Phase relations of aluminum‐undepleted and aluminum‐depleted komatiites at pressures of 4–12 GPa

Abstract

The anhydrous phase relations of natural Al‐undepleted komatiite (AUK) with 25% MgO and Al‐depleted komatiite (ADK) with 32% MgO have been investigated in the pressure range from 4 to 12 GPa, using samples from Munro Township, Ontario, and the Barberton Mountains, South Africa. The experiments were carried out with graphite or molybdenum capsules in 18‐mm octahedral pressure cells in a uniaxial split‐sphere multianvil apparatus. The thermal gradient within the stepped resistance heaters of graphite and lanthanum chromite was about 50°C/mm. For both compositions, the liquidus phase is olivine at low pressure (up to 5.6 and 9.6 GPa for the AUK and ADK, respectively) and garnet at high pressure. Pyroxene is the only liquidus phase in an intermediate pressure range for the ADK (9.6–11.5 GPa) and reaches the liquidus at a multiple saturation point with olivine and garnet at 5.6 GPa pressure for the AUK. The subsolidus assemblage consists of olivine, pyroxene, and garnet in the entire pressure range for both of the compositions. The pyroxenes have variable CaO contents, and most of them are subcalcic clinopyroxenes, but minor amounts of orthopyroxene have been observed in some of the run products. If AUK and ADK compositions represent near primary magmas, the melt separation most likely occurred at pressures corresponding to the three phase saturation point at 5.6 GPa for AUK and one of the two cosaturation points (olivine‐pyroxene at 9.6 GPa and pyroxene‐garnet at 11.6 GPa) for the ADK. The low Ca/Al ratio of the AUK requires a nearly complete melting of garnet. The high Ca/Al ratio of the ADK indicates that garnet was a residual phase at the time of melt separation, or that the mantle source experienced some premelting or synmelting garnet fractionation. The clinopyroxene liquidus interval between 9.6 and 11.6 GPa excludes the coexistence of garnet and olivine in the residue, and the following alternative petrogenetic models are suggested for the ADK: 1. The initial melt was more magnesian than the source peridotite, and the melting progressed to the stage of disappearance of the residual olivine. The melting progressed further along the pyroxene‐garnet cotectic before melt separation. 2. Garnet fractionation took place from an ascending and partially molten peridotite diapir. Further melting of the upper portion of the diapir produced a melt that was less magnesian than the source diapir. This could lead to total melt consumption of any remaining residual garnet, and further melting could proceed along the olivine‐pyroxene cotectic before final melt separation.