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A. J. T. Jull D. J. Donahue E. Cielaszyk F. Wlotzka 《Meteoritics & planetary science》1993,28(2):188-195
Abstract We report on a series of 27 14C terrestrial ages of meteorites from four states in the central and southwestern USA. These results were compared to the earlier work of Boeckl (1972). Our results showed that the weathering rate for destruction of meteorites is lower than suggested by Boeckl (1972). We estimated a “half-life” for removal of meteorites of about 10 to 15 ka, similar to that derived for Roosevelt County meteorites. We also studied the weathering of these meteorites compared to terrestrial age. Only a weak correlation was observed, and for these meteorites the degree of weathering can only be taken as a weak indicator of terrestrial residence time. We also measured the δ 13C and 14C and amount of weathering-product carbonates which show some interesting variations with the length of time the meteorites have been exposed to weathering. 相似文献
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It is shown that under present conditions the Jeans escape flux of hydrogen from Mars in the form of H and H2 is constrained to be the same as twice the non-thermal (McElroy, 1972) escape of O atoms. The mediation of the chemical chain that recombines CO2 plays an essential role in regulating the escape of hydrogen to match that of oxygen, confirming a mechanism postulated by McElroy and Donahue (1972). It is also shown that if the oxygen flux changes, a change in the O2 mixing ratio results and the consequence is to induce a large change in the odd hydrogen concentration, and consequently in H2 production and hydrogen escape. The effect is to stabilize the hydrogen escape flux at twice the O flux. It is shown that surface chemistry should not change the operation of this mechanism but has consequences for the eddy coefficient variation at low altitudes. There is a strong correlation between low humidity, large solar zenith angles and large O3 abundances. The effect of argon in a mixing ratio as large as 0.3 on these results is also investigated. 相似文献
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Roberto Santacroce Raffaello Cioni Paola Marianelli Alessandro Sbrana Roberto Sulpizio Giovanni Zanchetta Douglas J. Donahue Jean Louis Joron 《Journal of Volcanology and Geothermal Research》2008
A review of compositional data of the major explosive eruptions of Vesuvius is presented, comparing compositions (major elements) of whole rock with glass shards from the proximal deposits, hopefully useful for long-distance correlation. A critical review of published and new geochronological data is also provided. All available 14C ages are calibrated to give calendar ages useful for the reconstruction of the volcanological evolution of the volcanic complex. The pyroclastic deposits of the four major Plinian eruptions (22,000 yr cal BP “Pomici di Base”, 8900 yr cal BP “Mercato Pumice”, 4300 yr cal BP “Avellino Pumice”, and A.D. 79 “Pompeii Pumice”) are widely dispersed and allow a four-folded, Plinian to Plinian, stratigraphic division: 1. B–M (between Pomici di Base and Mercato); 2. M–A (between Mercato and Avellino); 3. A–P (between Avellino and Pompeii); 4. P–XX (from the Pompeii Pumice to the last erupted products of the XXth century). Within each interval, the age, lithologic and compositional features of pyroclastic deposits of major eruptions, potentially useful for tephrostratigraphic purposes on distal areas, are briefly discussed. The Vesuvius rocks are mostly high Potassic products, widely variable in terms of their silica saturation. They form three groups, different for both composition and age: 1. slightly undersaturated, older than Mercato eruption; 2. mildly undersaturated, from Mercato to Pompeii eruptions; 3. highly undersaturated, younger than Pompeii eruption. For whole rock analyses, the peculiar variations in contents of some major and trace elements as well as different trends in element/element ratios, allow a clear, unequivocal, easy diagnosis of the group they belong. Glass analyses show large compositional overlap between different groups, but selected element vs. element plots are distinctive for the three groups. The comparative analysis of glass and whole rock major element compositions provides reliable geochemical criteria helping in the recognition, frequently not obvious, of distal products from the different single eruptions. 相似文献
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Thin sections from 44 sherds representing eight prehistoric sites on four islands (Barbuda, Montserrat, Anguilla, St. Martin) in the northern Lesser Antilles (West Indies) were examined using a petrographic microscope. Point counting distinguished three temper associations: (1) exclusively volcanic,(2) volcanic and carbonate, and (3) volcanic, carbonate, and grog. Exclusively volcanic or dominantly volcanic (with low carbonate) temper associations occur in all Saladoid and many post-Saladoid sherds, with plagioclase feld-spar and volcanic rock fragments being most abundant. Thin sections with significant carbonate content or with grog were restricted to post-Saladoid sherds from Barbuda and to a lesser extent Anguilla. the presence of volcanic grains in sherds from the limestone islands of Barbuda and Anguilla indicates that volcanic islands in the region served as sources of pottery (or volcanic temper) for these two islands. 相似文献