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Michael L. Bender 《Geochimica et cosmochimica acta》1973,37(5):1229-1247
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W.A. Russell D.A. Papanastassiou T.A. Tombrello 《Geochimica et cosmochimica acta》1978,42(8):1075-1090
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Determinations of ages are reported for seven severely shock-heated chondrites. Shaw gives a plateau age of 4.29 Gyr. Louisville, Farmington, and Wickenburg give well-defined intercept ages of 0.5–0.6 Gyr. Orvinio, Arapahoe, and Lubbock show complex release curves, with age minima of 0.7–1.0 Gyr. Degassing times of 0.5–1.0 Gyr are suggested for these meteorites. Most severely shocked chondrites were apparently not totally degassed of 40Ar by the event, but retained from ~ 2 to ~45% of their 40Ar. When calculated values of the diffusion parameter, , for Ar are examined in Arrhenius plots, they show two distinct linear relationships, which apparently correspond to the degassing of different mineral phases with distinct ratios and different average temperatures for Ar release. The experimentally determined values of for the high temperature phase of several severely shocked chondrites are ~10?7 to 10?5sec?1 for their determined shock-heating temperatures of ~950°C to ~ 1200°C. The inferred reheating temperatures, values, and fraction of 40Ar loss during the reheating event for these seven chondrites suggest post-shock cooling rates and burial depth of ~ 10?2 10?4°C/sec and ~0.5–2m, respectively. For three chondrites these cooling rates agree with those determined from Ni diffusion in metal grains: for five chondrites the cooling rates derived from 40Ar and Ni disagree by a factor of ~105. It is suggested that five of these severely shocked chondrites were part of large ejecta blankets containing hot material and cold clasts with a distribution of sizes and that the cooling rate of this ejecta appreciably decreased as a function of time. 相似文献
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The Rameka Gabbro, emplaced 367 Ma ago, experienced a well documented reheating on intrusion of the Separation Point Batholith 114 Ma ago. age spectrum analyses of hornblende from the Rameka Gabbro show diffusion gradients which provide information on the 40Ar boundary concentration during reheating.Three samples of hornblende exhibit age spectra that conform to a model of 40Ar loss by diffusion, implying a zero 40Ar boundary concentration during heating. The calculated 40Ar loss from these samples, together with a model of heat flow in the aureole, provide estimates of diffusion coefficients of 40Ar in Mg-rich hornblende which correspond to an activation energy, E, of ~60 kcal-mol?1 and a frequency factor. D0, of ~ 10?3 cm2-sec?1. When combined with laboratory diffusion results, these data yield a well defined diffusion law (E = 63.3 ± 1.7 kcal-mol?1, ).The age spectra of the eight other samples record steep gradients of excess 40Ar over the first few percent of gas release. Although this effect causes high apparent conventional K-Ar ages, the plateau segments of many sampes still record the crystallization age of 367 ± 5 Ma. These measurements show that the excess 40Ar phase developed locally in the intergranular regions of the gabbro, following intrusion of the batholith. on time scales that varied from 104 to 106years. The minimum average ratio of this component was found to be 1300 ± 400. The partial pressure of Ar was at least 10?2 bars in some places.A single age spectrum analysis of plagioclase reveals a ‘saddle-shaped” release pattern with a minimum at 140 Ma.In conjunction with theoretical diffusion models and a diffusion law, age spectrum analysis of hornblende that has experienced a post-crystallization heating can provide close estimates of the maximum temperature of the thermal event as well as both age of crystallization and reheating. 相似文献
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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, . 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 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
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