Compatibility (geochemistry)
Contents
Introduction
In geochemistry, compatibility is a measure of how readily a particular trace element substitutes for a major element within a mineral.
Compatibility of an ion is controlled by two things: its valence and its ionic radius.[1] Both must approximate those of the major element for the trace element to be compatible in the mineral. For instance, olivine (an abundant mineral in the upper mantle) has the chemical formula (Mg,Fe)2SiO4. Nickel, with very similar chemical behaviour to iron and magnesium, substitutes readily for them and hence is very compatible in the mantle. The compatibility of an element in a rock is a weighted average of its compatibility in each of the minerals present. By contrast, an incompatible element is one that is least stable within its crystal structure.
Compatibility controls the partitioning of different elements during melting. If an element is incompatible in a rock, it partitions into a melt as soon as melting begins.
In general, when an element is referred to as being “compatible” without mentioning what rock it is compatible in, the mantle is implied. Thus incompatible elements are those that are enriched in the continental crust and depleted in the mantle. Examples include: rubidium, barium, uranium, and lanthanum. Compatible elements are depleted in the crust and enriched in the mantle, with examples nickel and titanium.
Distribution (Partition) Coefficient
In a mineral, nearly all elements distribute unevenly between the solid and liquid phase. This phenomenon known as chemical fractionation and can be described by an equilibrium constant, which sets a fixed distribution of an element between any two phases at equilibrium.[1] A distribution constant is used to define the relationship between the solid and liquid phase of a reaction. This constant is often times referred to as when dealing with trace elements, where
for trace elements
The equilibrium constant is an empirically determined value. These values depend on temperature, pressure, and composition of the mineral melt. values differ considerably between major elements and trace elements. By definition, incompatible trace elements have an equilibrium constant value of less than one because trace elements have higher concentrations in the melt than solids.[1] This means that compatible elements have a value of . Thus, incompatible elements are concentrated in the melt, whereas compatible elements tend to be concentrated in the solid.
Bulk Distribution Coefficient
The bulk distribution coefficient is used to calculate the elemental composition for any element that makes up a mineral in a rock. The bulk distribution coefficient, , is defined as
where is the element of interest in the mineral, and is the weight fraction of mineral in the rock. is the distribution coefficient for the element in mineral .[1]
- a b McSween, Harry Y.,. Geochemistry : pathways and processes. Richardson, Steven McAfee., Uhle, Maria E., Richardson, Steven McAfee. (Second edition ed.). New York. ISBN 9780231509039. OCLC 61109090.
See also
- Magma
- Mineral
- Isotope geochemistry
- Continental crust
- Mantle
- Igneous differentiation
- Fractional crystallisation
- Incompatible element
- Distribution constant
- Fractionation
References
External links
White, William M., 2005. Geochemistry (Online textbook)