Breyite inclusions in diamond: experimental evidence for possible dual origin
<p>Inclusions of breyite (previously known as walstromite-structured <span class="inline-formula">CaSiO<sub>3</sub></span>) in diamond are usually interpreted as retrogressed <span class="inline-formula">CaSiO<sub>3</sub></span&g...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-02-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/32/171/2020/ejm-32-171-2020.pdf |
| Summary: | <p>Inclusions of breyite (previously known as
walstromite-structured <span class="inline-formula">CaSiO<sub>3</sub></span>) in diamond are usually interpreted as
retrogressed <span class="inline-formula">CaSiO<sub>3</sub></span> 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 <span class="inline-formula">CaCO<sub>3</sub></span> <span class="inline-formula">+</span> <span class="inline-formula">SiO<sub>2</sub></span> <span class="inline-formula">=</span>
<span class="inline-formula">CaSiO<sub>3</sub></span> <span class="inline-formula">+</span> C <span class="inline-formula">+</span> <span class="inline-formula">O<sub>2</sub></span> was experimentally evaluated in the
CaO–<span class="inline-formula">SiO<sub>2</sub></span>–C–<span class="inline-formula">O<sub>2</sub></span> <span class="inline-formula">±</span> <span class="inline-formula">H<sub>2</sub>O</span> system at 6–10 GPa,
900–1500 <span class="inline-formula"><sup>∘</sup></span>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 (<span class="inline-formula">∼1500</span> <span class="inline-formula"><sup>∘</sup></span>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 <span class="inline-formula"><sup>∘</sup></span>C), and the melt had a
hydrous silicate composition. The reduction of melt resulted in graphite
crystallization in equilibrium with titanite-structured <span class="inline-formula">CaSi<sub>2</sub>O<sub>5</sub></span>
and breyite at <span class="inline-formula">∼1000</span> <span class="inline-formula"><sup>∘</sup></span>C. The maximum pressure of
possible breyite formation is limited by the reaction <span class="inline-formula">CaSiO<sub>3</sub></span> <span class="inline-formula">+</span>
<span class="inline-formula">SiO<sub>2</sub></span> <span class="inline-formula">=</span> <span class="inline-formula">CaSi<sub>2</sub>O<sub>5</sub></span> at <span class="inline-formula">∼8</span> 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.</p> |
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| ISSN: | 0935-1221 1617-4011 |