Diamond Annual Review 2019/20

66 67 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 The fate of carbonate subducted into Earth’s deepmantle Related publication: Drewitt J. W. E., Walter M. J., Zhang H., Mcmahon S. C., Edwards D., Heinen B. J., Lord O. T., Anzellini S. & Kleppe A. K. The fate of carbonate in oceanic crust subducted into earth’s lower mantle. Earth Planet. Sci. Lett . 511 , 213 (2019). DOI: 10.1016/j.epsl.2019.01.041 Publication keywords: Carbonate; Subduction; Lower mantle; Decarbonation; Diamond formation; X-ray diffraction T he global carbon cycle is key to Earth’s habitability, giving our planet a stable andhospitable climate, and an atmosphere relatively low in carbon dioxide. Carbonate can be transported into Earth’s interior by subduction, fuelling surface volcanoes. Carbonate minerals may also be transported much deeper into the lower mantle, but what happens to these minerals under such high pressure and temperature conditions is uncertain. Researchers wanted to understand if carbonate remains stable in the lower mantle and if not, what reactions take place and hence what mineral form the carbonate takes. Investigating the mineralogy of the deep Earth requires reproducing the extreme conditions of the Earth’s interior. The team took synthetic carbonate rocks and subjected them to very high pressures (up to 90 GPa) and temperatures (2200 K), using a laser-heated diamond anvil cell. Because crystalline minerals have very specific diffraction pattern ‘fingerprints’, using synchrotron X-ray diffraction on I15 (the extreme conditions beamline) allowed them to identify precisely which minerals formed. The results showed a barrier to subduction of carbonatedminerals at a depth of 1000–1500 km, halfway to the core-mantle boundary. At these depths, carbonate in a subducted slab reacts with surrounding silica to form silicate minerals and solid carbon dioxide. As the slab temperature increases, the carbon dioxide releases pure carbon, which forms ‘super-deep diamonds’. These super-deep diamonds may eventually return to the surface, providing the only direct evidence we have for the composition of Earth’s deep interior. The global carbon cycle is tightly connected to our planet’s habitability. Compared to Earth’s clement climate and low CO 2 atmosphere, the planet Venus is in a runaway greenhouse state, with high surface temperatures and a thick CO 2 atmosphere. One major difference between the planets is active plate tectonics, which has played a key role in making Earth’s environment unique within our solar system. Carbon is transported into Earth’s deep interior by subduction of carbonate-rich oceanic crust. Although much of this carbonate is re-released by volcanic activity as carbon dioxide (CO 2 ) into the atmosphere, the carbon isotope signature of so-called ‘super-deep diamonds’ and minerology of their inclusions reveal that crustal carbonate can reach the deep mantle 1,2 . Carbon and other volatiles in the deep mantle may promote chemical mass transfer through melting and may potentially have a large influence on the rheological properties of the mantle. However, despite its fundamental importance for understanding the evolution of the Earth, our knowledge of the minerology of carbonated rocks at the high pressure ( P ) and temperature ( T ) conditions of the lower mantle is extremely limited. To investigate carbonate stability and reactions in the presence of silica (SiO 2 ), the building block of mantle minerals, synthetic carbonate rocks in the systems FeO-MgO-SiO 2 -CO 2 or CaO-MgO-SiO 2 -CO 2 were subjected to deep mantle P - T conditions of up to 90 GPa ( ≈ 900,000 atmospheres) and 2200 K using a laser-heated diamond anvil cell (LHDAC) setup at the University of Bristol. Sample compositions were designed to lie on the ternary plane between carbonate and SiO 2 . In this way, the system will remain ternary if carbonate is stable, but will be non-ternary if decarbonation occurs. The key advantage of making systems on this ternary plane is that recognition of a decarbonation reaction requires only identification of the presence of non- ternary phases in the quenched run products (e.g. FeSiO 3 , CaSiO 3 , diamond, CO 2 ). Angle dispersive synchrotron X-ray diffraction (XRD) measurements were made on the P - T quenched run products at beamline I15, where reaction products were identified from their crystalline Bragg diffraction peaks. A selection of typical XRD patterns obtained are shown in Fig. 1. At the lowest P-T conditions stishovite (SiO 2 ) and carbonate phases are identified. At P beyond ~ 40 to 70 GPa the XRD patterns reveal the formation of (Mg,Fe)SiO 3 (bridgmanite) or CaSiO 3 (Calciumperovskite) with a complete exhaustion of the carbonate phase. In situ confocal micro-Raman spectroscopymeasurements on the T -quenched samples at high- P reveal the appearance of this silicate phase is accompanied by the formation of solid carbon dioxide (CO 2 -V phase 3,4 ). At T above ~ 1900 K diamond is detected in both XRD and Raman measurements of the run products. The experimental results across the full P-T range shown in Fig. 2 reveal three general reactions common to both Fe- and Ca-bearing systems. Reaction boundary (i) with a negative Clapeyron slope is the breakdown of carbonate in the presence of SiO 2 to form bridgmanite ± Ca-perovskite + CO 2 -V. Decarbonation reaction (ii) involves the formation of C (diamond) + O 2 directly from reaction of carbonate with SiO 2 . Reaction (iii) is the breakdown of CO 2 formed in reaction (i) to diamond + O 2 . Seismic tomography surveys indicate that most subducting slabs eventually sink to the base of the mantle and reside there over long geological time-scales. This research reveals that these regions will be free from carbonate, since the carbonate phase cannot be transported to depths greater than 1500 km (halfway to the core-mantle boundary). At these depths, CO 2 will be released into the subducted slab in the form of its solid CO 2 -V phase, where carbon and oxygen atoms are arranged in CO 4 tetrahedral units linked by oxygen atoms at each corner 5 . This is a structure very similar to that of silica suggesting the distinct possibility of rock-forming minerals consisting of solid CO 2 deep within Earth’s lower mantle. However, as the temperature of the typically cooler subducting slab rises as it equilibrates with the ambient mantle at these depths, the CO 2 dissociates to form diamond, releasing oxygen into a nominally oxygen deficient lower mantle in the process. The ultimate fate of Crystallography Group Beamline I15 carbonate in the lower mantle therefore is diamond, whichmay be stored in the deep Earth over geological time-scales representing a long-term sink of carbon before eventually returning to the surface via upwelling mantle plumes. This process could represent one of the sources of super-deep diamonds found at the surface, providing the only direct evidence for the composition of the deep Earth. References: 1. Walter M. J. et al. Primary carbonatite melt from deeply subducted oceanic crust. Nature 454 , 622–625 (2008). DOI: 10.1038/nature07132 2. Walter M. J. et al. Deep Mantle Cycling of Oceanic Crust: Evidence from Diamonds and Their Mineral Inclusions. Science 15 , 1209300 (2011). DOI: 10.1126/science.1209300 3. Litasov K. D. et al. Crossover from melting to dissociation of CO 2 under pressure: Implications for the lower mantle. Earth Planet. Sci. Lett. 309 318–323 (2011). DOI: 10.1016/j.epsl.2011.07.006 4. Santoro M. et al. Partially collapsed cristobalite structure in the non molecular phase V in CO 2 . PNAS (2012). DOI: 10.1073/pnas.1118791109 5. Datchi F. et al. Structure of Polymeric Carbon Dioxide CO 2 -V. Phys. Rev. Lett. 108 , 125701 (2012). DOI: 10.1103/PhysRevLett.108.125701 Funding acknowledgement: This work was funded by NERC grant NE/M000419/1 to M. J. Walter. Corresponding author: Dr James Drewitt, University of Bristol, james.drewitt@bristol.ac.uk Figure 1: Selected X-ray diffraction patterns for the P-T quenched run products in the FeO- MgO-SiO 2 -CO 2 (FMSC) system at (a) 51 GPa, 1755 K and (b) 83 GPa, 1780 K, and CaO-MgO- SiO 2 -CO 2 (CMSC) system at (c) 46 GPa, 1780 K and (d) 89 GPa, 2160 K. The diffraction lines are labelled according to identified phases; Brd = bridgmanite, St = stishovite, Mag =magnesite, as well as the NaCl thermal insulation and Re gasket. Figure 2: Pressure-temperature plots showing the decarbonation reaction boundaries in the systems (a) FeO-MgO-SiO 2 -CO 2 (black lines) and (b) CaO-MgO-SiO 2 -CO 2 (dashed lines with boundaries from (a) in grey). Open symbols denote experiments where carbonate and silica are stable. Filled symbols denote experiments where non-ternary phases formed in decarbonation reactions as labelled. Diamond symbols indicate the presence of diamond. Broken diamonds denote run products where the presence of diamond remains ambiguous. carb = carbonate; st = stishovite (SiO 2 ); brd = bridgmanite; CaPv = calcium perovskite.