Аннотация:EXPERIMENTAL STUDY OF MELTING OF FLUORITE-BEARING TRACHYRHYOLITE FROM MONGOLIA
SUK N.I.1, KOTELNIKOV A.R.1, PERETYAZHKO I.S.2, SAVINA E.A.2
1Institute of Experimental Mineralogy RAS, Chernogolovka Moscow district
2A.P. Vinogradov Institute of Geochemistry SB RAS, Irkutsk
Trachyrhyolite of Central Mongolia melting was produced in high gas pressure vessel in presence of 10 wt% H2O at two regimes: 1) at T=1250oC and P=5.5 kbar during of 6 h; 2) melting at T=1250oC and P=5.5 kbar (duration of 2 h) and then parameters were reduced to T=900oC and P=1 kbar (duration of 4 days) and then isobaric quenching was produced. To study trachyrhyolites of three types were selected. They differ in F content (0.58, 2.45 and 15.0 wt %). Experiments to study phase distribution of REE have been produced at T=1250oC and P=5.5 kbar during of 6 h with addition of REE (La, Ce, Y, Gd, Dy) oxides (1 mg for each). The experimental samples were analyzed on a digital scanning electron Tescan Vega II XMU microscope.
At T=1250oC and P=5.5 kbar in the sample with maximum F content (15 wt %) it has been obtained liquid immiscibility between silicate and fluoride-calcium melts, which produce drops of one liquid in another. Trachyrhyolites with F content of 0.58 and 2.45 wt % were melted forming homogeneous glass. Experiments with mixtures of trachyrhyolites with minimum and maximum F contents in ratio 1:1, 2:1 and 1:2 show that liquid immiscibility can arise when the system contain F content >5 wt %.
Second series to study trachyrhyolite melting was produced in conditions imitated volcanic process i.e. in decrease of temperature and pressure which is typical for the process of the magma rise to the earth surface and its eruption (T=1250oC, P=5.5 kbar => T=900oC, P=1 kbar). In the sample of trachyrhyolite with maximum F content liquid immiscibility between silicate and fluoride-calcium melts appeared too, but segregations of immiscible phases were smaller which approached experimental samples to natural trachyrhyolite image.
Investigation of REE (La, Ce, Y, Gd, Dy) distribution between layered phases at T=1250oC and P=5.5 kbar showed that they predominantly concentrate in fluoride melt. Partition coefficients of REE between aluminosilicate melt and fluoride-calcium phase (KiREE=CiLF/CiSil) are for Y ~ 20, La – 20-40, Ce –15-30, Gd – 20-30, Dy – 16-20.
Petrographical observations and data of study of melting inclusions in minerals from trachyrhyolites evidence about coexistence of trachyrhyolite (aluminosilicate) and fluoride-calcium melts as at the stage of mineral phenocrysts growth in magma chamber so at lava eruption. This fact is proved out by our experimental investigations.
