Experimental investigation of reduction in the CaO-SiO2-CO2 system with application to the formation of Breyite inclusions in diamonds
Final Report Abstract
The motivation for this project lies with the occurrence of Ca-rich minerals, including breyite-(CaSiO3), as inclusions in diamond that are often interpreted being the products of retrogressed-Ca-perovskite, and thus indicating a sub-lithospheric or “superdeep” origin from the transition-zone or even lower mantle. However, considering that breyite is actually stable at upper mantle-conditions, an important question remains if breyite can crystallize directly at shallower depths-from former sediments in subducted slabs. Thus, we experimentally simulated conditions in-the upper mantle where SiO2-saturated carbonated sediment in subducting oceanic crust-or overlying mélange zones can react to co-crystallize breyite together with diamond. We-evaluated two potential mechanisms for forming breyite and diamond together at upper mantle-conditions via this equilibrium by conducting experiments in the system 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 under (i) anhydrous and (ii) hydrous conditions. The most direct way to form breyite in a metasediment would be via the equilibrium:-CaCO3 + SiO2 = CaSiO3 + CO2. However, this reaction is metastable in the presence of pure CO2-due to melting (i.e. CaCO3 + SiO2 = melt). Our anhydrous experiments tested whether under-reducing conditions, breyite could crystallize via the reaction: CaCO3 + SiO2 = CaSiO3 + C + O2.-Even at ΔlogƒO2 ≈ IW–0.5 and CO2 undersaturated conditions, no breyite was found and this-equilibrium must be metastable relative to the melting reaction.-In the hydrous experiments, melt was always present as a major phase. The hydrous melt-contained abundant suspended graphite crystals, indicating carbon saturation and-graphite was also present as inclusions in the silicate phases. In addition, breyite was observed-in minor amounts at 6-8 GPa as prismatic or isometric grains. Thus, our experiments-demonstrate that simultaneous crystallization of breyite and diamond (solid carbon phase)-is feasible as a result of interaction between CaCO3 and SiO2 in a metasedimentary source-in the presence of reduced H2O-rich fluid or melt. At ~8 GPa, the equilibrium CaSiO3 + SiO2 =-CaSi2O5 is reached, placing an upper pressure limit on breyite stability. Our experiments-constrain the pressure range where breyite could be incorporated as an inclusion in diamond to-be between the diamond–graphite transition at ~ 4 GPa and 1100°C and the CaSi2O5 -forming-reaction at ~ 8 GPa and 1100°C. Such conditions are characteristic of cold subducted slabs. This-upper limit is ~0.7 GPa higher than calculated from the thermodynamic data of Holland & Powell (2011), representing a significant extension to the pressure range of possible breyite-crystallization in SiO2-saturated bulk compositions. In terms of the observed occurrences of breyite inclusions in diamond, we consider-that those coexisting with larnite or stishovite most probably had a perovskite precursor and imply-a “superdeep” origin, while monomineralic breyite or breyite occurring with CaSi2O5 can be-trapped in diamond at upper mantle pressures of ~8 GPa or somewhat less.
Publications
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(2020) Breyite inclusions in diamond: experimental evidence for a possible dual origin, European Journal of Mineralogy, 32,171-185
Woodland AB, Girnis AV, Bulatov VK, Brey GP, Höfer HE
