The Chelyabinsk meteorite: New insights from a comprehensive electron microscopy and Raman spectroscopy study with evidence for graphite in olivine of ordinary chondrites     

David Kaeter  Martin A. Ziemann  Ute Böttger  Iris Weber  Lutz Hecht  Sergey A. Voropaev  Alexander V. Korochantsev  Andrey V. Kocherov 《Meteoritics & planetary science》2018,53(3):416-432

We present results of petrographic, mineralogical, and chemical investigations of three Chelyabinsk meteorite fragments. Three distinct lithologies were identified: light S3 LL5, dark S4–S5 LL5 material, and opaque fine-grained former impact melt. Olivine–spinel thermometry revealed an equilibration temperature of 703 ± 23 °C for the light lithology. All plagioclase seems to be secondary, showing neither shock-induced fractures nor sulfide-metal veinlets. Feldspathic glass can be observed showing features of extensive melting and, in the dark lithology, as maskelynite, lacking melt features and retaining grain boundaries of former plagioclase. Olivine of the dark lithology shows planar deformation features. Impact melt is dominated by Mg-rich olivine and resembles whole-rock melt. Melt veins (<2 mm) are connected to narrower veinlets. Melt vein textures are similar to pegmatite textures showing chilled margins, a zone of inward-grown elongated crystals and central vugs, suggesting crystallization from supercooled melt. Sulfide-metal droplets indicate liquid immiscibility of both silicate and sulfide as well as sulfide and metal melts. Impact melting may have been an important factor for differentiation of primitive planetary bodies. Graphite associated with micrometer-sized melt inclusions in primary olivine was detected by Raman mapping. Carbon isotopic studies of graphite could be applied to test a possible presolar origin.  相似文献   

Nepheloid layer dynamics in the northern Portuguese shelf   总被引：1，自引：0，他引：1  

A. Oliveira  J. Vitorino  A. Rodrigues  J. M. Jouanneau  J. A. Dias  O. Weber 《Progress in Oceanography》2002,52(2-4)

A general hydrographic and nephelometric survey of the NW Portuguese continental shelf and upper slope was undertaken, under winter and spring conditions. The nepheloid layer dynamics along the shelf during three cruises were controlled, principally by the following factors: (a) the hydrography of the shelf-slope waters, i.e. the nepheloid layers followed isopycnals and water masses; (b) prevalence of upwelling or downwelling circulation over the shelf; (c) dispersion of material by river discharge (mainly the Douro river); (d) resuspension of mid-shelf fine deposits induced by swell; (e) peculiar morphology with the presence of Porto Canyon and outer shelf rock outcrops.The general circulation controls the seaward extension of the nepheloid layers. In winter, under dominant downwelling conditions, an intense bottom nepheloid layer (BNL) was observed on the shelf, due to river borne particle supply and remobilization of mid-shelf muddy sediments (depth ≈100 m). In this period the BNL increased in thickness to the top of the slope, following the isopycnals. Near the shelf-break the BNL detached to form intermediate nepheloid layers (INL). Small INLs appear deeper in some areas of the slope. A surface nepheloid layer (SNL) appears in the surface water over the shelf and slope. In spring, biological particles mainly contribute to the SNL, which is separated from the BNL by a zone of clear water. In winter the SNL is restricted to the inner shelf where there are high inputs of fluvial particles.Downwelling circulation probably induces transport of shelf particles to deeper waters in the BNL. In spring, the predominant circulation was southward (upwelling), the water column was highly stratified, and dispersion of particles in the SNL was offshore.  相似文献