Some general regularities of REE distribution in the minor and accessory minerals of ultramafic, mafic and some other rocks - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 220

have very low values and slightly increase in the row from LREE to HREE. Further
research will enhance understanding of the regularities of REE distribution in ilmen-
ites from the rocks of different composition and origin.
Zircon is one of the best concentrators of isomorphous REE impurity in the con-
tent of rocks of different composition and genesis, including ultramafites and gabbros.
In addition to the REE they contain in their structure many other impurities including
Hf, Y, Ta, U, Th, Pb, Fe, Ti and Ca, the total content of which can reach the first wt%.
Chondrite-normalized HREE content in this mineral is almost always much higher
than the content of MREE and particularly LREE. Published data on the geochemis-
try of REE in zircons characterize their samples from many petrographic rock types
and the largest number of tests was performed on zircons from granites, tonalites,
granodiorites, while the least data is available for the samples from kimberlites, gab-
bros, minettes, basanites, syenites, eclogites, charnockites, amphibolites, ultramafic
rocks and other rock types. The overall level of REE accumulation in zircons increases
in a row of samples from ultramafites to gabbros followed by alkaline, granitoids and
other rocks. On REE patterns of almost all zircons there are Ce anomalies of varying
intensity and negative Eu anomalies. For those zircons that have undergone various
transformations including melting under the influence of melts the configuration of
REE patterns in one way or another changes, in particular, due to the leveling of Ce
and Eu anomalies. Based on the crystal-chemical assumptions, we can assume that the
observed Ce and Eu anomalies in the patterns of zircon are the result of crystallization
of minerals under conditions of relatively high oxygen fugacity. Based on materials
of experimental studies it was established that for all REE and for all of their mater-
nal melts the K d (zircon/melt) values are greater than 1, and thus increase from the
first units (for LREE) to many hundreds and first thousands (for HREE). There is a
reason to believe that REE accumulation in zircons structure was carried out through
heterovalent isomorphous substitution of tetravalent Zr ions by trivalent REE ions.
Such a substitution could be carried out with the participation of Ti, Fe, Li and Mo
ions, which performed the function of compensating charge. In connection with the
strengthening of the isotope-geochronological studies of various magmatic including
gabbro and ultramafic rocks by using the U-Pb method on zircons, obviously, more
attention will be given to detailed geochemical studies of this mineral, with special
emphasis on its REE composition.
Apatite as an accessory mineral is presented in so many varieties of rocks of
magmatic and metasomatic origin, including gabbros and some high-Mg rocks. They
are able to concentrate in its structure very significant amounts of REE, which are
usually heavily fractionated with a tendency to a significant enrichment with LREE.
Depending on the composition of maternal rocks and conditions of their formation
the total REE content in apatites ranges from 10-100 to 10000-35000 ppm. Higher
REE contents of apatite are characteristic of granite and various high-alkaline rocks,
the lower contents were defined in the mineral from granitic pegmatites, gabbros,
pyroxenites. In most cases, the chondrite-normalized REE patterns of apatites are
close to a straight line with a steep negative slope. The values of (La/Yb) n parameter
for them vary between 4 and 370. The patterns of apatites are often complicated by
negative Eu anomalies. The (Eu/Eu*) n parameter for apatites typically ranges from
0.17-0.38. The nature of Eu anomalies in apatites is supposed to be due to crystal-
lization at high oxygen fugacity, when in an environment is dominated by Eu 3+ ions