Metamorphism in Archaean greenstone belts: calculated fluid compositions and implications for gold mineralization

An inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions. For mafic and ultramafic sequences, the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits (X_CO2= 0.2–0.3). This result follows from the calculation of mineral equilibria in the model system CaO–MgO–FeO–Al₂O₃–SiO₂–H₂O–CO₂, using a new, expanded, internally consistent dataset. Greenstone metamorphism cannot have involved much crustal over-thickening, because very shallow levels of greenstone belts are preserved. Such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere. In this case, the observed metamorphism, which was contemporaneous with deformation, is of the low-P high-T type. For this type of metamorphism, the metamorphic peak should have occurred earlier at deeper levels in the crust; i.e. the piezothermal array should be of the ‘deeper-earlier’type. However, at shallow crustal levels, the piezothermal array is likely to have been of ‘deeper-later’type, as a consequence of erosion. Thus, while the lower crust reached maximum temperatures, and partially melted to produce the observed granites, mid-crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde. We propose that these fluids are responsible for the gold mineralization. Thus, the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution, and does not mean that the mineralization has to be a consequence of igneous processes. Upward migration of metamorphic fluid, via appropriate structurally controlled pathways, will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower X_CO2. These assemblages are therefore grossly out of equilibrium with the fluid. In the case of infiltrated metabasic rocks, intense carbonation and sulphidation is predicted. If, as seems reasonable, gold is mobilized by the fluid generated by devolatilization, then the combination of processes proposed, most of which are an inevitable consequence of the metamorphism, leads to the formation of greenstone gold deposits predominantly from metamorphic fluids.     

Real-time difference imaging analysis of MOA Galactic bulge observations during 2000

We describe observations carried out by the MOA group of the Galactic bulge during 2000 that were designed to detect efficiently gravitational microlensing of faint stars in which the magnification is high and/or of short duration. These events are particularly useful for studies of extrasolar planets and faint stars. Approximately 17 deg² were monitored at a sampling rate of up to six times per night. The images were analysed in real time using a difference imaging technique. 20 microlensing candidates were detected, of which eight were alerted to the microlensing community whilst in progress. Approximately half of the candidates had high magnifications (≳10), at least one had very high magnification (≳50), and one exhibited a clear parallax effect. The details of these events are reported here, together with details of the on-line difference imaging technique. Some nova-like events were also observed and these are described, together with one asteroid.     

A ROSAT WFC observation of SW UMa: the EUV behaviour of dwarf novae in superoutburst explained

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations data base, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ∼ 450 d, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT . The WFC observations began just ≈13 hr after the optical rise was detected. With a peak count rate of ∼ 4.5 count s⁻¹ in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ≈19 hr the measured EUV flux dropped to < 2 count s⁻¹, while the optical brightness remained essentially static at m _v∼11 . Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst, as reported by Mauche & Raymond. In contrast, U Gem-type dwarf novae show no such early EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki and Mauche & Raymond and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities that affect the accretion discs in these systems.     

A model independent approach to future solar neutrino experiments

We discuss here what model independent information about properties of neutrinos and of the sun can be obtained from future solar neutrino experiments (SNO, Super-Kamiokande). It is shown that in the general case of transitions of solar ν_e's into νμ and/or ντ the initial ⁸B neutrino flux can be measured by the observation of NC events. From the CC measurements the ν_e survival probability can be determined as a function of neutrino energy. The general case of transitions of solar ν_e's into active as well as sterile neutrinos is considered. A number of relations between measurable quantities the test of which will allow to answer the question whether there are sterile neutrinos in the solar neutrino flux on the earth are derived. Transitions of solar ν_e's into active and sterile states due to neutrino mixing and Dirac magnetic moments or into active left-handed neutrinos and active right-handed antineutrinos due to neutrino mixing and Majorana transition magnetic moments are also considered. It is shown that future solar neutrino experiments will allow to distinguish between the cases of Dirac and Majorana magnetic moments.     

The Flared Disc Project: RXTE and ASCA observations of X 1822−371

We present archival Rossi X-ray Timing Explorer ( RXTE ) and simultaneous Advanced Satellite for Cosmology and Astrophysics ( ASCA ) data of the eclipsing low mass X-ray binary (LMXB) X 1822−371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822−371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M_⊙ neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822−371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M_⊙) black hole and is in a transitional state of duration shortward of 10⁷ yr.     

Dhofar 081: A new lunar highland meteorite

Abstract— The lunar meteorite Dhofar 081, found as a single fragment of 174 g in the Dhofar region of Oman, is a shocked feldspathic fragmental highland breccia dominated by anorthosite‐rich lithic and mineral clasts embedded into a fine‐grained mostly shock melted clastic matrix. Major mineral phases in the bulk rock are Ca‐rich plagioclase (An_96.5–99.5), pyroxene (FS_21.9–46.2Wo_3.0–41.4), and olivine (Fa_29.3–47.8); accessory phases include Fe‐Ni metal, ilmenite, and Ti‐Cr‐rich spinel. Dhofar 081 contains subordinate crystalline fragments of large anorthosites, intersertal impact‐melt rocks, microporphyritic impact‐melt breccias, dark fine‐grained impact‐melt breccias, large cataclastic feldspars, and irregularly shaped brown glass clasts. Mafic components are rare and no genuine regolith components were found in the sections studied. Minerals in Dhofar 081 show homogeneously distributed shock features: intergranular recrystallization, strong fracturing and mosaicism in feldspar as well as a high density of mostly irregular fractures in pyroxene and olivine. Localized impact melting caused by one or several impacts led to a strong lithification. Based on these effects an equilibration shock pressure of about 15–20 GPa is estimated for the strongest shock event in Dhofar 081. Devitrification of the “glassy” material in the rock indicates thermal annealing after shock melting suggesting that the 15–20 GPa shock event predated the ejection event. According to the concentrations of implanted solar noble gases Dhofar 081 represents a polymict clastic breccia deposit with possibly a minor regolith component. A similar noble gas record of Dhofar 081 and MacAlpine Hills 88104/05 suggests the possibility of a source crater pairing of both meteorites. As indicated by noble gas measurements pairing of Dhofar 081 with the other lunar meteorites found in Oman, Dhofar 025 and Dhofar 026, is unlikely.     

Non Gaussian and Non Local Transport in the Earth's Distant Magnetotail

We study the ion dynamics in a magnetic field reversal with a constant electric field and with a model of three dimensional magnetic turbulence. By computing the mean square displacements in the plane of the current sheet we find superdiffusive and superballistic transport regimes. Since velocity increases with the length of the free path, we have accelerated Lévyflights. The possibility to generate power law velocity distribution functions is pointed out, as well as the long memory effects and non local properties of ion transport. This revised version was published online in July 2006 with corrections to the Cover Date.     

An analytical expression for apsidal superhump precession and comparisons with numerical simulations and dwarf nova observations

We compare analytical expressions of precession rates from apsidal (positive) superhumps in close binary systems with numerical disc simulation results and observed values. In the analytical expressions, we include both the dynamical effects on the precession of the disc and effects caused by pressure forces that have been theorized to provide a retrograde effect (i.e. slowing) on the prograde disc precession. We establish new limits on density wave pitch angle to a normalized disc sound speed 60≥Ω_orb  d  tan  i / c >2.214 . Using average values for the density wave pitch angle i and speed of sound c , we find good correlation between numerical simulations and the analytical expression for the apsidal superhump period excess, which includes both the prograde and retrograde effects, for mass ratios of 0.025≤ q ≤0.33 . We also show good correlations with the four known eclipsing systems, OY Car, Z Cha, HT Cas, and WZ Sge. Our analytical expression for apsidal superhump period excess as a function of orbital period is consistent with the trend found in observed systems.     

The chronology of a large ice-dammed lake and the Barents–Kara Ice Sheet advances, Northern Russia

Beach and shoreface sediments deposited in the more than 800-km long ice-dammed Lake Komi in northern European Russia have been investigated and dated. The lake flooded the lowland areas between the Barents–Kara Ice Sheet in the north and the continental drainage divide in the south. Shoreline facies have been dated by 18 optical stimulated luminescence (OSL) dates, most of which are closely grouped in the range 80–100 ka, with a mean of 88±3 ka. This implies that that the Barents–Kara Ice Sheet had its Late Pleistocene maximum extension during the Early Weichselian, probably in the cold interval (Rederstall) between the Brørup and Odderade interstadials of western Europe, correlated with marine isotope stage 5b. This is in strong contrast to the Scandinavian and North American ice sheets, which had their maxima in isotope stage 2, about 20 ka. Field and air photo interpretations suggest that Lake Komi was dammed by the ice advance, which formed the Harbei–Harmon–Sopkay Moraines. These has earlier been correlated with the Markhida moraine across the Pechora River Valley and its western extension. However, OSL dates on fluvial sediments below the Markhida moraine have yielded ages as young as 60 ka. This suggests that the Russian mainland was inundated by two major ice sheet advances from the Barents–Kara seas after the last interglacial: one during the Early Weichselian (about 90 ka) that dammed Lake Komi and one during the Middle Weichselian (about 60 ka). Normal fluvial drainage prevailed during the Late Weichselian, when the ice front was located offshore.