No abstract

The water content in the garnet and clinopyroxene in the mantle eclogites from the V. Grib kimberlite pipe (Arkhangelsk Diamondiferous Province, NW Russia) was analysed using Fourier transform infrared spectrometry. The results show that all clinopyroxene grains contained structural water at concentrations of 39 to 247 ppm, whereas two garnet samples contained detectable water at concentrations of 211 and 337 ppm. The low-MgO eclogites with oceanic gabbro precursors contained significantly higher water concentrations in the omphacites (70–247 ppm) and whole rock (35–224 ppm) compared to those with oceanic basalt protoliths (49–73 ppm and 20–36 ppm, respectively). The incorporation of water into the clinopyroxene may be associated with vacancies at the M2 site, Al in the tetrahedral position, and the elements that filled the M2 site (mostly Na and Ca). The highest water content in the omphacite was detected in a nonmetasomatised sample and was assumed to represent residual water that survived during subduction. Other eclogite samples showed signs of modal and/or cryptic metasomatism and contained less water in the omphacites compared to the nonmetasomatised sample. The water content was heterogeneous within the eclogite section of the sampled lithospheric mantle. The lack of distinct and uniform correlations between the indices of eclogite modification and their water content indicated that the saturation with water was disturbed during their residence within the lithospheric mantle.

<p>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 <em>in situ</em> 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<strong> </strong>craton.</p> <p>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.</p> <p>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.</p> <p>The following mineral species were identified among daughter phases of melt inclusions: nyerereite (Na,K)<sub>2</sub>Ca(CO<sub>3</sub>)<sub>2</sub>, shortite Na<sub>2</sub>Ca<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>, gregoryite/natrite (Na,K,Ca)<sub>2</sub>CO<sub>3</sub>, eitelite Na<sub>2</sub>Mg(CO<sub>3</sub>)<sub>2</sub>, dolomite CaMg(CO<sub>3</sub>)<sub>2</sub>, calcite CaCO<sub>3</sub>, magnesite MgCO<sub>3</sub>, unidentified Ba-carbonate, bradleyite Na<sub>3</sub>Mg(PO<sub>4</sub>)(CO<sub>3</sub>), northupite Na<sub>3</sub>Mg(CO<sub>3</sub>)<sub>2</sub>Cl, burkeite Na<sub>6</sub>CO<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>, aphthitalite K<sub>3</sub>Na(SO<sub>4</sub>)<sub>2</sub>, arcanite K<sub>2</sub>SO<sub>4</sub>, halite NaCl, sylvite KCl, tetraferriphlogopite KMg<sub>3</sub>FeSi<sub>3</sub>O<sub>10</sub>(OH), an unidentified Fe-Ni-Cu- sulfide, Fe-Ti-Mg-spinel-group mineral (Fe,Mg)(Fe,Al,Ti)<sub>2</sub>O<sub>4</sub> and apatite Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(F,Cl,OH).</p> <p>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.</p>

No abstract