Experimental studying of
carbonatization of silicate melts at partial melting of peridotite
Experimental Mineralogy RAS, Chernogolovka, Russia
Association of carbonates
with silicate glass and sulphides in the metasomatized mantle peridotite
and eclogite nodules indicate the presence of alkali-carbonate-sulphide
fluids in upper mantle and their important role in mantle metasomatism
and in melting of metasomatized mantle [1-3]. The study of the
physicochemical conditions of formation of immiscible carbonate,
silicate and sulphide melts at melting of the mantle peridotite and
distribution of the major macro and microelements between coexisting
silicate and carbonate melts is of great scientific interest and belongs
to the actual petrology problems.
Experimental modelling of
silicate-carbonate-sulphide liquid immiscibility of mantle magmas was
studied in the presence of alkali carbonates. The use of alkali
carbonate (K,Na)2CO3 is due to the important role
of the alkali-carbonate fluid in the mantle metasomatism, as well as the
close positional and genetic relationship between alkaline rocks and
carbonatite complexes (such as, Okorusu (Namibia), Stjernoy (Norway),
Phalaborwa (S.Afrika), Fernando de Noronha Island (Brazil), etc.).
Phase relations in the
system peridotite – alkali carbonates with the addition of accessory
minerals (apatite, pyrrhotite, ilmenite, zircon) have been studied
experimentally at a pressure of 3.9 GPa and temperature of 1250ºC.
Experiments were carried out in the IEM RAS on the "anvil with hole" in
Fe-bearing platinum capsules using a quenching technique. The
temperature is measured by a Pt30Rh/Pt6/Rh thermocouple. At high
temperature, pressure is calibrated using a curve of balance quartz -
coesite. Uncertainties are ± 5ºC for temperature and ± 0.1 GPa for
pressure measurements. Duration of experiments were from 6 to 8 hours.
Products of experiments were studied by PC-controlled scanning electron
microscope Tescan VEGA TS 5130MM with
detector of secondary and backscattered electron on the YAG-crystals and
energy dispersive X-ray microanalyzer with semi-conductor Si(Li)
detector INCA Energy 350.
The powders of spinel
peridotite xenoliths from kimberlite pipes Obnazhennaya  used as
starting materials. Accessory minerals are apatite, ilmenite, zircon,
and pyrrhotite was added to the experiments to determine the
distribution coefficients of titanium, phosphorus, sulphur and zirconium
between coexisting silicate and carbonate melts. Starting materials were
used in the ratio: peridotite (70%) - (K,Na)2CO3
(10%) - ilmenite (5%) - apatite (5%) - zircon (5%) - sulphide (FeS)
At partial (10%) melting of
peridotite liquidus association of phlogopite–clinopyroxene–zircon–X-phase
(undiagnosed, similar in composition to the Ti–Cpx) cemented by
intergranular silicate glass with inclusions of carbonate and sulphide
phases (Fig. 1). The morphology, structure and relationships of glass,
carbonate and sulphide globules indicate the existence of immiscible
silicate, carbonate and sulphide melt under the experimental conditions.
Fig. 1. Micrograph of
polished sample after the experiment in the system peridotite - alkali
carbonates (T = 1250ºC, P = 3.9 GPa). The sample represented by
immiscible silicate (LSil), carbonate (LCarb) and
sulphide (LSulph) melts coexisting with clinopyroxene (Cpx)
and phlogopite (Phl).
The composition of the
silicate melt was phonolites with concentration of sulphur <0.1wt.%.
Calcium carbonate melt contained small amounts of alkali metal and
silicate components. The solubility of zircon in silicate melt reached
0.70 ± 0.23 wt.% ZrO2, in coexisting carbonate melt 1.28 ±
0.31 wt.% ZrO2.
The absence of ilmenite and
apatite in the experimental samples can be explained by the high
solubility of titanium and phosphorus in the coexisting phases. The
concentration of TiO2 in the silicate melt was 1.2 ± 0.35 wt.
%, in the carbonate melt 0.42 ± 0.16 wt.%. The concentration of P2O5
was 9.67 ± 1.90 wt.% in the carbonate melt and 2.33 ± 0.31 in the
silicate melt. The concentration of sulphur in these melts do not exceed
Thus, in the system
peridotite - alkali carbonates, silicate-carbonate-sulphide
stratification of alkali silicate melts observed at T = 1250ºC and P =
3.8 GPa. There was a complete dissolution of ilmenite and apatite;
silicate, carbonate and sulphide melts coexisting with zircon,
phlogopite, and clinopyroxene. The distribution coefficients of macro
and microelements (including titanium, zirconium, phosphorus, sulphur)
between coexisting silicate and carbonate melts showed that the main
concentrator of Na, Mn, Zr, S, Ca, P is the carbonate melt, the main
concentrator of Si, Al, K, Mg, Fe, Ti is silicate melt.
Fig. 2. The distribution of
the major macro and micro elements between coexisting silicate and
The data on the
distribution coefficients of macro and micronutrients (Ti, Zr, S, P,
etc.) between the silicate and carbonate melts and coexisting phases are
interesting for the construction of genetic models of carbonate and
sulphide magmas and associated deposits of rare elements,
platinum-copper sulphide and nickel deposits.
Supported by grant RFBR
Kogarko L.N., Henderson C.M.B., Pacheco H. Primary Ca-rich
carbonatite magma and carbonate-silicate-sulphide liquid immiscibility
in the upper mantle. Contrib. Mineral. Petrol. In 1995. V.121. P.
Ionov D. Trace element composition of mantle-derived carbonates
and coexisting phases in peridotite xenoliths from alkali basalts. J.
Petrol. 39. In 1998. P. 1931-1941.
Kogapko L.N. Role of deep fluids in the genesis of mantle
heterogeneities and alkaline magmatism. Geology and Geophysics, 2005, v.
46, № 12, P. 1234-1245.
Ukhanov A.V., Ryabchikov I.D., Kharkiv A.D. Lithospheric mantle of the
Yakutian kimberlite province. Moscow. Publishing House "Nauka", 1988.