     

Electron microprobe monazite geochronology of granitic intrusions from the Montes de Toledo batholith (central Spain)

U–Th–Pb monazite dating by electron microprobe has been applied to three peraluminous granitic intrusions of the western Montes de Toledo batholith (MTB). Back scattered electron images of monazite crystals reveal a variety of internal textures: patchy zoning, overgrowths around older cores and unzoned crystals. On the basis of their zoning pattern and chemical composition, two monazite domains can be distinguished: (1) corroded cores and crystals with patchy zoning, exhibiting relatively constant Th/U ratios and broadly older ages, and (2) unzoned grains and monazite rims, with variable Th/U ratios and younger ages. The first monazite group represents inherited domains from metamorphic sources, which accounts for pre‐magmatic monazite growth events. Two average ages from Torrico and Belvís de Monroy granites (333 ± 18 and 333 ± 5 Ma, respectively) relate these cores to a Viséan extensional deformation phase. The second group represents igneous monazites which have provided the following crystallization ages for the host granite: 298 ± 11 Ma (Villar del Pedroso), 303 ± 6 Ma (Torrico) and 314 ± 3 Ma (Belvís de Monroy). Two main magmatic pulses, the first about 314 Ma and the second at the end of the Carboniferous (303–298 Ma), might be envisaged in the western MTB. While Belvís de Monroy leucogranite is likely a syn‐ to late‐tectonic intrusion, the Villar del Pedroso and Torrico plutons represent post‐tectonic magmas with emplacement ages similar to those of equivalent intrusions from nearby Variscan magmatic sectors. Copyright © 2011 John Wiley & Sons, Ltd.     

浙江临安石室寺NYF型伟晶岩中稀有稀土金属的矿物学行为与成矿过程

浙江临安石室寺伟晶岩位于河桥岩体西北面，属于典型的Nb-Y-F （NYF） 型伟晶岩，富含大量稀有稀土矿物。本文在野外考察和显微镜观察的基础上，结合电子探针背散射电子图像观察与矿物化学成分分析，系统鉴定了石室寺NYF 型伟晶岩中的稀有稀土矿物，揭示了稀有稀土元素的富集、迁移、结晶与成矿过程。研究结果表明：（1） 石室寺伟晶岩中的稀有稀土矿物有铌钽矿物（铌铁矿、铌锰矿、重钽铁矿、细晶石等）、钇矿物（褐钇铌矿、黑稀金矿）、钨矿物（黑钨矿、 白钨矿、铌钨矿物）、铈矿物（独居石、氟铈矿、氟碳铈矿） 和钍矿物等。（2） 铌钨系列矿物的WO3含量在8.30~70.51 wt%之间呈规律变化，可能为铌铁矿与黑钨矿之间形成的一系列多体矿物。（3） 铌铁矿LA-ICP-MS U-Pb 定年结果显示，石室寺伟晶岩的形成年龄为133±2 Ma，与河桥花岗岩具有成因联系。（4） 石室寺NYF 型伟晶岩中稀有稀土元素的成矿过程与其岩浆的结晶演化密切相关：岩浆阶段，锆石、钍石与独居石等矿物最早晶出；岩浆—热液阶段，黑稀金矿、铌铁矿、褐钇铌矿、氟铈矿等稀有稀土矿物逐渐结晶；热液阶段，黑钨矿、铌钨矿物相继形成，同时早期的独居石、氟铈矿受晚期热液交代形成次生铈矿物。     

浙江临安石室寺NYF型伟晶岩中稀有稀土金属的矿物学行为与成矿过程

浙江临安石室寺伟晶岩位于河桥岩体西北面,属于典型的Nb-Y-F （NYF） 型伟晶岩,富含大量稀有稀土矿物。本文在野外考察和显微镜观察的基础上,结合电子探针背散射电子图像观察与矿物化学成分分析,系统鉴定了石室寺NYF 型伟晶岩中的稀有稀土矿物,揭示了稀有稀土元素的富集、迁移、结晶与成矿过程。研究结果表明：（1） 石室寺伟晶岩中的稀有稀土矿物有铌钽矿物（铌铁矿、铌锰矿、重钽铁矿、细晶石等）、钇矿物（褐钇铌矿、黑稀金矿）、钨矿物（黑钨矿、 白钨矿、铌钨矿物）、铈矿物（独居石、氟铈矿、氟碳铈矿） 和钍矿物等。（2） 铌钨系列矿物的WO3含量在8.30~70.51 wt%之间呈规律变化,可能为铌铁矿与黑钨矿之间形成的一系列多体矿物。（3） 铌铁矿LA-ICP-MS U-Pb 定年结果显示,石室寺伟晶岩的形成年龄为133±2 Ma,与河桥花岗岩具有成因联系。（4） 石室寺NYF 型伟晶岩中稀有稀土元素的成矿过程与其岩浆的结晶演化密切相关：岩浆阶段,锆石、钍石与独居石等矿物最早晶出;岩浆—热液阶段,黑稀金矿、铌铁矿、褐钇铌矿、氟铈矿等稀有稀土矿物逐渐结晶;热液阶段,黑钨矿、铌钨矿物相继形成,同时早期的独居石、氟铈矿受晚期热液交代形成次生铈矿物。     

Capabilities and constraints of the Pb-Pb dating of metamorphogenic minerals using the step-leaching method

Based on particular examples, this paper considers the capabilities and constraints of the step-leaching Pb-Pb dating (PbSL) of metamorphogenic minerals. It was shown that stepwise leaching allows the separation of fractions enriched in uranogenic and thorogenic Pb isotopes, which can be used for the determination of mineral ages and the time of crystallization of coexisting equilibrium and, occasionally, disequilibrium monazites. The presence of monazite in a mineral is indicated by a high Th/U ratio similar to that of monazite. The main limitation of the method is related to the presence in minerals of both disequilibrium domains of the mineral matrix and disequilibrium monazite microinclusions. By the example of minerals studied, we discussed three scenarios for the development of the U-Th-Pb isotopic systems of metamorphogenic minerals.     

Aureoles of Pb(II)-enriched feldspar around monazite in paragneiss and anatectic pods of the Napier Complex,Enderby Land,East Antarctica: the roles of dissolution-reprecipitation and diffusion

Extraordinarily high Pb content in K-feldspar and plagioclase has been found contiguous to monazite in two occurrences in the ultrahigh-temperature Napier Complex of Antarctica. Monazite shows a variety of textures and compositions. In a garnet-sillimanite-orthopyroxene paragneiss at Mount Pardoe (Amundsen Bay), grains range 80–150 μm across and are anhedral; two grains are Th- and Si-dominant. In pods that crystallized from anatectic melts at 2500 Ma at Zircon Point, Casey Bay, monazite grains range 0.05 mm–1 cm in length and are highly variable in texture. The coarsest grains (>0.7 cm) are skeletal and euhedral, whereas the smallest grains are anhedral and associated with fine- to medium-grained quartz, K-feldspar, plagioclase, garnet, sillimanite and rutile in aggregates that form interstitial veinlets interpreted to be a second generation of anatexis during an event at 1100 Ma. The huttonite component (ThSiO₄) reaches 30 mole% in the cores of the coarsest skeletal grains, whereas other grains, particularly smaller ones, show complex and irregular zoning in Th and U. The latter zoning is attributed to dissolution-reprecipitation, which also resulted in complete Pb loss during the 1100 Ma event. In the paragneiss at Mount Pardoe, K-feldspar and myrmekitic plagioclase (An16) are found in a 70–80 μm band between monazite and orthopyroxene and contain up to 12.7 wt.% and 2.7 wt.% PbO, respectively, corresponding to 18.5% and 3.4% PbAl₂Si₂O₈ component, respectively. Cathodoluminescence of both feldspars increases with distance from a nearby monazite grain and is not correlated with Pb content. Incorporation of Pb in K-feldspar and plagioclase could be a result of diffusion, even though the monazite adjacent to feldspar apparently lost little Pb, i.e., Pb could have been transported by fluid from the Th-rich grains, which did lose Pb. In contrast to the paragneiss, cathodoluminescence correlates with Pb content of K-feldspar in aureoles surrounding skeletal monazite grains 0.7–1 cm across in anatectic pods at Zircon Point. Pb content of K-feldspar decreases monotonically to near detection limits within several millimetres of monazite grains; the greatest PbO concentration is attained in K-feldspar inliers and embayments in monazite, 8.8 wt.%, corresponding to 11.7% PbAl₂Si₂O₈ component. Fine-grained quartz in the K-feldspar suggests that the mechanism for Pb incorporation involved breakdown of feldspar: Pb²⁺ + 2(K,Na)AlSi₃O₈ → PbAl₂Si₂O₈ + 4SiO₂ + 2(K,Na)⁺ . The smooth decrease of Pb in the aureoles is not characteristic of dissolution-reprecipitation, which is characterized by abrupt changes of composition, and it seems more likely that Pb was incorporated in K-feldspar by diffusion at 1100 Ma. We suggest a model whereby fluid introduced during the 1100 Ma event flowed along grain boundaries and penetrated mineral grains. Temperatures were sufficiently high, i.e., 700°C, assuming burial in the mid-crust, for the fluid to induce localized melting of quartzofeldspathic matrix of the anatectic pods. Loss of radiogenic Pb was complete. Some penetration of K-feldspar by aqueous fluid is suggested by the presence of scattered galena specks and by rays of turbidity emanating from monazite. Aqueous fluid or water-rich granitic melt may have mediated the diffusion of Pb in feldspar, but it did not cause dissolution-reprecipitation. Although Pb was mobilized by aqueous fluid or water-rich granitic melt, it was not entirely flushed from the immediate vicinity of the monazite, but nearly half was incorporated in adjacent feldspar. Fluid activity that could cause Pb loss in monazite does not always leave an obvious trace, i.e., hydrous minerals, such as sericite, are very sparse, and biotite is absent in the anatectic pods at Zircon Point. Nonetheless, electron microprobe dating of monazite from the pods could not detect the 2500 Ma age of original crystallization determined by isotopic dating.     

Mineralogy,geochemistry, radioactivity and environmental impacts of Gabal Marwa granites,southeastern Sinai,Egypt

The Gabal Marwa area is located in the southeastern part of Sinai,Egypt.It comprises gneisses and migmatites,granodiorites and monzogranites.Field,petrographic,mineralogic and chemical investigations indicated that the Gabal Marwa granites are classified as granodiorites and monzogranites.The monzogranites constitute the most predominant rock unit exposed in the study area.They have been subjected to hydrothermal alterations,especially along the shear zones.Sericitization,desilicification,nametasomatism and development of spotty or dendritic manganese oxides are the most pronounced alteration features.These alterations resulted in the increase of TiO2,Al2O3,FeOt,MnO,CaO,MgO,Na2O,K2O and Cr,Zr,Rb,Y and Sr and the decrease of SiO2,P2O5,Ni,Zn,Ba and Nb.Radiometric studies indicated that the altered granites belong to high thorium,high uranium granites.The U,Th,U and Th,Th/U,Th and U-K variation diagrams suggested that magmatic processes controlled the distribution of these elements but the effect of hydrothermal alteration processes were clearly distinct.The Scanning Electron Microscope and X-ray Diffraction analyses indicated that the most important radioactive minerals include uranothorite,thorite,zircon,monazite and samarskite.Cinnabar and Mn minerals were also found.From the U,Th,Ra and K activity concentrations obtained for all the studied granitic samples,radium equivalent activity(Raeq),external hazard index(Hex),and internal hazard index(Hin),were calculated to assess the radiation hazard to human beings living in dwellings made of the studied granites.Altered monzogranites have radioactivity above the proposed acceptable level and,therefore,caution must be taken when these granites are used as building materials.     

独居石成因矿物学特征及其对U-Th-Pb年龄解释的制约

独居石U-Pb定年在岩浆活动、变质作用和沉积作用等方面发挥着日益重要的作用,但是由于独居石成因复杂,因此从成因矿物学角度对不同成因独居石的特征进行总结将有助于解释独居石年代学数据.总结了不同成因独居石的矿物共生关系和组构特征、外部形貌-内部结构、化学元素特征,依托Th、U、Y、Ca、Pb、REE含量及其比值关系进行化学...     

Nanoscale resetting of the Th/Pb system in an isotopically-closed monazite grain: A combined atom probe and transmission electron microscopy study

Understanding the mechanisms of parent-daughter isotopic mobility at the nanoscale is key to rigorous interpretation of Ue The Pb data and associated dating. Until now, all nanoscale geochronological studies on geological samples have relied on either Transmission Electron Microscope(TEM) or Atom Probe Microscopy(APM) characterizations alone, thus suffering from the respective weaknesses of each technique. Here we focus on monazite crystals from a ~1 Ga, ultrahigh temperature granulite from Rogaland(Norway). This sample has recorded concordant UeP b dates(measured by LA-ICP-MS) that range over 100 My, with the three domains yielding distinct isotopic Ue Pb ages of 1034 ± 6 Ma(D1; Srich core), 1005 ± 7 Ma(D2), and 935 ± 7 Ma(D3), respectively. Combined APM and TEM characterization of these monazite crystals reveal phase separation that led to the isolation of two different radiogenic Pb(Pb*) reservoirs at the nanoscale. The S-rich core of these monazite crystals contains Cae Srich clusters, 5 -10 nm in size, homogenously distributed within the monazite matrix with a mean interparticle distance of 40 -60 nm. The clusters acted as a sink for radiogenic Pb(Pb*) produced in the monazite matrix, which was reset at the nanoscale via Pb diffusion while the grain remained closed at the micro-scale. Compared to the concordant ages given by conventional micro-scale dating of the grain,the apparent nano-scale age of the monazite matrix in between clusters is about 100 Myr younger, which compares remarkably well to the duration of the metamorphic event. This study highlights the capabilities of combined APM-TEM nano-structural and nano-isotopic characterizations in dating and timing of geological events, allowing the detection of processes untraceable with conventional dating methods.     

新疆拜城波孜果尔碱性岩中副矿物的特征

在野外地质调查的基础上,结合室内显微镜观察及电子探针分析测试,对新疆拜城波孜果尔碱性岩中的副矿物的矿物学特征和化学成分进行了研究.发现这些副矿物常以共生组合的形式产在碱性岩中,主要分布在石英二长闪长岩和石英二长岩中.烧绿石中U、Th和REE替代Ca、Na.独居石富含LREE,Th和LREE相互替代;根据独居石中w(La+ Ce) ＞40％和La/Nd比值在1.6～4.5,推断独居石为热液成因.磷钇矿中富含REE,且以HREE为主;w(Th)＞w(U).锆石中Zr/Hf比值在60％以上,符合碱性岩特征;其Th/U比值为0.6,属于岩浆锆石.星叶石中w(Rb2O)、w(Cs2O)较高.萤石中Y、Ce替代Ca.锆石中的钍石w(U)明显高于磁铁矿中钍石w(U).在石英二长岩中,烧绿石的w(CaO)、w(TiO2)、w(ZrO2)、w(U3O8),磷钇矿的w(Y2O3),星叶石的w(TiO2),萤石的w(Ca),氟碳铈镧矿的w(CaO)较丰富;而在石英二长闪长岩中,烧绿石的w(Ce2O3),磷钇矿的REE含量,星叶石的w(Nb2O5)、w(Rb2O),萤石w(Ce)、w(Y)和氟碳铈镧矿的w(La2O3)较高.     

An improved EPMA analytical protocol for U-Th-Pbtotal dating in xenotime: Age constraints from polygenetic Mangalwar Complex,Northwestern India

EPMA U-Th-Pb_total dating in U- and Th bearing minerals (e.g., monazite, zircon, and xenotime) is a low-cost and reliable technique used for retrieving age information from detrital, diagenetic and low to high-T metamorphic, as well as magmatic rocks. Although, the accuracy on measured ages obtained using EPMA is considered to be poor compared to isotopic ages, the superior spatial resolution, ability to integrate textural and age information by in-situ measurement, lack of sample damage and easier and cheaper data generation in EPMA make chemical dating a very valuable tool to decipher diverse petrological processes.This contribution presents an improved analytical protocol to obtain precise estimates of U, Th and Pb concentrations in xenotime. Results were tested on monazite standard (Moacyr pegmatite, Brazil; TIMS age: 487 ± 1 Ma) as the reference material. The proposed analytical protocol has been successfully applied to achieve an analytical uncertainty of less than 10% in U, Th and Pb measurements in xenotime. The protocol was further used to resolve polygenetic xenotime ages (ca. 1.82, 1.28 and 0.93 Ga) in metapelite samples from the Mangalwar Complex, Northwestern India. Monazites in the same samples were also analyzed and found to preserve the two younger ages (i.e., ca. 1.28 and 1.0 Ga). The obtained ages from the xenotime and monazite very well corroborate with the earlier published ages from the area validating the proposed analytical protocol.     

新疆拜城波孜果尔东矿区侵入岩矿物学特征

新疆拜城县波孜果尔东矿区侵入岩为富含铌、钽、铀、钍、稀土、锆、铷、铯、锂等有用元素的含矿岩体.通过偏光显微镜、电子探针(EPMA)分析, 对拜城波孜果尔东矿区侵入岩的矿物学特征进行了详细的研究, 并对岩浆形成的构造背景进行了初步探讨, 这对成岩、成矿作用研究有一定参考意义.研究结果表明, 波孜果尔东矿区侵入岩为黑云母碱性花岗岩, 主要造岩矿物包括石英、钠长石、钾长石、钠铁闪石、锂云母和黑云母等.其中, 黑云母为锂铁叶云母, 以富Si富Fe、低Al贫Mg为特征; 锂云母以富Si、高Li低Al为特征, 它们为黑云母-锂铁云母系列的不同成员, 均属三八面体型.钠铁闪石以富含Fe2+为特征.副矿物包括烧绿石、星叶石、氟铈矿、独居石、钍石、萤石、锆石、铌铁矿等.岩石形成于非造山的板内构造环境, 且具高温、无水、低氧逸度的成岩特点.      

数据不协调时独居石EPMA CHIME定年计算方法的改进

Th-U-Pb系统数据不协调是独居石电子探针化学定年(EPMA CHIME Dating)中一种很常见的问题。独居石矿物产生数据不协调的主要原因包括:1)蚀变或重结晶造成的铅丢失;2)不同年龄域在空间上的重叠或者存在于很小颗粒上的小年龄域。独居石EPMA年龄必大于U等于0时的极端情况给出的值,即当U为0时,EPMA CHIME年龄给出的是~(208)Pb/~(232)Th年龄,这是测量区域内最老年龄的下限。当Th为0时,EPMA CHIME年龄值介于~(206)Pb/~(238)U和~(207)Pb/~(235)U年龄值之间,这是EPMA法所能得到的最老年龄的上限。分析表明,当独居石EPMA数据出现不协调时,传统等时线方法计算的年龄值误差较大。本文提出了一种处理数据不协调情况下的优化算法。该算法考虑了测量误差,并根据剩余铀的总量剔出大的离散数据。利用已公开的数据进行算法对比的结果表明,本文提出的优化算法计算结果可靠。