Metasomatic processes in the lithospheric mantle beneath the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province, Russia)

Abstract

New data on metasomatic processes in the lithospheric mantle in the central part of the Arkhangelsk diamondiferous province (ADP) are presented. We studied the major- and trace-element compositions of minerals of 26 garnet peridotite xenoliths from the V. Grib kimberlite pipe; 17 xenoliths contained phlogopite. Detailed mineralogical, petrographic, and geochemical studies of peridotite minerals (garnet, clinopyroxene, and phlogopite) have revealed two types of modal metasomatic enrichment of the lithospheric-mantle rocks: high temperature (melt) and low-temperature (phlogopite). Both types of modal metasomatism significantly changed the chemical composition of the peridotites. Low-temperature modal metasomatism manifests itself as coarse tabular and shapeless phlogopite grains. Two textural varieties of phlogopite show significant differences in chemical composition, primarily in the contents of TiO2, Cr2O3, FeO, Ba, Rb, and Cs. The rock-forming minerals of phlogopite-bearing peridotites differ in chemical composition from phlogopite-free peridotites, mainly in higher FeO content. Most garnets and clinopyroxenes in peridotites are the products of high-temperature mantle metasomatism, as indicated by the high contents of incompatible elements and REE pattern in these minerals. Fractional-crystallization modeling gives an insight into the nature of melts (metasomatic agents). They are close in composition to picrites of the Izhmozero field, basalts of the Tur’ino field, and carbonatites of the Mela field of the ADP. The REE patterns of the peridotite minerals make it possible to determine the sequence of metasomatic enrichment of the lithospheric mantle beneath the V. Grib kimberlite pipe.

Diamond Exploration Potential of the Northern East European Platform

“…To our knowledge, extremely Mg-rich olivine (Mg# 96) has been identified in association with kimberlites only in the form of several groundmass olivine grains in the Udachnaya kimberlites [33]. Olivine from the ADP kimberlites, mantle peridotites, and diamonds is characterized by Mg# values between 88 and 94, notable trace amounts of NiO, and Cr 2 O 3 contents of >0.01 wt % (see Figure 5A-C; [11,[34][35][36][37][38][39]). Therefore, olivine with extremely Mg-rich composition is ruled out as being potentially kimberlitic in the current stage of this study.…”

“…In the Cr 2 O 3 -Al 2 O 3 discrimination diagram [42], 16% of the grains do not match the "on-craton" garnet-peridotite field and, therefore, are ruled out as potentially kimberlitic Cr-diopside ( Figure 6A). In the Cr 2 O 3 -CaO diagram ( Figure 6B), all the potentially kimberlitic Cr-diopside plots within the field of Cr-diopside from the Lomonosov and Grib peridotites and kimberlites [11,35,37,38,43] and the field representative of Cr-diopside inclusions in diamonds [44], with the exception of 20 grains ( Figure 6B). In total, the percentage of kimberlitic Cr-diopside is relatively high in samples from all of the areas and varies in the range of 64-78%.…”

“…Diamond association, high-chromium G9 and G10 pyrope [28][29][30]; N, number of grains analyzed. (E) Mg# vs. Cr 2 O 3 , lherzolites of the ADP diamondiferous kimberlites [11,30,35,37,38,43]. (F) Distribution of T Ni [32] for the pyrope of 'K', 'P', and 'S' areas.…”

“…[40], Lomonosov and Grib kimberlites and peridotites [11,35,[37][38][39], DI (inclusion in diamonds) from the Lomonosov and Grib [11,36], ADP melilitites [11]. [40], Lomonosov and Grib kimberlites and peridotites [11,35,[37][38][39], DI (inclusion in diamonds) from the Lomonosov and Grib [11,36], ADP melilitites [11]. [11,35,[37][38][39], diamond inclusion field [44], DI (inclusion in diamonds) from the ADP [11].…”

“…[40], Lomonosov and Grib kimberlites and peridotites [11,35,[37][38][39], DI (inclusion in diamonds) from the Lomonosov and Grib [11,36], ADP melilitites [11]. [11,35,[37][38][39], diamond inclusion field [44], DI (inclusion in diamonds) from the ADP [11].…”