Garnets - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 8

in alkaline basalts, and 3) inclusions in diamonds, and 4) eclogites from xenoliths in
kimberlites and alkaline basalts, and 5) eclogites from high-pressure metamorphic
complexes, and 6) some types of gabbros, and 7) some varieties of effusive rocks, and
8) skarns, and 9) gneisses, schist and other metamorphic mesobaric rocks.
High-pressure garnet peridotites, mainly harzburgites and lherzolites. Garnets
from these ultramafic rocks are studied in detail, particularly regarding the REE dis-
tribution in samples from diamondiferous and non-diamondiferous deep-seated xeno-
liths presented in kimberlites (Table 1.1, Figure 1.1). By chemical composition they
are predominantly high-chromium ( 5% Cr 2 O 3 ) and subcalcium pyropes. The obser-

vations indicate significant heterogeneity of the REE distribution in them, including
their total compositions and the levels of accumulation of certain elements, the degree
of fractionation of REE and configuration of the patterns. Based on the total sample
of analysis of garnets from these rocks, compiled from our database, we found that
the total REE content in them is from the first grams per ton to 35 ppm.
In the garnets from harzburgite xenoliths presented in the Roberts Victor kimber-
lite pipe, the total REE concentrations vary in the range of 1.5-18.6 ppm, while for
garnets from lherzolite xenoliths - 0.8-10.9 ppm. In the minerals from harzburgites
we can observe wider fluctuations in concentrations for medium and heavy REE,
while in the minerals from lherzolites - for light elements. Some samples of garnets
from harzburgites show depletion in heavy elements, starting with Sm and ending
with Yb that is why their patterns have a sinusoidal configuration (see Figure 1.1, 1),
they show smooth peaks in Pr area, as well as smooth minima in the range between
Gd and Er. Almost all analyzed garnets from lherzolite xenoliths of Roberts Victor
pipe are depleted by LREE, and in the medium and heavy elements area their patterns
have a flattened shape (see Figure 1.1, 2). Evidence of heterogeneity of REE distribu-
tion in garnets from xenoliths of this pipe is wide variations in (La/Yb) n values - from
0.01 to 14.2. According to the studies, REE composition of garnets from diamondif-
erous peridotite xenoliths of Roberts Victor pipe corresponds to the composition of
REE in this mineral from the primitive mantle. It is assumed that garnets from lherzo-
lite parageneses characterize the event of its enrichment in LREE, whereas the samples
of garnets from harzburgite parageneses have not been subjected to such enrichment
[Stachel et al ., 1998; Nixon et al ., 1987].
Having examined the representative collection of samples of garnets from peridotite
xenoliths of Udachnaya kimberlite pipe (Yakutia), Pokhilenko et al. [1993] and Shimizu
et al . [1997a] determined that the total REE content in them varies in a very wide range
(0.7-39.8 ppm). A significant part of the studied garnets had sinusoidal REE pattern (see
Figure 1.1, 13, 15). Such patterns are often observed in garnets from peridotite xenoliths
of Yubileynaya pipe (see Figure 1.1, 16). Apart from the garnets with sinusoidal patterns
in the xenoliths from Udachnaya pipe, there were identified certain varieties, signifi-
cantly depleted by LREE and at the same time enriched in heavy elements. These garnets
are characterized by more simple in form, positively inclined patterns, flattened in the
heavy REE (see Figure 1.1, 14). An overwhelming majority of the garnets from xeno-
liths of Udachnaya pipe is characterized by very low values of (La/Yb) n

>

= 0.01-0.20,

rarer are the samples with (La/Yb) n 2.1-9.4. By the configuration of garnet patterns of

Udachnaya pipe are comparable with some samples from peridotite xenoliths of South
African kimberlite from Jagersfontein, Kimberley, Lourensia, Premier (see Figure 1.1, 4,
5, 7, 8) and some other manifestations (see Figure 1.1 9-12). Relatively simple in form,

=