TRACE-ELEMENT AND ISOTOPIC ZONING IN MINERALS - MODELS OF COMPOSITIONAL FRACTIONATION BY MINERAL SEPARATION PROCEDURES

Data on compositional zoning of minerals are required to apply thermodynamic models to mineral chemistry and to constrain kinetic models of diffusion and grain growth. Trace elements and isotopes have not been utilized fully in these applications because in situ methods of analysis (e.g. SIMS and PIXE) have not become routine. Trace element and isotopic compositions of minerals are usually determined from mineral separates. However, routine mineral separation procedures are not designed to detect and characterize zoning. Mineral separates are formed by segregating simple (monomineralic) particles of a mineral from other minerals and compound particles. Models of the formation of simple and compound particles during rock comminution show how the size and shape of mineral grains and the size of the particles formed affect the constitution of the separates. The interior portions of minerals are strongly over-represented in mineral separates, while the margins of the grains are preferentially included in compound particles. The particle size/grain size ratio is the dominant factor in the segregation of the rims of minerals from the cores. The particle size/grain size ratio can be controlled through regulation of rock comminution and sieving procedures. Separation protocols are suggested to gain information about the nature of chemical and isotopic heterogeneities in minerals. In particular, chemical and isotopic effects associated with thin zones near grain boundaries can be investigated effectively.