Early Cretaceous volcaniclastic and quartzose sandstones from north central Nepal: composition,sedimentology and geotectonic significance     

Sören B. Dürr  Martin R. Gibling 《International Journal of Earth Sciences》1994,83(1):62-75

Early Cretaceous sandstones and shales from the Thakkhola region (Chukh and Tangbe Formations) were deposited on the subsiding northern (Tethyan) margin of Gondwana; current directions indicate a source area to the south. Whereas the Berriasian Chukh Formation is dominated by quartz arenites, volcaniclastic sandstones and minor conglomerates dominate the Tangbe Formation of Valanginian to Aptian age. Upsection, the composition of the volcaniclastic components changes from mainly basaltic to dominantly dacitic/rhyolitic. Magmatism was preceded and accompanied by strong uplift in the source area of the Chukh and Tangbe clastics, resulting in the erosion of probably more than several hundred meters of pre-Cretaceous sedimentary, metamorphic and plutonic rocks. A geochemical whole rock analysis of basaltic pebble fragments shows their alkaline character and suggests a within-plate geotectonic setting for the volcanic source. This volcanotectonic event is probably related to rifting between Australia and Greater India, where seafloor spreading began in late Valanginian/Hauterivian times.  相似文献   

Site occupation and electric field gradient in acentric neptunite-measurements and evaluations concerning powder- and single crystal – Mössbauer spectroscopy and x-ray diffractometry     

Werner Lottermoser  Karl Forcher  Georg Amthauer  Martin Kunz  Thomas Armbruster 《Physics and Chemistry of Minerals》1997,24(1):2-6

Mössbauer measurements on neptunite (KNa₂Li(Fe,Mn,Mg)₂Ti₂Si₈O₂₄) at 400?K reveal the distribution of Fe-ions on the crystallographic sites in agreement with neutron diffraction results published elsewhere. Even the previously postulated small amount of Fe-ions on the Ti(2) site has been detected, combined with a charge transfer which is in agreement with optical absorption investigations by other authors. A qualitative site occupation model is able to explain the different features of our observations. Single crystal Mössbauer measurements with the k-vector of incident γ-rays parallel to the crystallographic b-axis (space group Cc) of neptunite at different temperatures yield the angle β between the main component of the electric field gradient (EFG), V _zz and b. This angle is in close accordance with a calculated value of β for the Fe(1) position from electron density maps. The latter also reveal an absolute value of V _zz which is in satisfactory agreement with V _zz derived from Mössbauer spectroscopy.  相似文献   

Channel overflows of the Pōhue Bay flow,Mauna Loa,Hawai'i: examples of the contrast between surface and interior lava     

Zinzuni Jurado-Chichay  Scott K. Rowland 《Bulletin of Volcanology》1995,57(2):117-126

A number of overflows from a large lava channel and tube system on the southwest rift zone of Mauna Loa were studied. Initial overflows were very low viscosity gas-rich phoehoe evidenced by flow-unit aspect ratios and vesicle sizes and contents. Calculated volumetric flow-rates in the channel range between 80 and 890 m³/s, and those of the overflows between 35 and 110 m³/s. After traveling tens to hundreds of meters the tops of these sheet-like overflows were disrupted into a surface composed of clinker and phoehoe fragments. After these 'a' overflows came to rest, lava from the interiors was able to break out on to the surface as phoehoe. The surface structure of a lava flow records the interaction between the differential shear rate (usually correlated with the volumetric flow-rate) and viscosity-induced resistance to flow. However, the interior of a flow, being better insulated, may react differently or record a later set of emplacement conditions. Clefts of toothpaste lava occurring within fields of clinker on proximal-type 'a' flows also record different shear rates during different times of flow emplacement. The interplay between viscosity and shear rate determines the final morphological lava type, and although no specific portion of lava ever makes a transition from 'a' back to phoehoe, parts of a flow can appear to do so.  相似文献   

The circumstellar shells of carbon miras     

Martin Groenewegen 《Astrophysics and Space Science》1995,224(1-2):321-324

The relation between mass loss rate and pulsation period in carbon Miras is discussed. The dust mass loss rate is very low (about 2 × 10^–10 M/yr) up to aboutP = 380 days, where there is a sudden increase. ForP > 400 days there is a linear relation between logM andP. The change in the mass loss rate near 380 days may be related to radiation pressure on dust becoming effective in driving the outflow.  相似文献   

Temperature fluctuations in interstellar dust grains     

Per A. Aannestad  Scott J. Kenyon 《Astrophysics and Space Science》1979,65(1):155-166

Temperature fluctuations in interstellar dust grains caused by absorption of ultraviolet photons are discussed. Temperature probability distributions are presented for various assumptions about the grain specific heat and far infrared emissivity. Grains smaller than or about 0.01 may suffer large temperature fluctuations and would spend most of their time at very low temperatures. The equilibrium temperature is not a good measure for most average temperatures of such grains. It is shown that, although small grains spend only a small fraction of their time at the higher temperatures, the emitted far infrared spectrum is significantly shifted towards shorter wavelengths than predicted for grains assumed to be at the equilibrium temperature.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.  相似文献   

Study of the post-flare loops on 29 July 1973     

Sara F. Martin 《Solar physics》1979,64(1):165-176

Bright and dark curvilinear structures observed between the two major chromospheric ribbons during the flare of 29 July 1973 on films from the Big Bear Solar Observatory are interpreted as a typical system of coronal loops joining the inner boundaries of the separating flare ribbons. These observations, made through a 0.25 Å H filter, only show small segments of the loops having Doppler shifts within approximately ± 22 km s^–1 relative to the filter passband centered at H, H -0.5 Å or H +0.5 Å. However, from our knowledge of the typical behavior of such loop systems observed at the limb in H and at 5303 Å, it has been possible to reconstruct an appoximate model of the probable development of the loops of the 29 July flare as they would have been viewed at the limb relative to the position of a prominence which began to erupt a few minutes before the start of the flare. It is seen that the loops ascended through the space previously occupied by the filament. On the assumption that H fine structures parallel the magnetic field, we can conclude that a dramatic reorientation of the direction of the magnetic field in the corona occurred early in the flare, subsequent to the start of the eruption of the filament and prior to the time that the H loops ascended through the space previously occupied by the filament.  相似文献   

The study of comets: Parts 1 and 2. Proceedings of a conference held at Goddard Space Flight Center,Greenbelt, Md., October 28–November 1, 1974. Edited by B. Donn,M. Mumma,W. Jackson,M. A'Hearn,and R. Harrington. NASA SP-393, U.S. Government Printing Office,Washington, D.C. 1083 pp. $11.25     

Martin Harwit 《Icarus》1977,30(2):435-436

  相似文献   

Petrogenesis and paleotectonic history of the Wild Bight Group,an Ordovician rifted island arc in central Newfoundland   总被引：1，自引：0，他引：1  

H. Scott Swinden  G. A. Jenner  B. J. Fryer  J. Hertogen  J. C. Roddick 《Contributions to Mineralogy and Petrology》1990,105(2):219-241

The Wild Bight Group (WBG) is a sequence of early and middle Ordovician volcanic, subvolcanic and epiclastic rocks, part of the Dunnage Tectonostratigraphic Zone of the Newfoundland Appalachians. A detailed geochemical and Nd-isotopic study of the volcanic and subvolcanic rocks has been carried out to determine the geochemical characteristics of the rocks, interpret their palcotectonic environments and constrain their petrogenetic history. The lower and central stratigraphic levels of the WBG contain mafic volcanic rocks with island-arc geochemical signatures, including LREE-enriched are tholeiites with _Nd(t) =-0.1 to +2.2 (type A-I), LREE-depleted arc tholeiites with _Nd(t) =+5.6 to +7.1 (type A-II) and an unusual suite of strongly incompatible-element depleted tholeiites in which _Nd(t) ranges from-0.9 to +4.6 and is negatively correlated with¹⁴⁷Sm/¹⁴⁴Nd (type A-III). High-silica, low-K rhyolites occur locally in the central part of the stratigraphy, associated with mafic rocks of arc affinity, and have _Nd(t) =+4.7 to +5.4. The upper stratigraphic levels of the WBG dominantly contain rocks with non-arc geochemical signatures, including alkalic basalts with _Nd(t) =+4.6 to +5.5 (type N-I), strongly LREE- and incompatible element-enriched tholeiites that are transitional between alkalic and non-alkalic rocks with _Nd(t) =+4.4 to +7.0 (type N-II) and rocks with flat to slightly LREE-enriched patterns and _Nd(t) =+5.1 to +7.4 (type N-III). Rocks with non-arc and arc signatures are locally interbedded near the stratigraphic type of the WBG. Nd-isotopic data in the type A-I and A-II rocks are generally compatible with mixing/partial melting models involving depleted mantle, variably contaminated by a subducted crustally-derived sediment. The petrogenesis of type A-III rocks must involve source mixing and multi-stage partial melting, but the details are not clear. The geochemistry and Nd isotope data for types N-I, N-II and N-III rocks are compatible with petrogenetic models involving variable partial melting of a source similar to that postulated for modern oceanic island basalts. Comparison of the WBG with modern analogues suggests a 3-stage developmental model: stage 1) island-arc volcanism (eruption of type mafic volcancs); stage 2) arc-rifting (continued eruption of type A-I, A-I, eruption of types A-II and A-III mafic volcanics and high-silica, low-K rhyolites); and stage 3) back-arc basin volcanism (continued minor eruption of type A-I basalts, eruption of types N-I, N-II, N-III basalts). Stages 1 and 2 volcanism involved partial melting of subduction contaminated mantle, while stage 3 volcanism utilized depleted-mantle sources not affected by the subducting slab. This model provides a basis for interpreting coeval sequences in central Newfoundland and a comparative framework for some early Paleozoic oceanic volcanic sequences elsewhere in the Appalachian orogen.  相似文献   

Primary fractionation of elements among iron meteorites     

Edward R.D. Scott 《Geochimica et cosmochimica acta》1978,42(9):1447-1458

The primary fractionation process in iron meteorites is that responsible for the distribution of elements between the groups, most notably Ga and Ge, which show concentration ranges of 10³ and 10⁴ respectively. To investigate the cause of the primary fractionation, concentrations of 16 elements were converted to relative abundances by dividing the element/Ni ratio by the CI chondrite ratio. These abundances were plotted on logarithmic graphs with data for each group (except IB and IIICD) and each cluster of closely related anomalous irons averaged.Co, P, Au, As, Cu, Sb, Ge and Zn are positively correlated with Ga. For most groups (except IA, IC and IIAB) relative abundances of these elements tend to decrease from about 1 in approximately the order listed above. This is the expected order in which these elements will condense into Fe, Ni during equilibrium nebular condensation. Mean relative abundances of refractory elements in groups generally lie within a narrow range of 0.5–2, and are uncorrelated with Ga. Although the equilibrium model may be only a gross approximation, it suggests that most primary fractionation did occur during nebular condensation.The anomalous irons are essential for defining many of the primary fractionation trends. On several element-Ga graphs the displacements of the anomalous irons from the primary curves indicate that these irons experienced the same secondary fractionation process (probably fractional crystallization) that produced the trends within most groups. The anomalous irons appear to be samples from over 50 minor groups, which have similar histories to the 12 major groups.  相似文献   

Relationship of intertidal surface sediment chlorophyll concentration to hyperspectral reflectance and chlorophyll fluorescence     

Jacco C. Kromkamp  Edward P. Morris  Rodney M. Forster  Claire Honeywill  Scott Hagerthey  David M. Paterson 《Estuaries and Coasts》2006,29(2):183-196

Estimating biomass of microphytobenthos (MPB) on intertidal mud flats is extremely difficult due to their patchy occurrence, especially at the scale of an entire mud flat. We tested two optical approaches that can be applied in situ: spectral reflectance and chlorophyll fluorescence. These two approaches were applied in 4 European estuaries with different sediment characteristics. At each site, paired replicate measurements of hyperspectral reflectance, chlorophyll fluorescence (after 15 min dark adaptation, F_o ¹⁵), sediment water content, and chlorophylla concentrations were taken (including breakdown products: [chla+phaeo]). Sediments were further characterized by grain size and organic content analysis. The spectral signatures of tidal flats dominated by benthic microalgae, mainly diatoms, could be easily distinguished from sites dominated by macrophytes; we present a 3 waveband algorithm that can be used to detect the presence of macrophytes. The normalized difference vegetation index (NDVI) was found to be most strongly correlated to sediment [chla+phaeo], except for the predominantly sandy Sylt stations. F_o ¹⁵ was also significantly correlated to sediment [chla+phaeo] in all but one grid (Sylt grid A). Our results suggest that the functional relationships (i.e., the slopes) between NDVI or fluorescence and [chla+ phaeo] were not significantly different in the muddier grids, although the intercepts could differ significantly, especially for F_o ¹⁵. This suggests a mismatch of the optical depth seen by the reflectometer or fluorometer and the depth sampled for pigment analysis. NDVI appears to be a robust proxy for sediment [chla+phaeo] and can be used to quantify MPB biomass in muddy sediments of mid latitude estuaries.  相似文献