Minerals from the pegmatites of the Lovozersky massif. The study and experimental investigation.

Kovalskaya T.N^*, Kotelnikova Z.A.^**, Kotelnikov A.R.^*, Kovalsky A.M.^*, Goettlicher J.^***

^*IEM RAS, Chernogolovka, Moscow district, Russia, tatiana76@iem.ac.ru

^**IGEM RAS, Moscow. Russia

^*** Insitute for Synchrotron Radiation, Karlsruhe, Germany 

 

               Lovozersky alkaline massif is located in the central part of Kola peninsula and includes gigantic deposits of rare and radioactive elements, related with pegmatites. Data of alkalinity of postmagmatic mineralgenetic solutions are necessary for understanding the features of mineralogenesis process for alkaline complexes. Sodalite (hakmanite), natrolite and microcline are a typical mineral of pegmatites of Lovozersky massif. Investigation of fluid inclusions in hakmanite (as it composition) and natrolite supposes the conditions of their formation. The samples for investigations were collected during the fieldwork seasons 2005, 2007 and 2008. We investigated sodalites, natrolite and microcline from pegmatite №62 from Lovozersky massif, located near mountain Karnasurt.

The compositions of minerals were determined by X-ray microanalyzer Tescan Vega II XMU with X-ray energy dispersive spectrometry INCAx-sight. The relationships between the minerals are shown in Fig. 1. The composition of hakmanite is showed two types of hakmanite in pegmatite №62 of Lovozersky massif – with Cl^- and Cl^-  and SO₄^2- anion groups. A slice of hackmanite has been cut, and sulfur K-edge X-ray absorption spectra have been measured in the fluorescence mode at the bulk X-ray absorption spectroscopy beamline (XAS) at ANKA using a five element Ge solid state detector (Canberra). A silicon 111 crystal pair was used as monochromator. Higher harmonics were eliminated by passing a mirror behind the monochromator. To minimize absorption caused by the atmosphere and by the windows of the ionization chamber a combined ionization-sample-chamber has been used at a nitrogen gas pressure of about 80 mbar. We have detected two types of sulfur K-edge XANES spectra in a sodalite (hackmanite) sample from Lovozersky massif: one indicates reduced sulfur species, the other one mainly sulfur as sulfate.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 Fig. 1. The relationships between the minerals in pegmatite № 62. 1, 4, 5 - Cl-sodalite, 2 - S-containing sodalite, 3, 6 - natrolite.

 

We also investigated fluid inclusions in hakmanite and natrolite. It is shown, that fluid inclusions in hakmanite contain water, crystal of salt and gas bubble. The thermometry of 3-phase fluid inclusions showed that the temperature of inclusion homogenization is about 200^о. The temperature of ice-melt in inclusions is from -6.5 to -4.4 ^оС. On the basis of these data we calculated the concentration of NaCl_eq in the fluid – 9.9 wt.%. For the Lovozersky massif were calculated C_NaCl-eq for sodalite-bearing parageneses (Suk et al., 2007, in russian) - C_NaCl-eq = 10%. We calculated that at this concentration of NaCl in the fluid inclusions trapping temperature was 400^oC . It is correlated well with the mineral geothermometers, according to which the formation of pegmatites (with sodalite-bearing paragenesises) of the Lovozersky massif occurred at the temperatures of 400 450^оС (Ustinov et al, 2006, in russian).

Fluid inclusions in natrolite contain water and gas bubble. The temperature of homogenization of these inclusions is 150^oС.  The concentration of of NaCl_eq in the fluid – 7.7 wt.%. It is known, that natrolite is stable up to 250 ^oС, thus we suggested, that pegmatite №62 had several stages of formation.

We are carried out some experiments on synthesis the fluid inclusions in hakmanite. 1 M solution Na₂SO₄ and 2 M NaСl solution were used as a fluid component. High temperature gas pressure apparatus and hydrothermal apparatus UVD 10000 were used for the carrying out the experiments. The slices of hakmanite about 0,8*0,5 cm were used as starting materials. These slices were heated at 500°С and then were cooled in alcohol at room temperature. The ratio shot/fluid was 1:6. The charge of the starting materials was placed inside platinum capsule. Synthesis was carried out at Т=450°С and Р=1.5 kbar. The duration of the experiments was 31 and 40 days. Synthesized fluid inclusions were investigated by thermo-cryometric method with Linkam-TMHSG-600.

We checked new fluid inclusions in the products of the experiments. The investigation of these inclusions is shown that they consist of two immiscible liquids, gas bubble and crystal of salt. It is typically for the solution of  II type -  the lot of critical occurrences. (Ravich, 1974, in russian).

The results of our experiments are shown, that heterophase fluid was involved during Lovozersky massif pegmatite formation at 400 - 450°С and homogenius fluid - at 250°С, which is shown in the composition of minerals and their occurrence in the rocks.

 

This study was financially supported by RFBR grant 10-05-00508.

 

               References:

Ravich M.I. Systems salt-water at high temperatures and pressures. Moscow: Nauka, 1974, 150 p. (in Russian).

Suk N.I. Mineralogy of carbonatites. 1971. 288 p (in Russian).

Suk N.I., Kotelnikov A.R., Kovalskii A.M. Mineral thermometry and fluid composition of sodalite syenites of the Lovozersky alkaline massif / / Petrology, 2007, vol. 15, №5, pp. 474-492. (in Russian).

Ustinov V.I., Grinenko V.A., Kotelnikov A.R., Suk N.I., Kovalskaya T.N., Smirnova E.P. Thermometry sodalit-bearing associations rocks of the Lovozersky and Tiksheozersky alkaline massifs / / Thesises of All-Russian Meeting "Geochemistry, petrology, mineralogy and genesis of alkaline rocks”, Miass, 18-23 September 2006, pp. 267-272. (in Russian)