On origin of lower-mantle diamonds and their primary inclusions

Abstract

Knowledge of mineralogy and petrology of unattainable lower mantle material is usually founded on high-pressure experiments with pyrolite (‘in situ’ material) and oceanic MORB basalt (subducted material). Primary inclusions in transition zone and lower-mantle ‘super-deep’ diamonds represent heterogeneous fragments of diamond-parental medium (not the unaltered lower mantle material). Inclusions of magnesiowustite and stishovite intergrowths (‘stishovite paradox’) give experimentally-supported evidence that stishovite, similarly to magnesiowustite, is not subducted but in situ lower mantle mineral. Primary Ca-, Mg-, Na-carbonate inclusions are symptomatic for multicomponent carbonatite (carbonate-oxide-silicate) parental melts for the lower-mantle diamonds and inclusions. We investigated melting phase relations of simple carbonates of Ca, Mg, Na and multicomponent Mg-Fe-Na-carbonate up to 60GPa and 3500–4000K (using multianvil press and diamond-anvil cell with laser heating) and determined a congruent melting of the carbonates and stability of PT-extended phase fields of the carbonate melts. ‘Super-deep’ diamonds are experimentally crystallized in melts of the lower mantle diamond-parental carbonate - magnesiowustite – Mg-perovskite – carbon system. Based on experimental and mineralogical evidence for the lower mantle diamonds inclusions, genetic links between diamonds and inclusions are determined and a generalized composition diagram of parental media for lower mantle diamonds and inclusions is constructed.