Pierre-Jean Gauthier

Timescales of Magma Chamber Processes and Dating of Young Volcanic Rocks

One of the initial goals of measuring U-series disequilibria in young volcanic rocks was the development of new dating methods in an age range (about 0–300 ka), where few other methods were applicable at that time (Cerrai et al. 1965; Cherdyntsev et al. 1967; Kigoshi 1967; Taddeucci et al. 1967; Allegre 1968). In the following years, several studies showed that the 238U-230Th method, the first to be applied, was indeed able to give reliable ages on rocks from various volcanoes (e.g., Allegre and Condomines 1976; Condomines et al. 1978). But it was soon realized (e.g., Oversby and Gast 1968; Allegre and Condomines 1976; Capaldi et al. 1976) that U-series disequilibria could not only be used to date volcanic rocks, but also to infer the timescales of magma transfer and evolution in the crust, and that these methods could give rather unique information on such important parameters. During the last twenty years, besides continuing efforts to develop and discuss the U-series dating methods, many studies have addressed the problem of the timescales of magmatic processes, from melting in the mantle, to crystallization and differentiation of the magmas in their reservoir(s), and magma degassing near the surface. Moreover, as far as studies on volcanic minerals are concerned, determining the eruption age and the timescales of crystallization are intimately linked. It is the purpose of this chapter to review some of the recent advances in the fields of magma chamber processes and dating (the timescales of partial melting in the mantle are treated in other chapters of this book: Lundstrom 2003; Bourdon et al. 2003; Turner et al. 2003). General reviews of U-series disequilibria in volcanic rocks have been published by Condomines et al. (1988), Gill et al. (1992), …

Precise determination of Rb, Sr, Ba, and Pb in geological materials by isotope dilution and ICP-quadrupole mass spectrometry following selective separation of the analytes

A new method was developed for the high precision measurement of Rb, Sr, Ba, and Pb concentrations in silicate rocks. This method is based on the combined use of isotope dilution and ICP quadrupole mass spectrometry, with matrix element removal. The concomitant separation of the analytes is achieved by extraction chromatography (Ba, Sr, Pb) and inorganic cation exchange chromatography (Rb), on two small tandem columns filled with 0.1 ml of “Sr resin” and 1 ml of zirconium (or titanium) phosphate, respectively. Compared to previous separation schemes based on cation- and anion-exchange, this method allows the rapid, concomitant isolation of four trace elements of great interest in geochemistry, with a greatly reduced volume of reagents. The capabilities of the method in terms of precision (equal to or better than 1% RSD) and accuracy are demonstrated by duplicate analyses of four international reference materials (BIR-1, BHVO-1, AGV-1 and RGM-1, from the US Geological Survey) representative of various silicate rock bulk compositions and spanning a large range (0.2–1000 µg g−1) of analyte concentrations.