Derivation of K-rich ultramafic magmas from a peridotitic mantle source

Abstract

THE genesis of a wide spectrum of basaltic magmas by partial melting of a peridotitic upper mantle with small amounts of H₂O as its only volatile component is well established^1–3, as is the role of CO₂ in magma genesis in the upper mantle^4,5. However, the origin of K-rich mafic to ultramafic magmas has not been satisfactorily explained by mechanisms involving partial melting of commonly accepted upper mantle sources. We report here experiments on an olivine ugandite from Bufumbira, south-west Uganda⁶, in conditions of H₂O and H₂O–CO₂ mixtures ($x_{{\text{CO}}_{\text{2}}}$ ∼0.75) up to 40 kbar, and at an $f_{{\text{O}}_{\text{2}}}$⩾Ni–NiO buffer⁷. These are the first experiments reported on a lava of the ultramafic alkaline type with K₂O > Na₂O using both H₂O and CO₂ as volatiles and showing that K-rich ultramafic magmas can be derived by partial melting of a ‘normal’ peridotitic mantle containing both H₂O and CO₂. Olivine ugandite is believed to be the most primitive lava occurring in this K-rich province^6,8–10 and may reasonably be considered as a model composition for K-rich ultramafic magmas generally.