Origin of water in the mantle eclogites from the V. Grib kimberlite pipe, NW Russia

The study of water content in the rock-forming minerals of mantle xenoliths, entrained in kimberlites, provides information about the water storage of the lithospheric mantle of ancient cratons. In mantle xenoliths, the water can be identified as several percentages by weight in hydrous minerals (e.g. phlogopite and amphibole) and up to 2000 ppm in nominally anhydrous minerals (NAMs; olivine, pyroxene, and garnet). Since the hydrous phases occur sporadically in mantle xenoliths, their NAMs reserve the main water content in the lithospheric mantle.

The water content in garnet and clinopyroxene from the mantle eclogites from the V. Grib kimberlite pipe (Arkhangelsk Diamondiferous Province, NW Russia) was analysed using Fourier transform infrared spectrometry. The studied samples are coarse-grained (grain sizes from 0.5–1.3 cm) bimineralic (garnet and clinopyroxene) eclogites with accessories of phlogopite, ilmenite, and rutile. The samples include high-MgO (three samples) and low-MgO (six samples) groups. The eclogites are interpreted as metamorphosed fragments of oceanic crustal rocks (basalt and gabbro for low-MgO eclogites and picritic/MgO basalt and troctolite for high-MgO eclogites) emplaced into the lithospheric mantle via a subduction event at 2.8 Ga. Based on pressure-temperature estimates (44–78 kbar; 940°C–1275°C), eclogites were transported by kimberlite from the range of depths of about 160 to >200 km.

The results show that all clinopyroxene grains contain structural water in the amount of 39 to 111 ppm, whereas only two garnet samples have detectable water in the amount of 211 and 337 ppm. The water incorporation into the clinopyroxene is mostly linked to M2 sites and aluminium in the tetrahedral position. The water content in the majority of eclogite clinopyroxene positively correlates with the jadeite component. The low-MgO eclogites with oceanic gabbro precursor contain significantly higher water concentrations in omphacites (70–111 ppm) and whole rock (35–224 ppm) compared to those with the oceanic basalt protolith (49–73 ppm and 20–36 ppm, respectively). The proposed observation is also confirmed by the negative correlations of water content in clinopyroxenes with a La/Yb ratio in clinopyroxene and WR water content versus the WR Yb concentration. The equilibrium pressure could be an additional factor that controls the water incorporation into the clinopyroxene of the high-MgO group.

Our results show that water content in the V. Grib pipe eclogites is not from the mantle metasomatism and therefore can reflect the water saturation of their protoliths. The eclogite portion of the lithospheric mantle beneath the V. Grib kimberlite pipe can have at least twice the water enrichment compared to peridotite sections, indicating that an Archean subduction event played an essential role in the water saturation of the mantle.

This work was supported by the Russian Science Foundation under grant no. 16-17-10067