Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Мelting of garnet-bearing carbonatite of Tromsø area, Norvway

N.S. Gorbachev, A.N. Nekrasov, A.V. Kostyuk, D.M. Sultanov

Institute of experimental mineralogy RAS, Chernogolvka, Russia

gor@iem.ac.ru.

In connection with the problem of the genesis of carbonatites at T-P of upper mantle (950-1450 º C, 3.8-4.0 GPa) studied the melting of garnet-bearing ultra-high pressure carbonatite UHPC Tromsø area, Norway in "dry" conditions and with H₂O+CO₂ fluid.

Experiments were carried out in the IEM RAS on the "anvil with hole" in Au and 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 ± 10ºC for temperature and ± 0.1 GPa for pressure measurements. Duration of experiments were from 6 to 18 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.

Temperature T "dry" liquidus UHPC ~ 1270 º C. It is established full miscibility between silicate and carbonate melts. At quenching carbonatite melt the mix of microlites of variable composition, from the carbonate-silicate to silicate-carbonate is formed.

In the range of T 1250-1200ºС with carbonatite melt garnet Grt co-exists. This garnet differing from initial garnet UHPC by absence of zonality, higher content of CaO, TiO₂, lower Al₂O₃, FeO.

Temperature "dry" solidus is ~1150ºС. In the range Т from 1150 to 950ºС near soliduse and subsolidus associations are presented calcite Сс, garnet Grt, clinopyroxene Сpx, flogopite Flog and accessory minerals - apatite Apt, ilmenite Ilm, rutile Rt.  

In experiments with Н₂О+СО₂ fluid temperature liquidus goes down to 1250ºС. At Т=1450ºC, Р=4 GPa stratification carbonatite melt on highly and low-carbonate-silicate liquids with graphite allocation was observed (fig. 1).

Fig. 1. Stratification carbonatite melt on highly and low- carbonate-silicate liquids with graphite allocation: microphoto an composition of the quenching carbonatite melt sample. 1450ºC, 4 GPa.

In range T=1250-1150ºС with carbonatite melt the zonality garnet co-exists. Its reactionary border in comparison with its central part is enriched  in FeO, MgO, MnO and depleted in . CaO, TiO₂, SrO (fig. 2). Zonality of garnet testifies to interaction of carbonatite melt with garnet at which the reactionary garnet is enriched in CaO, TiO₂, SrO.

Fig. 2. Microphoto and composition of the quenching sample with zonality garnet. T 1150ºС, Р=4 GPa.

Zonality of garnet testifies to interaction of carbonatite melt with garnet at which the reactionary garnet is enriched in CaO, TiO₂, SrO.

At melting of UHPC with Н₂О+СО₂ a fluid temperature solidus goes down to Т ~ 950ºС. At Т=950ºС, P=3.8 GPa carbonatite melt co-exists with Сс, Flog, Grt, Cpx, Apt. At increase Т up to 1050ºС the portion of carbonatite melt increases, disappears Cpx, on liqudus present Сс, zonality Grt, Flog,

Results of experiments testify to formation characteristic for UHPC associations a carbonate-zonality garnet in the range of Т =1150-1250ºС.

This study was financially supported by RFBR grant 09-05-0113-a, 12-05-00777-a.-