Melt inclusions from the deep Slave lithosphere: implications for the origin and evolution of mantle-derived carbonatite and kimberlite

Abstract

Melt inclusions in clinopyroxenes from lherzolitic xenoliths from the deep lithospheric mantle beneath the Slave Craton (Lac de Gras area, Canada) reveal multiple origins for carbonatitic melts. One type of inclusions consists of a series of silicate–carbonate–silicate concentric layers, interpreted to have unmixed under disequilibrium conditions during rapid ascent to the surface. Bulk major- and trace-element compositions are typical of Group 1 kimberlites and quantitative nuclear microprobe imaging of the globules reveals fractionation of related elements (e.g. F–Br, Nb–Ta) between the silicate and carbonate components. The globules probably formed by partial melting of carbonated peridotite, consistent with results of melting experiments and some models for the generation of kimberlite magmas. They provide evidence for a genetic relationship between some carbonate-rich magmas and ultramafic silicate magmas, and for the possibility of unmixing processes of these melts during their evolution.

The second inclusion type comprises carbonate-rich globules interpreted as samples of Mg-carbonatite melt that quenched on ascent to the surface. Bulk major- and trace-element compositions indicate that the melts were derived from a carbonate-rich source and oxygen, carbon, and strontium isotope data are consistent with the involvement of recycled crustal material and suggest that some mantle-derived carbonatites are unrelated to kimberlites.

Introduction

Kimberlites and carbonatites are relatively rare but important silica-undersaturated igneous rocks that provide insights into the nature of geochemical processes that shape the subcontinental lithospheric mantle. Some hypotheses of carbonatite genesis favour crustal processes involving fractionation/immiscibility from carbonate-bearing alkaline silicate parents (e.g. Tuttle and Gittins, 1966, Heinrich, 1966); others that favour a primary mantle origin (e.g. Gittins, 1989, Bailey, 1993, Harmer and Gittins, 1998). Wallace and Green (1988) demonstrated that carbonatites can be produced by partial melting of carbonated mantle peridotite. Javoy (1997) estimated that the primitive upper mantle contains about 0.15% CO₂ (hosted in carbonate minerals) and Dalton and Presnall (1998) demonstrated that carbonatitic melts can be produced by small-degree partial melting of mantle with this CO₂ content. The Dalton and Presnall (1998) experiments illustrated further that as the temperature is raised to ca. 70–100 °C above the solidus, the melts grade into ultramafic silicate compositions similar to Group I kimberlites. Important consequences of their study include the demonstration of a genetic relationship between carbonatites and kimberlites. The silicate- and carbonate-rich melt inclusions hosted in mantle diopside of this work provide more information on carbonatite and kimberlite genesis in the deep lithosphere and may be used to place constraints on some aspects of the experimentally derived models.

The carbonate-rich globules were described in detail by Van Achterbergh et al., 2002. They are interpreted as partial melts involving subducted crustal material (based on oxygen, carbon and strontium isotope data). The ultramafic silicate globules are described here for the first time and their composition, origin and relationship to the carbonate-rich globules are explored.

The kimberlite containing the xenoliths of this study (the Eocene Pipe A154N) occur within the Slave Structural Province, an Archean granite-greenstone block within the North American craton. Xenocryst and xenolith data from the kimberlite pipes and alluvial deposits in this area have been used to map a strongly layered lithospheric mantle structure Pearson et al., 1999, Griffin et al., 1999. An upper ultradepleted layer (35 mW/m² conductive geotherm) is underlain from ∼140 to 150 km depth by a less depleted lower layer extending to about 200 km depth. Eclogite is largely confined to the lower layer (Griffin et al., 1999). Temperature estimates for the megacrystalline lherzolites discussed here indicate that they were derived from the lower, more fertile layer. More details regarding the regional geology and lithosphere structure can be obtained from Graham et al. (1999), Pearson et al. (1999) and Griffin et al. (1999).