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Trace Elements in Magmatic Systems

- behavior of trace elements during evolution of magmas may be considered in terms of their partitioning between crystalline and liquid phases, expressed as the partition coefficient, D, such that for any trace element:

conc. of element in mineral (from phenocryst)
conc. of element in liquid (from glass or aphanitic matrix)

- elements of D << 1 are incompatible
(preferentially conc. in liquid phase during melting and crystallization
- those elements which are incompatible with respect to normal mantle minerals (ol, pyx, spinel, garnet) are termed lithophile or large-ion lithophile (LIL e.g.: K, Rb, Sr, Ba, Zr, Th, light Rare Earth Elements, LREE)

- elements with D > 1 (eg Ni, Cr) are termed compatible and are preferentially retained in the residual solids during partial melting and extracted in the crystallizing solids during fractional crystallization

- trace element concentration and trends in a rock series may tell us something about 1) the source region for the primary/parental magma, 2) the degree of partial melting, and 3) the conditions at the time of partial melting (presence of CO2, H2O, or other volatile phase)

- table 2.1 summarizes key trace element parameters

Rare Earth Elements (REE) - especially useful trace elements

- group of 15 elements:
La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu with Z = 57 (La) to 71 (Lu), 14 of which occur naturally
- have very similar outer shell electronic configuration ( = 5d06s2 )

- for a long time thought to behave identically geochemically, but now with sensitive, modern analytical techniques we can detect fractionation during normal igneous processes

- those with lower Z = light REE (LREE_
- those with higher Z = heavy REE (HREE)
- all except for Eu and Ce are trivalent under most geologic conditions
- Eu is both +2 and +3 in igneous systems, the ratio Eu2+/Eu3+ depending upon Fo2
- Eu2+ geochemically similar to Sr
- Ce may be tetravalent under highly oxidizing conditions

- in order to compare REE abundance for rocks graphically must first eliminate the Oddo-Harkins effect ( = existence of higher concentrations of those elements with even atomic #’s as compared with those of odd atomic #)
- achieved by normalizing the concentrations of individual REE in a rock to their abundance in chondritic meteorites
- smooths out the concentration variations from element to element
- chondrites (stony meteorites of peridotitic composition containing chondrules) are used because they are primitive solar system material which may have been parental to the Earth

- particular minerals have characteristic effect upon shape of REE pattern of the melt during partial melting and fractional crystallization, depending on their D values

- allows identification of their role in magmatic differentiation processes
- magnitude of the effect of a particular mineral depends on its abundance in the rock and the magnitude of its D value for a particular element

Some Examples of REE Behavior in Minerals:
1) Feldspars: D<1.0; repel all REE except for Eu, which behaves like Ca

2) Garnet: repel LREE, attract (concentrate) HREE. Its presence in equilibrium with a magma leads to a depletion in HREE

3) OPX/CPX: generally D<1.0; repel REE; values fro LREE slightly lower than for HREE, which may lead to LREE enrichment in the melt

4) Hornblende: D>1.0 or >>1.0; usually attracts (becomes enriched in) REE

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