Magmatic graphite in dolomite carbonatite at Pogranichnoe,North Transbaikalia,Russia

A recently discovered dolomite carbonatite at Pogranichnoe, North Transbaikalia, Russia, dated at 624 ± 3 Ma, contains xenoliths of calcite-bearing dolomite carbonatite with graphite spherulites. Apatite and aegirine are the other rock-forming minerals. Chemically the carbonatites are ferrocarbonatite and ferruginous calciocarbonatite. The graphite forms <1 mm up to 1.5 mm diameter spherulites, with Raman spectra similar to published spectra of microcrystalline, amorphous carbon and disordered graphite, with G and D bands at 1,580−1,600 cm⁻¹ and at around 1,350 cm⁻¹. Alteration has formed Fe-bearing calcite to Ca-bearing siderite compositions not previously reported in nature around the graphite along cracks and fractures. Mineral and stable isotope geothermometers and melt inclusion measurements for the carbonatite all give temperatures of 700°–900°. It is concluded that the graphite precipitated from the ferrocarbonatite magma. There are three candidates to control the precipitation of graphite (a) a redox reaction with Fe^II in the magma, (b) potential presence of organics in the magma (c) seeding of, or dissolution in, the magma of graphite/diamond from the mantle, and further work is required to identify the most important mechanism(s). Graphite in carbonatite is rare, with no substantial published accounts since the 1960s but graphite at other localities seems also to have precipitated from carbonatite magma. The precipitation of reduced carbon from carbonatite provides further evidence that diamond formation in carbonate melts at high mantle pressures is feasible.     

Titan's diverse landscapes as evidenced by Cassini RADAR's third and fourth looks at Titan

Cassini's third and fourth radar flybys, T7 and T8, covered diverse terrains in the high southern and equatorial latitudes, respectively. The T7 synthetic aperture radar (SAR) swath is somewhat more straightforward to understand in terms of a progressive poleward descent from a high, dissected, and partly hilly terrain down to a low flat plain with embayments and deposits suggestive of the past or even current presence of hydrocarbon liquids. The T8 swath is dominated by dunes likely made of organic solids, but also contain somewhat enigmatic, probably tectonic, features that may be partly buried or degraded by erosion or relaxation in a thin crust. The dark areas in T7 show no dune morphology, unlike the dark areas in T8, but are radiometrically warm like the dunes. The Huygens landing site lies on the edge of the T8 swath; correlation of the radar and Huygens DISR images allows accurate determination of its coordinates, and indicates that to the north of the landing site sit two large longitudinal dunes. Indeed, had the Huygens probe trajectory been just 10 km north of where it actually was, images of large sand dunes would have been returned in place of the fluvially dissected terrain actually seen—illustrating the strong diversity of Titan's landscapes even at local scales.     

Geochemistry of carbonatite–silicate pairs in nature: A case history from Central Italy

At Oricola (Aquila-Abruzzo, Italy) carbonatite is associated with phonolitic foidite tuff. The Oricola carbonatite contains fresh silicate glass of kamafugitic foidite composition which, compared with carbonate, shows similar trace element patterns but lower concentrations. As a whole, the mineralogy of the Oricola rocks matches that of the neighbouring Grotta del Cervo kamafugitic foidite and carbonatitic foidite and is in the range of the Intramountain Ultralkaline Province (IUP) of melilitites and carbonatites of Italy.

The IUP carbonatites and kamafugitic foidites definitely form intra-outcrop conjugate pairs. All these co-eruptive rocks have parallel trace element patterns, namely REE, which implies a dilution–concentration relationship among them but always with higher contents in primary calcite. Based on current textural and compositional criteria, as well as comparable experimental data, we attribute this feature to liquid immiscibility dominant over crystal fractionation at crustal pressure. This relatively late immiscibility phenomenon is superimposed on co-magmatic features shown by inter-outcrop conjugate rock couples. In fact if we consider San Venanzo kamafugite and Polino Ca-carbonatite, or Grotta del Gervo kamafugite and Oricola Ca-carbonatite, we note couple by couple that they are chemical heteromorphs erupted in isolation in different place. The REE distribution is another distinctive feature of these couples and shows a marked crossover at MREE level. A fact we interpret as produced by near mantle-solidus immiscibility. After this early phenomenon the two members of the couple can erupt in a near primary state carrying mantle xenoliths or undergo some evolution including settling out mantle xenoliths and crystals and fractionation and finally exsolve a carbonatitic residuum by immiscibility.     

Cassini RADAR observations of Enceladus, Tethys, Dione, Rhea, Iapetus, Hyperion, and Phoebe

Cassini 2.2-cm radar and radiometric observations of seven of Saturn's icy satellites yield properties that apparently are dominated by subsurface volume scattering and are similar to those of the icy Galilean satellites. Average radar albedos decrease in the order Enceladus/Tethys, Hyperion, Rhea, Dione, Iapetus, and Phoebe. This sequence most likely corresponds to increasing contamination of near-surface water ice, which is intrinsically very transparent at radio wavelengths. Plausible candidates for contaminants include ammonia, silicates, metallic oxides, and polar organics (ranging from nitriles like HCN to complex tholins). There is correlation of our targets' radar and optical albedos, probably due to variations in the concentration of optically dark contaminants in near-surface water ice and the resulting variable attenuation of the high-order multiple scattering responsible for high radar albedos. Our highest radar albedos, for Enceladus and Tethys, probably require that at least the uppermost one to several decimeters of the surface be extremely clean water ice regolith that is structurally complex (i.e., mature) enough for there to be high-order multiple scattering within it. At the other extreme, Phoebe has an asteroidal radar reflectivity that may be due to a combination of single and volume scattering. Iapetus' 2.2-cm radar albedo is dramatically higher on the optically bright trailing side than the optically dark leading side, whereas 13-cm results reported by Black et al. [Black, G.J., Campbell, D.B., Carter, L.M., Ostro, S.J., 2004. Science 304, 553] show hardly any hemispheric asymmetry and give a mean radar reflectivity several times lower than the reflectivity measured at 2.2 cm. These Iapetus results are understandable if ammonia is much less abundant on both sides within the upper one to several decimeters than at greater depths, and if the leading side's optically dark contaminant is present to depths of at least one to several decimeters. As argued by Lanzerotti et al. [Lanzerotti, L.J., Brown, W.L., Marcantonio, K.J., Johnson, R.E., 1984. Nature 312, 139-140], a combination of ion erosion and micrometeoroid gardening may have depleted ammonia from the surfaces of Saturn's icy satellites. Given the hypersensitivity of water ice's absorption length to ammonia concentration, an increase in ammonia with depth could allow efficient 2.2-cm scattering from within the top one to several decimeters while attenuating 13-cm echoes, which would require a six-fold thicker scattering layer. If so, we would expect each of the icy satellites' average radar albedos to be higher at 2.2 cm than at 13 cm, as is the case so far with Rhea [Black, G., Campbell, D., 2004. Bull. Am. Astron. Soc. 36, 1123] as well as Iapetus.     

Titan's surface from Cassini RADAR SAR and high resolution radiometry data of the first five flybys

The first five Titan flybys with Cassini's Synthetic Aperture RADAR (SAR) and radiometer are examined with emphasis on the calibration and interpretation of the high-resolution radiometry data acquired during the SAR mode (SAR-radiometry). Maps of the 2-cm wavelength brightness temperature are obtained coincident with the SAR swath imaging, with spatial resolution approaching 6 km. A preliminary calibration shows that brightness temperature in these maps varies from 64 to 89 K. Surface features and physical properties derived from the SAR-radiometry maps and SAR imaging are strongly correlated; in general, we find that surface features with high radar reflectivity are associated with radiometrically cold regions, while surface features with low radar reflectivity correlate with radiometrically warm regions. We examined scatterplots of the normalized radar cross-section σ⁰ versus brightness temperature, finding differing signatures that characterize various terrains and surface features. Implications for the physical and compositional properties of these features are discussed. The results indicate that volume scattering is important in many areas of Titan's surface, particularly Xanadu, while other areas exhibit complex brightness temperature variations consistent with variable slopes or surface material and compositional properties.     

水氡与井水位短临前兆信息的一种提取方法

工作中使用了三口水位观测井及五个水氡观测泉点的观测资料。三口观测井均位于南北地震带北段的六盘山地震带上,大地构造位置属于祁吕贺兰山字型构造体系的马蹄型盾地也(图1)。     

The carbonate fraction in carbonatitic Italian lamprophyres

Alkaline and ultramafic lamprophyres represent the majority of pre-Pleistocene alkaline mafic magmatic activity in Italy and have been described from several localities. The age of magmatism ranges from Triassic to Lower Oligocene. Some contain appreciable amounts of carbonate. The primary carbonate of the Italian carbonatitic lamprophyres is mainly Sr- or Mn-rich calcite that occurs mostly as immiscible ocelli or as groundmass. Its textural occurrence, composition, and relationship with co-precipitating silicate phases is taken as evidence of an igneous origin. Low BaO and REE contents in the carbonate are explained by early crystallization of essential mica and subordinate apatite. Whole rock analyses and isotopic data (Rukhlov, A.S., Bell, K., Vichi, G., Stoppa, F., submitted for publication. The heterogeneous deep mantle: the Sr, Pb and Nd isotopic evidence from Early Cretaceous alkaline lamprophyres of Southern Tuscany, Italy. Lithos.) suggest a mantle origin for these rocks and rule out contamination in either high or low pressure regimes. The bulk compositions of the carbonatitic lamprophyres have high HFSE / LILE and LREE / HREE ratios and although the abundances of these elements are generally lower than for carbonatites s.s., they are comparable with the abundances in other ‘carbonate-free’ Italian lamprophyres and Italian carbonatites, suggesting similar mantle sources. Moreover, the age of the Italian lamprophyres, ranging from Middle Triassic to Lower Oligocene, is much greater than the Pleistocene age of Italian carbonatites and indicates that the source remained similar over a long time span.     

地震弱短临前兆信息的提取方法研究

地震短临预报是地震预报的重大难题,它有赖于好的前兆手段和数据算是方法,本提出“弱前兆信息”的观点,并研究了提取弱短临前兆信息的方法和定量指标。     

Radio and millimeter continuum surveys and their astrophysical implications

We review the statistical properties of the main populations of radio sources, as emerging from radio and millimeter sky surveys. Recent determinations of local luminosity functions are presented and compared with earlier estimates still in widespread use. A number of unresolved issues are discussed. These include: the (possibly luminosity-dependent) decline of source space densities at high redshifts; the possible dichotomies between evolutionary properties of low- versus high-luminosity and of flat- versus steep-spectrum AGN-powered radio sources; and the nature of sources accounting for the upturn of source counts at sub-milli-Jansky (mJy) levels. It is shown that straightforward extrapolations of evolutionary models, accounting for both the far-IR counts and redshift distributions of star-forming galaxies, match the radio source counts at flux-density levels of tens of μJy remarkably well. We consider the statistical properties of rare but physically very interesting classes of sources, such as GHz Peak Spectrum and ADAF/ADIOS sources, and radio afterglows of γ-ray bursts. We also discuss the exploitation of large-area radio surveys to investigate large-scale structure through studies of clustering and the Integrated Sachs–Wolfe effect. Finally, we briefly describe the potential of the new and forthcoming generations of radio telescopes. A compendium of source counts at different frequencies is given in Supplementary Material.