Composition of garnet and clinopyroxene in peridotite xenoliths from the Grib kimberlite pipe, Arkhangelsk diamond province, Russia: Evidence for mantle metasomatism associated with kimberlite melts

• Published in: Lithos Vol. 262; pp. 442 - 455
• Main Authors: Kargin, A.V., Sazonova, L.V., Nosova, A.A., Tretyachenko, V.V.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-10-2016
• Subjects: Clinopyroxene; Garnet; Kimberlite; Lithospheric mantle; Mantle metasomatism; Peridotite xenolith; Lithospheric mantle; Mantle metasomatism; Garnet; Peridotite xenolith; Kimberlite; Clinopyroxene
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2016.07.015

Here we present major and trace element data for garnet and clinopyroxene from mantle-derived peridotite xenoliths of the Grib kimberlite, the Arkhangelsk diamond province, Russia, and provide new insights into the metasomatic processes that occur within the subcontinental lithospheric mantle (SCLM) during the kimberlite generation and ascent.
The mantle xenoliths examined in this study are both coarse and sheared garnet peridotites and consist of olivine, orthopyroxene, clinopyroxene, garnet with minor ilmenite, magnetite, and Cr-spinel. Based on garnet and clinopyroxene composition, two groups of peridotite are recognized. One group contains high-Ti, light rare earth elements (LREE) enriched garnets and low-Mg# clinopyroxenes with low (La/Sm)n (C1 chondrite-normalized) values. This mineral assemblage was in equilibrium with a high-temperature carbonate–silicate metasomatic agent, presumably, a protokimberlite melt. Pressure–temperature (P–T) estimates (T=1220°C and P=70kbar) suggest that this metasomatic event occurred at the base of the SCLM. Another group contains low-Ti garnet with normal to sinusoidal rare earth elements (REE) distribution patterns and high-Mg# clinopyroxenes with wide range of (La/Sm)n values. The geochemical equilibrium between garnet and clinopyroxene coupled with their REE composition indicates that peridotite mantle experienced metasomatic transformation by injection of a low-Ti (after crystallizations of the ilmenite megacrysts) kimberlite melt that subsequently percolated through a refractory mantle column. Peridotites of this group show a wide range of P–T estimates (T=730–1070°C and P=22–44kbar).
It is suggested that evolution of a kimberlite magma from REE-enriched carbonate-bearing to carbonate-rich ultramafic silicate compositions with lower REE occurs during the ascent and interaction with a surrounding lithospheric mantle, and this process leads to metasomatic modification of the SCLM with formation of both high and low-Ti garnets and clinopyroxene widely varying in Mg# and (La/Sm)n values.
The sequence of metasomatic events in the subcontinental lithospheric mantle associated with the generation of the Grib kimberlite. [Display omitted]
•Peridotites are divided into two groups formed by different metasomatic processes.•The first group contains high-Ti garnets and low-Mg# clinopyroxenes.•The second group contains low-Ti garnets and high-Mg# clinopyroxenes.•The metasomatic agent changes its composition during evolution of kimberlite magma.•The sequence of metasomatic events is proposed.