Mantle Processes and Isotopic Analysis of Diamond Formation

Diamonds form deep within the Earth’s mantle under pressures above 5 GPa and temperatures exceeding 900 °C. Their genesis is governed by interactive processes within peridotitic and eclogitic domains, where metasomatic infiltration by carbon- and water-rich fluids or melts drives precipitation of crystallographic carbon allotropes. Isotopic compositions of carbon, hydrogen, helium and trace elements encapsulated in fluid inclusions and daughter minerals provide direct insights into redox conditions, fluid–rock reactions and the timing of diamond growth. Advances in experimental high-pressure petrology have unveiled mechanisms such as pH-modulated precipitation during fluid–rock interactions, methane-driven crystallisation under reduced conditions, and surface-induced isotopic fractionation at crystal faces. Combined with high-resolution geochronology based on U–Th–He systematics, these methods now permit reconstruction of fluid sources, evolution of carbon transfer pathways and the chronology of mantle metasomatic events that underpin diamond formation and ascent in kimberlitic magmas.

Research from Nature Portfolio

Studies have demonstrated that a drop in pH during interaction of aqueous fluids with eclogitic rocks at around 900 °C and 5 GPa can induce diamond nucleation and growth without any change in oxygen fugacity, providing a quantitative model for fluid-mediated diamond formation. Experimental work has also shown that nearly pure methane fluids at lithospheric pressures and temperatures can spontaneously crystallise diamond via mechanisms involving hydroxylation of silicate minerals and hydrogen-induced “etching” of graphite under relatively oxidised conditions. High-precision U–Th–He geochronology of fluid-bearing diamonds has resolved multiple metasomatic episodes over billion-year timescales in the Kaapvaal craton, directly linking discrete fluid infiltration events to orogenic cycles and kimberlite activity.

Mantle Processes and Isotopic Analysis of Diamond Formation publication trend

The graph below shows the total number of articles in mantle processes and isotopic analysis of diamond formation across all publications each year (not limited to Nature Index journals).

Technical terms

Mantle metasomatism: Alteration of mantle peridotite or eclogite by infiltration of fluids or melts, modifying mineral chemistry.

Isotopic fractionation: Partitioning of stable or radiogenic isotopes between coexisting phases due to mass-dependent processes.

Fluid inclusion: Microscopic pocket of fluid trapped within a growing mineral, preserving chemical and isotopic information.

Oxygen fugacity (fO₂): A quantitative measure of the oxidising potential of a system, influencing redox-sensitive reactions.

Carbonatite: An igneous rock or melt rich in carbonate minerals, often associated with mantle metasomatic fluids.

U–Th–He geochronology: Dating method based on accumulation and diffusion of He from radioactive decay of U and Th in mineral inclusions.

References

1. An almost universal CO2 - CO3 2− carbon isotope fractionation function for high temperatures. Earth and Planetary Science Letters (2024).
2. The importance of carbon to the formation and composition of silicates during mantle metasomatism. Geochimica et Cosmochimica Acta (2023).
3. Diamond formation due to a pH drop during fluid–rock interactions. Nature Communications (2015).
4. Origin and Evolution of High-Mg Carbonatitic and Low-Mg Carbonatitic to Silicic High-Density Fluids in Coated Diamonds from Udachnaya Kimberlite Pipe. Minerals (2019).
5. Reduced methane-bearing fluids as a source for diamond. Scientific Reports (2020).
6. Helium in diamonds unravels over a billion years of craton metasomatism. Nature Communications (2021).
7. Experimental and Theoretical Evidence for Surface-Induced Carbon and Nitrogen Fractionation during Diamond Crystallization at High Temperatures and High Pressures. Crystals (2017).