Volatiles in Kimberlitic Magmas: Forced Multiple Saturation with a Mantle Source

Abstract:

The geochemistry and mineralogy of the mantle source for primary kimberlite melts is still very much debated, the primary melt is argued to be either of carbonatitic or kimberlitic nature and proposed melting mechanisms range from low-degree partial melting of a carbonated peridotite to high-degree melting of strongly metasomatized veins. Experimental multiple saturation of a proposed close-to-primary kimberlitic composition from Jericho (Kopylova et al. 2007, GCA) at 7 GPa shows that saturation of a lherzolitic mineral assemblage occurs at 1300-1350 °C resulting in a carbonatitic melt with less than 8 wt% SiO₂ and >35 wt% CO₂. At higher temperatures, where the Jericho melt stays kimberlitic, it is only saturated in opx and garnet.

We hence forced the close-to-primary Jericho kimberlite into multiple saturation with a lherzolitic assemblage (7 GPa, 1400-1650 °C) by adding a volatile-free peridotite with the aim to saturate the system in olivine, opx, cpx and garnet. This mineral assemblage is obtained over a wide temperature range (1400-1600 °C) for a starting Jericho composition with 20-22.5 wt% CO₂, H₂O was kept at 0.46 wt% corresponding to the K:H ratio of phlogopite. The transition from a carbonatitic melt with ~10 wt% SiO₂ and >35 wt% CO₂ to a kimberlitic melt with ~27 wt% SiO₂ and <25 wt% CO₂ occurs from 1450 to 1600 °C. Compared to the Jericho composition, these melts have higher Na₂O and lower X_Mg. At lower CO₂ contents (10 wt%) opx was absent, while at higher CO₂ (30 wt%) olivine and cpx were not stable.

Kimberlitic melts in equilibrium with a lherzolite are obtained for temperatures of >1500 °C, requiring a few hundred degrees more than estimated for the base of the cratonic lithosphere (1200-1400 °C at a heat flux of 40-45 mW/m²). If lower temperature carbonatites intrude into the base of the lithosphere it is questionable how these should develop into kimberlites within the lithosphere.