The Earth’s deepest diamonds are commonly made up of former living organisms

The Earth’s deepest diamonds are commonly made up of former living organisms that have effectively been recycled more than 400 kilometers below the surface, new Curtin research has discovered.

The research, published in Nature’s Scientific Reports, found that both diamonds found in oceanic rocks and the so-called super-deep continental diamonds shared a common origin of recycled organic carbon deep within the Earth’s mantle.

Lead author Dr. Luc Doucet, from Curtin’s Earth Dynamics Research Group within the School of Earth and Planetary Sciences, said the findings offered a fascinating insight into the world’s most expensive gemstones.

“Bringing new meaning to the old trash to treasure adage, this research discovered that the Earth’s engine actually turns organic carbon into diamonds many hundreds of kilometers below the surface,” Dr. Doucet said.

“Ballooning rocks from the Earth’s deeper mantle, called mantle plumes, then carry the diamonds back up to the Earth’s surface via volcanic eruptions for humans to enjoy as sought-after gemstones.

“While recycling is becoming a modern-day necessity for our sustainable survival, we were particularly surprised to learn, through this research, that Mother Nature has been showing us how to recycle with style for billions of years.”

The three main types of natural diamonds include oceanic, super-deep continental and lithospheric diamonds, formed at different levels of the mantle with a varying mixture of organic and inorganic carbon.

Co-lead author John Curtin Distinguished Professor Zheng-Xiang Li, the Head of the Earth Dynamics Research Group, said the research provided a model that explains the formation and locations of all three major types of diamonds.

“This is the first time that all three major types of diamonds have been linked to mantle plumes, ballooning hot rocks driven by plate tectonics and the supercontinent cycle from deeper Earth,” Professor Li said.

“This research not only helps to understand Earth’s carbon cycle, but also has the potential to unlock more secrets of the Earth’s dynamic history through tracking the past locations of mantle plumes and superplumes. This can be achieved by mapping out the distribution of both continental and oceanic diamonds.”

However, Professor Li said it remained a mystery as to why diamonds formed in the so-called “mantle transition zone,” 400 to 600 kilometers deep, utilized recycled organic carbon only.

“This might have something to do with the physical-chemical environment there,” Professor Li said. “It is not uncommon for a new scientific discovery to raise more questions that require further investigation.”

Carbon is circulated around the surface and interior of the Earth with subducting slabs and volcanic eruptions; subduction carries carbon-bearing rocks to the Earth’s interior and volcanic eruption expels carbon-bearing gas, lavas and rocks from the interior of the Earth1.

The flux of subducted carbon within oceanic plates is estimated to be more than 5 Tmol/yr, almost twice as large as the expelled-carbon flux, 2–3 Tmol/yr, through arc magmatism2. This difference suggests the existence of carbon reservoirs in the deep Earth1,2.

One source of direct evidence for deep carbon is carbon-bearing samples originating from the Earth’s interior. Diamond is evidence of quite a deeper-origin carbon. In particular, some diamonds, called ‘super-deep diamond’, are thought to arise from the mantle transition zone or the lower mantle3,4,5,6,7,8.

The inclusions in super-deep diamond may supply information on the lithology, water content, and/or elemental distribution in deep parts of the Earth3,4,5,6,7,8.

Subducting slabs play a key role for carrying carbon-bearing phases into the deep Earth and forming super-deep diamond3,4,5. Altered rocks in the oceanic crust contains the large amount of carbon such as organic carbon or carbonate minerals, which can be the deep reservoir of subducted carbon9,10. These carbonate minerals or melts may change to diamond if they become unstable in the Earth during the subduction process3,5,11,12,13.

The stability of carbonates has been investigated using high-pressure experiments and ab initio calculations, and MgCO3 magnesite is determined to be stable under the high-pressure and high-temperature conditions expected during subduction11,12,13,14,15,16,17,18,19,20.

The existence of carbonate (or carbonatite melt) in the deep mantle is supported by the discovery of carbonate as inclusions in diamonds originating from the mantle transition zone and/or the lower mantle3,5.

Since SiO2 is one of the abundant components and also is an important phase in deeply subducted slabs21,22,23, the MgCO3-SiO2 system may be applied to the slabs descending into the lower mantle. SiO2 phases may change to its high-pressure polymorph, such as coesite, stishovite, CaCl2-type phase and seifertite24,25.

Magnesite is expected to break down to CO2 or diamond by reacting with silica minerals in the MgCO3-SiO2 system in subducting process12,13,26,27 although the detail of its phase relation has not yet been clarified. Knowledge of the reactions in this system at high pressure and high temperature may provide important insights into the carbon-related processes in the deep mantle, such as the origin of super-deep diamond and melting or oxidation by release of volatile components13,28.

We used a laser-heated diamond anvil cell (LHDAC) combined with a high-pressure and high-temperature in situ synchrotron X-ray diffraction (XRD) technique to quantify the phase relations of the MgCO3-SiO2 system down to the lowermost-mantle conditions. Our objective is to clarify the behavior of carbon in the lower mantle and to model the origin of super-deep diamond.

reference link :https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5233982/

reference link: More information: Luc S. Doucet et al, Oceanic and super-deep continental diamonds share a transition zone origin and mantle plume transportation, Scientific Reports (2021). DOI: 10.1038/s41598-021-96286-8