Isotopic evidence on the origin of the Shamrocke copper mine,Rhodesia

The Shamrocke ore body is a stratiform deposit of disseminated copper-iron sulphides found within lenses of calcareous meta-arkose occurring in Lomagundi Group graphitic schist (Late Precambrian) at Karoi, Rhodesia. Both sulphides and sediments were subjected to high grades of regional metamorphism. Argon/argon isochron ages indicate a major metamorphic event at 550 m. y. (Damaran orogeny) with later minor argon losses. Ore sulphide ³⁴S values range from +3.0 to +14.8 CDT and a general decrease from footwall to hanging-wall reflects an original sedimentary environment where sulphides formed about the sea bed from hydrothermal fluids progressively mixing with sea water. Isotopically lighter sulphides formed syngenetically in the host rocks from bacterial reduction of sulphate. The pyrrhotite was probably formed from pyrite during metamorphism, and owing to reducing conditions maintained similar ³⁴S values to the original pyrite. Oxygen and carbon isotopic analyses of mineralised and unmineralised carbonate lenses are consistent with deposition as marine limestones in an evaporitic environment and/or near hot spring vents.I. N. S. Contribution No 734     

Zircon U–Pb age, trace element and Hf isotope composition of Kongling terrane in the Yangtze Craton: refining the timing of Palaeoproterozoic high-grade metamorphism

U–Pb age, trace element and Hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the Kongling terrane in the northern part of the Yangtze Craton. The zircon shows distinct morphological and chemical characteristics. Most zircon in an amphibolite shows oscillatory zoning, high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, high formation temperature, high trace element contents, clear negative Eu anomaly, as well as HREE-enriched patterns, suggesting that it is igneous. The zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2857 ± 8 Ma, representing the age of the magmatic protolith. The zircon in the other two samples is metamorphic. It has low Th/U ratios, low trace element concentrations, variable HREE contents (33.8 ≥ Lu_N≥2213; 14.7 ≤ Lu_N/Sm_N ≤ 354) and ¹⁷⁶Lu/¹⁷⁷Hf ratios (0.000030–0.001168). The data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions. The metamorphic zircon yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2010 ± 13 Ma. These results combined with previously obtained Palaeoproterozoic metamorphic ages suggest a c. 2.0 Ga Palaeoproterozoic collisional event in the Yangtze Craton, which may result from the assembly of the supercontinent Columbia. The zircon in two samples yields weighted mean two-stage Hf model ( T _DM2) ages of 3217 ± 110 and 2943 ± 50 Ma, respectively, indicating that their protoliths were mainly derived from Archean crust.     

Aragonite in California Glaucophane Schists, and the Kinetics of the Aragonite--Calcite Transformation

Coleman & Lee's discovery that aragonite is a widespreadmetamorphic mineral in California glaucophane schists is confirmedand amplified. Methods of microscopic distinction between aragonite,calcite, and dolomite, including a universal-stage technique,are described. Further data are recorded regarding the paragenesisof aragonite-bearing glaucophane and lawsonite schists. Carbonatesin the greenschist facies are found to be exclusively calciteand dolomite. In many Californian metamorphic aragonites partialinversion to calcite has been observed. This appears to be anequal-volume replacement in which an a axis and an edge [f:f] of calcite—both directions of Closest spacing of Ca⁺⁺ions—commonlyare parallel to of the crystal axes a, b, and c of aragonite. The problem of survival of metamorphic aragonite through a necessarilylong period of post-metamorphic unloading is approached by experimentalexploration of the kinetics of the aragonite calcite transformation.It is found that Californian aragonite could survive unloadingfrom a depth of 20 km if the linear temperature gradient were10? C per km, but not appreciably higher. Available experimentaldata are consistent with crystallization of aragonite, jadeite,and lawsonite at depths of 20–30 km if a gradient of 10?C per km is assumed. The corresponding conditions of the glaucophane-schistfacies (T = 2OO?-3OO? C, P = 6000-9000 bars) are attributedessentially to deep burial in regions (subsiding geosyn-clines)of unusually low temperature gradient (10? C per km).     

Thermodynamics of Co3O4: a possible electron spin unpairing transition in Co3+

The free energy of the reaction: $$Co_3 O_4 \rightleftarrows 3C_O O + \tfrac{1}{2}O_2$$ has been studied between 890 and 1,240 K using an e.m.f. technique. There is a phase transition in Co₃O₄ at 1,120±20 K which is accompanied by a large change in entropy (～47 JK^?1 mol^?1 of Co₃O₄), and a rapid increase in unit cell volume and in electical conductivity. This is interpreted to be due to a partial change in electronic spin states in Co^{3 +} from the spin-paired (low spin) configuration observed at room temperature to the spin-unpaired (high spin) state. The transition is probably not first order.     

The five-minute oscillations in the solar atmosphere

The acoustic overstability in a polytropic plane-parallel atmosphere with superadiabatic temperature gradient and radiative dissipation is demonstrated for optically thick disturbances. The periods of oscillation are found to be in the range 250–480 s and the associated wavelength of the order of 4000 km. The five-minute oscillations in the solar surface are attributed to self-excited sound waves in a layer in the subphotospheric convection zone of about 1000 km thickness.     

IUE observations of faint comets

Ultraviolet spectra of seven comets taken with the same instrument are presented. Comets P/Encke (1980), P/Tuttle (1980 h), P/Stephan-Oterma (1980 g), and Meier (1980 q) were observed during November-December 1980 with the International Ultraviolet Explorer (IUE) satellite observatory, while comets P/Borrelly (1980 i) and Panther (1980 u) were observed with IUE on 6 March 1981. The spectra of these comets are compared with those of comet Bradfield (1979 X), studied extensively earlier in 1980 with IUE, as well as with each other. In order to simplify the interpretation of the data and to minimize the dependence upon a specific model, the spectra are compared at approximately the same value of heliocentric distance whenever possible. Effects due to heliocentric velocity, geocentric distance, and optical depth are also discussed. All of the cometary spectra are remarkably similar, which suggests that these comets may have a common composition and origin.     

Crustal thicknesses in Fennoscandia

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As a supplement to seismic profiling surveys, crustal thicknesses have been estimated for 11 Fennoscandian seismograph stations equipped with three-component long period instruments, using the so-called spectral ratio technique of Phinney. The largest Moho depths, of the order of 45 km, were found for stations located in the north-east areas of Norway and Sweden and in Finland, with a local maximum in the Bothnian Bay. The coastal area of south-east Norway and Zealand, Denmark exhibit crustal thicknesses in the range 28–33 km. The agreement between our results and those obtained by conventional refraction profiling is good, when this comparison is restricted to profiles of lengths 300 km or more, and when the associated crustal thickness estimate is averaged over the central parts of the profiles in question. Also, a comparison between our results and other available geophysical information gives that the oldest tectonic provinces of the Baltic Shield also are characterized by relatively modest heat flow, and exhibit the greatest crustal thicknesses. Post-glacial uplift data and large wavelength free air gravity data appear to be uncorrelated with crustal thickness. The same partly applies to Bouguer gravity anomalies, thus implying that the isostatic compensation mechanism in Fennoscandia is of both Airy and Pratt type.     

Cosmological implications of BL Lac objects

It has become clear in recent years that relativistic beaming is a good explanation for the BL Lac phenomenon. Of studies based on the relativistic beaming model of BL Lac objects, we note that the orientation of jet's axis to the line-of-sight is very small and, therefore, the observed flux emitted from a rapidly moving source is orders of magnitude higher than the flux in its rest-frame:F _obs = ^{3 +} F _intr, where is the bulk relativistic Doppler factor. Then the observed apparent magnitudem _v must be corrected for this effect. For our 39 samples, the corrected apparent magnitudem _v ^corr and logZ have a good correlation.     

湖南省柿竹园矽卡岩矿床中石榴石特征

通过野外与显微镜观察和电子探针分析, 对柿竹园多金属矿床矽卡岩中石榴石的特征进行了研究.根据石榴石的产出状态、矿物的共生组合, 矽卡岩可分为4个带: 磁铁矿-辉石-石榴石带、辉石-石榴石带、符山石-石榴石带、矽卡岩化大理岩带.从成分上的变化, 探讨了石榴石在各矽卡岩带中的特征.柿竹园矿床矽卡岩中的石榴石可分为早、晚两期, 早期形成的石榴石颜色为暗棕色, 并且在垂直和水平方向上有明显的变化规律.从磁铁矿-辉石-石榴石带到矽卡岩化大理石带, 随着石榴石中Fe₂O₃含量的减少, Al₂O₃含量的增加, 由钙铁榴石向钙铝榴石变化; 石榴石晶体具有从核部向边缘由均质性向非均质性变化的规律.早期石榴石形成于较氧化的条件下, 温度为520~620℃, 压力为1000×105Pa, 由富含Si, Al, Fe, Cl, F组分的热液和泥盆纪佘田桥组灰岩反应交代形成.当温度降至450~ 540℃, pH, Eh值降低时, 晚期石榴石形成的同时使白钨矿沉淀.晚期石榴石颜色比早期形成的石榴石浅, 为红色.结晶颗粒较大, 并且, 普遍可以观察到石榴石环带结构.      

Tentative Classification and Correlation of Silurian Rocks of South China

PROLEGOMENA The presence of Silurian rocks in China was first made known by Richthofen [106] more than sixty years ago. Since then no further discovery has been added for a long time. It was in 1920 that Yabe and Hayasaka [150] described the second collection of Silurian fossils from China. After that date Silurian stratigraphy and paleontology have made rapid progress.