Serpentine Mineral Association, Texture and Composition as Keys to Serpentine Origin in Kimberlites

Abstract

Syn-emplacement serpentine is one of the most abundant late minerals in kimberlites; its multiple generations can be distinguished by various textural positions and parageneses. Composition of the primary kimberlite melt cannot be accurately determined if we do not recognize distinct origins for several textural varieties of serpentine. This study aims to find compositional indicators of the serpentine origin by characterizing millimetre-sized serpentine domains in hypabyssal kimberlites. Serpentine forms as segregations in the groundmass or when serpentine replaces olivine or metasomatized silicate xenoliths. The latter textural variety of serpentine has not been recognized previously; it develops in Si-rich basement xenoliths ranging from basalt to granite. This serpentine is associated with abundant diopside, pectolite, phlogopite and chlorite and less prominent amphibole, hydrogarnet, wollastonite, xonotlite and other rare Ca hydrosilicates. We report petrography and textures of reacted silicate xenoliths in Renard 65, Orapa AK15, BK1, Gahcho Kué 5034 and Jericho kimberlites and provide a global summary of the phase compositions in the xenoliths. This study discovered that NiO content < 0.05 wt %, Al₂O₃ content > 1.3 wt % and MnO > 0.3 wt % in serpentine are clear signs of formation after felsic xenoliths. Serpentine/chlorite replacing olivine always have 1.5–4 wt % more FeO than serpentine after silicate xenoliths. The compositional contrast results from the immobile behaviour of conserved Al, Ni and Mn. The proposed criteria were tested on a pyroclastic kimberlite with an enigmatic origin of round serpentinized clasts overgrown by fibrous clinopyroxene and identified the precursor of these clasts as felsic. We also determined mineralogical characteristics of serpentine parageneses that can be used for reconstruction of the initial xenolith lithology. Serpentine coexists with the more abundant calcic hydrosilicates (hydrogarnet, xonotlite, amphiboles) in reacted mafic xenoliths. There, serpentine and chlorite crystal structures show less ideal stoichiometry indicative of a higher volume of nanometre-scale interstratification with smectites. Serpentine-rich assemblages in reacted xenoliths formed metasomatically at T < 600°C due to skarn-like mass transfer with the host kimberlite that controlled the gain of Ca and Mg and desilication. These metasomatic assemblages are remarkably identical to rodingites. Serpentine production appeared to be limited by the availability of Si in and around silicate xenoliths, but by the H₂O availability in pseudomorphed olivine/monticellite.