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distribution in the minor and
accessory minerals of ultramafic,
mafic and some other rocks
Mineral is a structural sieve for elements and this sieve is dirty.
D. Shaw
In recent years the development of modern microprobe methods of REE determin-
ing, and the lowering of the limits of REE detection, have expanded the study on the
regularities of distribution of these impurities not only on rock-forming, but also on
minor and accessory minerals of the ultramafic and mafic rocks, which include gar-
nets, chrome-spinels, ilmenites, zircons, apatites, titanites, perovskites and micas. As
it was shown in previous chapters, these minerals are significantly different from each
other in the general chemical composition, crystal-chemical properties and formation
conditions, have different abilities to accumulate REE and other impurities. There are
reasons to believe that an appropriate use of data on the REE distribution in these
minerals can help to gain additional geochemical criteria and typomorphic signs for
their diagnosis, discrimination, systematization and genetic interpretation.
Geochemical studies conducted on this issue suggest that the same types of acces-
sory minerals that are presented in ultramafic and mafic rocks of different composi-
tion and origin and in magmatic complexes composed of them, to some extent, differ
in the level of REE accumulation, the degree of fractionation and in a number of
other features. There are reasons to believe that these differences were due to various
factors, such as an initial concentration of REE in the maternal melts of these min-
erals, as well as differences in temperature, pressure and redox conditions of melts
generation and minerals crystallization. Along with the general level of accumulation
and other parameters of REE distribution in minor and accessory minerals there is a
very important characteristic of them which is the calculated on the basis of data on
natural and experimental systems distribution coefficients of REE between minerals
and their parent melts, as well as between coexisting mineral phases. Given the ability
to concentrate in its structure one or another amount of isomorphous REE impurity,
the accessory and minor minerals from the ultramafic, mafic and related rocks can be
divided into the following five groups: 1) garnets and zircons that accumulate signifi-
cant amounts of REE with substantial prevalence of chondrite-normalized contents
of HREE; 2) apatites and perovskites that can accumulate significant amounts of
REE with substantial predominance of LREE; 3) titanites that accumulate medium
amounts of light and heavy REE at near their chondrite-normalized concentrations,
4) micas that accumulate medium amounts of REE with a slight predominance
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