Apatites - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 189

For example, on the REE patterns of apatites from some lherzolites and
carbonatites the negative Eu anomalies are absent (Figure 5.5, 1 and 5), which accord-
ing to the model of Eu isomorphism presupposes the crystallization of apatites in
relatively reducing conditions. The presence of rather intense negative Eu anomalies
on the REE patterns of many apatites from granitoids (Figure 5.3) dolerites, gra-
nitic pegmatites, larvikites, jacupirangites, rocks from iron-ore deposits (Figure 5.5,
2, 4, 6, 8) and from the products of experiments with granitic and andesite-tholeiite
melts (Figure 5.8) might indicate that these apatites were crystallized in a relatively
oxidizing conditions. At the same time, the presence of positive Eu anomalies on the
patterns of apatites from granite pegmatites from the State of South Dakota (USA)
(Figure 5.1, 5, 6) also suggests their crystallization under reducing conditions.
* * *
Summarizing the above, we emphasize again that the apatites are able to con-
centrate in their structure very significant amounts of those REE that were present
in crystallizing environments, and almost always LREE are dominating among them.
Depending on the composition of melts and their crystallization conditions the total
amount of REE in apatites can range from 10-100 ppm to 10000-35000 ppm. Inten-
sive fractionation of REE, which is expressed in a rather steep negative slope of REE
patterns and also corresponding high values of (La/Yb) n , is characteristic for most of
the studied apatites. Negative Eu anomalies that are commonly observed in the REE
patterns of apatites are supposed to be the result of crystallization under oxidizing
conditions. Negative Eu anomalies observed in the patterns of apatites from some
lherzolites, kimberlites, basanites, hawaiites and other rocks are characterized by rela-
tively low intensity and in some cases they are either absent or replaced by positive
anomalies, which might indicate the crystallization of mineral in a poorly oxidizing or
even reducing environment. The values of K d (apatite/melt) for most of REE are usually
greater than 1. This suggests that when isomorphic occurrence of these elements into
the structure of apatites takes place, they have the properties inherent in compatible
trace elements.