Isochrons mineral

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. 

U-Th mineral isochrons to date ancient volcanic rocks and thus deduce their ( Th/ Th)o ratios at the time of eruption (if isochron ages are close to eruption ages, see section 3.5 below), or alternatively rely on samples dated by another method (e g., K-Ar, Ar-" °Ar) to recalculate their initial Th isotope ratios. On the shorter timescale covered by Ra- Th disequilibria (0-8 ka), it can be even more difficult to find a suite of well dated samples, because eruption ages should be obtained from historical accounts... 

Data on mineral s arates in present day volcanic rocks. Since every dating method (including the K-Ar or C systems) can be affected by several geochemical perturbations which may lead to erroneous ages, the best test for the °Th- U mineral isochrons consists in the analysis of presently erupted lavas or historic lavas of well known eruption dates. Rather surprisingly the data obtained on such samples are not so numerous (some examples are illustrated in Fig. 10). Early data showed that, in some cases, there were interlaboratory analytical discrepancies, especially in Th isotope ratios measured on mineral separates extracted from the same lava flows (this was the case for the 1971 lava from Mt. Etna and 1944 lava from Mt. Vesuvius Capaldi and Pece 1981 Hemond and Condomines 1985 Capaldi et al. 1985). This emphasizes the fact that °Th- U mineral analyses... 

Figure 10 (on facing page). Several examples of mineral isochrons obtained on near zero age volcanic rocks. The figure compares the isochron diagrams obtained from both and data. 

Schaeffer et al. (1993) analyzed mineral phases extracted from the 1985 dacitic pumice erupted by Nevado del Ruiz (Colombia) pyroxene, plagioclase, magnetite, glass and whole rock define an U-Th isochron with a near-zero age (7 6 ka, 2g Fig. 10b). 

Figure 12. Several examples of complex mineral isochrons, or isochrons giving crystallization ages older than eraption ages (see text for a detailed discnssion). (a) data from a Santorini andesite (Pyle et al. 1988) interpreted as evidence of...

Figure 12 continued, (d)-(g) Mineral isochrons for rhyolites from the Olkaria volcanic center (Kenya), (d) and (f) alpha spectrometry resnlts from Black et al. (1997). (e) and (g) TIMS resnlts from Henmaim and Davies (2002). All the rhyolites have eraption ages between 3.3 and 9.2 ka. Note that the same sample (570) analyzed in both stndies gives rather different ages (f and g). Same abbreviations of mineral names as in Fignre 10 + Qz qnartz KF alkali feldspar Amph amphibole Bt biotite. 

It should be noted that, even when good U-Th mineral isochrons indicate old crystallization ages, Ra- Th disequilibria can be found in minerals, groundmass and whole rocks. It is the case for Mt. St Helens and Mt. Shasta (Volpe et al. 1991, 1992) and Vesuvius (Black et al. 1998) (see Fig. 10c,d) this requires either a selective Ra enrichment in the magma shortly before eruption accompanied by further crystallization... 

DATING YOUNG VOLCANIC ROCKS 4.1. Mineral isochrons... 

Th- U dating of samples from subaerial volcanoes. When several volcanic rocks covering a significant period of the eruptive activity of a volcano can be dated (either by mineral isochrons or by other dating methods), the evolution through time of the ( °Th/ Th)o or ( °Th/ U)o initial ratios will be revealed (see section 3.4). If these ratios remain nearly constant, then they may be used to calculate the ages of other lavas... 

Figure 23. Measured ( °Th/ Th) ratios in basalts from Piton de la Fournaise (Reunion Island) as a function of their eraption ages deduced from mineral isochrons. These ratios decrease with increasing emption ages as a result of post-eraptive radioactive decay. The curve shows the theoretical evolution by radioactive decay for a rock with a Th/U ratio of 3.95 and a ( °Th/ Th) ratio of 0.93, similar to the values measured in presently erapted lavas. An approximate age can thus be obtained from the measured ( °Th/ Th) ratio of an old sample. Part of the dispersion around the theoretical curve are due to small source heterogeneities (slightly variable ( °Th/ rh) and Th/U ratios), also evidenced by Sr/ Sr ratios (Condomines et al. 1988, and unpublished results).

Whole rock U-Th isochrons can also provide an age for the differentiation process, but they are rare because of the difficulty of fractionating the U/Th ratio through crystallization of major minerals. In the two reported examples (Laacher See phonolites, and Vico ultrapotassic volcanics), whole-rock isochrons show that differentiation occurred shortly before eruption. A similar conclusion was reached in the case of the Longonot volcano (Kenya), whose trachytes define the only reported ( Ra)/Ba-(230Th)/Ba whole-rock isochron. 

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