Whole Rock

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. 

Pb 206p j/204p 5 207p 5 204p 208p /204p  

Evaluation. The method of determining Pb- Pb age on whole-rock crystalline rocks shows great promise as a dating technique. The procedure has some capacity to see through metamorphisms and has 

Although some data suggest optimism in the use of isochrons on shales, limestones, and authigenic portions of clastic sediments, other information presents severe problems. The technique is not yet perfected. 

Primary growth equations (all in atomic mass units)  

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If, hypothetically, the whole rock was taker to be porous, a /a would be 1 (volume sorption) this is typical for the migration in a chromatographic column. In reality, only part of the rock matrix could be available for sorption, typically giving aj/ap = 10-3 (surface sorption) for the conditions used in our studies (66). Other representative values are p =2500 kg/m3, p =1000 kg/n and e 0.001. r W... 

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).

Figure 1.28. Whole-rock 0 values of Miocene volcanic and sedimentary rocks from the Hokuroku district, grouped by alteration zones. Each square represents one sample. Mont. = montmorillonite, Ser. = sericite, Chi. = chlorite, av. = average (Green et ah, 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. 

Watanabe et al. (1993) have determined a whole rock isochron age of 107 15 Ma (yi b = 1.42 X 10 Vy) for well-preserved pillowed basalts in Western Shikoku with their basalt initial ratio of 0.70401. 

Halbach, H.P., Hansmann, W., Koppel, V. and Proajus, B. (1997) Whole-rock and sulfide lead-isotope data from the hydrothermal JADE field in the Okinawa back-arc trough. Mineralium Deposita, 32, 70-78. 

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. 

Figure 7. Two examples of whole-rock isochrons, (a) Isochron obtained on a lava nnit of the Vico volcano (Central Italy) (Villemant and Flehoc 1989). (b) Isochron defined by glass and groundmass analyses of the Laacher See phonolite (Bonidon et al. 1994). In both examples, differentiation occurred shortly before eraption since calculated ages are indistinguishable from eraption ages.

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). 

In the Taupo volcanic zone of New Zealand, the 26.5 ka Oruanui eruption was studied by Charlier and Zellmer (2000). Three fractions of zircons (sub 63 pm 63-125 pm 125-250 pm) were extracted from the rhyolitic pumice, which together with the whole rock respectively define three ages from 5.5 to 12.3 ka before eruption (Fig. 12b). Microscopic observation of the zircons showed that they are composed of a core surrounded by euhedral rims, and the preferred explanation of the authors is that zircons represent mixtures in variable proportions of old crystal cores crystallized 27 ka before eruption and crystal rims crystallized just before eruption. 

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 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. 

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. 

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. 

Ghaleb et al. 1990 Goetz 1990 Hillaire-Marcel et al. 1990 Dequincey et al. 2002). The models retained in the various studies differ from each other in the relative chronology assumed for U gains and losses. All the models have assumed, however, first order kinetics for U losses, which is supported by a few U leaching experiments on both whole rocks and separated minerals (Latham et al. 1987a b). 

Langmuir D (1997) Aqueous Environmental Geochemistry. Prentice Hall, New Jersey, USA Latham AG, Schwarcz HP (1987a) On the possibihty of determining rates of removal of uranium from crystalline igneous rocks using U-series disequilibria - 1 a U-leach model, and its apphcabihty to whole-rock data. Appl Geochem 2 55-65... 

Fig. 1. Distribution of whole rock geochemical data in Australia (plus signs) extracted from the OZCHEM national database as at June 2006, overlain on bedrock (pink) and regolith (green) coverages.

The country-wide dataset of stream sediment analyses in Austria consists of 36,136 samples analyzed for 34 chemical elements (Fig. 1), (Thalmann et al. 1989). Complemented by local surveys of hydrochemistry, whole rock geochemistry, soil chemistry and mineralogical phase analyses, these data are used to derive natural background levels of different rock units, investigate chemical fluxes between soil, rock and groundwater, and evaluate the emission risks of historical mine waste. 

While the first method gives a quick visual overview of ranges, the second, more sophisticated method leads to a more detailed correlation. In both cases, the correspondence between stream sediment and whole rock geochemistry is not perfect since sediments represent only the weathered product of rocks (Pfleiderer et al. 2008). Lithologically homogeneous areas away from mining sites or mineralization are used to derive natural background levels. 

XRF is widely used in industrial applications where a large number of elements need to be determined quantitatively. It is used for continuous quality control in the steel industry (e.g., the determination of Mn, Cr, Ni, Co, etc., in the production of stainless steels), and also for casting quality of coins in the Royal Mint (where Cu, Ni, and Zn are continuously monitored). Geological applications include whole rock analyses and clay identification. The power industry uses it as pollution control management, measuring sulfur and heavy metal concentrations in fuels (coal, oil) and ash. 

Although less compatible than Mg, Al apparently is still well retained by the residue during melting, which supports the presence of an aluminous phase in the residue as suggested by Johnson et al. (1990). If we knew the Al203/Mg0 ratio of the whole rock prior to melting, it would be possible to calculate that ratio in the last liquid extracted and compare its value with mid-ocean ridge basalt (MORB) values. <=... 

Figure 1.13 Incremental water/rock interaction change of the whole rock 5180 with the integrated water/rock ratio Q as given by equation (1.5.40). Water-rock fractionation is...

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