Diffuse band shapes in both extinction and polarization are calculated for interstellar coremantle particles for varying size distributions of mantle thickness. It is shown that no matter whether the source of the bands is in the silicate cores or the accreted icy mantles the polarization shapes are highly asymmetric for all mantel thicknesses. The extinction band shapes are significantly less asymmetric although the effect is clearly present. The only apparent possibility for producing symmetric band shapes in the dust grains is in the very small bare particles in interstellar space which, if they are aligned and produce the 2200 band, must exhibit a strong polarization effect in this region.Work supported in part by NASA Grant NGR-33-011-043.Paper presented at the Symposium on Solid State Astrophysics, held at the University College, Cardiff, Wales, between 9–12 July, 1974. 相似文献
A multiproxy approach (textural characteristics, heavy metal concentrations and benthic foraminifera data) was adopted to study the OMEX core KSGX 40, collected at the Galicia Mud Deposit (NW Iberian outer continental shelf) and recording the last ca. 4.8 ka cal BP. Geochemical profiles of Fe, Mn, Zn, Co, Cr, Cu, Ni, Pb, Al and Ca show significant temporal variations. Benthic foraminifera productivity proxy suggests that the recorded muddy events were contemporaneous of a higher Corg flux to the marine benthic environment. The higher flux of organic matter, following a long-term intensification of the upwelling and coinciding with a finer sedimentation, led to depressed levels of oxygen beneath the water-sediments interface and to early diagenetic processes of several redox-sensitive elements. 相似文献
The eastern part of the Guiana Shield, northern Amazonian Craton, in South America, represents a large orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga), which consists of extensive areas of Paleoproterozoic crust and two major Archean terranes: the Imataca Block, in Venezuela, and the here defined Amapá Block, in the north of Brazil.
Pb-evaporation on zircon and Sm–Nd on whole rock dating were provided on magmatic and metamorphic units from southwestern Amapá Block, in the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80–2.79 Ga) and during the Neoarchean (2.66–2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26–2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian orogenic cycle, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated at 2.22 Ga, 2.18 Ga and 2.05–2.03 Ga. Most of the εNd values and TDM model ages (2.52–2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components.
The Archean Amapá Block is limited in at southwest by the Carecuru Domain, a granitoid-greenstone terrane that had a geodynamic evolution mainly during the Paleoproterozoic, related to the Transamazonian orogenic cycle. In this latter domain, a widespread calc-alkaline magmatism occurred at 2.19–2.18 Ga and at 2.15–2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the eastern Guiana Shield. Nevertheless, TDM model ages (2.50–2.38 Ga), preferentially interpreted as mixed ages, and εNd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. In addition, the Carecuru Domain contains an oval-shaped Archean granulitic nucleus, named Paru Domain. In this domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism are recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga.
The lithological association and the available isotopic data registered in the Carecuru Domain suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian orogenic cycle, which was accreted to the southwestern border of the Archean Amapá Block. 相似文献
Mössbauer spectra (MS) of blue, green and yellow beryl (ideally Be3Al2Si6O18) containing approximately 1% of iron were obtained at 295 and 500 K. Room temperature (RT) spectra of both blue and green samples showed the presence of an asymmetric Fe2+ doublet (ΔEQ~2.7 mm/s, δ~1.1 mm/s), with a very broad low-velocity peak. There is no clear evidence for the presence of a ferric component. The MS of the yellow sample at RT consists of an intense central absorption with parameters typical for Fe3+ (ΔEQ~0.4 mm/s, δ~0.29 mm/s), plus an apparently symmetrical Fe2+ doublet. This sample acquires a light-blue shade upon heating in air at about 620 K. Thermal treatments at high temperatures caused no significant changes in the MS, but the green and yellow beryl acquire a blue colour. All these results are interpreted in relation to the existence of channel water and the distribution of iron among the available crystallographic sites. 相似文献
A detrital zircon aged 4.1 Ga is discovered by the SHRIMP U-Pb method in a quartzite in Burang County, western Tibet. This is presently the oldest single-grain detrital zircon in China. The Th-U ratios of the two testing points of the 〉4.0 Ga zircon are between 0.76 and 0.86, indicating their magmatic origin. This discovery has offered an important age for investigating the geological evolution of the Qinghai-Tibet Plateau. 相似文献