Geochemical examples

The measurement of diffusion rates as functions of various activities, in particular oxygen fugacity, is extremely important to clarifying the diffusion mechanisms and aid in extrapolation. The work of Ryerson et al. (1989) is the only geochemical example of such type of work. Questions of concern include under what conditions does the /02-dependence occur, and what does it tell us about the main mechanism of O2 transport (e.g. permeation versus diffusion, network versus molecular oxygen diffusion etc.). 

Mineral polymorphs of this type are the usual sort of geochemical examples used to define metastable equilibria, and the usual definition centers on the fact that the metastable phase has a higher energy content than the stable phase. By focusing on the constraint that prevents the metastable phase from changing to the stable phase, we have not changed the concept of metastability but have simply broadened its applicability, as shown in the other examples. The perfect analogy between the partitioned gas example and the polymorph example should demonstrate the utility of the extra constraint definition we have used. 

Geochemical studies of sulfate ia streams have beea approached ia various ways. For example. Ref. 17 is a review of Hterature oa sources of sulfate ia the dissolved loads of streams, especially ia areas where developmeat effects are likely to be stroag. It was coacluded that for North America as much as 60% of the average yield of sulfate carried to the ocean is related to human activities. This study assigned a higher proportion of the total yield of sulfate ia European streams to human sources, but for other continents it was thought the effects of human activities to be relatively minor. 

Geochemically significant mixing and transport can sometimes be accomplished by biological processes. An interesting example is redistribution of sediment material caused by the movements of worms and other organisms (bioturbation). 

Sulfur exists naturally in several oxidation states, and its participation in oxidation/reduc-tion reactions has important geochemical consequences. For example, when an extremely insoluble material, FeS2, is precipitated from seawater under conditions of bacterial reduction, Fe and S may be sequestered in sediments for periods of hundreds of millions of years. Sulfur can be liberated biologically or volcanically with the release of H2S or SO2 as gases. 

These examples do not eover all possible variations of the solvents or column types described in the geochemical literature. The combinations of solvent separations, ion-exchange chromatography, and adsorption chromatography have also been proposed. 

Examples of the use of INAA in homogeneity testing of RMs are numerous and therefore only a few illustrative examples can be mentioned. More than ten elements can frequently be used for the accurate estimation of homogeneity of several types of biological materials (Tu and Lieser 1984), while a much higher number of elements can usually be precisely assayed in geochemical and environmental RMs. 

Examples of using reference samples for calibration can be found in several chapters of the uses Methods for Geochemical Analysis (Baedecker 1987). Solid reference sample powders are used in cahbrating the dc arc emission, energy-dispersive X-ray and instrumental neutron activation analyses described, while acid-dissolved rock reference samples are used for IGP emission analyses and fused reference samples are used for wavelength-dispersive X-ray analyses. 

Frequently, many laboratories around the world are joint participants in major geochemical studies. One example already mentioned is the lunar program of almost... 

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... 

The complex interactions amongst geological, biological, and geochemical processes at the land-sea margin control the delivery and fate of radionuclides, contaminants, and other natural elements in coastal environments (Swarzenski et al. 2003). For many such constituents, there is at least a fundamental understanding of major source and sink functions and their potential estuarine transformation reactions. For example, rivers can be monitored quite easily for discharge rates into estuaries as well as for elemental... 

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