Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Phase composition and geochemistry of carbonatites from Tromsø area, Norway.

N.S. Gorbachev*, E. Ravna^** , A.N. Nekrasov^*., K. Kullerud^**

^*Institute of experimental mineralogy RAS, Chernogolovka Russia

* Tromsø, University, Tromsø, Norway

gor@iem.ac.ru

For explore of genesis of mantle carbonate magmas have been studied the phase relations and geochemistry of garnet-bearing ultra-high pressure carbonatites (UHPC) Tromsø area, Norway. The matrix of UHPC is represented by carbonate C calcite-dolomite composition with inclusions of garnet Grt and accessory minerals - apatite, sphene, ilmenite, rutile. The samples were studied by 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.microprobe, trace elements were determined by ICP MS.

On structure and composition it is allocated three generation of Grt: early, Grt1, of pyr15-gros25-alm60 composition, depleted in REE (<2.10^-2 wt.%); reacting Grt2-3 of pyr5-gros65-alm30 composition, anomalously enriched (up to 4-8 wt.%) in LREE .

The abundances of trace elements in the UHPC, its carbonate C and silicate Si fraction were examination. Compared to silicate, carbonate fraction is enriched in Ba, Rb, Nb, Sr, P, Zr, depleted in Hf, Ti, Th, Ta. Carbonate and silicate fraction of UHPC differ in REE concentrations and the character of its dependence from atomic number N REE. Silicate fraction enriched in REE, the dependence of the normalized for chondrite REE concentrations C_N from N REE has negative slope with slight Eu minimum. In the carbonate fraction to the dependence of C_N-N REE has an extreme character with  maximum at Gd-Tb. The largest differences are observed in the distribution of LREE. In the silicate fraction C_N REE reduced from 500 at La to 50 for Sm, in carbonate fraction C_N REE rise from 30 to 45 at La to Sm. Starting from Gd to Lu REE behavior is similar in silicate fraction C_N REE reduced to 8, and in carbonate C fraction up to 5 (fig. 1).

Fig. 1. Normalize for chondrite concentration of trace elements in UHPC, its carbonate C and silicate Si fractions

Since the silicate fraction to UHPC dominates Grt, then we can assume that D REE - partition coefficients between silicate and carbonate fraction to characterize the partition coefficients REE between Grt and carbonatite melt C.

Fig. 3. Partition coefficients REE between silicate and carbonate fraction of UHPC (D REE Si/C) and partition coefficients REE between Grt and carbonatite melt (D REE Grt/Cm) (for our experimental datas).

However, D REE Si/C~D REE Grt/C in the UHPC differ from the experimental D Grt/C. In UHPC dependence of D REE Si/C is an extreme character, decreasing from 18 to 1.2 for La to Gd-Tb, and then increasing to 2 at Tm-Lu. According to our experimental data, the dependence of D REE Grt / C is positive, increasing from 0.07 at La to 10 at Lu (fig. 3).

Anomalous REE distribution in UHPC indicates a lack of equilibrium between Grt and carbonatite melt at formation of carbonatite melt at melting of uper mantle.

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