Spinel peridotite xenoliths from the Atsagin-Dush volcano, Dariganga lava plateau, Mongolia: a record of partial melting and cryptic metasomatism in the upper mantle

Spinel peridotite xenoliths from the Atsagin-Dush volcanic centre, SE Mongolia range from fertile lherzolites to clinopyroxene(cpx)-bearing harzburgites. The cpx-poor peridotites typically contain interstitial fine-grained material and silicate glass and abundant fluid inclusions in minerals, some have large vesicular melt pockets that apparently formed after primary clinopyroxene and spinel. No volatile-bearing minerals (amphibole, phlogopite, apatite, carbonate) have been found in any of the xenoliths. Fifteen peridotite xenoliths have been analysed for major and trace elements; whole-rock Sr isotope compositions and O isotope composition of all minerals were determined for 13 xenoliths. Trace element composition and Sr-Nd isotope compositions were also determined in 11 clinopyroxene and melt pocket separates. Regular variations of major and moderately incompatible trace elements (e.g. heavy-rare-earth elements) in the peridotite series are consistent with its formation as a result of variable degrees of melt extraction from a fertile lherzolite protolith. The Nd isotope compositions of LREE (light-rare-earth elements)-depleted clinopyroxenes indicate an old (≥ 1 billion years) depletion event. Clinopyroxene-rich lherzolites are commonly depleted in LREE and other incompatible trace elements whereas cpx-poor peridotites show metasomatic enrichment that can be related to the abundance of fine-grained interstitial material, glass and fluid inclusions in minerals. The absence of hydrous minerals, ubiquitous CO₂-rich microinclusions in the enriched samples and negative anomalies of Nb, Hf, Zr, and Ti in primitive mantle-normalized trace element patterns of whole rocks and clinopyroxenes indicate that carbonate melts may have been responsible for the metasomatic enrichment. Low Cu and S contents and high δ³⁴S values in whole-rock peridotites could be explained by interaction with oxidized fluids that may have been derived from subducted oceanic crust. The Sr-Nd isotope compositions of LREE-depleted clinopyroxenes plot either in the MORB (mid-ocean-ridge basalt) field or to the right of the mantle array, the latter may be due to enrichment in radiogenic Sr. The LREE-enriched clinopyroxenes and melt pockets plot in the ocean island-basalt field and have Sr-Nd isotope signatures consistent with derivation from a mixture of the DMM (depleted MORB mantle) and EM (enriched mantle) II sources. Received: 18 January 1996 / Accepted: 23 August 1996     

The cities of Russia: some recent trends and experience of regional and urban planning

The author examines the recent situation in the development of cities in Russia. Recent trends include a lack of growth and even a decrease in population; extensive economic restructuring; a grave ecological situation in cities; and recent peculiarities in the geopolitical situation and population migration. Due to the new economic, demographic, geopolitical and social situation in Russia it is necessary to re-evaluate the prospects for the development and growth of the big cities in this country. The system of basic documents for regional and city planning in the former USSR and in Russia includes different stages: from The General Scheme for the Settlement Systems of the Country and The Regional Territorial Complex Schemes of Environmental Protection in different regions to the detailed plans for the different parts of cities and towns. The shortcomings of this system and in the process of the realization of the schemes of regional planning and general plans for cities are documented.     

Sulfate-coated dust particles in the free troposphere over Japan

Airborne aerosol collections were performed over Wakasa bay (36°00′N, 135°30′E) in March and Kumano open sea (34°00′N, 136°50′E) and Seto (35°10′N, 137°10′E) in July 2001 at altitudes between 1.0 and 5.8 km. The particles were individually analyzed using transmission electron microscopy (TEM). Relatively large mineral-dust (mostly clay) particles were abundant in the March samples. They also dominated in July in the mid-troposphere higher than 4 km altitude, whereas sea salt and ammonium sulfate were more abundant at lower altitudes. Ca-coated grid samples show many traces of aqueous sulfate droplets. The proportions of former sulfate droplets to the total collected particles apparently increased with increasing relative humidity at the time of sampling. TEM analysis revealed that a significant fraction of these former droplets enclose mineral-dust particles as well as sea salt, soot, and fly ash. Some enclose mixtures of mineral-dust, sea-salt, soot, and fly ash particles. The results provide evidence that mineral dust from the Asian continent could acquire coatings of sulfate while being transported in the free troposphere. The mineral-dust particles probably acquired the sulfate coatings either through heterogeneous uptake of gaseous SO₂ and subsequent oxidation or through coagulation with cloud or fog droplets. The presence of the mixed particles in sulfate droplets also indicates that aggregation of particles of different origins occurred through cloud processing. Such sulfate-coated dust particles would affect cloud formation, precipitation, and chemistry of the free troposphere.     

Trace element distribution in peridotite xenoliths from Tok, SE Siberian craton: A record of pervasive, multi-stage metasomatism in shallow refractory mantle

Spinel peridotite xenoliths in alkali basalts at Tok, SE Siberian craton range from fertile lherzolites to harzburgites and wehrlites; olivine-rich (70-84%) rocks are dominant. REE patterns in the lherzolites range from nearly flat for fertile rocks (14-17% cpx) to LREE-enriched; the enrichments are positively correlated with modal olivine, consistent with high-permeability of olivine-rich rocks during melt percolation. Clinopyroxene in olivine-rich Tok peridotites typically has convex-upward trace element patterns (La/Nd_PM < 1 and Nd/Yb_PM ? 1), which we consider as evidence for equilibration with evolved silicate liquids (with higher REE and lower Ti contents than in host basalts). Whole-rock patterns of the olivine-rich xenoliths range from convex-upward to LREE-enriched (La/Nd_PM > 1); the LREE-enrichments are positively correlated with phosphorus abundances and are mainly hosted by accessory phosphates and P-rich cryptocrystalline materials. In addition to apatite, some Tok xenoliths contain whitlockite (an anhydrous, halogen-poor and Na-Mg-rich phosphate), which is common in meteorites and lunar rocks, but has not been reported from any terrestrial mantle samples. Some olivine-rich peridotites have generations of clinopyroxene with distinct abundances of Na, LREE, Sr and Zr. The mineralogical and trace element data indicate that the lithospheric mantle section represented by the xenoliths experienced a large-scale metasomatic event produced by upward migration of mafic silicate melts followed by percolation of low-T, alkali-rich melts and fluids. Chromatographic fractionation and fractional crystallisation of the melts close to the percolation front produced strong LREE-enrichments, which are most common in the uppermost mantle and are related to carbonate- and P₂O₅-rich derivatives of the initial melt. Reversal and gradual retreat of the percolation front during thermal relaxation to ambient geotherm (“retrograde” metasomatism) caused local migration and entrapment of small-volume residual fluids and precipitation of volatile-rich accessory minerals. A distinct metasomatic episode, which mainly produced “anhydrous” late-stage interstitial materials was concomitant with the alkali basaltic magmatism, which brought the xenoliths to the surface.     

Compositional variations and heterogeneity in fertile lithospheric mantle: peridotite xenoliths in basalts from Tariat,Mongolia

Clinopyroxene-rich, poorly metasomatised spinel lherzolites are rare worldwide but predominate among xenoliths in five Quaternary basaltic eruption centres in Tariat, central Mongolia. High-precision analyses of the most fertile Tariat lherzolites are used to evaluate estimates of primitive mantle compositions; they indicate Mg#_PM = 0.890 while lower Mg# in the mantle are likely related to metasomatic enrichments in iron. Within a 10 × 20 km area, and between ~45 and ≥60 km depth, the sampled xenoliths suggest that the Tariat mantle does not show km-scale chemical heterogeneities and mainly consists of residues after low-degree melt extraction at 1–3 GPa. However, accessory (<1%) amphibole and phlogopite are unevenly distributed beneath the eruption centres. Ca abundances in olivine are controlled by temperature whereas Al and Cr abundances also depend on Cr/Al in coexisting spinel. Comparisons of conventional and high-precision analyses obtained for 30 xenoliths show that high-quality data, in particular for whole-rocks and olivines, are essential to constrain the origin of mantle peridotites. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Weibel Instability and Deformation of an External Magnetic Field in the Region of Decay of a Strong Discontinuity in a Plasma with Hot Electrons

Geomagnetism and Aeronomy - In this paper, for a number of significantly different parameters of a localized plane layer of collisionless electron–proton plasma and an external magnetic field...     

Elaboration of a Regional Concept for Developing a Water Disposal System for the Central Ecological Zone of the Baikal Natural Territory

Water Resources - A unified concept of the elaboration of a water disposal system has been developed stage-by-stage for a large area in the central ecological zone of Baikal region with the use of...     

An Analytical Model for the Current Structure of the Magnetosheath Boundary in a Collisionless Plasma

Astronomy Letters - We have constructed an analytical model of a quasi-stationary current sheet in a collisionless plasma that describes the boundary of the magnetosheath formed by the solar...     

Global photometric analysis of Galactic HII regions

Total infrared fluxes are estimated for 99 HII regions around massive stars. The following wavebands have been used for the analysis: 8 and 24 μm, based on data from the Spitzer Space Telescope(IRAC and MIPS, respectively); 70, 160, 250, 350 and 500 μm, based on data from the Herschel Space Observatory(PACS and SPIRE). The estimated fluxes are used to evaluate the polycyclic aromatic hydrocarbon(PAH) mass fraction(qPAH) and the intensity of ultraviolet emission in the studied objects.It is shown that the PAH mass fraction, qPAH, is much lower in these objects than the average Galactic value, implying effective destruction of aromatic particles in HII regions. Estimated radiation field intensities(U) are close to those derived for extragalactic HII complexes. Color indices [F_(24)/F_8], [F_(70)/F_(24)],[F_(160)/F_(24)] and [F_(160)/F_(70)] are compared to criteria proposed to distinguish between regions of ionized hydrogen and planetary nebulae. Also, we relate our results to analogous color indices for extragalactic complexes of ionized hydrogen.