Abstract
Abstract. Inclusions of breyite (previously known as walstromite-structured CaSiO3) in diamond are usually interpreted as retrogressed CaSiO3 perovskite trapped in the transition zone or the lower mantle. However, the thermodynamic stability field of breyite does not preclude its crystallization together with diamond under upper-mantle conditions (6–10 GPa). The possibility of breyite forming in subducted sedimentary material through the reaction CaCO3 + SiO2 = CaSiO3 + C + O2 was experimentally evaluated in the CaO–SiO2–C–O2 ± H2O system at 6–10 GPa, 900–1500 ∘C and oxygen fugacity 0.5–1.0 log units below the Fe–FeO (IW) buffer. One experimental series was conducted in the anhydrous subsystem and aimed at determining the melting temperature of the aragonite–coesite (or stishovite) assemblage. It was found that melting occurs at a lower temperature (∼1500 ∘C) than the decarbonation reaction, which indicates that breyite cannot be formed from aragonite and silica under anhydrous conditions and an oxygen fugacity above IW – 1. In the second experimental series, we investigated partial melting of an aragonite–coesite mixture under hydrous conditions at the same pressures and redox conditions. The melting temperature in the presence of water decreased strongly (to 900–1200 ∘C), and the melt had a hydrous silicate composition. The reduction of melt resulted in graphite crystallization in equilibrium with titanite-structured CaSi2O5 and breyite at ∼1000 ∘C. The maximum pressure of possible breyite formation is limited by the reaction CaSiO3 + SiO2 = CaSi2O5 at ∼8 GPa. Based on the experimental results, it is concluded that breyite inclusions found in natural diamond may be formed from an aragonite–coesite assemblage or carbonate melt at 6–8 GPa via reduction at high water activity.
Highlights
Since the discovery by Joswig et al (1999), CaSiO3 inclusions in diamond have been reported by a number of researchers
In the subsolidus experiments at temperatures ≤ 1300 ◦C, the three layers remained separated with only minor interaction between Fe metal and SiO2 that produced ferrosilite and fayalite, whereas no interaction was observed between aragonite and SiO2
At higher temperatures, quenched melt appeared at the contacts between aragonite and SiO2 (Fig. 3)
Summary
Since the discovery by Joswig et al (1999), CaSiO3 inclusions in diamond have been reported by a number of researchers. Monomineralic CaSiO3 inclusions as well as CaSiO3 coexisting with larnite, titanite-structured CaSi2O5, calcite (aragonite), perovskite, silica polymorphs, orthopyroxene (possibly converted from bridgmanite), clinopyroxene, ringwoodite, ferropericlase, monticellite and cuspidine have been described in diamonds from Kankan, Guinea (Joswig et al, 1999; Stachel et al, 2000; Nasdala et al, 2003); Juína, Brazil (Hayman et al, 2005; Brenker et al., 2007; Walter et al, 2008; Wirth et al, 2009; Bulanova et al, 2010; Pearson et al, 2014; Anzolini et al, 2016; Kaminsky et al, 2016); Machado River, Brazil (Burnham et al, 2016); and the Slave province, Canada (Davies et al, 2004; Tappert et al, 2005). The name breyite was recently proposed for the latter phase and approved by IMA in 2018 (IMA no. 2018-062; Brenker et al, 2018)
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