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991.
Brian R. Jicha Clark M. Johnson Wes Hildreth Brian L. Beard Garret L. Hart Steven B. Shirey Brad S. Singer 《Earth and Planetary Science Letters》2009,277(1-2):38-49
A suite of 23 basaltic to dacitic lavas erupted over the last 350 kyr from the Mount Adams volcanic field has been analyzed for U–Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (230Th/238U) > 1 and span a large range in (230Th/232Th) ratios, and most basalts have higher (230Th/232Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U–Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20–25 ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350 ka consists of lavas that have small to moderate 230Th excesses (2–16%), which are likely inherited from melting of a garnet-bearing intraplate (“OIB-like”) mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by 187Os/188Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os–Th isotope variations suggest that unusually large 230Th excesses (25–48%) and high 187Os/188Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic’ processes has been decoupled from shallow-level crystallization. 相似文献
992.
西藏林芝至通麦一带出露大面积变质岩,变质岩中含长柱状锆石。长柱状锆石晶体形态特征、矿物包裹体、CL图像及Th/U比值在0.1~0.4之间,表明锆石主要来自火山岩,受后期变质流体改造。长柱状锆石ELA-ICP-MSU-Th-Pb年龄为(40.7±0.3)Ma,MSWD=2.35。变质流体改造边缘部分年龄和核部年龄基本一致。长柱状锆石特征及年龄表明长青温池至通麦一带片麻岩原岩为新生代含火山物质的沉积岩而不是元古宙变质基底。 相似文献
993.
Martyn P. Clark David E. Rupp Ross A. Woods Xiaogu Zheng Richard P. Ibbitt Andrew G. Slater Jochen Schmidt Michael J. Uddstrom 《Advances in water resources》2008
This paper describes an application of the ensemble Kalman filter (EnKF) in which streamflow observations are used to update states in a distributed hydrological model. We demonstrate that the standard implementation of the EnKF is inappropriate because of non-linear relationships between model states and observations. Transforming streamflow into log space before computing error covariances improves filter performance. We also demonstrate that model simulations improve when we use a variant of the EnKF that does not require perturbed observations. Our attempt to propagate information to neighbouring basins was unsuccessful, largely due to inadequacies in modelling the spatial variability of hydrological processes. New methods are needed to produce ensemble simulations that both reflect total model error and adequately simulate the spatial variability of hydrological states and fluxes. 相似文献
994.
The process of uptake, metabolism and eventual biliary excretion of xenobiotics by the teleost liver affords an opportunity for alteration within at least the following cells: sinusoidal endothelial, perisinoidal fat-storing (vitamin A-containing),1 hepatocytes and biliary epithelial (of preductules, ductules and intrahepatic ducts). Our understanding of the existence and role(s) in physiologic and pathobiologic phenomena of non-hepatocytic populations of cells in teleost liver has been hampered by the use of immersion fixation procedures obviating resolution and analysis of sinusoidal lining cells and by the subjective nature of light and electron micrographs. The purpose of this study was to quantitatively define the rainbow trout (Salmo gairdneri, Richardson) liver. Computer-assisted morphometry2 was conducted on livers of actively spawning 5 year old male and female trout (Wytheville strain). 相似文献
995.
A major task in developing a management strategy for the Baltic Sea will be the establishment of a program that adheres to the philosophy of sustainable use and is capable of implementation. For many nations this has proven to be a very difficult task marked with successes and failures. This paper discusses North Carolina's attempts to move toward sustainability in its Albemarle and Pamlico estuaries. The paper concludes with some lessons learned that could be helpful to the Baltic region as it strives for sustainability. 相似文献
996.
Allen P. Nutman Vickie C. Bennett Clark R. L. Friend Marc D. Norman 《Contributions to Mineralogy and Petrology》1999,137(4):364-388
In the Itsaq Gneiss Complex south of the Isua supracrustal belt (West Greenland) some areas of early Archaean tonalite and
quartz-diorite are non-gneissic, free of pegmatite veins, and in rarer cases are undeformed with relict igneous textures and
hence were little modified by heterogeneous ductile deformation under amphibolite facies conditions in several Archaean events.
Such well-preserved early Archaean rocks are extremely rare. Tonalites are high Al, and have bulk compositions close to experimental
liquids. Trace element abundances and modelling suggest that they probably originated as melts derived from basaltic compositions
at sufficiently high pressures to require residual garnet + amphibolites ± clinopyroxene in the source. The major element
characteristics of the quartz-diorites suggest these were derived from more mafic magmas than the tonalites, and underwent
either igneous differentiation or mixing with crustal material. As in modern arc magmas, high relative abundances of Sr, Ba,
Pb, and alkali elements cannot be generated simply from a basaltic source formed by large degrees of melting of a depleted
mantle. This may indicate an important role for fluids interacting with mafic rocks in generating the earliest preserved continental
crust. The high Ba/Th, Ba/Nb, La/Nb and low Nb/Th, Ce/Pb, and Rb/Cs ratios of these tonalites are also observed in modern
arc magmas. SHRIMP U-Pb zircon geochronology was undertaken on seven tonalites, one quartz-diorite, a thin pegmatitic vein
and a thin diorite dyke. Cathodoluminescence images show the zircon populations of the quartz-diorite and tonalites are dominated
by single-component oscillatory-zoned prismatic grains, which gave ages of 3806 ± 5 to 3818 ± 8 Ma (2σ) (quartz-diorite and
5 tonalites) and 3795 ± 3 Ma (1 tonalite). Dating of recrystallised domains cutting oscillatory-zoned zircon indicates disturbance
as early as 3800–3780 Ma. There are rare ca. 3600 Ma and 3800–3780 Ma (very high U and low Th/U) ≤ 20 μm wide partial overgrowths
on the prismatic grains. Given likely Zr-undersaturation of precursor melts and evidence of zircon recrystallisation and metamorphic
regrowth as early as 3800–3780 Ma, the age determinations on the prismatic oscillatory-zoned zircon populations give the igneous
crystallisation age of the tonalite and quartz-diorite protoliths. When the coherency of the geochemistry is considered, these
samples represent the best preserved suites of ca. 3800 Ma felsic igneous rocks yet documented.
Received: 1 December 1998 / Accepted: 23 July 1999 相似文献
997.
998.
Realistic simulation of the internal variability of the climate system is important both for climate change detection and
as an indicator of whether the physics of the climate system is well-represented in a climate model. In this work zonal mean
atmospheric temperatures from a control run of the second Hadley Centre coupled GCM are compared with gridded radiosonde observations
for the past 38 years to examine how well modelled and observed variability agree. On time scales of between six months and
twenty years, simulated and observed variability of global mean temperatures agree well for the troposphere, but in the equatorial
stratosphere variability is lower in the model than in the observations, particularly at periods of two years and seven to
twenty years. We find good agreement between modelled and observed variability in the mass-weighted amplitude of a forcing-response
pattern, as used for climate change detection, but variability in a signal-to-noise optimised fingerprint pattern is significantly
greater in the observations than in a model control run. This discrepancy is marginally consistent with anthropogenic forcing,
but more clearly explained by a combination of solar and volcanic forcing, suggesting these should be considered in future
`vertical detection' studies. When the relationship between tropical lapse rate and mean temperature was examined, it was
found that these quantities are unrealistically coherent in the model at periods above three years. However, there is a clear
negative lapse rate feedback in both model and observations: as the tropical troposphere warms, the mid-tropospheric lapse
rate decreases on all the time scales considered.
Received: 11 August 1998 / Accepted: 20 July 1999 相似文献
999.
1000.
Richard Dame Merryl Alber Dennis Allen Michael Mallin Clay Montague Alan Lewitus Alice Chalmers Robert Gardner Craig Gilman Björn Kjerfve Jay Pinckney Ned Smith 《Estuaries and Coasts》2000,23(6):793-819
Estuaries of the southeastern Atlantic coastal plain are dominated by shallow meso-tidal bar-built systems interspersed with shallow sounds and both low flow coastal plain and high flow piedmont riverine systems. Three general geographical areas can be discriminated: the sounds of North Carolina; the alternating series of riverine and ocean dominated bar-built systems of South Carolina, Georgia, and northeast Florida, and the subtropical bar-built estuaries of the Florida southeast coast. The regional climate ranges from temperate to subtropical with sea level rise and hurricanes having a major impact on the region's estuaries because of its low and relatively flat geomorphology. Primary production is highest in the central region. Seagrasses are common in the northern and southern most systems, while intertidal salt marshes composed ofSpartina alterniflora reach their greatest extent and productivity in South Carolina and Georgia. Nuisance blooms (cyanobacteria, dinoflagellates, and cryptomonads) occur more frequently in the northern and extreme southern parts of the region. Fishery catches are highest in the North Carolina and Florida areas. Human population growth with its associated urbanization reaches a maximum in Florida and it is thought that the long-term sustainability of the Florida coast for human habitation will be lost within the next 25 years. Tidal flushing appears to play an important role in mitigating anthropogenic inputs in systems of moderate to high tidal range, i.e., the South Carolina and Georgia coasts. The most pressing environmental problems for the estuaries of the southeastern Atlantic coast seem to be nutrient loading and poor land use in North Carolina and high human population density and growth in Florida. The future utilization of these estuarine systems and their services will depend on the development of improved management strategies based on improved data quality. 相似文献