Geochemical peculiarities of apatite in alkaline rocks from the Ukrainian Shield

Dubyna A.V., Kryvdik S.G., BondarenkoS.N.

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

Apatite is an ubiquitous mineral in alkaline and subalkaline rocks of the Ukrainian shield (USd), and its features of composition reflects the chemistry of parental magmas and processes of their differentiation. Within USd we distinguish two main complexes of alkaline and subalkaline rocks - alkaline-ultrabasic (carbonatitic) and gabbro-syenitic. Their parental magmas may be melanephelinite and subalkaline basalts correspondly. These melts have various trends of differentiation: the final derivates of alkaline-ultrabasic complexes are carbonatites and nepheline syenite, and of gabbro-syenitic one - alkaline and nepheline syenites, including their agpaitic varieties.

In the rocks of these complexes significant differences in the composition of apatite are revealed. In the carbonatites and associated rocks apatites are usually enriched in Sr (from 0.18 to 3.20% SrO) and REE_Ce (from 0,85 to 8,55% REE₂O₃), while in agpaitic nepheline syenites and phonolites (Oktyabrsky massif) apatites is poor in Sr (0,12-0,42% SrO) by very wide variation in the concentration of REE and Y. In addition, REE patterns of apatite from rocks of gabbro-syenitic complexes are characterized by negative Eu-anomalies, while in the carbonatites these patterns are more «flat» or with negligible anomalies. It is considered that the negative Eu-anomalies in apatites and rocks containing them due to feldspar fractionation.

In carbonatite complexes of USd positive correlation between content of REE and Sr are observed. However, by very high concentrations of REE₂O₃ (up to 11.0%) contents of Sr is no more than 2.4-4.5%. That is not so high, as typical e.g. for apatites from Khibiny and Lovozero massifs. In ordinary ores of these massifs apatite contains 2.02-8.44% SrO, but there is very enriched in strontium varieties (up to 62.86% SrO) (Chakhmouradian et at., 2002). It is considered (Kogarko, 1977) parental magmas for these massifs have been of alkaline-ultrabasic composition (melanephelinite type).

The Ce/Y ratio in apatites from alkaline-ultrabasic rock complexes of USd varies from 9 to 53. At the same time, with increase of REE content this ratio increases.

In apatite of agpaitic rocks in Oktyabrsky massif Ce/Y varies from 5 to 17 and as in carbonatites rise with increasing REE amount. Whereas, apatites in South Kalchyk quartziferous rocks Ce/Y varies between 0.46 and 12. It is not certain regularity of REE amount and this ratio sharply reduces in the yttrium-rich (up to 7.90% Y₂O₃) apatites. In alkaline differentiates of acidic and basic composition and associated quartz-feldspar šmetasomatites, there are also yttrium rich as well as Y-apatite (Marchenko, 1995).

REE, Sr, Y replace, as it is well known, calcium in the apatite, but there are two main schemes of heterovalent isomorphism: britholitic (REE → Ca = Si → P) and belovitic (Na + REE → 2Ca).

Britholitic scheme of isomorphism was found in apatites from SiO₂ saturated and quartziferous melanocratic carbonatites and ringites in Chernigovka complex (Azov area). These apatites have dissoluble SiO₂ (up to 3.70 wt.%), and to 11.0% REE₂O₃. Exsolution inclusions of britholite and products of its alternation (orthite, quartz, calcite, bastnasite, strontianite) (Kryvdik et al., 1990; Vorobiev, 2000) in these apatites are observed. These inclusions may overfill apatites and they become brown (“sealing wax”).

Belovitic scheme of isomorphism is typical for apatites in SiO₂ unsaturated rocks of USd: in agpaitic nepheline syenites and phonolites (Oktyabrsky massif), as well as in olivine calcite-dolomite carbonatites (beforsite, Chernigovka complex). In apatite from last carbonatites REE₂O₃ content reaches to 7.34%, and Na₂O – 1.41%. These apatites also have high concentration SrO (up to 2.5, sometimes 4.3%), as it is typical for carbonatites in general. At the same time, apatites with belovitic scheme of isomorphism in Oktyabrsky agpaitic phonolites are characterized by relatively low contents of SrO (0.12-0.40%) by very high concentrations of Na₂O (up to 2,7%) and REE₂O₃ (Ce₂O₃ - up to 7.19 %, La₂O₃ – 2.55%, Nd₂O₃ – 3.96%) and moderate Y₂O₃ (0.05-0.67%). According to the chemical composition, these apatites are very similar to Na-REE-apatite (belovitic) in agpaitic feldspar syenite of Ilimaussaq massif (Ronsbo, 1989) and significantly differ from belovite apatite of Lovozero massif (Chakhmouradian et al., 2002; Bussen et al., 1972) (above all by the strontium content). In previous publications (Kryvdik, Tkachuk, 1988) the similarity of agpaitic phonolitic and feldspar syenites in Oktyabrsky and Illimaussak massifs had been noted.

Apatite from alkaline and other magmatic rocks of Ukraine are presented by F-variety, but the contents of fluorine in these minerals differ among themselves. In a deep eroded massifs (abyssal facies) highly fluorine apatite (up to 3.6-4.0% F) occur, and in hypabyssal ones – low fluorine (1.5% F and less) varieties and they often associate with fluorite. The point is that in abyssal conditions fluorine does not form its own minerals (fluorite, REE-F-carbonate, villiaumite) and belongs to apatite (up to full saturation), as well as in amphiboles and micas (up to 2.6% F). Whereas, in hypabyssal massifs fluorite and low fluorine apatite as well as poor and fluorineless amphiboles and mica are crystallized. However, this rule does not apply to agpaitic rocks where apatite is always high fluorine and can associate with villiaumite (NaF) and/or fluorite.

Consequently, apatite is a very sensitive mineral-indicator of chemistry of mineral-forming environment.

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