Garnets - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 70

The K d (Yb)/K d (Ce) parameter values, calculated from the experimental studies on
the distribution of REE between coexisting garnets and melts of different composi-
tion, describing the fractionation intensity of these impurities during the crystalliza-
tion of garnet, vary within very wide limits (Table 1.13). Thus, for ultramafic melts
the range of variation of this parameter is 120-500, for basaltic melts it is much
wider - 30-1320, and for melts of dacite, rhyodacite and rhyolite composition, by
contrast, it is much narrower - 25-64.
Available data on K d (garnet/melt) values are often used in solving inverse prob-
lems, i.e. determining the REE compositions of model parental melts, from which
garnets were crystallized. Similar calculations were performed using the average REE
compositions of garnets from the ultramafic xenoliths from Roberts Victor, Udach-
naya, Aykhal and Mir kimberlite pipes, and have shown that the REE compositions
of the model parental melts for ultramafites of these xenoliths were very similar, both
by the overall level of REE accumulation and by the relations between individual ele-
ments; the greatest similarity is observed for the contents of Sm (Figure 1.14, a, b).
Assuming that the calculated model REE composition is close enough to the real
one, we can conclude that the REE in these melts were rapidly fractionated. Level of
accumulation of elements in them repeatedly decreased from La (30-100 t.ch.) to Lu
(0.02-0.2 t.ch.). Such calculations of REE distribution in the model melts, which are
crystallized into garnets presented in ultramafic xenoliths from basalts of Vitim prov-
ince, suggest that the REE in the parental melt of these ultramafites were relatively
low fractionated (Figure 1.14, b, 2). Unlike all the previous ones, REE compositions
of model melts for the garnets from eclogites presented in the xenoliths from kimber-
lites of Bobbejaan pipe, in gabbros from Ivrea Verbano complex, as well as from bas-
anites of Dutsen Dushowo, are characterized by a certain Eu excess (Figure 1.14 c).
Let us emphasize once again that the estimates of REE distribution coefficients
between garnets and coexisting melts of different composition are an important geo-
chemical characteristic of both the garnet and those of magmatic systems, in which

a b c

Figure 1.14 Chondrite-normalized REE patterns for model melts from which presumably crystallized
garnets, presented in various types of ultramafic and mafic rocks. a - ultramafic rocks from
xenoliths in kimberlite pipes: (1) Roberts Victor (n = 23), (2) Udachnaya (n = 50) and (3)
Aykhal (n = 8); b - (1) ultramafic rocks from xenoliths in Mir kimberlite pipe (n = 25), (2)
ultramafic rocks from xenoliths in alkaline basalts of Vitim province (n = 18); c - (1) eclog-
ites from xenoliths in Bobbejaan kimberlite pipe (n = 8); (2) gabbros from Ivrea Verbano
complex (n = 15), (3) phenocrysts from basanites of Dutsen Dushawo complex (n = 1).
In the calculations were used the average values of K d (garnet/ultramafic melt), as well as
K d (garnet/basaltic melt) “Kakan-1” (see Table 1.13).