The composition of primary fluid inclusions in olivine of ultramafic xenoliths

from Avacha volcano (Kamchatka)

Timina T. Yu., Tomilenko A. A., Sokolova E. N.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

 

We studied harzburgite samples affected by processes of recrystallization and melting in different degrees and samples of primary harzburgites. The chemical compositions of peridotitic and pyroxenitic minerals and their primary melt inclusions were described in (Tomilenko et al., 2010; Timina et al., 2012). Primary fluid inclusions were found only in the minerals of metasomatically recrystallized harzburgites and pyroxenites.

Olivine from metasomatized peridotite xenoliths contains a lot of primary fluid inclusions. Their phase composition: gas bubble + high saline aqueous solution + daughter phases. Talc, actinolite and magnesite have been detected by Raman spectroscopy among daughter phases. Partial homogenization of the crystal-rich inclusions into the liquid occurs at 330°C, complete homogenization occur at 960-980°C.

Primary olivine-hosted fluid inclusions from pyroxenite xenoliths occur much rarer. Their phase composition is represented by gas bubble, carbonate-water-salt solution and several daughter crystalline phases (magnesite, calcite, talc, anhydrite amphibole and clinopyroxene).

It was suggested in previous studies that pyroxenites are derivates of primary harzburgites (Timina et al., 2012).  In this case the olivine from pyroxenite and olivine from metasomatized peridotites should be compositionally similar. Chemistry of olivine from metasomatized peridotite is featured by high magnesium number (Mg# 0.88-0.85), NiO contents (0.32–0.25 wt. %) and low CaO (0.07-0.1 wt.%). Olivines from clinopyroxenite xenoliths differ from them by lower Mg# (0.84-0.8), NiO (0.1-0.2 wt.%) and CaO (0.05-0.08 wt.%) contents.

The studied olivines differ from each other in fluid inclusion compositions. Mg-bearing minerals represent majority of daughter phases of fluid inclusions in peridotitic olivine. Daughter minerals of fluid inclusions in clinopyroxenitic olivine are represented by Ca-bearing phases. It is suggested that Ca was extracted mantle wedge rocks by metasomatism. The data obtained suggest that the clinopyroxenites were formed as a result of metasomatic alteration and melting of mantle lherzolites rather than mantle wedge harzburgites beneath Avacha volcano.

This work is supported by the grant of President of Russian Federation (No. MK-5459.2013.5), RFBR (grant No. 12-05-00888-а).