An ESR experimental study of artificial optical bleaching of sedimentary quartz has shown that the aluminum center was maximally bleached after a 6-month illumination equivalent to natural light. This duration seems too long to apply in natural conditions. Nevertheless, the measurement of the ESR intensity of aluminum centers in quartz extracted from modern sediments and deposited in sandy bars shows that the maximum bleaching has effectively been reached.
In order to determine the relationship between the bleaching and the distance covered by a quartz grain in a river, samples were collected along the Creuse River (France) from its spring to about 170 km downstream, where maximum bleaching levels were observed in previous studies. The ESR intensities of the aluminum and titanium centers in quartz were measured, using X-band spectroscopy, before and after artificial bleaching. The difference measured between these sub-samples shows that the maximum bleaching level is obtained in the course of the first kilometer. Hence, the assumption that ESR dating of fluvial sediment is based on the optical bleaching was validated. 相似文献
Six samples were collected from a section of Peoria Loess in Eustis, North America, for optically stimulated luminescence (OSL) dating of quartz, and all except one (LV90) produced narrow dose distributions. A comparative study was conducted on this sample and on a ‘well-behaved’ sample (LV91), involving other dating methods and examination of the quartz OSL. These investigations revealed differences in the quartz OSL growth with dose, OSL response to thermal treatments and the range of components within the OSL signals. An ultra-fast component was found in LV90 that displayed a higher rate of sensitivity change than the fast component and this had a malign influence on the determination of the equivalent dose. The distinctive luminescence characteristics of LV90 imply either a change in wind dynamics and/or the source area for the silt. 相似文献
Sediments attributed to flooding events of River Danube concerning the bleaching of the optical stimulated luminescence (OSL) signal were investigated. It is demonstrated that the OSL signal in both quartz and feldspar is not completely but differentially bleached in the sediment grains. Partial bleaching of the samples is clearly indicated by the scatter of equivalent dose determined for several individual single aliquots. It is also shown that residual ages in feldspars are significantly higher than those calculated for quartz. It is furthermore demonstrated that analysing measurement-time dependent equivalent dose estimates is not a suitable method to identify partial bleaching in the investigated sediment grains. However, the transport and deposition process of the investigated samples was probably disturbed by an artificial input of sediment, and this case study may thus not be representative of undisturbed high flood events in the past. 相似文献
Non-steady state deformation and annealing experiments on vein quartz are designed to simulate earthquake-driven episodic deformation in the middle crust. Three types of experiments were carried out using a modified Griggs-type solid medium deformation apparatus. All three start with high stress deformation at a temperature of 400 °C and a constant strain rate of 10− 4 s− 1 (type A), some are followed by annealing in the stability field of α-quartz for 14–15 h at zero nominal differential stress and temperatures of 800–1000 °C (type A + B), or by annealing for 15 h at 900 °C and at a residual stress (type A + C).The quartz samples reveal a very high strength > 2 GPa at a few percent of permanent strain. The microstructures after short-term high stress deformation (type A) record localized brittle and plastic deformation. Statisc annealing (type A + B) results in recrystallisation restricted to the highly damaged zones. The new grains aligned in strings and without crystallographic preferred orientation, indicate nucleation and growth. Annealing at non-hydrostatic conditions (type A + C) results in shear zones that also develop from deformation bands or cracks that formed during the preceding high stress deformation. In this case, however, the recrystallised zone is several grain diameters wide, the grains are elongate, and a marked crystallographic preferred orientation indicates flow by dislocation creep with dynamic recrystallisation. Quartz microstructures identical to those produced in type A + B experiments are observed in cores recovered from Long Valley Exploratory Well in the Quaternary Long Valley Caldera, California, with considerable seismic activity.The experiments demonstrate the behaviour of quartz at coseismic loading (type A) and subsequent static annealing (type A + B) or creep at decaying stress (type A + C) in the middle crust. The experimentally produced microfabrics allow to identify similar processes and conditions in exhumed rocks. 相似文献
The Taparko gold deposit, located in the eastern branch of the Proterozoic Birimian Bouroum-Yalogo greenstone belt (Burkina
Faso) consists of a network of quartz veins developed in a N 170° trending shear zone (250 m wide, 4 km long) superimposed
on the regional Birimian structural pattern. The quartz vein network is composed of: (a) a dominant array of quartz veins
(type 1), parallel to the shear zone and comprising strongly deformed dark quartz exhibiting foliation, layering, ribbon,
tension gashes, etc.; (b) oblique and subparallel related veins (type 2) of gray to white weakly deformed quartz crosscutting
the dominant quartz veins resulting in breccia structures; and (c) shallow dipping veins (type 3), cross-cutting veins types
1 and 2 and filled by undeformed white buck structure quartz. Cross-cutting relationships and different quartz types in different
veins and within individual veins imply a concomitant filling of the veins during the progressive deformation. Initial sinistral
transcurrent shearing evolved with time to sinistral reverse shearing. Metallic minerals occur only in type 1 and 2 veins
and were deposited in two stages, with native gold being related to second stage sulfides. Gold (and chalcopyrite) precipitated
preferentially upon the surfaces of fractured pyrite grains in low-pressure sites (pressure shadow zones) around and/or within
the sulfide grains (along subsequently annealed fractures). The formation of the South Taparko deposit can be divided into
a succession of events: (a) during the first event, N 170°-directed sinistral transcurrent shearing resulted in a N 20° mylonitic
foliation and fractured rock which allowed H2O-, CO2- and SiO2-rich fluids to circulate and deposit quartz with buck texture; (b) during the second event, type 1 quartz was strongly deformed
and type 2 veins formed with sigmoidal shapes as viewed on a horizontal plane; and (c) during the third event, the sinistral
transcurrent shearing evolved to sinistral reverse shearing and the deformation style evolved correspondingly from ductile
to brittle-ductile. During the last phase of deformation gold nucleated and deposited in low-pressure zones.
Received: 9 July 1997 / Accepted: 23 March 1998 相似文献