| Silica-undersaturated potassic rocks, kamafugites, are believed to be products of melts
generated in veined mantle source [e.g. 1] resulted from the influx of alkalic fluids into
peridotites. Close association of the kamafugitic rocks with carbonatites as well as data on
fluid and melt inclusions in the minerals of these rocks [e.g. 2] indicate that generation and
evolution of such melts was accompanied by complex salt liquids enriched in alkali
carbonates and halides, sulfur, phosphorus, etc., which served as metasomatic agents
promoting the deep-seated melting. These components could be also responsible for liquid
immiscibility phenomena during formation of the alkalic melts. In particular, alkali chlorides
cause immiscibility in silicate and carbonate-silicate melts at pressures above 4 GPa
[3].
In order to evaluate the role of chlorides and carbonates in generation of the alkalic
silica-undersaturated melts, experiments on interaction of a model garnet peridotite
Fo63En30Prp5Di2 with CaCO3+Na2CO3+KCl melt were performed at 7 and 2
GPa.
At 7 GPa and temperatures 1200OC via melting of the assemblage forsterite+clinopyroxene+kalsilite. This
subsolidus assemblage is resulted from interaction of pyroxene and garnet constituents
of peridotite with the chloride-carbonate liquid: Prp + 2En + [KCl + Na2CO3 +
CaCO3] = 2Ks + 7/3Fo + 1/3Di + [2NaCl + 4/3CO2]. The following sequence
of phase assemblages is observed within the temperature interval 1200-1300OC:
Fo+Cpx+Ks+LCC - Fo+Cpx+LS+LCC - Fo+LS+LCC. All these assemblages are
accompanied by the CO2-rich fluid. In presence of H2O, kalsilite is absent being replaced by
phlogopite. The obtained temperature of solidus (i.e. slightly above 1200OC) is in
a good agreement with temperatures estimated for some kamfugitic rocks [e.g.
4].
Present experiments show that presence of carbonates results in the undersaturation of the
melts in silica, while immiscibility caused by chlorides assists to a strong enrichment of the
melts in potassium. Being formed within metasomatised mantle peridotite in equilibrium with
chloride-carbonate liquids, the melts loose chlorine during percolation through the
peridotitic mantle. Nevertheless, they preserve high potassium content during their
ascent.
The study is supported by the RFBR (10-05-00040), Russian President Grant
(MD-130.2008.5) and Russian Science Support Foundation.
References: [1] Foley (1992), Lithos, 28, 435-453; [2] Panina & Motorina (2008),
Geochem. Int., 46, 448-464; [3] Safonov et al. (2009), Lithos, 112S, 260-273; [4]
Kamenetsky et al. (1995), Contrib. Mineral. Petrol., 120, 186-196. |