Zircons - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 147

[Zhang et al ., 2005], from ultramafic xenoliths in kimberlites of Gibeon province
(Southern Namibia) [Liati et al ., 2004], as well as from ultramafic xenoliths of some
diatremes of Hinyang province (North China) [Zheng et al ., 2006a]. According to the
research, in the zircons from Hinyang province the total REE contents, except for Tb
and Tm, vary in the range from 435 to 1483 ppm (Table 4.4). Also there have been
identified the contents of Y (572-1933 ppm), P (202-586 ppm), Pb (1.35-23.5 ppm),
Fe (10-2535 ppm) Mn (0.22-5.40 ppm) and HfO 2 (1.2-2.49 wt%). Judging by mul-
tielemental patterns, these zircons are characterized by a certain deficit of Fe and Mn.
The zircons from ultramafites of Hinyang province by the values of (La/Yb) n parame-
ter can be divided into two geochemical types - with lower (0.0001-0.0003) and with
relatively higher (0.0016-0.0074) values of this parameter. On REE patterns of the
first type of zircons the positive Ce anomalies have a higher intensity (Figure 4.5, 1)
compared to zircons of the second type (Figure 4.5, 2). It can be assumed that the
relative enrichment of the second type of zircon with LREE and specificity of their
patterns are due to the presence in the microcracks of their crystals of small amounts
of unstructured LREE impurities, brought by epigenetic processes. The configurations
of the multielemental patterns of the observed zircons also indicate their anomalous
enrichment with uranium and thorium (Figure 4.5, 3). The average value of U/Th
parameter in them is 3.3.
Saltykova et al . [2008] analyzed the REE composition and identified a U-Pb iso-
topic age of zircons from garnet-spinel lherzolites, presented in the xenoliths from
alkaline basalts of Vitim province (Russia) (Table 4.4). These zircons form elongate
colorless prismatic crystals, which quite often contain melt, fluid and solid (apatite,
sillimanite, quartz) microinclusions. Some of these crystals contained round or irregu-
larly shaped relict 'core', which indicates their zonal structure. It should be noted
that on the surface and in microcracks of some crystals there were signs of corrosion,
which colored them brownish.
Based on the data collected by isotope dating, the authors divided the collection of
zircons from ultramafic xenoliths of Vitim province into four groups. The first group
consists of three samples, which are defined as the oldest ones. Two of these zircons
with ages of 1694 and 1506 million years show a significant depletion by La. On their
patterns there are intense positive Ce anomalies, as well as negative Eu anomalies.
The third sample of zircon with the age of 1088 million years showed a significant
enrichment with light elements - La, Ce, Pr and Nd. On its spectrum the negative
Eu anomaly has a low intensity, and the positive Ce anomaly is completely absent
(Figure 4.5, 4). Compared with the first group, the samples from the second, third and
fourth groups showed significantly lower values of the isotopic age (277-139 Ma). All
of them have patterns complicated by intense positive Ce anomalies, while the inten-
sity of negative Eu anomalies consistently decreases in them from the second group
to the fourth group, which has Eu anomaly completely neutralized (Figure 4.5, 5-7).
We assume that such a 'reduction' of the isotopic ages of zircons and the accompany-
ing changes of their REE composition were caused by later processes of redistribution
of impurities and disturbances in the original U-Pb isotopic system in the process of
thermal and isotopic-geochemical influence of those fluids that were separated from
basaltic melts, subtracting xenoliths of garnet-spinel lherzolites from the upper-mantle
level to the surface and seeped into the xenoliths and their minerals by the systems of
microcracks. Such an assumption correlates with the presence of fluid microinclusions