Garnets - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 32

observed for the garnets from xenoliths from Aykhal pipe (Yakutia province, Russia),
Venetia, and Namibia provinces (South Africa), as well as Slave craton (Canada)
(Figure 1.3, 5-13). All of these garnets in one way or another differ in the (La/Sm) n
parameter values, by using these values we divided the garnets from diamond micro-
inclusions of Udachnaya pipe into two groups: for the first, (La/Sm) n 0.008-0.097

and for the second - 0.220-0.407. The garnets from microinclusions in the diamonds
of Mir pipe are divided into three groups with different (La/Sm) n values: 0.013-0.109,
0.268-0.726 and 1.346-1.444, respectively. Similar differences in REE composition
were observed in garnets from some other kimberlite manifestations and provinces.
As emphasized above, the garnets from the microinclusions in diamonds of eclog-
ite parageneses differ significantly in the REE composition from those that are present
in peridotite parageneses. Within the whole operational sample of analysis of these
garnets, the total REE compositions varies in the range of 6.4-56 ppm with an aver-
age of 22.6 ppm; La/Yb) n values are ranging from 0.003 to 0.050 with an average
value of 0.015. Their REE patterns often have a steep positive slope, which is most
clearly observed in samples from the Slave craton (Canada) and Namibia province.
However, it should be noted that in contrast to the pattern of garnets from the Slave
craton, which have a close to straight shape of line (Figure 1.3, 14), the patterns of the
samples from Namibia province are characterized by a subhorizontal location in the
area of heavy elements (Figure 1.3, 15), more rarely they have a negative slope due to
the relative depletion of a mineral by heavy elements (Figure 1.3, 16).
Eclogites from xenoliths. The REE composition of garnets from eclogite xenoliths
is characterized by the example of samples from Roberts Victor, Bobbejaan and Koidu
pipes (Table 1.4). Garnets of this type contain a relatively low amount of pyrope
minal and have a high content of grossular minal. The total REE contents in garnets
from eclogites of Roberts Victor pipe vary in the range of 3-13 ppm. The exception
is sp. HRV-175, for which the total REE is as high as 35 ppm which is mainly due
to the anomalous enrichment with medium elements. In the garnets from Bobbejaan
and Koidu pipes the total REE contents are comparable to those for samples from
Roberts Victor pipe - 3.3-18.1 ppm, the only exception is the sp. KEC 86-19 for
which the total REE is 52 ppm due to enrichment with medium and heavy elements.
Chondrite-normalized HREE contents in all garnets from eclogites parageneses are
much higher than that of light elements, the values of (La/Yb) n range from 0.001 to
0.47 (Figure 1.5). This pattern is most clearly expressed in samples from xenoliths of
Roberts Victor and Koidu pipes. By their configuration of REE pattern the garnets
from Roberts Victor pipe are divided into two types: 1) garnets that are moderately
depleted by LREE and, accordingly, have a flattened form of the patterns in the area
of medium and heavy elements, and 2) garnets intensively depleted by LREE, par-
ticularly by cerium. In one of the samples the garnet is anomalously enriched with
Eu. To add more, some garnets from eclogites, provided in the xenoliths from kim-
berlites of Udachnaya pipe, are characterized by a zonal distribution of REE, which
becomes apparent in a gradual increase of the content of light elements moving from
the central zone of garnet grains to the marginal, as well as in the depletion by HREE
in the same direction.
Eclogites of high-pressure metamorphic complexes. Eclogites of this type are spread
in tectonically exhumed high-pressure metamorphic formations among the ancient
folded structures. Some features of the REE distribution in garnets of these areas are

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