In:
European Journal of Mineralogy, Copernicus GmbH, Vol. 32, No. 1 ( 2020-02-11), p. 171-185
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.
Type of Medium:
Online Resource
ISSN:
1617-4011
DOI:
10.5194/ejm-32-171-2020
Language:
English
Publisher:
Copernicus GmbH
Publication Date:
2020
detail.hit.zdb_id:
2039451-2
