THE THERMODYNAMICS OF TRACE ELEMENT DISTRIBUTION

Lamont-Doherty Geological Observatory of Col umbia University, Palisades, New York 1096U  

Department of Earth Sciences, University of Leeds, Leeds. 

Conventionally, elements are categorized as major, minor or trace depending upon their relative abimdances in the system of interest. The divisions between these three categories have not been rigidly fixed, but the term trace element has frequently been used for any element whose abundance is 1000 ppm or less. At very low concentrations of a trace solute constituent, the thermodynamic behaviour of the solution may be expected to approach that of an ideal dilute solution. The concentration range over which Henry s Law is obeyed by a given trace constituent cannot as yet be predicted and must be determined by experiment. It is unfortunate that the term trace element as currently used encompasses concentration ranges where departures from Henry s Law may occur. As more data become available on the solution behaviour of trace elements some qualifications of the term may prove worthwhile. 

The present article is primarily concerned with a discussion of the thermodynamics of typical solid-liquid and some solid-gas equilibria which may have been important in controlling the distribution of trace elements in the earth, moon and meteorites. 

In this respect attention is drawn to the limitations of the conventional Berthelot-Nemst and Henderson-Kracek distribution coefficients in geochemistry. Particular attention is paid to the thermodynamics of gas-solid reactions since these are becoming increasingly important as evidence accrues for fractionation of rare-earth elements by solid-gas reactions prior to meteorite formation. 

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O Nions, R. K. Powell, R. (1977). The thermodynamics of trace-element distribution. In Thermodynamics in Geology, ed. D. G. Fraser, pp. 349-63. Dordrecht Reidel. 

Though several studies (5-9) have described the concentrations of trace elements in surface waters, most of them did not differentiate between species of a particular element. Some (10) have considered the distribution between dissolved and particulate forms. However, few attempts (11) have been made to evaluate the distribution of metals between various solid phase components of the suspended material. The present study provides quantitative estimates of dissolved (dissolved is defined as those aquatic components that could not be removed by centrifugation from liquid phase) and various solid phase associated metal fractions in southeastern United States streams. Between November 83 and August 85, 46 bi-weekly samples were taken from six SRP associated watersheds to determine dissolved (filterable) and total element concentrations. As one of several goals of the study was to assess the impact of natural and production related activities on trace element behavior in these aquatic systems, knowledge of speciation within solid and dissolved phases was essential for data interpretation. The research described herein used sequential extraction and a thermodynamics approach to define solid and dissolved phase species of Cu, Cd, Fe, Mn, Ni and Zn. The study also evaluated the effects of natural and production related processes on the distribution of metals in aquatic systems at SRP. 

The factors which control the distribution of trace elements [defined arbitrarily in geochemistry as those elements present at less than 0.1 weight percent (wt %)] can be discussed under a number of headings - structural, thermodynamic, kinetic and, in the sedimentary environment, solubility and speciation. 

This is a study of the non-convergent ordering of Mg and Ni in synthetic olivines by means of the neutron powder diffraction and X-ray absorption spectroscopy (EXAFS). The implications of the intracristalline ordering to olivine crystal chemistry and their application to the consideration of thermodynamic relations, geothermometry, geo-speedometry, and minor or trace element distribution control is discussed in detail. 

The interpretation and modelling of trace element abundance variations in rocks and minerals have employed Berthelot-Nernst distribution coefficients or less frequently compounded coefficients such as the Renderson-Kracek coefficients. Although from the standpoint of thermodynamics these are inadequate and may differ considerably from time equilibrium constants, their use has met with considerable success. 

The distribution of trace elements between solid and gas phases during, for example, the condensation of the solar nebula, is also amenable to thermodynamic treatment, and the results of... 

As measurements in these protocols are made in equilibrium conditions, only thermodynamic information is obtained. However, kinetic extraction-desorption studies are a more correct approximation to the distribution of species in natural media (Aulitiia and Pickering, 1988 Bermond et al., 1998 Ortiz-Viana et al., 1999 Fangueiro et al., 2002 Gismera et al., 2004). The desorption rate constants of the trace element in sediments and soils can be related to its mobility and toxicity. 

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