The thermal significance of potassium feldspar K-Ar ages inferred from ${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectrum results     

T.Mark Harrison  Ian McDougall 《Geochimica et cosmochimica acta》1982,46(10):1811-1820

${}^{40}\text{Ar}{}^{39}\text{Ar}$ age spectrum analyses of three microcline separates from the Separation Point Batholith, northwest Nelson, New Zealand, which cooled slowly (~5°C-Ma^?1) through the temperature zone of partial radiogenic ⁴⁰Ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages (~80 Ma) corresponding to the time that the samples cooled below about 100°C. The last 35 percent of ³⁹Ar released from the microclines yields plateau ages (103,99 and 93 Ma) which reflect the different bulk mineral ages, and correspond to cooling temperatures between about 130 to 160°C. Theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates ≤5°C-Ma^?1. Diffusion parameters calculated from the ³⁹Ar release yield an activation energy, E = 28.8 ± 1.9 kcal-mol^?1, and a frequency factor/grain size parameter, $\text{D}{}_0\text{l}{}^2\text{= 5.6}{}_{\mathrm{?3.9}}{}^{\mathrm{+14}}\text{sec}{}^{\mathrm{?1}}$. This Arrhenius relationship corresponds to a closure temperature of 132 ± 13°C which is very similar to the independently estimated temperature. From the observed diffusion compensation correlation, this $\text{D}{}_0\text{l}{}^2$ implies an average diffusion half-width of about 3 μm, similar to the half-width of the perthite lamellae in the feldspars. The range in microcline K-Ar ages from the Separation Point Batholith is the result of relatively small temperature differences within the pluton during cooling. Comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous K-feldspars (E = 40 to 52 kcal-mol^?1) reveals that Ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state. Previously published U-shaped age spectra are probably the result of the superimposition of excess ⁴⁰Ar upon diffusion profiles of the kind described here.  相似文献   

Diffusion of 40Ar in hornblende   总被引：8，自引：0，他引：8  

T. Mark Harrison 《Contributions to Mineralogy and Petrology》1982,78(3):324-331

Measured radiogenic ⁴⁰Ar loss from two compositionally contrasting hornblendes following isothermal-hydrothermal treatment have provided model diffusion coefficients in the temperature range of 750° C to 900° C. Eight experiments using a hornblende (77–600) with a Mg/(Mg +Fe) ratio of 0.72 yield a linear array on an Arrhenius plot with a slope corresponding to an activation energy of 66.1 kcal-mol^–1 and a frequency factor of 0.061 cm²-sec^–1, assuming spherical geometry for the mineral aggregate. Five experiments undertaken on a hornblende (M Mhb-1) with a Mg/(Mg+Fe) ratio of 0.36 show similar behavior to the Mgrich sample, suggesting that the diffusivity of Ar in hornblendes is not sensitive to the Mg/Fe ratio.These data are consistent with kinetic information obtained from a geological experiment using the thermal effect of a granitoid intrusion. Together these data yield an activation energy of 64.1±1.7 kcal-mol^–1 and a frequency factor of 0.024± _0.011 ^0.053 cm²-sec^–1. For a hornblende with an effective diffusion radius of 80 m, these diffusion parameters predict closure temperatures between 578° C and 490° C for cooling rates in the range 500 to 5° C-Ma^–1.  相似文献   

Excess⁴⁰Ar in metamorphic rocks from Broken Hill,New South Wales: implications for⁴⁰Ar/³⁹Ar age spectra and the thermal history of the region     

T. Mark Harrison  Ian McDougall 《Earth and Planetary Science Letters》1981,55(1):123-149

⁴⁰Ar/³⁹Ar age spectrum analyses of samples from Broken Hill, New South Wales, indicate that the region has experienced a complex thermal history following high-grade metamorphism, 1660 Ma ago. The terrain cooled slowly (～3°C Ma^?1) until about 1570 Ma ago, when the temperature fell below about 500°C. Following granitoid emplacement ～1500 Ma ago, the region remained relatively cold until affected by a thermal pulse 520±40Ma ago, causing temperatures to rise to～350°C in some places. During this event, accumulated⁴⁰Ar was released from minerals causing a significant Ar partial pressure to develop. Laboratory Ar solubility data combined with the⁴⁰Ar/³⁹Ar age spectra gives a local estimate of this partial pressure of ～10^?4atm. The region finally cooled below 100°C about 280 Ma ago.⁴⁰Ar/³⁹Ar age spectrum analyses of hornblende, plagioclase and clinopyroxene containing excess⁴⁰Ar are characterized by saddle-shaped age spectra. Detailed analysis of plagioclase samples reveals a complex diffusion behaviour, which is controlled by exsolution structures. This effect, in conjunction with the presumed different lattice occupancy of excess⁴⁰Ar with respect to radiogenic⁴⁰Ar, appears to be responsible for the saddle-shaped age spectra.  相似文献   

Stellar luminosities and radii fromuvbyβ photometry     

Terry Moon 《Astrophysics and Space Science》1985,117(2):261-270

A linear relation is found to exist between the Barnes-Evans visual surface brightness parameterF _V and the (b-y)₀ colour index ofuvby photometry for spectral types later than G0. Using this relation, and a previously determined relation for spectral types B0-G0, tables of intrinsic colours and indices, absolute magnitude and stellar radius are given for the ZAMS and luminosity classes Ia-V over a wide range of spectral types. Comparison of absolute magnitudes and radii calculated fromuvby photometry with values obtained using the visual magnitude, parallax and angular diameter indicates thatuvby photometry can be used to determineM _V to ±0 ^m .2 and log(R/R ) to ±0.05.  相似文献   

The Caesura: remarks on Wittgenstein’s interruption of theory, or, why practices elude explanation     

Paul Harrison 《Geoforum》2002,33(4):487-503

This paper aims to bring the work of Ludwig Wittgenstein into contact with the growing interest and concerns over the status of practice, performance and non-representational ‘theory’ within human geography. Drawing predominantly on Wittgenstein’s later work, the aim is to use Wittgenstein’s comments to illuminate how certain presuppositions and idealisations over the nature of understanding and meaning are or have been built into our (social scientific) modes and methods of explanation. Thus Wittgenstein’s work is used as a diagnosis––a diagnosis of how the modus operandi of giving an explanation can, and often does, prevent us from acknowledging the practical and the performative, from witnessing the taking-place of meaning and understanding. The paper carries out this task by focusing first on Wittgenstein’s critique of the role of ‘rules’ and ‘rule-following’ in the construction of social scientific accounts and secondly, through a consideration of the implications of Wittgenstein’s ‘scenic’ style of writing through which he attempts to deconstruct the epistemo-methodological idealisations and representationalist desires of social analysis. The claim here is not that Wittgenstein’s work provides the solution to the problematics which confront us in considering the status (or otherwise) of practice, but rather that his work may provide us with other ways of going-on, ones more sensitive to the eventful, creative, excessive and distinctly uncertain realms of action.  相似文献   

Sources and bioavailability of polynuclear aromatic hydrocarbons in Galveston Bay, Texas     

Yaorong Qian  Terry L. Wade  Jose L. Sericano 《Estuaries and Coasts》2001,24(6):817-827

Oyster and sediment samples collected from six sites in Galveston Bay from 1986 to 1998 were analyzed for polynuclear aromatic hydrocarbons (PAHs). Total concentrations of parent PAHs in oysters ranged from 20 ng g⁻¹ at one site to 9,242 ng g⁻¹ at another and varied randomly with no clear trend over the 13 year period at any site. Concentrations of alkylated PAHs, which are indications of petroleum contamination, varied from 20 to 80,000 ng g⁻¹ in oysters and were in higher abundance than the parent PAHs, indicating that one source of the PAH contaminants in Galveston Bay was petroleum and petroleum products. Four to six ring parent PAHs, which are indicative of combustion source , were higher than those of 2–3 ring parent PAHs, suggesting incomplete combustion generated PAHs was another source of PAHs into Galveston Bay. Concentrations of parent PAHs in sediments ranged from 57 to 670 ng g⁻¹ and were much lower than those in oysters. Sediments from one site had a high PAH concentration of 5,800 ng g⁻¹. Comparison of the compositions and concentrations of PAHs between sediment and oysters suggests that oysters preferentially bioaccumulate four to six ring PAHs. PAH composition in sediments suggests that the sources of PAH pollution in Galveston Bay were predominantly pyrogenic, while petroleum related PAHs were secondary contributions into the Bay.  相似文献   

Variations in Capture-Zone Geometry of a Partially Penetrating Pumping Well in an Unconfined Aquifer   总被引：1，自引：0，他引：1  

E. Scott Bair  Terry D. Lahm 《Ground water》1996,34(5):842-852

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An experimental marine ecosystem response to crude oil and Corexit 9527: Part 2—Biological effects     

T.R. Parsons  P.J. Harrison  J.C. Acreman  H.M. Dovey  P.A. Thompson  C.M. Lalli  K. Lee  Li Guanguo  Chen Xiaolin 《Marine environmental research》1984,13(4):265-275

Three experimental ecosystems were employed to test the effect of Corexit 9527, with and without Prudhoe Bay crude oil, on the ecology of a temperate pelagic ecosystem. The results indicated that Corexit 9527 alone enhanced biological productivity without changing the structure of the ecosystem. The mixture of Corexit and crude oil caused a major change in the ecology of the ecosystem which resulted in large numbers of bacteria and zooflagellates, but a depression of all other zooplankton phyla.  相似文献   

Variable deposition rates on the slope and rise off the Mid-Atlantic States     

Terry A. Nelsen  Daniel Jean Stanley 《Geo-Marine Letters》1983,3(1):37-42

Pronounced variations in slope topography and offshelf spillover have produced a three-fold difference (9 to 25 cm/1,000 yr) in Late Quaternary deposition rates among slope cores southeast of the Mid-Atlantic States. Upper rise cores revealed a high rate but lower core-to-core variability (20 to 44 cm/1,000 yr), largely the result of slope bypassing, i.e., effects of gravity-controlled downslope transport. A transport model suggests temporary slope sediment storage and higher accumulation of mud in more distal rise regions. Eustatic change accounts for the four-fold decrease in slope deposition rate between the Late Pleistocene and Holocene.  相似文献   

Partitioning of copper among copper-binding proteins in the mussel Mytilus edulis exposed to soluble copper     

F.L. Harrison  J.R. Lam 《Marine environmental research》1985,16(3):151-163

Partitioning of copper among copper-binding proteins was evaluated in digestive glands of Mytilus edulis exposed to soluble copper. Groups of mussels were held in flow-through bioassay systems and exposed to 25 μg Cu liter^?1 for up to 21 weeks. At three-week intervals, groups of 25 mussels were removed and the digestive glands were analyzed for copper-binding proteins by gel-permeation chromatography and atomic absorption spectrometry.Chronic exposure to copper resulted in increased amounts of copper in the low molecular weight (LMW) protein fraction, which contains metallothioneins, and in the high molecular weight (HMW) protein fraction, which contains metalloenzymes. Concentrations of copper in the LMW protein fraction increased and then appeared to plateau with long exposure times, whereas those in the HMW protein fraction continued to increase with exposure time.  相似文献