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881.
882.
E.Bruce Watson T.Mark Harrison F.James Ryerson 《Geochimica et cosmochimica acta》1985,49(8):1813-1823
Diffusion coefficients for Sm, Sr, and Pb in fluorapatite at 900°–1250°C were obtained by measuring experimentally-induced diffusional uptake profiles of these elements in the margins of gem-quality apatite crystals. The crystals were immersed in synthetic melts enriched in the trace elements of interest and presaturated in apatite, and the resulting diffusion gradients were characterized by electron microprobe analysis. Except in the case of Pb, the diffusivities define good Arrhenius lines for the respective elements: DSm = 2.3 × 10?6exp(?52,200/RT) DSr = 412 exp(?100,000/RT). (Diffusion perpendicular to and parallel to c is measurably different in the case of Sr; the Arrhenius equation given above is an average for the two directions). Results on Pb were erratic, probably because extremely Pb-rich melts were used for some of the experiments. Data believed to be reliable define the following Arrhenius line: DPb = 0.035 exp(?70,000/RT). Constraints based on closure of natural apatites with respect to Pb suggest that the experimental data can be extrapolated, with sizeable uncertainty, to temperatures as low as 550°C.When applied to the question of isotopic and trace-element equilibration of residual or entrained apatites with crustal melts, the measured diffusivities indicate that 0.05-cm crystals will rarely preserve the original Pb-isotope characteristics of the source; the same is not true, however, of Sr (and, under some conditions, the REE), which may be unaffected at crystal cores during typical melting events. 相似文献
883.
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 (). 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
884.
885.
Mark D. Kurz John J. Gurney William J. Jenkins Dempsey E. Lott III 《Earth and Planetary Science Letters》1987,86(1)
The distribution and isotopic composition of helium has been measured in a suite of well-characterized one-carat diamonds from the Orapa kimberlite, Botswana. Crushing of the diamonds in vacuo indicates that most of the helium is contained by the matrix (generally greater than 90%), rather than by the inclusions. Step-heating experiments, performed on inclusion-free fragments remaining after crushing, indicate that the3He/4He ratio is variablewithin individual diamonds. The fragments, as small as 10 mg, were heated in two timed steps, both at 2000°C. In every case, lower3He/4He ratios are observed in the first graphitization step (0.05–3 × atmospheric), while the last heating step releases helium with systematically higher3He/4He ratio (30–80 × atmospheric). We suggest that this internal isotopic variability is the result of stepwise graphitization: the first heating step initiates graphitization, which nucleates around defects, and the second heating step graphitizes the relatively defect-free regions of the diamond. The3He/4He ratio measured, using the partial graphitization technique, differs by up to a factor of 100 within a single specimen. The inclusion-free fragments release small quantities of helium below 2000°C, which suggests that helium release is obtained only by graphitization. The3He contents of the monocrystalline diamonds are relatively constant (at 3 × 10−13 cm3 STP/gram) and indicate that most of the isotopic variability is due to radiogenic4He. The variations in4He content are either related to zoning of Th and U in the diamonds (i.e., in-situ decay), to zoning of inherited4He, or to implantation of α-particles from a Th and U rich environment (i.e., kimberlite). Because the Orapa diamonds were mined from roughly 40 m depth in the kimberlite, spallation reactions from cosmic ray interactions are not a significant source of3He. However, calculations based on the age of the kimberlite (90 m.y.) and reasonable Th and U abundances suggest that most of the3He in the Orapa diamonds could be produced by6Li(n, α)T in the diamond. Although this may not be true of all diamonds, nuclear reactions in the crust and mantle (including spallation reactions at the surface) can explain many of the high3He/4He ratios previously reported for diamonds. 相似文献
886.
William J. Brennan Mark J. Hamilton Richard K. Kilbury Richard L. Reeves Laura J. Covert 《Earth and Planetary Science Letters》1984,70(2):363-372
A secular variation record of the geomagnetic declination has been obtained from glaciolacustrine varved clays and postglacial sediments, sampled both from outcrop and from piston cores from Lake Ontario. It appears to be uninterrupted and covers the interval 14,000 years B.P. (14C) to the present. The record for the interval 14,000–12,300 years B.P. consists of true declination and was obtained from samples collected from outcrop. The record for the interval 12,500 + years B.P. to the present consists of relative declination and was obtained from piston cores. The secular variation record is internally consistent and is compatible with the chronology of events known to have occurred during and following the deglaciation of western New York. Use of the record for magnetic correlation within the study area is demonstrated. 相似文献
887.
Francis P. Miknis Arthur W. Lindner A. John Gannon Mark F. Davis Gary E. Maciel 《Organic Geochemistry》1984,7(3-4)
Solid state 13C NMR techniques of cross polarization with magic-angle spinning, and interrupted decoupling have been employed to examine the nature of the organic matter in eight kerogen concentrates representing five Tertiary deposits in Queensland, Australia. The NMR results show that five of the kerogens have high proportions of aliphatic carbon in their organic matter and correspond to Type I–II algal kerogens. Three of the kerogens, derived from carbonaceous shales, have a high proportion of aromatic carbon in their organic matter and correspond to Type III kerogens. The fractions of aliphatic carbon in all the kerogens, regardless of type, are shown to correlate with the conversion characteristics of the corresponding raw shales during Fischer assay. Interrupted decoupling NMR results show the presence of more oxygen-substituted carbon in the carbonaceous shales, which may account for the greater CO2 evolution and phenolic materials found in the pyrolysis products of the carbonaceous shales. 相似文献
888.
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
889.
Activity/composition relations in the ternary feldspars 总被引:6,自引:0,他引:6
Mark S. Ghiorso 《Contributions to Mineralogy and Petrology》1984,87(3):282-296
Activity/composition relations are presented for high-structural state feldspars whose bulk compositions lie within the ternary system NaAlSi3O8 CaAl2Si2O8-KAlSi3O8. The expressions are parameterized from the data for coexisting feldspars of Seck (1971a) using an asymmetric regular solution approximation for the excess Gibbs free energy of mixing and an Al-avoidance model for the configurational entropy of solution. The solution properties of the plagioclase and alkali-feldspar binaries have been made to conform to the recent work of Thompson and Hovis (1979) and Newton et al. (1980). Using the proposed model the ternary feldspar solvus is extrapolated in temperature (up to 1,500° C) and pressure (up to 5kbars). A new two-feldspar geothermometer is presented which provides somewhat more reasonable estimates of crystallization temperatures than the equations and graphs of Stornier (1975), Powell and Powell (1977), Brown and Parsons (1981) and Haselton et al. (1983). In conjunction, some criteria are suggested for establishing the existence of equilibrium tie-lines between coexisting ternary feldspars in rhyolites and trachytes. Calculated values of the activity of KAlSi3O8 in plagioclase are examined in some detail. These compare favorably with independent estimates obtained from experimentally grown plagioclases precipitating at liquidus temperatures from igneous rocks of widely varying alkali contents. 相似文献
890.
Scale-dependent fractal dimensions of topographic surfaces: An empirical investigation, with applications in geomorphology and computer mapping 总被引:6,自引:0,他引:6
Fractional Brownian surfaces have been widely discussed as an appropriate model for the statistical behavior of topographic surfaces. The fractals model proposes that topographic surfaces are statistically self-similar, and that a single parameter, the fractal dimension, applies at all scales. This paper presents the results of empirical examinations of 17 topographic samples. Only one of these samples shows the statistical behavior predicted by the fractals model; however, in 15 of the 17 samples, the surfaces' variograms could be adequately described by ranges of scales having constant fractal dimension, separated by distinct scale breaks. For scale ranges between adjacent breaks, surface behavior should be that predicted by the fractals model; the breaks represent characteristic horizontal scales, at which surface behavior changes substantially. These scale breaks are especially important for cartographic representations and digital elevation models, since they represent scales at which there is a distinct change in the relation between sampling interval and the associated error. 相似文献