The small, elliptical Vuorijarvi massif (3.5 x 5.5 km) belongs to Kola Alkaline and Carbonatitic Province. It is a complex multi-phased intrusion classically interpreted as resulting from successive magma pulses. The core of the massif is mainly represented by ultramafic cumulates while alkaline silicate rocks of the ijolite-melteigite series are found at the periphery. Two carbonatitic stocks with associated phoscorites occur at Vuorijarvi: an early one in the eastern part of the massif and a later thick stock (c.a. 800m) in the south-eastern part. Apatite is a common liquidus phase in the three kinds of rocks (ultramafic cumulates, ijolites-melteigites and carbonatites/phoscorites). Apatites from representative clinopyroxenite, ijolite and carbonatites have been analysed by LA-ICP-MS. All apatites are rich to extremely rich in REE (SREE: 730 to 13800 ppm), with LREE enrichment: La/Yb ratio varies from 70 up to 467. Apatite REE composition does not define a regular geochemical evolution trend with emplacement sequence. The crystallisation/fractionation of other REE-rich accessory phases (pyrochlore, perovskite, …) has partly controlled the REE pattern of the apatites. There is an increase in REE content from the clinopyroxenites to the early carbonatites. Intensive crystallisation of apatite and other REE-rich phases deplete the residual melt in REE, the late magnesiocarbonatite are indeed poor in REE (SREE: 120 ppm) and devoid of apatite. REE partition coefficients for apatite crystallising in a carbonatite melt are poorly known. They have been tentatively estimated for some African carbonatites (Homa mountain, Kalkfeld, Ondurakorume, Otjisazu, Okorusu and North Ruri) by comparing the trends of the apatite and of the whole rocks (Bühn et al, 2001). The partition coefficients increase regularly from La to Yb. We use the same approach in this study by combining whole rocks analyses and LA-ICP-MS on apatites. The Vuorijarvi carbonatites display consertal (intergrowth) and polygonal textures. The carbonatites occur as cm to m thick veins suggesting quick cooling of the melt. Calcite crystals (1-2mm) are zoned (CaO: 54.5-56.3wt%; MgO: 0.23-1.9 wt%; SrO: 1.06-0.14wt%) suggesting rapid growth leading to closed-system behaviour. As the zoning of calcite is easily erased at high to medium T° (800-500°C; Cherniak, 1998), the presence of zoned calcite at Vuorijarvi testifies a rapid cooling. Moreover, deformation in calcite involves mechanical twinning rather than pressure-dissolution, so the deformation process has not homogeneised the composition of the grains. We will consider that the REE composition of the liquid in equilibrium with the apatite can be approximated by subtracting the apatite contribution to the whole rocks carbonatite REE content. The apatite/carbonatite liquid partition coefficients for the Vuorijarvi carbonatites are given in Table 1, they range from 7.6 to 3.5 for La, 8.4 to 4.9 for Eu and 5.2 to 1.0 for Yb. The REE distribution coefficient patterns are convex-upwards as the apatite/silicate liquid partition coefficients (Watson and Green, 1981). The REE patterns obtained for Vuorijarvi are completely different from those by Bühn et al (2001). This discrepancy can be explained either by variations of melt composition or by the fractionation of a co-existing LREE enriched phase (i.e. perovskite, pyrochlore).Yttrium and holmium contents are high in apatite from early silicate carbonatites (775 and 33 ppm respectively) and lower in later carbonatites (72 and 2.8 ppm). As Y and Ho have the same charge and nearly the same ionic radius, they do not commonly fractionate. Nevertheless, the calculated DY/DHo ratio (0.89 and 0.68) implies a slight fractionation. Whole rocks analyses plot along a well-defined linear array (R2=0.9884), corresponding to an Y/Ho ratio of 27.5, close to the chondritic value (28.7). Apatite crystallisation would result in an increase of the Y/Ho ratio of the residual melt. The apatite Y/Ho ratio decreases in the sequence clinopyroxenite (27)-ijolite (22.5)-carbonatite (19). The nearly constant Y/Ho ratio of whole rocks implies that apatite alone does not control the Y/Ho fractionation. Two other processes could be invoked: 1) the fractionation of an Y-rich mineral (i.e. monazite) and/or 2) the depletion of Y by exsolution of CO2/H2O fluids (Bau, 1996; Bühn et al, 2001).The apatites of the early clinopyroxenites display slight negative Ce anomaly (Ce/Ce*: 0.7 to 1.0), this anomaly decreases progressively during differentiation, suggesting a decrease in oxygen fugacity. For all the analysed apatites, the Ca (p.f.u) content decreases while the La+Ce+Na (p.f.u) content increases, this feature has already been interpreted in terms of isomorphic substitution 2Ca2+ = REE3++Na+ (Brassinnes et al, 2003), related to fractional crystallisation of a single batch of melt from the ultramafic cumulates to the carbonatites. This is confirmed by the similar Sr and Nd isotopic compositions (0.70303-0.70318; eNd380Ma: +3.5 to 6.1 of all the Vuorijarvi rocks.
REFERENCES:
Brassinnes S.et al (2003) Period. di Mineral, in press; Bau M. (1996) Contrib. Minera. Petrol 123, 323-333; Bühn B. et al (2001), Contrib. Mineral. Petrol. 141, 572-592; Cherniak D.J. (1998), Earth Planet. Sci. Lett. 160, 273-287; McDonough, W.F. and Sun, S.-s. (1995), Chemical Geology 120, 223-253; Watson E.B. and Green T.H. (1981), Earth Planet. Sci. Lett. 56, 405-421. top...

The Lovozero alkaline massif (Gerasimovsky et al, 1966; Kogarko et al, 1995) is composed of three major petrographic units; the main central unit (80% of the volume) is a differentiated layered complex that consists of numerous (29) rhythms composed of several layers: the ideal sequence is urtite-juvite-foyaite-lujavrite. A detailed textural, mineralogical and geochemical study of the complete rhythm II-7 allows to better constrain the mode of emplacement and the differentiation process.
Two distinct types of nepheline have been recognised: 1) inclusion-free euhedral nepheline occurs mainly in urtites and as a minor phase in foyaites and 2) anhedral to subhedral nepheline containing numerous inclusions of aegirine and arfvedsonite occurs in all rock types. The inclusion-free nephelines display rather homogeneous compositions (Ne56-68Ks14-19Q15-25), characterised by high SiO2 (44 to 47.5 %) and Fe2O3 (1.5 to >2 %) contents in comparison with natural nephelines. The nepheline inclusions in aegirine and alkali feldspar are comparable to the inclusion-free nephelines. The inclusion-rich nephelines also show quite homogeneous composition (Ne69-79Ks20-25Q0-11) but are clearly distinct from the inclusion-free nephelines; they are more potassic and less enriched in SiO2 (<43.5 %) and Fe2O3 (< 0.55%). The excess of Si over that required by stoichiometry can be used as a temperature indicator. The inclusion-free nephelines crystallise at significantly higher temperatures (800-1100°C) than the inclusion-rich nephelines (<700°C). The high T° of crystallisation (already noted by Kogarko et Romanchev, 1977) of the inclusion-free nepheline is probably higher than expected for slowly cooled plutons and is attributed to the rapid cooling in a subvolcanic environment (Woolley and Platt, 1986).
The morphological and textural characteristics of each mineral phase in the urtite-lujavrite rocks sequence can be discussed in terms of order of crystallisation and cumulate (solid) and intercumulate (liquid) origin. Alkali feldspar laths are ubiquitous in the sequence; they are dominantly euhedral but have been variously affected by a deformation that occurred contemporaneously with the layering development. Alkali feldspar can be considered, without ambiguity, as a cumulate phase. Inclusion-free high T° nepheline is also a cumulate phase whereas subhedral to anhedral inclusion-rich low T° nepheline is of intercumulate origin. In the rhythm II-7, high T° nepheline is rare, less than 10% of the total nepheline modal proportion; the small (0.2-0.4mm) crystals with cubic-like morphology suggest that nepheline results from the destabilisation of a high T° polymorphic phase (carnegieite?). The inclusion-rich nepheline is common in all the rocks of the rhythm II-7; it represents the main mineral phase of the basal urtite layer of the rhythm. Aegirine-augite constitutes the core of the large poikilitic augite crystals that contain aegirine inclusions, it growths after alkali feldspar and inclusion-free nepheline but before inclusion-rich nepheline. It is tentatively interpreted as a cumulate phase, characterised by a high nucleation rate (numerous crystals have been observed) but a low growth rate. Aegirine and arfvedsonite crystallise lately around the aegirine-augite cores or as large interstitial or poikilitic grains.
Inside the rhythm II-7, strong variations of the cumulate-liquid (L) proportions and of the nature of the liquid have been observed. The liquid proportion, estimated by the proportion of intercumulate phases, decreases dramatically from the basal urtite (L ~ 90%) to the uppermost lujavrite (L ~ 10%). In the urtite, the cumulate phases are represented by rare feldspar laths and inclusion-free nepheline; in lujavrite, euhedral feldspar laths are abundant. The intercumulate assemblage of the urtite is represented by the inclusion-rich nepheline and rare aegirine and arfvedsonite, whereas in the lujavrite, the proportion of the latter increases and are roughly equal to the inclusion-rich nepheline. The modal proportion of alkali feldspar and their size increase from the bottom to the top in the urtite-juvite-foyaite-lujavrite sequence of rhythm II-7. Structural features suggest that each rhythm was formed by in situ differentiation of a sill-like sheet of liquid. The layering could result from a sill-in-sill mode of emplacement rather than from magma chamber processes.

REFERENCES:
Gerasimovsky V. et al (1966) Geochemistry of the Lovozero alkaline massif, Nauka (Moscow), 365pp; Kogarko L. and Romanchev B. (1977) Geokh., 2, 199-216; Kogarko L. et al (1995) Alkaline rocks and carbonatites of the world. Pt2. Former USSR, Chapman et Hall, 225pp; Woolley A.R. and Platt R.G. (1986), Mineral. Mag. 50, 597-610. top...