From dust to diamonds: Studying the geological carbon cycle

 

Experimental study sheds light on the fate of carbon in Earth’s mantle

The global carbon cycle is key to Earth’s habitability, giving our planet a stable and hospitable climate, and an atmosphere relatively low in carbon dioxide. Textbook diagrams of the carbon cycle typically show carbon repositories in the atmosphere, the oceans, and the soil, but not what’s going on deeper into the planet. Earth’s mantle, which holds at least 75% of the planet’s total volume, potentially holds more carbon than all of the other reservoirs combined. Uniquely in the solar system, Earth has active plate tectonics, which move materials between the mantle and the crust. However, investigating the mantle is not easy. It extends from about 35 to 2,890 km below the surface, with temperatures which rise from 200 °C at the upper boundary with the crust to approximately 4,000 °C at the core. A team of experimental geoscientists from the University of Bristol has used diamond anvil cells (DAC) and laser heating to recreate the extreme conditions in Earth’s mantle. Their research, published in Earth and Planetary Science Letters, investigated what happens to carbonate minerals when they are subducted from the oceanic crust into the mantle. They found that decarbonation reactions prevent subduction of carbonate deeper than around 1500 km, and that the mantle stores carbon in the form of diamond.

     

Super-deep diamonds

Pressure–temperature diagram showing phase relations in the MgO–SiO2–CO2 (MSC) system at lower mantle pressures. The black solid reaction boundaries are from Maeda et al. (2017) and the grey boundaries are from Seto et al. (2008). The short-dashed line is the CO2 breakdown reaction in the C–O system as determined by Litasov et al. (2011). The long-dashed line is the decarbonation reaction in the CaO-SiO2–CO2 (CSC) system as determined by Li et al. (2018). Brd=bridgmanite; Mgs=magnesite; MgsII=magnesite II; Cc=calcite; CaPv=Ca-perovskite; St=stishovite; CS=CaCl2-structured SiO2; Se=seifertite; Ppv=post-perovskite; Dia=diamond.
Pressure–temperature diagram showing phase relations in the MgO–SiO2–CO2 (MSC) system at lower mantle pressures. The black solid reaction boundaries are from Maeda et al. (2017) and the grey boundaries are from Seto et al. (2008). The short-dashed line is the CO2 breakdown reaction in the C–O system as determined by Litasov et al. (2011). The long-dashed line is the decarbonation reaction in the CaO-SiO2–CO2 (CSC) system as determined by Li et al. (2018). Brd=bridgmanite; Mgs=magnesite; MgsII=magnesite II; Cc=calcite; CaPv=Ca-perovskite; St=stishovite; CS=CaCl2-structured SiO2; Se=seifertite; Ppv=post-perovskite; Dia=diamond.

Conditions in the mantle are extreme, and samples from this region are rarely found on the surface. Most of the carbon that is subducted into the mantle is returned to the surface via volcanic activity. However, analysis of mined diamonds shows that about 1% are formed much deeper in the mantle than is normal, at depths of up to 800 km. The carbon in these ‘superdeep’ diamonds comes from Earth’s surface, telling us that some of the subducted carbon is stored, and transformed, in the mantle.

In order to study the stability of carbonate in oceanic crust subducted into the lower mantle, the team from the University of Bristol subjected synthetic carbonate rocks to very high pressures and temperatures comparable to deep Earth conditions of up to 90 GPa (about 900,000 atmospheres) and 2000 K using a laser-heated diamond anvil cell (DAC), in their own lab. Bringing the samples to Diamond, they used beamline I15 to collect Angle-dispersive X-ray diffraction (XRD) measurements from pressure–temperature quenched samples recovered from the DAC, or temperature-quenched samples at high pressure. These measurements were used to identify the reaction products created.

High pressure reactions

Pressure–temperature plots showing decarbonation reaction boundaries in a) the FMSC and b) the CMSC systems. In panel b) the dashed lines are reaction boundaries in the CMSC system and the grey lines are from the FMSC system for comparison. Open symbols indicate experiments where carbonate and silica are stable and filled symbols indicate experiments where the presence of non-ternary phases formed in decarbonation reactions, which are labelled. Symbols encased in a diamond indicate the presence of diamond in run products, and broken diamonds denote samples where the presence of diamond remains ambiguous.
Pressure–temperature plots showing decarbonation reaction boundaries in a) the FMSC and b) the CMSC systems. In panel b) the dashed lines are reaction boundaries in the CMSC system and the grey lines are from the FMSC system for comparison. Open symbols indicate experiments where carbonate and silica are stable and filled symbols indicate experiments where the presence of non-ternary phases formed in decarbonation reactions, which are labelled. Symbols encased in a diamond indicate the presence of diamond in run products, and broken diamonds denote samples where the presence of diamond remains ambiguous.
The team found that the carbonate was stable until about 40-50 GPa, a pressure which corresponds to a depth of 1000-1300 km in the mantle. It then reacts with silicon dioxide in the surrounding subducted slab, forming the perovskite minerals that constitute the majority of the lower mantle, and solid carbon dioxide. As the slab of subducted crust heats up to ambient mantle temperatures, the carbon dioxide disassociates, forming oxygen and diamonds.

Lead author Dr James Drewitt explains:

"Eventually these super-deep diamonds could be returned to the surface in upwelling mantle plumes, and this process could represent one of the sources of super-deep diamonds that we find at the surface and which provide the only direct evidence we have of the composition of the deep earth."

Dr Drewitt and his team are regular visitors to Diamond. He says:

"To generate extreme high pressures in a diamond anvil cell, our samples are very small, approximately 50 microns in diameter. Synchrotron facilities like Diamond generate the very bright, high-energy and highly focused X-ray beams that we need to analyse the bulk atomic-scale structure of these tiny samples, synthesised under extreme conditions, using X-ray diffraction. It’s fantastic to have a synchrotron essentially on our doorstep. We can literally drive down with the samples, and it makes it very easy to collaborate closely with the beamline staff."

The next step for this research is to combine these high pressure and high-temperature experiments with advanced computer simulation techniques to analyse other mantle materials. Dr Drewitt says:

 There is potentially several ocean's worth of water transported deep into the mantle, and when released this will induce melting of Earth's upper and lower mantle. At the moment we can’t adequately test or understand current models of the dynamic behaviour of this water rich molten rock because we do not know their composition or their physical properties. Further experiments at extreme conditions including the in-situ high-pressure laster heating facilities in commissioning at beamline I15, plus the advanced computer simulations that we are currently working on will help to resolve these problems.

 
To find out more about the I15 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Christine Beavers: Christine.beavers@diamond.ac.uk.
 

Related Publications:

Drewitt, JW et al. The fate of carbonate in oceanic crust subducted into earth's lower mantle. Earth and Planetary Science Letters511, 213-222 (2019). DOI: 10.1016/j.epsl.2019.01.041.

Anzellini, S et al. Laser-heating system for high-pressure X-ray diffraction at the Extreme Conditions beamline I15 at Diamond Light Source. Journal of Synchroton Radiation, vol 25 (2018). DOI: 10.1107/S1600577518013383