Trace element partitioning between immiscible silicate and carbonate melts from primary melt inclusions

Solovova Irina P., Girnis Andrey V. and Ryabchikov Igor D.

Institute of Ore deposits, petrography, mineralogy and geochemistry RAS (IGEM),

 Moscow, Russia

We investigated silicate-carbonate melt inclusions (Fig. 1) in clinopyroxene phenocrysts from the fergusite of the Dunkeldyk potassic alkaline basaltoid complex in the Eastern Pamirs (Tajik Republic). The presence of silicate and carbonate melts in coexisting hermetic inclusions allowed us to evaluate trace element partitioning between these melts. The pressures and temperatures of melt equilibration were constrained by heating and cooling experiments. Both carbonate and silicate melts homogenized at 1150–1180^оC, and their subsequent cooling resulted in the formation a fine-grained quench aggregate and a silicate glass. The maximum pressure of host clinopyroxene crystallization and melt inclusion entrapment was constrained as 0.4–0.6 GPa.

Fig. 1. Photomicrograph of a quenched melt inclusion in clinopyroxene containing two immiscible liquids, silicate melt (S) with carbonate melt globules (C), at 1150œC.

The silicate melt from inclusions is alkali-rich (up to 16.5 wt % Na₂O + K₂O) at K₂O/Na₂O up to 4.3. The least evolved melts (SiO₂ < 46 wt %) are agpaitic, (K₂O+Na₂O)/Al₂O₃ > 1. The composition of carbonate melt quenched from a temperature of 1180œC was estimated as (wt %) 10.4 SiO₂, 0.3 TiO₂, 2.6 Аl₂O₃, 5.0 FeO, 2.6 MgO, 16.2 CaO, 0.2 SrO, 5.8 Na₂O, 14.1 K₂O, 0.2 Cl, 1.1 F, 41.5 СО₂. Noteworthy is the very high content of K₂O, which is in line with the K-rich composition of coexisting silicate melt.

The silicate melt inclusions show high contents of REE (up to 1000 ppm in total), especially LREE, Ва, Th, U, Li, B, and Be. The primitive mantle-normalized (La/Yb)_N ratio is up to 68. Both the melt inclusion and bulk rock compositions have high Th/U (up to 5.0 and 6.7, respectively). The carbonatite melt exhibits the same geochemical features but has more pronounced negative anomalies of Zr, Nb, Ti, and V.

The partition coefficients of trace elements were estimated on the basis of the SIMS analyses of two carbonatite inclusions and 2 silicate inclusions quenched from 1150–1180œC. The compositions of the carbonate inclusions were similar (except for Zr) and their average was used for the calculation of partition coefficients.

Fig. 2. Primitive mantle-normalized trace element contents in silicate and carbonate melts.

Fig. 3. Trace element partition coefficients between carbonate and silicate melt inclusions homogenized at 1180œC.

The rare earth elements and most trace element (e.g., Nb, Hf, Y) show negligible fractionation between the two melts (D ≈ 1), i.e., the geochemical effect of silicate-carbonate liquid immiscibility under the P-T conditions of inclusion entrapment is very small. The carbonate melt is somewhat enriched in Gd, Ba, Sr, and U, whereas the silicate melt is enriched in Pb, Ti, and Li. The experiments of Hamilton et al. (1989) showed that the partition coefficients of REE between immiscible carbonate and silicate melt are close to 1 at 1150œC and 0.35–0.60 GPa. They suggested that carbonate melts could be enriched in trace elements at high pressures (>0.6 GPa), which makes them potent agents of mantle metasomatism. On the other hand, the enrichment of carbonate melts in trace elements was observed at low temperatures (Jones et al., 1995; Veksler et al., 1998). This trend may have a bearing on the formation of carbonatite-related rare metal deposits. In addition, the enrichment of natural carbonatites in rare elements can be related to the high mobility of carbonate melt and its separation from crystalline residue at very low melt fractions.

References:

Veksler I.V., Petibon C., Jenner G. A. et al. Trace element partitioning in immiscible silicate–carbonate liquid systems: an initial experimental study using a centrifuge autoclave // J. Petrol. 1998. V. 39. N 11/12. P. 2095–2104.

Jones J.H., Walker D., Pickett D.A et al. Experimental investigations of the partitioning of Nb, Mo, Ba, Ce, Pb, Ra, Th, Pa and U between immiscible carbonate and silicate liquids // Geochim. Cosmochim. Acta. 1995. V. 59. P. 1307– 1320.

Hamilton D. L., Bedson P., Esson J. The behaviour of trace  elements in the evolution of  carbonatites // In: Bell, K. (ed.) Carbonatites. Genesis and Evolution. 1989. London: Unwin Hyman, pp. 405–427.