Trace elements in magmatic processes

Trace elements are useful tracers of geochemical processes mostly because they are dilute their behavior depends primarily on the trace element-matrix interaction (e.g., Rb-host feldspar, Sr-calcite) and very little on the trace-trace interaction (e.g., Rb-Rb, Sr-Sr). Consequently, the distribution of trace elements among natural phases largely obeys the linear Henry s law. The modeling of trace elements in various geological environments (magmas, hydrothermal fluids, seawater.) relies on three different aspects 

This form of the partition coefficient, analogous to that used for Fe-Mg fractionation between olivine and melt (see Chapter 1), is necessary only for the rare cases where trace substitution affects Cj and Cp substantially. A number of reviews (O Nions and Powell, 1977 Michard, 1989) describe the various sorts of partition coefficients expressed either in mass-fractions, atom fractions, or normalized to a major element and their respective merits. If the discussion is restricted to a narrow range of chemical compositions (e.g., basaltic systems, Irving, 1978, Irving and Frey, 1984), enough experimental information exists on trace-element partitioning to resort to the wonderfully simple equation (9.1.1). 

Batch partial melting will hereafter be understood as equilibrium melting, which is in contrast to fractional melting discussed in Section 9.3.3. The foundation of this model is remarkably simple and was first laid down by Schilling and Winchester (1967). A number of more or less complex modifications enabling useful information to be extracted from the data were later introduced by Gast (1968), Shaw (1970) and Albarede (1983). Bulk equilibrium crystallization of a liquid batch can be handled with equations identical to those for batch-melting. 

We consider a molten multi-mineral assemblage, referred to as the source, which is presumed to give rise to an erupting magma. X is the mass-fraction of each phase j relative to the molten source (and not to the residue). Subscript j= 1 refers to the melt, j = 2. n to the n — 1 residual mineral phases. The sum of the Xj over all the n phases is unity. If Cliq is the concentration of the ith among m elements in the liquid, the concentration of element i in phase j will be K/C,iq . Let C0 be the concentration of i in the source prior to melting. Hence, mass balance requires 

The sum of the f over all the n — 1 residual mineral phases is unity. This leads to the equation known as the equilibrium, partial, or batch-melting equation 

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Carroll M. R. and Draper D. S. (1994) Noble gases as trace elements in magmatic processes. Chem. Geol. 117, 37—56. 

Bumard PG, Hu R, Turner G, Bi X (1999) Mantle, crustal and atmospheric noble gases in Ailaoshan gold deposits, YuiuianProvince, China. Geochim Cosmochim Acta 63 1595-1604 Carroll MR, Draper DS (1994) Noble gases as trace elements in magmatic processes. Chem Geol 117 37-56... 

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