Formation of Paleoproterozoic eclogitic mantle, Slave Province (Canada): Insights from in-situ Hf and U–Pb isotopic analyses of mantle zircons

In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date.

The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the 2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages.     

Low-Frequency Plasma Waves in the Outer Polar CUSP: A Review of Observations from Prognoz 8, Interball 1, Magion 4, and Cluster

This paper presents a review of the most interesting observations of low-frequency plasma waves together with plasma particles which were made by the Interball 1, Magion 4 and Prognoz 8 satellites in the outer polar cusp. Accelerated plasma particles, hot electron populations and very strong wave activity, particularly at low frequencies, are observed. A detailed study of the wave spectra together with the distribution function for electrons indicate the correlation between the presence of lower-hybrid waves and the population of the particles with higher energy than in the surrounding space. These experimental facts suggest that strong coupling between waves and particles is responsible for plasma heating. During polar cusp crossings by Interball 1 and Prognoz 8, FFT analysis of the wave form indicates many bursts of ULF emissions in both electric and magnetic components. These waves have highly non-stationary characteristics. To study the dynamics of changes in the spectral characteristics of the waves wavelet analysis has been used. Nonlinear interactions are studied using bispectral methods of analysis. This presentation gives the results of such an analysis for selected cusp crossings at different altitudes. An example of wave activity registered by the STAFF instrument onboard the CLUSTER spacecraft in the polar cusp is also presented.     

The migration of fluid-filled grain boundaries in recrystallizing synthetic bischofite: first results of in-situ high-pressure, high-temperature deformation experiments in transmitted light

The effect of fluids on recrystallization behaviour is well known; however, the detailed microscale distribution of fluid in grain boundaries and the influence of fluid on grain boundary migration are still unresolved. In this study, in‐situ deformation experiments in transmitted light microscopy were undertaken, as this allows continuous and direct observation of the whole range of processes involved in fluid‐assisted grain boundary migration. A new see‐through deformation apparatus was developed to enable the control of fluid pressure. Bischofite containing small amounts of aqueous fluid was deformed at temperatures between 50 and 90 °C, over a range of fluid pressure from 0.5 to 1 MPa, and strain rates of 5 × 10^?6 to 1 × 10^?4 s^?1. The rates of grain boundary migration were measured at different temperatures and strain rates. Detailed observations during and after the deformation illustrate the evolution of migrating fluid‐filled grain boundaries and show that the incorporation of fluids from inclusions as well as their pinch‐off is dependent on the grain boundary velocity, the thickness of the grain boundary and the size and shape of the inclusions. Direct evidence is presented for the contraction of the grain boundary fluids into isolated inclusions after equilibrium conditions are attained.     

Cultivation and human impact at 6000 cal yr B.P. in tropical lowland forest at Niah, Sarawak, Malaysian Borneo

This paper describes palynological evidence for what appears to be comparatively large-scale human impact in the catchment of the Sungai Niah in the wet tropical lowland swamp forests of Sarawak, Malaysian Borneo close to the Great Cave of Niah. Pollen associated with cleared landscapes and rice cultivation is evident in the sedimentary record from before 6000 cal yr B.P. Human activity seems to have been associated with changes in sedimentary regime, with peat-dominated environments being replaced diachronously by clay-dominated deposition. This may reflect anthropogenic soil erosion in the catchment of the Sungai Niah.     

Comparison of Proterozoic and Phanerozoic rift-related basaltic-granitic magmatism

This paper compares the 1.67–1.47 Ga rapakivi granites of Finland and vicinity to the 1.70–1.68 Ga rapakivi granites of the Beijing area in China, the anorogenic 130 Ma granites of western Namibia, and the 20–15 Ma granites of the Colorado River extensional corridor in the Basin and Range Province of southern Nevada. In Finland and China, the tectonic setting was incipient, aborted rifting of Paleoproterozoic or Archean continental crust, in Namibia it was continental rifting and mantle plume activity that led to the opening of southern Atlantic at 130 Ma. The 20–15 Ma granites of southern Nevada were related to rifting that followed the Triassic–Paleogene subduction of the Farallon plate beneath the southwestern United States. In all cases, extension-related magmatism was bimodal and accompanied by swarms of diabase and rhyolite–quartz latite dikes. Rapakivi texture with plagioclase-mantled alkali feldspar megacrysts occurs in varying amounts in the granites, and the latest intrusive phases are commonly topaz-bearing granites or rhyolites that may host tin, tungsten, and beryllium mineralization. The granites are typically ferroan alkali-calcic metaluminous to slightly peraluminous rocks with A-type and within-plate geochemical and mineralogical characteristics. Isotope studies (Nd, Sr) suggest dominant crustal sources for the granites. The preferred genetic model is magmatic underplating involving dehydration melting of intermediate-felsic deep crust. Juvenile mafic magma was incorporated either via magma mingling and mixing, or by remelting of newly hybridized lower crust. In Namibia, partial melting of subcontinental lithospheric mantle was caused by the Tristan mantle plume, in the other cases the origin of the mantle magmatism is controversial. For the Fennoscandian suites, extensive long-time mantle upwelling associated with periodic, migrating melting of the subcontinental lithospheric mantle, governed by heat flow and deep crustal structures, is suggested.     

Analysis of the color variability of ON 231 in 1994–2002

We analyze multicolor observations of the blazar ON 231 obtained during coordinated observations in 1994–2002. On average, the spectral energy distribution of the variable component in the optical range remains constant, and can be represented by the power law F _ν ∼ ν ^−0.85. Since the radiation of the blazar is strongly polarized, there is no doubt that the variable emission that is responsible for the activity of the blazar is synchrotron radiation. There are small but significant season-to-season variations in the spectral index.     

Hydrography of the eastern Arabian Sea during summer monsoon 2002

Hydrographic observations in the eastern Arabian Sea (EAS) during summer monsoon 2002 (during the first phase of the Arabian Sea Monsoon Experiment (ARMEX)) include two approximately fortnight-long CTD time series. A barrier layer was observed occasionally during the two time series. These ephemeral barrier layers were caused byin situ rainfall, and by advection of low-salinity (high-salinity) waters at the surface (below the surface mixed layer). These barrier layers were advected away from the source region by the West India Coastal Current and had no discernible effect on the sea surface temperature. The three high-salinity water masses, the Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW), and the Arabian Sea Salinity Minimum also exhibited intermittency: they appeared and disappeared during the time series. The concentration of the ASHSW, PGW, and RSW decreased equatorward, and that of the RSW also decreased offshore. The observations suggest that the RSW is advected equatorward along the continental slope off the Indian west coast.     

The possible escape of electron cyclotron maser radiation from hot stellar coronas through a window of transparency

The main argument against the idea that the intense radio emission observed from active regions on the Sun and flare stars is electron-cyclotron maser (ECM) radiation is that such radiation should be strongly absorbed in higher-lying layers where the condition for the cyclotron resonance at harmonics of the electron gyrofrequency is fulfilled. Cyclotron absorption lowers the efficiency of ECM radiation virtually to zero for a broad range of angles between the direction of propagation of the radiation and the magnetic field. Less severe absorption is possible only in narrow angular “windows” along (for ordinary and extraordinary waves) and perpendicular to (for ordinary waves) the magnetic field. However, the ECM radiation that is generated does not fall into these windows of transparency due to the kinematic conditions corresponding to coronal magnetic traps. We investigate the efficiency of induced scattering of ECM radiation on ions in the equilibrium plasma in the source. Under certain conditions, induced scattering leads to the formation of a condensate of ECM radiation with the direction of its wave vectors approximately along the magnetic field, enabling the escape of the radiation through windows of transparency. The most favorable conditions for this phenomenon are realized for ordinary waves. We estimate the optical depths of the sources of the ECM radiation to the scattering and the angular width of the condensate for ordinary and extraordinary waves for the cases of the flare radio emission of the star AD Leo and the sources of type I noise storms in the solar corona. In both cases, the polarization of the emergent radiation should correspond to the ordinary wave.     

Magmatic zircon oxygen isotopes of 1.88–1.87 Ga orogenic and 1.65–1.54 Ga anorogenic magmatism in Finland

Summary Oxygen isotope ratios of igneous zircon from magmatic rocks in Finland provide insights into the evolution and growth of the Precambrian crust during the Svecofennian orogeny. These data preserve magmatic δ¹⁸O values and correlate with major discontinuities in the lower crust. Oxygen isotope ratios of zircon across the 1.88–1.87 Ga Central Finland granitoid complex (CFGC) range from 5.50‰ to 6.84‰, except for three plutons in contact with the adjacent greenstone and metasedimentary belts (δ¹⁸O(Zrc) = 7.60‰–7.78‰). There is a systematic variation in δ¹⁸O(Zrc) with respect to geographic location in the CFGC, ranging from 6.60±0.23‰ (σ) in the northeast to 5.90±0.40‰ in the west-southwest. These values correlate with a change in crustal thickness and shift in geochemical composition. The oxygen isotope composition of the 1.65–1.54 Ga rapakivi granites and related rocks in southern Finland show a decreasing trend from north to south, independent of their emplacement age. The southern anorogenic granite group has an average δ¹⁸O in zircon of 6.14±0.07‰ and the northern anorogenic group has an average δ¹⁸O in zircon of 8.14±0.59‰. This difference reflects the boundary between island arc terrains accreted during the Paleoproterozoic. Deceased