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Moratti G. Benvenuti M. Santo A. P. Laurenzi M. A. Braschi E. Tommasini S. 《International Journal of Earth Sciences》2018,107(7):2491-2515
International Journal of Earth Sciences - This study is based upon a stratigraphic and structural revision of a Middle Jurassic–Upper Cretaceous mostly continental succession exposed between... 相似文献
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Palaeomagnetic study of the El Quemado complex and Marifil formation, Patagonian Jurassic igneous province, Argentina 总被引：1，自引：0，他引：1
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A rhyolitic lava flow from Basiluzzo islet (Aeolian Islands), has been analysed with the Fission tracks (FT) and 40Ar–39Ar methods on glass, and with the U/Th method on whole rock to constrain its age and to compare the behaviour of different dating methodologies on glass samples late Quaternary in age. Laser 40Ar–39Ar total fusion analyses were performed on populations of grains. Due to the low yields of radiogenic 40Ar the age data are characterised by very high errors. The weighted average of the ages of the whole population is 55.7 ± 8.7 ka (MSWD = 0.7). The isochron age calculated on all points is 40.6 ± 11.4 ka (MSWD = 0.6), with an initial 40Ar/36Ar ratio of 297.8 ± 1.8; the isochron is characterised by very little spread among points. The procedure named ‘point-counting technique’ was adopted in FT dating. Spontaneous track mean size resulted reduced by around 20% compared to induced tracks, which indicates that the determined FT age, 28.6 ± 3.6 ka, is a reduced age, due to a certain amount of track annealing. For this reason the plateau technique for correcting thermally lowered ages was applied. We determined a plateau age (commonly assumed as a reliable estimate of the glass formation age) of 43.4 ± 7.1 ka. Four sub-samples of whole rock from Basiluzzo lava flow have been analysed using U/Th isochron method. The 238U/232Th and 230Th/232Th activity ratios of sub-samples have been determined by alpha counting and plotted on an isochron diagram. The resulting age is 46 ± 8 ka and the 234U/238U activity ratios are always close to one, demonstrating that no significant processes of alteration have occurred. The relatively high error associated with the age is due to a low fractionation of U/Th ratio in the analysed whole rocks. The ages obtained with different methods, 43.4 ± 7.1 ka (FT plateau age), 40.6 ± 11.4 ka (40Ar–39Ar isochron age of all grains), and 46 ± 8 ka (U/Th isochron) agree at the 1σ level, excluding a Holocene age for this sample. This could be valuable information for the Department of Civil Protection because it seems to mitigate the potential risk for present volcanic activity in the area. All ages are affected by very high analytical errors, which are due to the characteristics of the material analysed. Young ages result in low tracks numbers (FT dating) and barely detectable amounts of radiogenic 40Ar in the presence of high atmospheric contamination (40Ar–39Ar dating). Stratigraphic successions without strict chronologic constraints might however benefit even from age data with low precision. 相似文献
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Marinella A. Laurenzi Maria Laura Balestrieri Giulio Bigazzi Julio C. Hadler Neto Pedro J. Iunes Pio Norelli Massimo Oddone Ana Maria Osorio Araya José G. Viramonte 《Geostandards and Geoanalytical Research》2007,31(2):105-124
New analyses have been performed in order to enhance the data-set on the independent ages of four glasses that have been proposed as reference materials for fission-track dating. The results are as follows. Moldavite - repeated 40 Ar/39 Ar age determinations on samples from deposits from Bohemia and Moravia yielded an average of 14.34 ± 0.08 Ma. This datum agrees with other recent determinations and is significantly younger than the 40 Ar/39 Ar age of 15.21 ± 0.15 Ma determined in the early 1980s. Macusanite (Peru) -four K-Ar ages ranging from 5.44 ± 0.06 to 5.72 ± 0.12 Ma have been published previously. New 40 Ar/39 Ar ages gave an average of 5.12 ± 0.04 Ma. Plateau fission-track ages determined using the IRMM-540 certified glass and U and Th thin films for neutron fluence measurements agree better with these new 40 Ar/39 Ar ages than the previously published ages. Roccastrada glass (Italy) - a new 40 Ar/39 Ar age, 2.45 ± 0.04 Ma, is consistent with previous determinations. The Quiron obsidian (Argentina) is a recently discovered glass that has been proposed as an additional reference material for its high spontaneous track density (around 100 000 cm-2 ). Defects that might produce "spurious" tracks are virtually absent. An independent 40 Ar/39 Ar age of 8.77 ± 0.09 Ma was determined and is recommended for this glass. We believe that these materials, which will be distributed upon request to fission-track groups, will be very useful for testing system calibrations and experimental procedures. 相似文献
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Palaeomagnetic and geochronological measurements have been carried out on the late Pleistocene basaltic–andesitic unit of Monte Chirica–Costa Rasa, on the island of Lipari (Aeolian Archipelago). The lava flow sequence is about 10 m thick and has been sampled in detail. Magnetic properties are rather uniform; Curie temperatures of 540° to 580 °C, and the saturation IRM reached at applied values of 0.1 T point to titanomagnetite as the main magnetization carrier. Thermal and AF demagnetization have shown the presence of secondary magnetization components. These were removed mostly at 450°–500 °C or 20–30 mT, indicating a highly stable ChRM with directions from transitional to reverse. Where a ChRM could not be isolated by application of the demagnetization techniques, the converging remagnetization circles method gave a mean ChRM value fully comparable with that obtained from other methods. 40 Ar/39 Ar determinations were performed on two lava flows, in the lower and upper parts of the sequence. The former shows a transitional ChRM direction and a whole-rock age of 157±12 ka, the latter a reverse direction, a whole-rock age of 143±17 ka and a ground-mass age of 128±23 ka. The radiometric data and the reconstructed stratigraphy, which indicate ages of 150±10 ka and 104±3.5 ka, respectively, for the volcanic units at the bottom and top of the Monte Chirica–Costa Rasa unit, suggest that the reverse directions recorded in Lipari are related to the Blake event. 相似文献
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D. Brocchini C. Principe D. Castradori M. A. Laurenzi L. Gorla 《Mineralogy and Petrology》2001,73(1-3):67-91
Summary The present review of data on the Trecase 1 well, including stratigraphy, updated 40 Ar/39Ar ages and the results of newly performed calcareous nannofossil studies, serves to resolve the chronological contradictions
pointed out by Bernasconi et al. (1981) and Balducci et al. (1983) concerning the onset of volcanic activity in the area now
occupied by the Somma-Vesuvius Volcanic Complex. New 40 Ar/39Ar data indicate that volcanic activity in this area started about 0.4 myr B.P. After such time, tephritic magmatic activity,
distributed in small scattered centers, developed and alternated with periods of volcanic quiescence and marine sedimentation.
This first phase of magmatic activity ended in the Vesuvian area about 0.3 myr B.P. and was followed by a period of marine
sedimentation in a marginal environment. Complete emergence of the shoreline occurred about 37,000 yr B.P. as a result of
sea level changes during the last glacial period and deposition of the 60 m thick Campanian ignimbrite (CI). Volcanic activity
reappeared in the Vesuvian area only after the CI eruption. Magma rising along and at the intersection of linear and curved
tectonic and volcano-tectonic elements (linked to the pre-existing Pleistocene tectonic trend and formation of the vast Phlegraean
Fields caldera) formed a number of small lava and scoria edifices. One of these tephritic centers lies above the CI deposits
under the Trecase 1 well area. The CI bottom in the Trecase 1 well is currently at an altitude of − 120 m a.s.l.; this allows
estimating the maximum tectonic subsidence over the last 37,000 yr. by the southern sector of the Vesuvian area to be about
30 m.
Received April 12, 2000; revised version accepted March 1, 2001 相似文献
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