Garnets - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 3

by the INAA technique, the authors studied geochemical properties of garnets and
clinopyroxenes from eclogites forming xenoliths from kimberlites of Bobbejaan and
Roberts Victor diamond deposits (South Africa). The contents of REE in these garnets
vary considerably depending on the variations in the contents of major components.
In particular, with increasing contents of Fe and Ca, the REE patterns of the mineral
become gentler relative to heavy elements. All the garnets from this collection are
depleted in LREE and enriched in HREE, the values of (La/Yb) n parameter change in
the range of 0.027-0.700. It was also shown that the REE distribution values in the
clinopyroxene-garnet system increase with the enrichment of minerals in Mg, and in
Ca-enriched samples the values of these coefficients are low. Based on the established
regularities in the variation of K d (clinopyroxene/garnet), the authors conclude that
REE in studied minerals have an equilibrium distribution. The distribution of major
and trace elements both in garnets and clinopyroxenes corresponds to their equilib-
rium at 3-5 GPa and temperature of 950-1150°C. Making reference to the previous
research efforts, Caporuscio and Smith suggested that in garnets REE isomorphously
replace bivalent cations Ca, Mg, and Fe, which occur in an octahedral coordination.
The REE distribution in zonal garnets from the contacting-metamorphosed rocks
of the Kwoiek Area (British Columbia, Canada) was studied using the IPMA tech-
nique [Hickmott & Shimizu, 1990]. The obtained data gives evidence that the con-
tents of MREE and HREE, as well as Y, decrease from the inner zones of garnet grains
toward the outer zones. Unlike other elements, the content of Dy remains nearly the
same within the inner zones of grains, but it decreases toward their peripheral zones.
The same analytical method was used in the detailed studies of the REE distribu-
tion in two types of garnets and coexisting clinopyroxenes from gabbros of the Ivrea
Verbano complex (Italy) [Mazzucchelli et al ., 1992]. The first type of garnets are large
porphyroblastic segregations and the second occur in reactional rims developed on
the borders of mineral grains. The REE patterns of garnets from porphyroblastic seg-
regations are positively sloping lines with negative Eu anomalies of low intensity. The
patterns of garnets from reactional rims at times display positive Eu anomalies. In the
mineral from porphyroblastic segregations the values of (Yb/Nd) n parameter equal
12-36, whereas in garnets from reactional rims the parameter is normally 6. In the

garnets from porphyroblastic segregations the K d (clinopyroxene/garnet) gradually
increases with increasing atomic number of REE, and in the mineral from reactional
rims this regularity is not so distinct. The authors report significantly wider variations
of K d (clinopyroxene/garnet) values for Ce compared to other REE.
The REE distribution in pyropes from megacrystals in peridotite xenoliths occur-
ring in kimberlites from the Udachnaya pipe (Yakutia, Russia) was studied in details by
Pokhilenko et al . [1993]. They showed that most of the described pyropes have a sinusoi-
dal shape of REE spectra, due to the more or less intense enrichment in LREE compared
to HREE. Following Shimizu & Richardson [1987], the authors draw a conclusion that
garnet-bearing peridotites from studied xenoliths underwent deep-level metasomatism
under the influence of fluids which were genetically related to carbonatite melt.
The study of the REE distribution between garnets and coexisting melts, con-
ducted by Beattie [1993] evidences that the values of K d (garnet/melt) somehow depend
on the concentration of these trace elements in the system. He believes that the studied
magmatic system in relation to REE does not obey the Henry's rule, the same as the
earlier established distribution of Ni in the “olivine-melt” system.

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