Late Neoproterozoic,Ordovician and Carboniferous events recorded in monazites from southern-central Madagascar

The evolution of the basement of southern Madagascar north and south of the Ranotsara shear zone was investigated using (U + Th)/Pb electron probe monazite age dating in combination with petrographic constraints. Several monazite grains show a stepwise progression of younger ages towards the rim indicating partial and complete resetting during tectonic, metamorphic and/or fluid events. The oldest ages, ranging from 630–2400 Ma, occur relatively rare in relic cores. A first, clear age-population is dated at 550–560 Ma. Most ages fall in two populations at 420–460 and 490–500 Ma, which in some samples overlap in error. We interprete these ages as dating low-pressure and high-temperature metamorphism. We have also clear evidence for Carboniferous (300–310 Ma) monazite overgrowth rims, which can not directly be related to macroscopic structures or metamorphic parageneses. In combination with literature data, we propose that the observed monazite age populations are related to Gondwana amalgamation and subsequent rifting events during the break up of Gondwana. Our study confirms that only the electron or ion microprobe yields sufficient spatial resolution to date individual shells of multiple zoned monazites in the polymetamorphic basement of Madagascar.     

模拟月壤研究进展及CUG-1A模拟月壤

利用我国东北部吉林省辉南县新生代火山岩为初始原料,初步研制了模拟月壤CUG-1A.在研究CUG-1A的化学性质的同时,重点研究了其物理力学性质.数据分析表明,CUG-1A与Apollo14采样点的月壤样品有着相似的化学成分、矿物组成和物理力学性质,是一种理想的低钛型月海月壤的模拟样品.     

Pb isotopic composition,colour, and microstructure of monazites from a polymetamorphic rock in Antarctica

A moderate- to high-grade regionally metamorphosed paragneiss from Antarctica contains monazites of several different colours — brown, yellow and grey. Each colour type has a distinctive U-Pb isotopic composition which appears to result from different proportions of radiogenic Pb loss. Isotopic differences are neither related to La, Nd, Ce, P, Ca, Ti (and/or Ba), nor to U or Th content. All colour types have similar structures at the submicron scale, as determined by both conventional and high-resolution transmission electron microscopy (TEM). These show that the grains are essenttially non-metamict but are composed of 100 Å crystalline domains misoriented from each other by no more than 2× 10^–3 radians, and separated by narrow confused boundary regions where misorientation is probably accommodated by imperfect atomic arrangements. These regions of mismatch form potential zones of high permeability/diffusivity which are believed to be fundamental to the isotopic and colour differences between grains. Colour type is apparently related to the capacity of different minerals to shield included monazite grains from fluids circulating in the rock system.The well aligned monazite U — Pb analyses produce concordia intercepts of 2429 _–16 ⁺¹⁷ Ma and 1087±29 Ma. Both ages are comparable to those of major geological events in this part of Antarctica. They are interpreted in terms of isotopic resetting through Pb loss, and original monazite crystallisation is thought to have occurred somewhat earlier, possibly at the time this terrain first underwent granulite-facies metamorphism, about 3070 Ma ago.     

Study of some allanites and monazites from the South Carpathians (Romania)

Summary Allanites and monazites from different rocks of the South Carpathians (Romania) are described, and 17 analyses are presented. The genetic relations are discussed.

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Untersuchungen an Allaniten und Monaziten aus den Südkarpaten (Rumänien)

Zusammenfassung Allanite und Monazite aus verschiedenen Gesteinen der Südkarpaten (Rumänien) werden beschrieben und 17 chemische Analysen werden gebracht. Die genetischen Beziehungen werden diskutiert.

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Beach monazites from Alleppey: A window to the Trivandrum Block,Southern India

Detrital monazites can be used to reconstruct the tectonothermal history of their provenance. Dating of beach monazites from Alleppey by EPMA U-Th-Pb_total technique using the centroid method demonstrates that the ages recorded by even a small number of 28 analyses from a single thin section grain mount can potentially reveal a considerable portion of the tectonothermal history of the Trivamdrum block. Three monazite populations were identified that yielded ages 605 ± 9, 575 ± 23 and 548 ± 11 Ma, which have their more or less exact counterparts reported from the Trivandrum Block. One monazite grain yielded random Paleoproterozoic ages ranging from 1756 Ma to 1345 Ma representing similar ages which earlier have been interpreted as due to differential Pb-loss from >2000 Ma monazites during the late Neoproterozoic-Cambrian metamorphism.     

Microstructure of 24-1928 Ma concordant monazites; implications for geochronology and nuclear waste deposits

The microstructure of monazite was studied using scanning electron microscopy (SEM), electron microprobe analysis (EMP), X-ray diffraction patterns (XRD), and transmission electron microscopy (TEM). Four well-characterized monazites were investigated, having very different concordant U-Pb ages (24 to 1928 Ma), and up to ∼15 wt.% ThO₂, and ∼0.94 wt. % UO₂. The SEM and EMP analyses of polished single crystal fragments reveal the absence of significant chemical zoning. XRD and TEM investigations show that the monazites are not metamict, despite their old ages, very high abundances of radionuclides, and hence, high time-integrated radiation doses. Except for the youngest one, the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains. This structure is responsible for the presence of (1) mottled diffraction contrasts on the TEM, and (2) a second structural phase (B), with very broad reflections in the XRD patterns. Older monazites receive higher self-irradiation doses, and hence, they contain higher amounts of this B-phase. For the 1928 Ma monazite, XRD reveals only the broad reflections of phase B, implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice. It is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because self-annealing heals the defects as they are produced by α-decay damage. The only memory of irradiation-induced defects is the presence of distorted domains. As the diffusion rate of Pb in an undisturbed monazite lattice is extremely low, Pb loss due to volume diffusion out of the monazite lattice is virtually impossible. This is considered as one reason why almost all monazites have concordant U-Th-Pb ages. Moreover, as long-term self-irradiation effects are limited in monazite, we consider this phase as a good candidate for the storage of high-level nuclear waste under the aspect of its high resistance to irradiation.     

月壤中的氦-3

除了极少数非常陡峭的山脉与撞击坑和火山通道的峭壁外，整个月球表面几乎都被一层厚度不等的月尘、岩屑和岩块的混合堆积物即月壤所覆盖。由于月球无大气层等特殊环境，太阳光长驱直入，太阳风粒子直接注入到月壤细小颗粒上使月壤中富含稀有气体等太阳风粒子组分。本文在系统阐述月壤的形成过程与形成机制的基础上，分析了月壤中稀有气体的来源及其浓度与月壤的成熟度、月壤颗粒大小、月壤矿物组成和化学成分的相关关系，进而利用已有的探测数据和分析结果，对月壤中氦-3资源的开发利用前景进行了初步评估。     

Lunar science: An overview

Before spacecraft exploration, facts about the Moon were restricted to information about the lunar orbit, angular momentum and density. Speculations about composition and origin were unconstrained. Naked eye and telescope observations revealed two major terrains, the old heavily cratered highlands and the younger mostly circular, lightly cratered maria. The lunar highlands were thought to be composed of granite or covered with volcanic ash-flows. The maria were thought to be sediments, or were full of dust, and possibly only a few million years old. A few perceptive observers such as Ralph Baldwin (Baldwin 1949) concluded that the maria were filled with volcanic lavas, but the absence of terrestrial-type central volcanoes like Hawaii was a puzzle. The large circular craters were particularly difficult to interpret. Some thought, even after the Apollo flights, that they were some analogue to terrestrial caldera (e.g., Green 1971), formed by explosive volcanic activity and that the central peaks were volcanoes. The fact that the craters were mostly circular was difficult to accommodate if they were due to meteorite impact, as meteorites would hit the Moon at all angles. The rilles were taken by many as definitive evidence that there was or had been, running water on the lunar surface. Others such as Carl Sagan thought that organic compounds were likely present (see Taylor 1975, p. 111, note 139).     

Flood Basalts and Lunar Petrogenesis

The popular interpretation of lunar maria, as sequences of primarypicritic flood basalts derived by remelting a mantle that hadaccumulated from a global magma ocean, has many unsatisfactoryaspects. The expertise of Keith Cox would have been valuablein their interpretation. KEY WORDS: europium anomaly; Io; magma ocean; mare basalt; picrite; quench crystal; regolith     

Two Lunar Mare Soil Simulants

Two new lunar mare soil simulants, NAO-2 and NAO-3, have been created in National Astronomical Observatories (NAO), Chinese Academy of Sciences. These two simulants were produced from low-titanium basalt and high-titanium basalt respectively. The chemical composition, mineralogy, particle size distribution, density, angle of internal friction, and cohesion of both simulants have been analyzed, indicating that some characteristics of NAO-2 and NAO-3 are similar to those of Apollo 14 and Apollo 11 landing site soils. NAO-2 and NAO-3 will be of great benefit to the scientific and engineering research on lunar soil.     

Photoelectric Observations of the Lunar Occultation of the Star SAO 79361 during a Total Lunar Eclipse

The results of photoelectric observation of the occultation of the visual binary star SAO 79361 = ADS 6038 by the Moon during the total lunar eclipse of January 9, 1982 are presented. The upper limit of the angular diameter of the brighter component A is estimated to be d_ul ? 0.0011″ based on direct measurements, which is in agreement with indirect estimates. Accurate estimates for the magnitude difference between components A and B, their angular separation, p ? 3.2″, and the inclination of the Moon’s edge’s at the point of occultation to the mean lunar horizon, θ ? +25°, are obtained. The time of the geometrical occultation of the center of the stellar disk of component A is determined with an accuracy of 1 ms. The results obtained for this star are unique among observations worldwide.     

月球同位素地质年代学与月球演化

同位素地质年代学是月球形成与演化研究的重要工具。目前对月岩和月球陨石的研究积累了大量的同位素年代学资料,其测试的主要方法包括K—Ar、Ar_Ar、Rh-sr、Sm—Nd、Lu—Hf、Re-0s、Pb—Pb和U—Pb等。根据目前获得的资料,本文对月球上不同类型岩石的形成年龄进行了总结,发现月球高地岩石主要形成于45～38亿年,而月海玄武岩相对年轻,大致形成于38～31亿年。根据这-年代资料,结合对月岩样品进行的短半衰期Hf-W与Sin-Nd同位素体系的研究结果,认为月球大致形成于45亿年,其后开始由岩浆海导致的内部核-幔-壳的分异,岩浆结晶的低密度斜长岩构成最初的月壳,而密度大的岩石构成月幔,而此月幔则成为后来月海玄武岩的主要岩浆源区。同时指出了当前月岩同位素地质年代学存在的问题,并根据技术进展的情况,对未来月岩同位素年代学的发展趋势作了全面的分析．     

Lunar deposits of possible pyroclastic origin

Glass droplets of possible pyroclastic origin are present in the lunar regolith at the Apollo 11, 15, and 17 sites. The droplets may be derived from deposits, interbedded with mare lava flows, which have been partly mixed into the regolith by impact processes. Orange glass droplets from the Apollo 17 site (spheres, ovoids, broken droplets) are both chemically and texturally homogeneous and have rare olivine phenocrysts. None of the droplets contain shock damaged crystals which are common in glass produced during meteorite impacts. The droplets are similar to those formed in terrestrial lava fountains and are here interpreted as tephra.The homogeneous glass droplets sampled at the Apollo 11, 15 and 17 sites are located on or close to mare basin rims. Vents for the youngest mare lava flows, located near basin rims, have been identified photogeologically. Dark mantle deposits, interpreted as pyroclastic blankets in some locations, and numerous rules are also present on the mare surface, near basin rims. The glass droplets, having ages nearly contemporaneous with the associated mare lavas, may be concentrated locally near such vent areas. This association is in accordance with the limited extent of ash deposits from terrestrial lava fountains (? km from the vent).     

Lunar ranging instrument for Chandrayaan-1

Lunar Laser Ranging Instrument (LLRI) proposed for the first Indian lunar mission Chandrayaan-1 is aimed to study the topography of the Moon’s surface and its gravitational field by precisely measuring the altitude from a polar orbit around the Moon. Altimetry data close to the poles of the Moon would also be available from the instrument, which was not covered by earlier missions. This instrument supplements the terrain mapping camera and hyperspectral imager payloads on Chandrayaan-1. The instrument consists of a diode pumped Nd:YAG pulsed laser transmitter having 10 nsec pulse width and a receiver system. The receiver system features 17 cm diameter Ritchey—Chrétien collecting optics, Si Avalanche Photo Detector (APD), preamplifiers, constant fraction discriminators, time-of-flight measurement unit and spacecraft interface. Altimeter resolution of better than 5 m is targeted. The received signal strength of LLRI depends on laser pulse backscatter from the Moon’s surface. Moon’s surface being a poor reflector, the choice of receiver size and its type and the selection of detector play an important role in getting a good signal-to-noise ratio and in turn achieving the target resolution. At the same time, the spacecraft puts a limitation on payload size and weight. This paper discusses the proposed LLRI system for Chandrayaan-1 and signal-to-noise ratio estimation.     

Lunar concrete: Prospects and challenges

The possibility of using concrete as a construction material at the Moon surface is considered. Dissimilarities between the Earth and the Moon and their possible effects on concrete are also emphasized. Availability of constituent materials for concrete at lunar surface is addressed. An emphasis is given to two types of materials, namely, hydraulic concrete and sulfur concrete. Hydraulic concrete necessitates the use of water and sulfur concrete makes use of molten sulfur in lieu of cement and water.     

月球起源研究进展

月球是地球唯一的天然卫星,早期学者认为地月系统是普遍存在的行星一卫星系统的一员,但是月球又有许多特征,如质量异常大,挥发分和Fe元素亏损等,传统理论难以解释这些特征.因此,针对月球的起源提出了四种学说:捕获理论,共增生理论,分裂理论和大碰撞假说.目前对大碰撞假说研究较多,但研究者们始终无法较好解释地月系统氧同位素的高度一致等特征.本文试探性提出新的月球起源分裂模型,能较好的解释某些月球特征,引起研究者们对月球起源新的思考.     

月壤中的稀有气体

月球极度亏损挥发分,但是月壤中赋存有大量的稀有气体,主要来源于太阳风注入、宇宙射线作用和放射性同位素衰变等过程。月岩和月壤样品的稀有气体研究,不仅是获取月球表面形成和演化历史、近地空间小行星撞击历史等的重要内容,更是解译40亿年以来太阳风演化的惟一可行途径。本文主要介绍月岩和月壤中的太阳风记录、宇宙射线暴露年龄、Ar-Ar定年以及稀有气体测试技术等方面研究的进展。