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111.
Yen-Hong Shau Melanie E. Feather Eric J. Essene Donald R. Peacor 《Contributions to Mineralogy and Petrology》1991,106(3):367-378
Electron microbeam techniques have been used to examine submicroscopically intergrown paragonite, phengite and chlorite from
the South Fork Mountain Schist of the Franciscan Terrane of northern California, which was subjected to blueschist facies
metamorphism. The sample also contains quartz, albite, lawsonite, and rutile. The subassemblage albite-lawsonite-rutile requires
metamorphic conditions on the low-temperature side of the equilibrium albite+lawsonite+rutile=paragonite+sphene+quartz+H2O (T<200° C and P<7.4 kbars based on thermodynamic data of Holland and Powell 1990). The white micas appear to be optically
homogeneous, but back-scattered electron images can distinguish two different micas by their slight difference in contrast.
Electron microprobe analyses (EMPA) of micas show Na/(Na+K) ranging from 0.2 to 0.8. The two micas are resolved by transmission
electron microscopy (TEM) as packets of phengite and paragonite that range from 20 to several hundred nm in thickness. The
compositions, determined by analytical electron microscopy (AEM), constrain the limbs of the phengite-paragonite solvus to
values of Na/(Na+K)=<0.02 and 0.97, representing less mutual solid solution than ever reported by EMPA. The textural relations
imply that the sheet silicates were derived from reactions between fluids and detrital clays and that they are in an intermediate
stage of textural development. We caution that microprobe analyses of apparently homogeneous sheet silicates may yield erroneous
data and lead to faulty conclusions using phengite barometry and paragonite-muscovite thermometry, especially in fine-grained
rocks that formed at relatively low temperatures.
Contribution no. 473 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann
Arbor, Michigan, USA 相似文献
112.
Eric J. Barron 《Surveys in Geophysics》1986,8(1):1-23
The two last decades of research in paleoceanography are characterized by extensive data collection and by the development of numerous hypotheses based on these data. Both the nature of the thermohaline circulation and the nature of the surface circulation during past times are currently being debated. Physical models of the ocean circulation are beginning to play an important role in these debates. The models range from highly simplified mathematical representations intended to illustrate physical principles to three-dimensional time-dependent global models intended to simulate temperature, salinity and flow fields. At the very least such models can be applied to explore the limitations or appropriateness of various assumptions in paleoceanography. At the very best, paleoceanographic data may be used as a verification of simulations of past oceans. 相似文献
113.
Résumé Les galènes et sulfoantimoniures de plomb des filons quartzeux, barytiques et fluorés du Haut-Allier (district de Brioude-Massiac) ont fait l'objet d'analyses isotopiques du plomb. Les résultats montrent l'existence de deux populations de plomb appartenant à chacune des deux phases minéralisatrices reconnues dans la région.La première population (18,108<206Pb/204Pb<18,181) est rapportée à la phase mésothermale permienne. Elle comprend les sulfosels précoces des filons antimonifères ainsi que la galène des filons plombo-stannifères. L'homogénéité isotopique confirme la liaison génétique existant entre ces différents types filoniens et leur appartenance à un même processus métallogénique. Le socle métamorphique régional apparaît comme la source possible du plomb.La relative dispersion des valeurs isotopiques de la seconde population (18,215<206Pb/204Pb<18,472), d'âge liasique, est corrélable avec la situation géographique et géologique des filons. Elle est due à deux phénomènes complémentaires: le lessivage d'un segment crustal comprenant le socle métamorphique avec une participation graduée du granite uranifère de la Margeride et la réassimilation partielle du plomb des filons permiens. Ce dernier phénomène d'héritage métallique se traduit au niveau des paragenèses par la formation de filons à sulfosels dominants.
Lead isotope analyses are reported for galena and lead sulfosalts from quartz, barite and fluorite vein deposits from the Haut-Allier (Brioude-Massiac district). The data define two lead populations reflecting the two mineralization episodes in this region.The first population (18,108<206Pb/204Pb<18,181) is related to the Permian mesothermal episode. It includes early sulfosalts from antimony veins and galena from lead-stanniferous veins. The Pb isotopic homogeneity confirms the relationship between the different vein types and their formation by a single metallogenic process. The metamorphic basement is the potential source of lead.The second population includes quartz-barite and fluorite veins. It shows much more isotopic variation (18,215<206Pb/204Pb<18,472) correlating with the geographical and geological location of the veins. Because the veins have a contemporaneous age, the isotopic dispersion therefore depends on two complementary phenomena: the leaching of a large volume of crust, including metamorphic basement, with a progressive contribution of the uraniferous granite of Margeride; and the partial remobilization of lead from Permian veins. The latter inheritance phenomena result in the genesis of major lead-sulfosalt vein deposits.相似文献
114.
Richard A. Ketcham Eric C. Beam Michelle A. Kominz 《Earth and Planetary Science Letters》1995,130(1-4):31-44
The effects of applying laboratory-derived material parameters to simple thermal basin subsidence models are examined. The temperature-dependent properties of thermal conductivity, specific heat and the coefficient of thermal expansion are considered, as well as conductivity contrasts between the crust and mantle and lithosphere-scale radiogenic heat production. The effects of conductivity and radioactivity are complementary, and can be predicted from the way in which they influence the initial and final steady-state geothermal gradient. A convex-up initial gradient, such as is generated by the conductivity and radioactivity functions and is also assumed to be the case based on independent evidence, will lead to more initial subsidence than in a simple constant-parameter model. The final subsidence will also be greater in the modified model, due to the fact that the stretching event will result in a lithospheric column which is intrinsically cooler than in the pre-deformation case. Because the coefficient of thermal expansion rises with increasing temperature, including its temperature dependence will result in a model with substantially less initial subsidence than one with a constant value. When these parameters are combined into a single model, the initial subsidence is approximately 15–20% less than in the constant-parameter model if radioactivity is not included, and 10% less if it is, while the final subsidence is about 5% greater without radioactivity and 7–9% greater with radioactivity included. Depending on the magnitude of extension, these effects can translate into differences of tens to hundreds of meters when compared to the constant-parameter model. 相似文献
115.
The chemistry of glycolaldehyde (hydroxyacetaldehyde) relevant to the troposphere has been investigated using UV absorption spectrometry and FTIR absorption spectrometry in an environmental chamber. Quantitative UV absorption spectra have been obtained for the first time. The UV spectrum peaks at 277 nm with a maximum cross section of (5.5± 0.7)×10–20 cm2 molecule–1. Studies of the ultraviolet photolysis of glycolaldehyde ( = 285 ± 25 nm) indicated that the overall quantum yield is > 0.5 in one bar of air, with the major products being CH2OH and HCO radicals. Rate coefficients for the reactions of Cl atoms and OH radicals with glycolaldehyde have been determined to be (7.6± 1.5)×10–11 and (1.1± 0.3)×10–11 cm3 molecule–1 s–1, respectively, in good agreement with the only previous study. The lifetime of glycolaldehyde in the atmosphere is about 1.0 day for reaction with OH, and > 2.5 days for photolysis, although both wet and dry deposition should also be considered in future modeling studies. 相似文献
116.
Eric K. Noji 《Global Change & Human Health》2001,2(1):46-53
Biological weapons represent a unique "environmental" hazard. The pathogens involved are natural in the sense that they are
risks that naturally occur in our environment. However they are unnatural in the way in which they are inflicted upon society.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
117.
Alan D. Ziegler Edwin P. Maurer Justin Sheffield Bart Nijssen Eric F. Wood Dennis P. Lettenmaier 《Climatic change》2005,72(1-2):17-36
We use diagnostic studies of off-line variable infiltration capacity (VIC) model simulations of terrestrial water budgets
and 21st-century climate change simulations using the parallel climate model (PCM) to estimate the time required to detect
predicted changes in annual precipitation (P), evapotranspiration (E), and discharge (Q) in three sub-basins of the Mississippi River Basin. Time series lengths on the order of 50–350 years are required to detect
plausible P, E, and Q trends in the Missouri, Ohio, and Upper Mississippi River basins. Approximately 80–160, 50, and 140–350 years, respectively,
are needed to detect the predicted P, E, and Q trends with a high degree of statistical confidence. These detection time estimates are based on conservative statistical
criteria (α = 0.05 and β = 0.10) associated with low probability of both detecting a trend when it is not occurring (Type
I error) and not detecting a trend when it is occurring (Type II error). The long detection times suggest that global-warming-induced
changes in annual basin-wide hydro-climatic variables that may already be occurring in the three basins probably cannot yet
be detected at this level of confidence. Furthermore, changes for some variables that may occur within the 21st century might
not be detectable for many decades or until the following century – this may or may not be the case for individual recording
station data. The long detection times for streamflow result from comparatively low signal-to-noise ratios in the annual time
series. Finally, initial estimates suggest that faster detection of acceleration in the hydrological cycle may be possible
using seasonal time series of appropriate hydro-climatic variables, rather than annual time series. 相似文献
118.
W. David Rust Donald R. MacGorman Eric C. Bruning Stephanie A. Weiss Paul R. Krehbiel Ronald J. Thomas William Rison Timothy Hamlin Jeremiah Harlin 《Atmospheric Research》2005,76(1-4):247
Balloon-borne electric field soundings and lightning mapping data have been analyzed for three of the storms that occurred in the Severe Thunderstorm Electrification and Precipitation Study field program in 2000 to determine if the storms had inverted-polarity electrical structures. The polarities of all or some of the vertically stacked charge regions in such storms are opposite to the polarities observed at comparable heights in normal storms. Analyses compared the charge structures inferred from electric field soundings in the storms with charges inferred from three-dimensional lightning mapping data. Charge structures were inferred from electric field profiles by combining the one-dimensional approximation of Gauss's law with additional information from three-dimensional patterns in the electric field vectors. The three different ways of inferring the charge structure in the storms were found to complement each other and to be consistent overall. Charge deposition by lightning possibly occurred and increased the charge complexity of one of the storms.Many of the cloud flashes in each case were inverted-polarity flashes. Two storms produced ground flash activity comprised predominantly of positive ground flashes. One storm, which was an isolated thunderstorm, produced inverted-polarity cloud flashes, but no flashes to ground. The positive and negative thunderstorm charge regions were found at altitudes where, respectively, negative and positive charge would be found in normal-polarity storms. Thus, we conclude that these storms had anomalous and inverted-polarity electrical structures. Collectively, these three cases (along with the limited cases in the refereed literature) provide additional evidence that thunderstorms can have inverted-polarity electrical structures. 相似文献
119.
Bridging understanding of local environmental change with regional and global patterns of land-use and land-cover change (LUCC) remains a key goal and challenge for our understanding of global environmental change. This meta-analysis attempts to bridge local and regional scales of LUCC by demonstrating the ways in which previously published case studies can be compared and used for a broader regional synthesis in the tropics. In addition to providing results from a meta-analysis, this paper suggests ways to make future case studies more widely comparable. 相似文献
120.
Yongjun Gao Jochen Hoefs Eric Hellebrand Anette von der Handt Jonathan E. Snow 《Contributions to Mineralogy and Petrology》2007,153(4):429-442
Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a
combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions,
trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics
in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous
modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling
could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt
during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal
mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration
of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation
or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3–4):191–233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges. 相似文献