Zircons - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 170

Hinton & Upton [1991] determined the values of K d (zircon/melt) on the basis
of the analysis of REE in the individual large crystals of zircon from basanites and
syenites. According to them the values of K d (zircon/syenite melt) are consistently
increasing in the row from La (0.0083) to Lu (472), and that these values correlate
with a successive decrease in the size of the radii of trivalent ions of these elements -
from 1.16 Å for La to 0.977 Å for Lu. However, such a sequence disrupted in case
of Ce, the mean value of K d for which amounted to 718, which is much higher than
that for Lu (Figure 4.14, 10). Given these observations, the authors concluded that
such an anomaly for Ce is due to the fact that in the zircons they studied this element
was in the form of Ce 4+ ions, which are more favorable for the occurrence in the
mineral structure as compared to Ce 3+ ions, because they have a much smaller radius
(0.970 Å) compared with Ce 3+ ions (1.143 Å). These data suggest that Ce 4+ ions in
the zircon structure would prevail over the Ce 3+ ions in cases where the zircons were
crystallized under high oxygen fugacity.
In conclusion, Rubatto [2002] presented data on the REE distribution between
coexisting zircons and garnets, obtained in the study of eclogitic micaschist from Ses-
ia-Lanzo Zone (Alps). According to the research for these rocks the highest values
of K d (zircon/garnet) are typical of Ce (69-90), for all subsequent elements are much
lower: Nd (5.7-7.0), Sm (2.1-2.4), Eu (3.2-4.0), Gd (0.94-1.60), Tb (0.93-1.8),
Dy (1.3-2.6), Ho (2.0-4.3), Er (3.2-7.1). For the other three elements K d values are
slightly rising: Tm (5.2-11.0), Yb (8.6-17.0), Lu (12.1-23.9).

4.3 ON THE ISOMORPHISM OF REE IN ZIRCONS
AND CONDITIONS OF THEIR CRYSTALLIZATION

The level of REE in zircons, as in all other minerals, is largely due to their crys-
tal-chemical properties. According to earlier data of Voytkevich et al . [1970], the
size of the radius of Zr 4+ ion as the main net-forming element of zircon is 0.82 Å,
which is comparable with the size of the radii of the trivalent ion of heavy REE,
especially the Yb 3+ (0.82 Å) and Lu 3+ (0.80 Å). Later estimates of the size of the ionic
radii of elements were slightly adjusted, resulting in the currently accepted values for
Zr 4+ -0.977 Å [Shannon, 1976]. It is likely that

similar ionic radii of Zr 4+ and HREE are an important determinant of particularly
high values of K d (zircon/melt) for them, and that these REE are characterized by a
high degree of compatibility with the crystal structure of zircons.
In the study of synthetic zircon by using cathodoluminescence method it has been
found that their patterns contain a series of narrow lines in the area of 200-500 nm
that were attributed to the REE such as Sm, Eu, Tb, Dy and Er [Cesbron et al ., 1993].
In addition, on a laser-fluorescent patterns of zircons from garnet amphibolites of the
Cape Kamchatka (Kamchatka, Russia) there were also determined the lines, which
were attributed to Er 3+ ions [Osipenko et al ., 2007].
It is assumed that in the structure of zircons there are two basic positions that
can potentially be replaced by cations: tetragonal position in which Si 4+ ions are, and
triangular (dodecahedral) position, which contains the Zr 4+ ions, and the latter is
the most favorable for the replacement of heavy REE by ions [Thomas et al ., 2002].
However, the relatively high level of compatibility with the zircon crystal structure of

-0.84 Å; for Yb 3+ -0.985 Å; for Lu 3+