P14-Chen Ming_abstract+photo

Ferromagnesian carbonate in a meteorite impact crater

Ming Chen1*, Jinfu Shu2, Xiande Xie1, Dayong Tan1, Ho-kwang Mao2,3*

1Guangzhou Institute of geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 2Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China; 3Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA. *Correspondence: mchen@gig.ac.cn or mao@gl.ciw.edu

Intensive research over the past half century shows that most diamonds from the Earth’s deep interior were formed via a redox reaction between C–O–H-bearing fluid or carbonate melt and rocks, where the formation of diamonds requires the reduction of carbon to its bare elemental form by the presence of extra reductant. In a study of ferromagnesian carbonate in the shocked gneiss at the Xiuyan impact crater, we found that diamond was produced directly from ankerite Ca(Fe2+,Mg)(CO3)2 via a subsolidus self-redox reaction without melting, fluid, and another reductant. The carbonate self-reduced to diamond by concurrent oxidation of Fe2+ to Fe3+ to form a high pressure form of MgFe3+2O4. Pressure and temperature conditions for the formation of diamond are constrained to 25–45 GPa and 800–900 °C, respectively. This mechanism for natural diamond formation provides valuable insights to deep carbon in the deep mantle. The old oceanic crust with abundant ferromagnesian carbonates might subduct into the deep mantle, and diamond could be produced from ferromagnesian carbonates via above self-redox reaction. In addition, ferromagnesian carbonates are chemical equivalent of ferromagnesian oxide plus CO2. The CO2 produced from ferromagnesian carbonate or from decomposition of iron-free carbonates can react with (Fe,Mg)O in the lower mantle to form diamonds. The subsolidus self-reduction mechanism for the diamond formation indicates that another reductant, liquid and melting conditions are no longer required. Diamonds could be ubiquitously present as a dominant host for carbon in the vast, solid deep lower mantle where the carbonates are abundant and pressures and temperatures are sufficiently high.