Garnets - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 7

the REE pattern is weakly convex upward and has a negative Ce anomaly. Khazan &
Ariasova [2007] studied the REE composition of garnets and coexisting clinopy-
roxenes from xenoliths carried out by kimberlites of the Kaapvaal province (Southern
Africa) and Somerset Island province (Arctic Canada). They established considerable
variations in REE composition of clinopyroxenes and, to a lesser degree, garnets from
pressure at 2-6 GPa. The authors think that the spatial fractionation of REE was the
result of their nonequilibrium differentiation between melt and restite, which was
accompanied by the episodes of scaled melting of the mantle. According to the model
proposed in the work, the spatial fractionation of REE could result from moderately
high (10-20%) melting of preliminarily depleted peridotite. This episode of melting
preceded the episode of a higher-degree melting that led to the 'wash-out' of a con-
siderable part of clinopyroxene and garnet from the mantle. The considerable differ-
ences between clinopyroxenes and garnets from the upper mantle in the character of
variations in REE composition depending on the pressure are explained by the dif-
ference in the activation energy of REE diffusion: 400-500 kJ/mole in clinopyroxene
and less than 300 kJ/mole in garnet. In their other work the authors emphasize once
again that the character of dependence of REE contents in garnets and clinopyroxenes
from xenoliths carried out by kimberlites on the pressure, regularly changes in the
series from La to Lu. The observed regularity is explained by the nonequilibrium
differentiation of these trace elements between the melt and polymineral restite owing
to the high degree of partial melting of mantle substrate and further segregation of
generated magmas [Ariasova & Khazan, 2007]. In the work of Spera et al . [2007] a
compilation of solid-fluid and melt-fluid distribution coefficients for about 30 trace
elements, including REE, among coexisting garnet, clinopyroxene, plagioclase and
some other minerals is provided. Study of the geochemistry of olivine-bearing rocks
with garnets and omphacite from high-temperature granulites of the Kandalaksha
massif (Kola Peninsula, Russia) showed positive Eu anomalies, which are not typ-
ical of most garnet-bearing formations [Skublov & Terekhov, 2009]. This led the
authors to assume that garnets from these metamorphic rocks are also characterized
by positive Eu anomalies, which is observed quite rarely. Li et al . [2010] studied the
REE composition of hydrogrossular from rodingites, included in ultramafic rocks of
the Changawuzi ophiolite in southwestern Tien Shan (China). The hydrogrossular
formed during rodingitization and does not display strong enrichment of HREE. The
crystals show very low total REE, with overall flat HREE patterns, a positive increase
in LREE with increasing atomic number and pronounced positive Eu anomaly. This
unusual Eu anomaly in garnet may reflect the special origin of Ca-rich garnet during
Ca metasomatism.
1.2 GENERAL REGULARITY OF REE DISTRIBUTION
IN GARNETS OF THE MOST COMMON OF THEIR
PARAGENESES
Features of geochemistry of REE in garnets are considered depending on those petro-
graphic types of rocks they participate in. The most common types of garnet-bearing
rocks are the following: 1) high-pressure peridotites, mainly harzburgites and lherzo-
lites from xenoliths in kimberlites, and 2) lherzolites and pyroxenites from xenoliths