Analysing geochemical data

The major part of this book discusses the four main types of geochemical data outlined above and shows how they can be used to identiiy geochemical processes. In addition, Chapter 5 has been included to show the way in which trace and major element chemistry is used to determine the tectonic setting of some igneous and sedimentary rocks. Chapter 2 discusses some of the particular statistical problems which arise when analysing geochemical data-sets, and some recommendations are made about permissible and impermissible methods of data presen tion. 

Geochemical data consist of analyses of 485 pond sediment samples collected as part of the 1970s National Uranium Resource Evaluation (NURE) program (http //tin.er.usgs.gov/geochemO. 

Sample description and preparation. The samples analysed in this study are listed in Table I together with available geochemical data. Table II provides general burial history information for the four sedimentary sequences. 

Different evolutionary histories of other terrestrial planets have influenced the relative concentrations of the transition elements compared to their cosmic abundances, as suggested by geochemical data for surface rocks on the Moon, Mars and Venus (Appendix 1). Chemical analyses of lunar samples returned from the Apollo and Luna missions show that minerals and glasses occurring on the Moon contain high concentrations of Fe and Ti existing as oxidation states Fe(II), Ti(III) and Ti(IV). Some lunar minerals, notably olivine and opaque oxides, also contain significant amounts of Cr(H), Cr(III) and Mn(H). The lack of an atmosphere on the Moon simplifies interpretation of remote-sensed reflectance spectra of its surface. 

The geochemical data come from 239 shallow probe (1.2 metre, 4 feet) soil-gas samples collected on 500 - 1000 m grids placed directly over these two fields, with 95 sites over Filo Morado and 144 sites over Loma de La Lata. The free soil gases were analysed for methane, ethane, ethylene, propane, propylene, iso-butane and normal butane by gas chromatography using a flame ionisation detector. 

On the basin flanks there have been several papers on the eastern margin, including geochemical data on cements in the Mountain View and Edison fields (Fischer Surdam, 1988) and Kem River, Poso Creek, Rosedale Ranch, Fruitvale and Mount Poso fields (Boles Ramseyer, 1987 Hayes Boles, 1993). On the western flank, studies of carbonate cementation Include the North Belridge field (Taylor Soule, 1993) and Kettleman North Dome (Merino, 1975 Lee Boles, 1996). The new analyses from the basinmargin include data from the Mount Poso field and a wildcat well Ohio KCLG-1, about 5 km southwest of the Fruitvale field. 

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... 

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