Carbonatitic melts along the solidus of model lherzolite in the system CaO-MgO-Al2O3-SiO2-CO2 from 3 to 7 GPa     

John A. Dalton  Dean C. Presnall 《Contributions to Mineralogy and Petrology》1998,131(2-3):123-135

We have experimentally determined the solidus position of model lherzolite in the system CaO-MgO-Al₂O₃-SiO₂-CO₂ (CMAS.CO₂) from 3 to 7 GPa by locating isobaric invariant points where liquid coexists with olivine, orthopyroxene, clinopyroxene, garnet and carbonate. The intersection of two subsolidus reactions at the solidus involving carbonate generates two invariant points, I_1A and I_2A, which mark the transition from CO₂-bearing to dolomite-bearing and dolomite-bearing to magnesite-bearing lherzolite respectively. In CMAS.CO₂, we find I_1A at 2.6 GPa/1230 °C and I_2A at 4.8 GPa/1320 °C. The variation of all phase compositions along the solidus has also been determined. In the pressure range investigated, solidus melts are carbonatitic with SiO₂ contents of <6 wt%, CO₂ contents of ˜45 wt%, and Ca/(Ca+Mg) ratios that range from 0.59 (3 GPa) to 0.45 (7 GPa); compositionally they resemble natural magnesiocarbonatites. Volcanic magnesiocarbonatites may well be an example of the eruption of such melts directly from their mantle source region as evidenced by their diatremic style of activity and lack of associated silicate magmas. Our data in the CMAS.CO₂ system show that in a carbonate-bearing mantle, solidus and near-solidus melts will be CO₂-rich and silica poor. The widespread evidence for the presence of CO₂ in both the oceanic and continental upper mantle implies that such low degree SiO₂-poor carbonatitic melts are common in the mantle, despite the rarity of carbonatites themselves at the Earth's surface. Received: 9 April 1997 / Accepted: 25 November 1997  相似文献   

西秦岭降扎地区金、铀矿床年代学对比研究   总被引：9，自引：0，他引：9       下载免费PDF全文

刘家军 《地质科学》1998,33(3):300-309

西秦岭降扎地区寒武、志留系中的金、铀矿床,是我国相当重要的碳硅泥岩型矿床。根据金矿床中岩石和矿石异常铅模式年龄、氩-氩同位素年龄和钾-氩同位素年龄等,获得了一批有益的年代学信息。金矿床的各种同位素年龄数据主要有2组:(1)242-186Ma;(2)137-47Ma.结合区域地质发展史、成矿热液脉与岩脉的穿切关系以及矿床产出的其它宏观特征等,确定金矿床的成矿时代为137-47Ma,与区域铀矿床形成的主要时代(117-55Ma)相比,二者成矿时间大体相同。  相似文献   

Stable isotopes and sediments from Pickerel Lake, South Dakota, USA: a 12ky record of environmental changes   总被引：2，自引：0，他引：2  

Antje Schwalb  Walter E. Dean 《Journal of Paleolimnology》1998,20(1):15-30

Sedimentological parameters and stable O- and C-isotopic composition of marl and ostracode calcite selected from a 17.7-m-long core from the 8-m-deep center of Pickerel Lake, northeastern South Dakota, provide one of the longest (ca. 12ky) paleoenvironmental records from the northern Great Plains. The late Glacial to early Holocene climate in the northern Great Plains was characterized by changes from cold and wet to cold and dry, and back to cold and wet conditions. These climatic changes were controlled by fluctuations in the positions of the Laurentide ice sheet and the extent of glacial Lake Agassiz. We speculate that the cold and dry phase may correspond to the Younger Dryas event. A salinity maximum was reached between 10.3 and 9.5 ka, after which Pickerel Lake shifted from a system controlled by atmospheric changes to a system controlled by groundwater seepage that might have been initiated by the final withdrawal of Glacial Lake Agassiz. A prairie lake was established at approximately 8.7 ka, and lasted until about 2.2 ka. During this mid-Holocene prairie period, drier conditions than today prevailed, interrupted by periods of increased moisture at about 8, 4, and 2.2 ka. Prairie conditions were more likely dry and cool rather than dry and warm. The last 2.2 ka are characterized by higher climatic variability with 400-yr aridity cycles including the Medieval Warm Period and the Little Ice Age.Although the signal of changing atmospheric circulation is overprinted by fluctuations in the positions of the ice sheet and glacial Lake Agassiz during the late Glacial-Holocene transition, a combination of strong zonal circulation and strong monsoons induced by the presence of the ice sheet and high insolation may have provided mechanisms for increased precipitation. Zonal flow introducing dry Pacific air became more important during the prairie period but seems to have been interrupted by short periods of stronger meridional circulation with intrusions of moist air from the Gulf of Mexico. More frequent switching between periods of zonal and meridional circulation seem to be responsible for increased climatic variability during the last 2.2 ka.  相似文献   

On conditional simulation to inaccurate data   总被引：5，自引：0，他引：5  

Dean S. Oliver 《Mathematical Geology》1996,28(6):811-817

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A natural resource damage assessment model for coastal and marine environments     

Thomas A. Grigalunas  James J. Opaluch  Deborah French  Mark Reed  Dean Knauss 《GeoJournal》1988,16(3):315-321

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Carbon dioxide,water vapor and sensible heat exchanges of a grain sorghum canopy   总被引：3，自引：0，他引：3  

Dean E. Anderson  Shashi B. Verma 《Boundary-Layer Meteorology》1986,34(4):317-331

With the aid of eddy correlation instrumentation, the components of the energy budget and CO₂ flux were measured over grain sorghum grown at Mead, Nebraska. Diurnal patterns of sensible heat, latent heat, CO₂ and momentum flux are examined for typical days. On a mostly clear day when the crop leaf area index was 3.7, net radiation reached a mid-day peak of 560 W m^-2, while sensible and latent heat fluxes peaked at 50 and 460 W m^-2, respectively. The peak CO₂ flux occurring just prior to solar noon was 1.5 mg m^-2(ground area) s^-1. CO₂ flux (respiration from plants, soil and roots) in the early evening was about -0.28 mg m^-2 (ground area) s^-1.A relationship between CO₂ flux and photosynthetically active radiation (PAR) was developed. Except during the late stage of growth (growth stage 8.5, toward the initiation of senescence), the crop showed no evidence of saturation up to PAR 1800 Ei m^-2s^-1. The light compensation point was found to be about 211 Ei m^-2s^-1. Examination of CO₂ flux-PAR relationships for selected days through the season indicated an aging effect in terms of a decrease in photosynthetic activity of the sorghum canopy. Measurements made on two consecutive days demonstrate the effects of weather conditions on CO₂ flux and carbon-water flux ratio (a measure of water use efficiency of the crop). The occurrence of regional sensible heat advection with concommitant high vapor pressure deficit and air temperature-limited CO₂ exchange reduced the carbon-water flux ratio.Published as Paper No. 7717, Journal Series, Nebraska Agricultural Research Division. The work was conducted under Regional Research Project 11-33 and Nebraska Agricultural Research Division Project 27-003.Formerly Post Doctoral Research Associate (now at the University of Connecticut Department of Renewable Natural Resources Storrs CT)  相似文献   

Fecundity of Atlantic menhaden,Brevoortia tyrannus     

Robert M. Lewis  Dean W. Ahrenholz  Sheryan P. Epperly 《Estuaries and Coasts》1987,10(4):347-350

Since the 1950’s the population structure of the Atlantic menhaden,Brevoortia tyrannus, has undergone dramatic changes. High rates of fishery exploitation during the 1960’s resulted in a drastic decline in the breadth of the age structure and numbers of spawning aged fish. The stock recovered under continued exploitation during the 1970’s in spite of these unfavorable conditions. Hence, the length and age at first spawning, the seasons and areas of spawning, and the potential number of ova produced as a function of length were reexamined during the 1978, 1979, and 1981 fishing seasons. No changes were detected in the reproductive biology of Atlantic menhaden. The size at first spawning, areas, and season of spawning were similar to results of previous studies. The minimum fork length of potential spawners was 180 mm. Few age-1 fish were mature but most females were mature at age-2. Analysis of the ovaries collected along the coast indicated some spawning occurred in the spring, a limited amount occurred in the summer, and most spawning occurred during the fall and winter. There was high intra-and interyear variability in the relationship between length and potential number of ova produced, and regression coefficients encompassed values from two previous studies, each based on a single fishing year. Data from this and the previous studies were combined and equally weighted to generate a representative predictive equation for the potential number of ova spawned as a function of length: eggs = 2563e^0.0150(FL).  相似文献   

Copper dissociation from estuarine humic materials     

Dean L. Olson  Mark S. Shuman 《Geochimica et cosmochimica acta》1985,49(6):1371-1375

Dissolved humic material from three locations on the Ogeechee River Estuary near Savannah, GA, was ultrafiltered into three size fractions and used for kinetic experiments with Cu(II). A Cu(II)-humic mixture was reacted with a colorimetric reagent for Cu(II) and absorbance observed from 50 msec to at least 1835 sec corresponding to rate constants from 0.001–40 sec^?1. The apparent dissociation rate constants were distributed over a wide range, with most bound Cu(II) having k > 1 sec^?1 ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{< 0.7 sec}$). Nearly all the variation seen in the kinetic distribution was among size fractions; as size fraction decreased, the distribution of bound Cu(II) shifted to larger rate constants. Location of sampling stations on the estuary had little effect on results.  相似文献   

The age and growth of young-of-the-year spot,Leiostomus xanthurus Lacépède,in South Carolina     

Daniel W. Beckman  John Mark Dean 《Estuaries and Coasts》1984,7(4):487-496

Young-of-the-year spot,Leiostomus xanthurus, were sampled in Town Creek of North Inlet Estuary, South Carolina, with towed plankton nets, seines, and stationary block nets from February 14 to May 2, 1981. Age estimates obtained by analysis of daily growth increments in the sagittal otoliths showed that these fish hatched between late November 1980 and mid-February 1981. The youngest spot was estimated to be 60 days old, indicating a prolonged migration period from offshore spawning grounds. Age-length regressions permitted calculations of growth and estimates of age from standard length measurements. An increase in growth rate was observed with an increase in size of fish; however, growth of fish of equal sizes was lower in April and early May than in February and March. Age estimates suggested that the hatching period for young-of-the-year spot was concentrated between late November and mid-January. Recruitment of three separate cohorts into Town Creek occurred at approximately monthly intervals, and immigration of each cohort into the estuary took place approximately 2 months after its estimated date-of-hatch.  相似文献   

Paleoamospheric temperature structure     

Dean A. Morss  William R. Kuhn 《Icarus》1978,33(1):40-49

Radiative equilibrium and radiative convective temperature profiles for the Earth's evolving atmosphere been have calculated. If the atmosphere evolved from one rich in carbon on dioxide, and deficient in oxygen, to its present composition, the temperature structure showed considerable change. The models of 3 to 4 billion years ago display steadily decreasing temperatures with altitude, being 185°K at pressures associated with the present-day upper stratosphere. A lapse rate feature similar to the present-day tropopause is not indicated until about 1 billion years ago; but the stratospheric region is approximately 15°K colder than presently found at comparable pressures. Surface temperatures approximately 10°K warmer than at present existed until nearly 1 billion years ago. When the oxygen content exceeded roughly 0.1 times the present level, surface temperatures began to decrease. If biological processes are important to carbon dioxide—ozone variations, such as has been suggested during the Ice Ages, then estimates of surface temperature should include the effects of both gases.  相似文献