Whole rock dating

Graham IJ, Palmer K (1987) New precise Rb-Sr mineral and whole-rock dates for I-type granitoids from Granite Harbor, south Victoria Land, Antarctica. New Zealand Antarctic Record 8(l) 72-80... 

Whole-rock dating is analytically difficult because of the lack of radiogenic enrichment in the lead, particularly ° Pb. Though some approaches to whole-rock dating appear to have been made previously, the first concerted attempts were made by Sobotovich (1961) on granites and gneisses about 3,000 m.y. old (for which ° Pb effects would be relatively large) and by Cobb (1961) on lower Paleozoic black shales and Swedish Kolm. 

Evaluation of U-Th-Pb whole-rock dating on Phanerozoic sedimentary rocks. Eclogae Geological Helvetica 63, in press (1970). 

Figure 1.27. Areal distribution of the whole-rock 5 0 values of footwall volcanic rocks in the Fukazawa area. The boundaries for the alteration zones are modified from Date et al. (1983) (Green et al., 1983).

Green, G.R., Ohmoto, H., Date, J. and Takahashi, T. (1983) Whole-rock oxygen isotope distribution in the Fukazawa-Kosaka area, Hokuroku district, Japan, and its potential application to mineral exploration. Econ. Geol. Mon., 5, 395—411. 

Dating of whole rocks by any U-series method is possible only if the initial (N2/Ni)o ratio is known (see Eqn. 1). In several cases, this condition can be met as will be shown in the examples below. 

Further advances in the interpretation of mineral data could come from in situ analyses (unfortunately only possible presently on U-Th rich accessory minerals like zircon), or from detailed studies of particularly large phenocrysts, whose successive growth zones could be sampled (e g., through microdrillings) and analyzed. Ra measurements in such crystals could allow a direct determination of their growth rates. A systematic study of successive, well-dated eruptions of a given volcano, combining U-series measurements in both whole rocks and minerals, should also help with the interpretation of mineral data. 

Dating of whole rocks from the measurements of their °Th- U or Pa- U disequilibria has been successful in the case of MORE, and can also be applied to volcanoes for which the evolution of these disequilibria through time has been studied in detail (e.g., Piton de la Foumaise). Similarly Ra- Th disequilibria may be useful dating tools in the age range 0 to 8 ka, for MORE or continental volcanoes where a detailed knowledge of their variations in well dated samples is available. 

The Lac Turgeon Granite was previously dated at >980 Ma, based on a U-Pb Concordia intercept (Rimsaite 1982), and 948 23 Ma from a whole-rock Rb-Sr isochron. It has an initial Sr/ Sr of 0.7090 0.0043 (Fowler Doig 1983) it is relatively young suggesting it has been partially reset or remained open. 

PI and P5 represent the age of the granite, and agree with previous dating (Rimsaite 1982). P3 represents the age of the pegmatite. P2 and P4 overlap and represent an event that reset monazite in both the granite and pegmatite, and corresponds to the age that Fowler Doig (1983) obtained by whole-rock Rb-Sr methods. 

After formation, the parent and daughter should have not been lost from or gained by the system to be dated. That is, the system must be a closed system after the event. In some ideal cases, for a system disturbed by later events, there can be two different isochrons, a mineral isochron, from which the age of the later disturbance (e.g., metamorphic event) can be calculated, and a whole rock isochron, from which the age of the original igneous event can be calculated. That is, the ages of both events may be inferred. 

The Re- Os system does not lend itself to the determination of internal isochrons for most meteorites. Chen et al. (1998) produced an internal isochron for the St. Severin (LL6) chondrite using metal separates and whole-rock samples (Fig. 8.21). The slope of the isochron gives a date of 4.60-0.15 Ga (X = 1.666 x 10 11 yr 1), consistent with expectations based on other chronometers, but not precise enough to improve upon other techniques. 

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