Melt inclusions within olivine in a garnet lherzolite xenolith from the V. Grib kimberlite pipe: the first finding of alkaline-carbonate melt snapshots in the lithospheric mantle beneath the East European craton.

Abstract

The generation of carbonate melts during low degree partial melting of a mantle rocks was experimentally and theoretically predicted, however now days there are few direct in situ observations of relicts such melts in the mantle rocks. Here, we present the first directly empirical evidence of existence of alkali-carbonate liquid in the lithospheric mantle beneath the East European craton.A xenolith of garnet lherzolite G1-25 from the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province, East European platform) was studied. The estimated P-T parameters of the last mineral equilibrium of the xenolith are 33 kbar and 750° C, which corresponds to the depth of ~ 100 km.  Olivine grains of the xenolith contain a large number of inclusions that arranged in groups along the randomly oriented healed fractures. Size of the inclusions varies from 2 to 50 μm. There are subhedral, elongate and vermicular inclusions shapes.The melt inclusions are composed of different daughter minerals, up to 10 within one inclusion, and rare bubble/bubbles. Mineral assemblages of the inclusions were studied by Raman spectroscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy.The following mineral species were identified among daughter phases of melt inclusions: nyerereite (Na,K)2Ca(CO3)2, shortite Na2Ca2(CO3)3, gregoryite/natrite (Na,K,Ca)2CO3, eitelite Na2Mg(CO3)2, dolomite CaMg(CO3)2, calcite CaCO3, magnesite MgCO3, unidentified Ba-carbonate, bradleyite Na3Mg(PO4)(CO3), northupite Na3Mg(CO3)2Cl, burkeite Na6CO3(SO4)2, aphthitalite K3Na(SO4)2, arcanite K2SO4, halite NaCl, sylvite KCl, tetraferriphlogopite KMg3FeSi3O10(OH), an unidentified Fe-Ni-Cu- sulfide, Fe-Ti-Mg-spinel-group mineral (Fe,Mg)(Fe,Al,Ti)2O4 and apatite Ca5(PO4)3(F,Cl,OH).Analysis of confocal Raman images (Raman-maps) and BSE images show that more than 55 vol. % of studied inclusions is consist of various carbonate minerals dominated (>70 %) by alkaline carbonates, which indicate that the melt preserved in the inclusions was an alkaline carbonate liquid. Also, the funds of numerous sulfates (8 vol.%), halides (4 vol.%), phosphates (3 vol.%) among daughter phases of inclusions indicate high concentrations of volatile components in the preserved melt.