Kaapvaal lamproite melts (aka orangeites): A new type of alkali-carbonate liquid? insights from olivine-hosted multiphase inclusions (Silvery Home, South Africa)

Kaapvaal lamproites (aka orangeites) are a group of volatile-rich (H₂O, CO₂), micaceous, ultrapotassic igneous rocks that are unique to the Kaapvaal craton in southern Africa. However, the composition of the melts that give rise to these rocks remains poorly understood due to overprinting effects of contamination by mantle and crustal material, volatile exsolution, fractional crystallisation and post-magmatic alteration. Consequently, this lack of reliable data on the initial composition of the Kaapvaal lamproite melts hampers our understanding of their source, petrogenesis and ascent mechanisms.

Olivine is a common mineral comprising the Kaapvaal lamproites that has both xenocrystic (i.e., mantle) and magmatic origins. Multiphase inclusions (melt/fluid) entrapped within olivine have been consistently demonstrated as an effective tool for gaining fundamental insights into the composition and evolution of melts that produce both kimberlites and lamproites, prior to processes, such as eruption, devolatization and syn-/post-magmatic alteration.

In this study, multiphase inclusions of both secondary and pseudosecondary origin hosted in olivine from a Kaapvaal lamproite (Silvery Home, South Africa) provide novel insights into the composition of the melt(s) that initially transported olivine to the surface and then crystallised after emplacement to form the lamproite groundmass. The inclusions in our study contain daughter mineral assemblages consisting of diverse Ca-Mg carbonates, including K-, Na-, Ba-, and Sr-bearing varieties, moderate K-rich silicates (phlogopite, tetraferriphlogopite), and subordinate oxides, phosphates, sulphides, sulphates, and halides. Based on these daughter mineral assemblages, we suggest that the composition of the melt entrapped by olivine was SiO₂-poor, Ca-Mg carbonate-rich and contained elevated concentrations of K, Na, Ba, Sr, P and Cl. The mineral and reconstructed melt compositions are in stark contrast to the mineral association of the groundmass and the bulk-rock composition of the Silvery Home lamproite, respectively. We suggest that alkali-/alkali-earth carbonates, phosphates, sulphides, sulphates, and halides represented a potentially significant, or even dominant, component of the melt that crystallised the Silvery Home lamproite but were likely removed by degassing and/or interaction with syn-/post-magmatic fluids. We show that olivine-hosted multiphase inclusions from the Silvery Home lamproite share many compositional similarities to melt inclusions hosted in olivine from kimberlites but are distinct from ‘classic’ cratonic olivine lamproites worldwide.