Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Dependence of Eu/Eu* in nepheline and alkaline syenites on crystallization fractionation degree  

Dubyna A.V., Kryvdik S.G.

N.P. Semenenko Institute of geochemistry, mineralogy and ore formation NAS of Ukraine, Kyiv, Ukraine

kryvdik@ukr.net

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 decreasing and intensive iron enrichment in femic minerals (this parameter 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.

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 (К_а) 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 syenite-pulaskites (К_а<1) 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 (К_а<1) 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.

Consequently different petrogenetic mechanisms (liquation, crystallization fractionation) of nepheline and alkaline syenites formation and different fractionation degrees have been caused divers chondrite-normalized REE patterns.