Petrology and origin of amoeboid olivine aggregates in CR chondrites     

Michael K. WEISBERG  Harold C. CONNOLLY  Denton S. EBEL 《Meteoritics & planetary science》2004,39(10):1741-1753

Abstract— Amoeboid olivine aggregates (AOAs) are irregularly shaped, fine‐grained aggregates of olivine and Ca, Al‐rich minerals and are important primitive components of CR chondrites. The AOAs in CR chondrites contain FeNi metal, and some AOAs contain Mn‐rich forsterite with up to 0.7 MnO and Mn:Fe ratios greater than one. Additionally, AOAs in the CR chondrites do not contain secondary phases (nepheline and fayalitic olivine) that are found in AOAs in some CV chondrites. The AOAs in CR chondrites record a complex petrogenetic history that included nebular gas‐solid condensation, reaction of minerals with the nebular gas, small degrees of melting, and sintering of the assemblage. A condensation origin for the Mn‐rich forsterite is proposed. The Mn‐rich forsterite found in IDPs, unequilibrated ordinary chondrite matrix, and AOAs in CR chondrites may have had a similar origin. A type A calcium, aluminum‐rich inclusion (CAI) with an AOA attached to its Wark‐Lovering rim is also described. This discovery reveals a temporal relationship between AOAs and type A inclusions. Additionally, a thin layer of forsterite is present as part of the Wark‐Lovering rim, revealing the crystallization of olivine at the end stages of Wark‐Lovering rim formation. The Ca, Al‐rich nodules in the AOAs may be petrogenetically related to the Ca, Al‐rich minerals in Wark‐Lovering rims on type A CAIs. AOAs are chondrite components that condensed during the final stage of Wark‐Lovering rim formation but, in general, were temporally, spatially, or kinetically isolated from reacting with the nebula vapor during condensation of the lower temperature minerals that were commonly present as chondrule precursors.  相似文献   

Oxidative stress in ecotoxicology: from the analysis of individual antioxidants to a more integrated approach     

Regoli F  Gorbi S  Frenzilli G  Nigro M  Corsi I  Focardi S  Winston GW 《Marine environmental research》2002,54(3-5):419-423

An integrate approach will be discussed for investigations on oxidative stress in xenobiotic toxicity. While the analysis of individual antioxidants is useful for their sensitivity and to understand the mode of action of a stressor, the integration with the analysis of the total antioxidant capacity provides a more holistic assessment of the overall biological significance of such variations. TOSC has a greater predictive value on the health condition of the organisms and allows to discriminate the different role of specific ROS in oxidative stress syndrome.  相似文献   

Variability of surface roughness and turbulence intensities at a coastal site in India   总被引：4，自引：0，他引：4  

Radhika Ramachandran  J. Winston Jeeva Prakash  K. Sen Gupta  K. Narayanan Nair  P. K. Kunhikrishnan 《Boundary-Layer Meteorology》1994,70(4):385-400

Surface-layer features with different prevailing wind directions for two distinct seasons (Southwest Monsoon and Northeast Monsoon) on the west coast of India are studied using data obtained from tower-based sensors at a site located about 500 m from the coast. Only daytime runs have been used for the present analysis. The surface boundary-layer fluxes have been estimated using the eddy correlation method. The surface roughnessz ₀ obtained using the stability-corrected wind profiles (Paulson, 1970) has been found to be low for the Southwest monsson season. For the other season,z ₀ is relatively high. The drag coefficientC _D varies with height in the NE monsoon season but not in the season with lowz ₀. This aspect is reflected in the wind profiles for the two seasons and is discussed in detail. The scaling behaviour of friction velocityu _* and the turbulence intensity of longitudinal, lateral and vertical winds _u, _v and _w, respectively) are further examined to study their dependence on fetch. Our study shows that for the non-dimensional case, _u/u_* and _v/u_* do not show any surface roughness dependence in either season. On the other hand, for _w/u_* for the season with lowz ₀, the values are seen to agree well with that of Panofskyet al. (1977) for homogeneous terrain whereas for the other season with highz ₀, the results seem to conform more to the values observed by Smedman and Högström (1983) for coastal terrain. The results are discussed in the light of observations by other investigators.  相似文献   

Formation of refractory inclusions by evaporation of condensate precursors     

Lawrence Grossman  Denton S Ebel  Steven B Simon 《Geochimica et cosmochimica acta》2002,66(1):145-161

Berman’s (1983) activity-composition model for CaO-MgO-Al₂O₃-SiO₂ liquids is used to calculate the change in bulk chemical and isotopic composition during simultaneous cooling, evaporation, and crystallization of droplets having the compositions of reasonable condensate precursors of Types A and B refractory inclusions in CV3 chondrites. The degree of evaporation of MgO and SiO₂, calculated to be faithfully recorded in chemical and isotopic zoning of individual melilite crystals, is directly proportional to evaporation rate, which is a sensitive function of P_H2, and inversely proportional to the droplet radius and cooling rate. When the precursors are partially melted in pure hydrogen at peak temperatures in the vicinity of the initial crystallization temperature of melilite, their bulk chemical compositions evolve into the composition fields of refractory inclusions, mass-fractionated isotopic compositions of Mg, Si, and O are produced that are in the range of the isotopic compositions of natural inclusions, and melilite zoning profiles result that are similar to those observed in real inclusions. For droplets of radius 0.25 cm evaporating at P_H2 = 10⁻⁶ bar, precursors containing 8 to 13 wt.% MgO and 20 to 23% SiO₂ evolve into objects similar to compact Type A inclusions at cooling rates of 2 to 12 K/h, depending on the precise starting composition. Precursors containing 13 to 14 wt.% MgO and 23 to 26% SiO₂ evolve into objects with the characteristics of Type B1 inclusions at cooling rates of 1.5 to 3 K/h. The relatively SiO₂-poor members of the Type B2 group can be produced from precursors containing 14 to 16 wt.% MgO and 27 to 33% SiO₂ at cooling rates of <1 K/h. Type B2’s containing 27 to 35 wt.% SiO₂ and <12% MgO require precursors with higher SiO₂/MgO ratios at MgO > 15% than are found on any condensation curve. The characteristics of fluffy Type A inclusions, including their reversely zoned melilite, can only be understood in the context of this model if they contain relict melilite.  相似文献   

Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions: experiments, theoretical considerations, and constraints on their thermal evolution     

Frank M. Richter  Andrew M. Davis  Denton S. EbelAkihiko Hashimoto 《Geochimica et cosmochimica acta》2002,66(3):521-540

Experiments exposing Type B calcium-, aluminum-rich inclusion (CAI)-like melts at high temperatures to high vacuum or reducing hydrogen-rich gas mixtures were used to determine the rates and consequences of elemental and isotopic fractionation by evaporation. Silicon and magnesium were found to evaporate much faster than calcium and aluminum, and the resulting residual liquid trajectories in composition space are reproduced via a thermodynamic model for the saturation vapor pressure of the evaporating species. Isotopic fractionations associated with evaporation were measured for magnesium. The resulting relationship between fraction of magnesium lost and enrichment of the residue in the heavy isotopes of magnesium follows a Rayleigh fractionation curve with a fractionation factor that is close to, but not exactly, the theoretically expected value. The rate of evaporation is found to be a strong function of temperature, oxygen fugacity, and melt composition, which can be understood and modeled in terms of the dependence of the saturation vapor pressures on these variables. The relationship between evaporation rate, which we measure, and calculated saturation vapor involves empirical evaporation coefficients that we find to be significantly less than one (∼0.1). Analytical and numerical models are used to characterize how diffusion in both the melt and in the surrounding gas affects evaporation rates and the degree of chemical and isotopic fractionation. The experimental data and theoretical considerations are combined to give a parameterization of the rates and consequences of evaporation of Type B CAI-like liquids, which is then used to translate the measured isotopic fractionation of Type B CAIs into constraints on their thermal history. Cooling rates of the order of 10°C per hour are indicated.  相似文献   

Oxygen isotope and geochemical variations in the Missouri River     

W. Winston  R. Criss 《Environmental Geology》2003,43(5):546-556

Oxygen isotope ratios in streamflow of the Missouri River basin vary geographically due to differences in source precipitation and the integration of waters from upstream regions. Average '¹⁸O values in the Missouri River main stem systematically increase from less than -17‰ in the headwaters to about -9‰ in the lower basin. Seasonal variations at a given location result from fluctuations in meteoric precipitation, residence time in reservoirs and groundwater systems, evaporation, and snowmelt. Average water chemistry values are successfully predicted for the upstream stations of two reaches on the lower Missouri River based on changes in discharge along each reach and water quality measurements collected at the downstream stations. Source regions for some dissolved ions found in the lower Missouri River are also identified. Sodium and sulfate originate predominantly from the basin above Sioux City, Iowa, while nitrate is largely derived from agricultural regions below Sioux City.  相似文献   

中国中西部地区地震的重新定位和三维地壳速度结构   总被引：8，自引：1，他引：8       下载免费PDF全文

杨智娴  于湘伟  郑月军  陈运泰  倪晓晞  Winston CHAN 《地震学报》2004,26(1):19-29

采用中国中西部地区(21~36N， 98~112E)193个地震台在1992～1999年间记录到的9 988次地震的Pg和Sg震相走时的读数资料，用Roecker的SPHYPIT90程序，反演了该地区三维地壳P波速度结构，并用SPHREL3D90程序进行了地震的重新定位．反演结果揭示了中国中西部地区地震P波速度结构明显的横向不均匀性，这些不同深度上波速的横向变化多以该地区的活动断裂为分界线．可以看出活动断裂两侧存在明显的速度反差．通过重新定位，得到了6 459次地震的震源参数，这些精确定位的地震震中明显沿该区活动断裂呈现条带状分布，其范围和尺度清晰地表示了这一地区地震活动与活动断裂的紧密关系．其中，82％重新精确定位的事件的震源深度在20 km以内. 这一结果与笔者用双差地震定位法得到的重新定位的震源深度分布相一致．   相似文献   

Earthquake relocation and 3-dimensional crustal structure of P-wave velocity in cen-tral-western China     

杨智娴  于湘伟  郑月军  陈运泰  倪晓晞  Winston CHAN 《地震学报(英文版)》2004,17(1):20-30

IntroductionUnderstandingthemechanismofcontinentalearthquakesisveryimportantforseismichaz-ardpreventionandearthquakeprediction.Themodernseismotectonictheoryandtheideaofearthquakepredictionaredevelopedmainlyfromthestudiesoninterplateearthquakes,whicharedifficulttoexplainthephenomenaofintraplateearthquakes,suchasthecontinentalearthquakesoccurredinChinesemainland.Whiletheinterplateearthquakesoccurredalongtheplatebounda-ries,theintraplateearthquakesdistributediffuselyintheinterioroftheplates.Thus…  相似文献   

Estimation of flood damage functions for river basin planning: a case study in Bangladesh     

Y. C. Ethan Yang  Patrick A. Ray  Casey M. Brown  Abedalrazq F. Khalil  Winston H. Yu 《Natural Hazards》2015,75(3):2773-2791

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Milankovitch theory of ice ages: Hypothesis of ice-sheet linkage between regional insolation and global climate     

George H. Denton  Terence J. Hughes 《Quaternary Research》1983,20(2):125-144

J. D. Hays, J. Imbrie, and N. J. Shackleton (1976, Science194, 1121–1132) showed that the astronomical theory explained many features of late Quaternary ice-age climates, but they did not specify the physical mechanisms involved. Here it is proposed that interlocked variations of ice-sheet heat sinks in both polar hemispheres amplified and transmitted Milankovitch summer half-year insolation changes (a version of the astronomical theory) between 45° and 75°N into the globally synchronous climate changes recorded in geologic records. It is suggested that late Quaternary ice sheets had terrestrial components (grounded above sea level, melting margins, fluctuations controlled by climate) and marine components (grounded below sea level, drained largely by ice streams, limited melting margins, fluctuations controlled primarily by sea level and secondarily by climate, interior surface elevations coupled to downdraw through ice streams). Northern Hemisphere ice sheets were largely marine (with minor melting margins) in the Arctic and terrestrial (with major melting margins) in the midlatitudes. West Antarctic and peripheral East Antarctic ice was marine-based and lacked melting margins. Because of their geographic array, these terrestrial and marine components formed an ice-sheet system whose variations were coupled on a global scale. Milankovitch summer isolation changes near midlatitude Northern Hemisphere melting margins controlled most variations of this system, because advance or retreat of melting margins initiated concurrent eustatic sea-level change. Such sea-level change afforded the critical interlocking mechanism between terrestrial and marine components because it forced simultaneous expansion or contraction of marine margins in both polar hemispheres. This initiated an amplifying feedback loop among all marine components and influenced interior downdraw through ice streams. Arctic summer insolation change was less important because northern melting margins were relatively minor. Its greatest influence was on surface ablation of ice streams that controlled interior downdraw. This affected eustatic sea level and activated global linkage of marine sectors. By analogy with present-day Antarctica, late Quaternary ice sheets were enormous planetary heat sinks due to their reflective and radiative surface characteristics. It is suggested that the effectiveness of these ice-sheet heat sinks varied with their areal extent and interior surface elevation. Thus, it is postulated that concurrent growth or decay of these interlocked ice-sheet heat sinks in both polar hemispheres served as the global amplifier of regional Milankovitch summer insolation.  相似文献