Isochronous studies

CRM suppliers such isochronous tests can be planned and can be repeated over time. For the study of new materials this approach is not suited as the probable instability is only known at the very end. For the development studies on storage and behaviour of the material rapid answers are necessary. 

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Similar isochronous studies have been carried out on polyethylene terephthalate sheets with orthorhombic symmetry by Ladizesky and Ward, on cold-drawn Nylon 6 and 66 by Owen and Ward and on oriented polypropylene by Owen and Ward. ... 

A more recent approach is based on isochronous studies in which samples are stored at different temperatures at different time intervals, performing all the analyses at the end of the study. This enables to plan the measmements well in advance and hence limits risks finked to an insufficient reproducibility of the method (which may be faced by the first approach). The drawback is that results are only available at the end of the study, which may generate a delay of the stability assessment and thus of the overall certification process. A flow chart illustrating this approach is shown in Figure 3. 

That there is hope of finding terrestrial rocks somewhat older than 3,500 m.y. is implied in the secondary isochron study of igneous rocks about l,900-m.y.-old from the Churchill province in Canada (Doe, 1967). The slope of leads from three feldspars (A Pb/ Pb)/( A Pb/ Pb) is 0.521 the source age (t) may then be 3,800 m.y. 

Long-term deformation such as shown by creep curves and/or the derived isochronous stress-strain and isometric stress-time curves, and also by studies of recovery for deformation. 

Fig. 2-5 Typical storage design for isochronous measurements — long term study (Lamberty et al. 1998)...

Lambeety a, Schimmel H, Pauwels J (1998) The study of the stability of reference materials by isochronous measurements. Fresenius J Anal Chem 360 359-361. 

In their study of the Laacher See eruption. Bourdon et al. (1994) obtained an isochron from their data on phonolitic pumice glasses (probably with U/Th ratios similar to those of the whole rocks) and glasses from cumulate nodules. The fractionation of U/Th ratios is attributed in that case to the crystallization of accessory U and Th-enriched phases such as sphene and apatite. The age of 14.3 6.5 ka is similar to the ages deduced from mineral isochrons (see section 3.5) and to the eruption age of 12.5 0.5 ka (Fig. 7b). Thus differentiation within the phonolitic magma occurred shortly before eruption. 

In a recent study, Zellmer et al. (2000) found a zero-age U-Th isochron for the 1940 Kameni dacite in Santorini (Aegean volcanic arc). The existence of ( Ra/ °Th) ratios lower than 1 in Kameni dacites suggests that plagioclase fractionation took place less than 1 ka before eruption, in agreement with estimates based on Sr diffusion profiles in plagioclases (Zellmer et al. 1999). 

Another example of Ra- Th isochron dating is provided by the study of Reagan et al. (1992) on phonolites erupted in 1984 and 1988 at Mt. Erebus (Antarctica). Glass-... 

Several volcanic centers from the Kenya Rift Valley in East Africa were studied by Black et al. (1997, 1998). Four U-Th isochrons were obtained from trachytes from Emuruangogolak and Paka volcanoes (Black et al. 1998) their crystallization ages range from 9.3 to 39.5 ka and are in full agreement with in situ Ar- Ar laser dating of... 

Age Stellar ages can be derived using isochrones. However, such data can not be used to select stars for studies of the thick disk as this must imply a prior knowledge of the age of the stellar population in this disk. This is knowledge that we seek, but do not have. 

Radial velocities were measured by cross-correlation, using a synthetic spectrum as template. Individual spectra were shifted to rest wavelength and coadded. Effective temperatures were derived from the (V — I)o colours by means of the Alonso calibration [8], We assumed log g = 2.0 for all stars (estimated from isochrones) and with these parameters we fed the spectra to our automatic procedure for the determination of abundances [9], We found that the S/N ratio was too low to be able to determine reliably the microturbulent velocities, the weak Fe I lines could not be measured on many spectra. This resulted in a marked dependence of derived abundances on microturbulent velocities. It is well known that microturbulence is not a truly independent parameter but correlates with surface gravity and, more mildly also with effective temperature. By considering the large sample of stars studied by [10] one can be convinced that for all stars with 1.5 < logg < 3.0 (20 stars) there is no marked dependence from either Tefi or log g, and the mean value of the microturbulent velocity is 1.6 kms 1. For this reason we fixed the microturbulent velocity at 1.6 kms-1. 

Stellar evolution has consequences in the development of luminosity and colours of stellar populations, as well as chemical enrichment. Boissier and Prantzos (1999) have produced a more-or-less classical model of the evolution of the Milky Way, making a detailed study of this aspect, known as chemo-photometric evolution , using an IMF similar to the Kroupa-Scalo function in Chapter 7 this detail is significant because the Salpeter(O.l) function often used has a smaller contribution from stars of around solar mass which dominate the light at late times. The chemical evolution results are combined with metallicity-dependent stellar isochrones, synthetic stellar spectra by Lejeune et al. (1997) and a detailed treatment of extinction by dust. Some of their results are shown in Fig. 8.39. 

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