Interaction of peridotite with Ca-rich carbonatite melt at 3.1 and 6.5 GPa: Implication for merwinite formation in upper mantle, and for the metasomatic origin of sublithospheric diamonds with Ca-rich suite of inclusions

• Published in: Contributions to mineralogy and petrology 2018-03, Vol.173 (3), p.1-16, Article 22
• Main Authors: Sharygin, Igor S., Shatskiy, Anton, Litasov, Konstantin D., Golovin, Alexander V., Ohtani, Eiji, Pokhilenko, Nikolay P.
• Format: Article
• Language: English
• Subjects: Calcium; Carbonates; Crystals; Diamonds; Earth and Environmental Science; Earth mantle; Earth Sciences; Experiments; Geology; Grain boundaries; Homogeneous mixtures; Inclusions; Magma; Mantle (Geology); Melts; Merwinite; Mineral Resources; Mineralogy; Oceanic crust; Olivine; Original Paper; Peridotite; Petrology; Silicates; Silicon; Single crystals; Upper mantle
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-017-1432-3

We performed an experimental study, designed to reproduce the formation of an unusual merwinite + olivine-bearing mantle assemblage recently described as a part of a Ca-rich suite of inclusions in sublithospheric diamonds, through the interaction of peridotite with an alkali-rich Ca-carbonatite melt, derived from deeply subducted oceanic crust. In the first set of experiments, we studied the reaction between powdered Mg-silicates, olivine and orthopyroxene, and a model Ca-carbonate melt (molar Na:K:Ca = 1:1:2), in a homogeneous mixture, at 3.1 and 6.5 GPa. In these equilibration experiments, we observed the formation of a merwinite + olivine-bearing assemblage at 3.1 GPa and 1200 °C and at 6.5 GPa and 1300–1400 °C. The melts coexisting with this assemblage have a low Si and high Ca content (Ca# = molar 100 × Ca/(Ca + Mg) > 0.57). In the second set of experiments, we investigated reaction rims produced by interaction of the same Ca-carbonate melt (molar Na:K:Ca = 1:1:2) with Mg-silicate, olivine and orthopyroxene, single crystals at 3.1 GPa and 1300 °C and at 6.5 GPa and 1400 °C. The interaction of the Ca-carbonate melt with olivine leads to merwinite formation through the expected reaction: 2Mg 2 SiO 4 (olivine) + 6CaCO 3 (liquid) = Ca 3 MgSi 2 O 8 (merwinite) + 3CaMg(CO 3 ) 2 (liquid). Thus, our experiments confirm the idea that merwinite in the upper mantle may originate via interaction of peridotite with Ca-rich carbonatite melt, and that diamonds hosting merwinite may have a metasomatic origin. It is remarkable that the interaction of the Ca-carbonate melt with orthopyroxene crystals does not produce merwinite both at 3.1 and 6.5 GPa. This indicates that olivine grain boundaries are preferable for merwinite formation in the upper mantle.