Dependence of Eu/Eu* in
nepheline and alkaline syenites on crystallization fractionation degree
N.P. Semenenko Institute of
geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine
It is considered that a
crystallization fractionation is one of the main factors in formation of
Eu anomalies (Eu/Eu*) in igneous rocks. As an especially effective, the
feldspar (plagioclase and alkali-feldspathic) fractionation can be.
The indicators of the
crystallization fractionation of alkaline magmatic melts can be
geochemical and mineralogical characteristics as magnesium
and intensive iron enrichment in femic minerals (this
is often referred as an fractionation coefficient), increasing the
agpaitic coefficient - (Na+K)/Al - with the appearance of such minerals
as aegirine , arfvedsonite, aenigmatite, eudialyte, sodalite, and
increase of incompatible rare metals (Zr, Nb, Rb) concentrations,
including REE. Simultaneously with this especially by the feldspar
fractionation a decrease of Sr and Ba concentration occurs.
correlation between Eu/Eu* and REE, Ba, Sr concentrations
and Ka ((Na+K)/Al) in alkaline and nepheline
syenites of the Ukrainian Shield.
All these indicators of
crystallization fractionation have been reflected on characteristics of
chondrite-normalized REE patterns in alkaline and nepheline syenites of
the Ukrainian Shield (USh). Especially contrasting they are in
gabbro-syenite massifs where feldspar fractionation has been intensive.
In these massifs, from early to late differentiates (alkaline and
nepheline syenites) generally REE content increases, rock's magnesity
reduces and an agpaitic coefficient
increases. In this
direction, Eu/Eu* reduces from 2.3 to 0.17. In this respect the
Oktyabrsky alkaline massif is especially representative: in earlier
with moderate amounts
of REE (~300 ppm) and high Sr (2000 ppm) and Ba the Eu/Eu* ratio reach
up to 2.3 whereas in the later aegirine mikrofoyaites, agpaitic
phonolites and mariupolites Eu/Eu* is from 0.35 to 0.45 with a high
content of REE (up to 1900 ppm) and low Sr (12 ppm) (Fig.). In one of
massifs (Velikoviskovsky) the lowest value of Eu/Eu* - 0,17 is fixed at
very low Sr concentrations - 9 ppm. In nepheline and alkaline syenites
where Sr content is more than 230 ppm Eu/Eu* is close to 1.0 (in some
syenites, reaching 2.3) (Fig.). There is such correlation between the Ba
content and Eu/Eu* (Fig.).
Quite distinct dependence,
although with some variations, exists between REE content and Eu/Eu*
(Fig.). All varieties of nepheline and alkaline syenites of
gabbro-syenite complexes, with REE>700 ppm, have Eu/Eu*<1. From this
general pattern the nepheline syenites of Pokrovo-Kyreevo massif which
are characterized by high contents of both REE and Sr, Ba are deviated.
The ratio Eu/Eu*~1
is in the nepheline syenites
of alkaline-ultrbasic (carbonatitic)
Chernigivka, Proskurivka and Antonivka massifs. These rocks are
characterized by high Sr and moderate REE concentrations. Probably the
formation of nepheline syenite of alkaline-ultrabasic complexes are not
have occured feldspar fractionation. Likely these rocks have been formed
by melt liquation (nephelinite + CO2 = carbonatite +
phonolite) in which high Sr content survived and without a significant
rare earth fractionation.Moreover, the REE concentration somewhat
decreases in nepheline syenites compared to those in more melanocratic
ijolite-melteigites and carbonatites.
petrogenetic mechanisms (liquation, crystallization fractionation) of
nepheline and alkaline syenites formation and different fractionation
degrees have been caused divers chondrite-normalized REE patterns.