The Case of the Missing Diamonds: New global and regional thermo-compositional models of cratonic lithosphere

Cratons are the ancient cores of continents, stable over billions of years. The thermochemical properties of their lithosphere are debated, with a number of open questions regarding their composition, the presence of volatiles and the degree of metasomatism.  Cratonic mantle lithosphere is thought to be dominated by depleted mantle peridotites, primarily harzburgites, which can provide chemical buoyancy and, therefore, long-term stability. Some recently proposed models, however, featured substantially metasomatised shallow mantle lithosphere, modified by the addition of volatiles (Eeken et al. 2018) or significant proportions of eclogite and diamond within the lithosphere (Garber et al. 2018). The broad range of the compositions proposed highlights the persisting uncertainty over what cratons are made of.

 

Arguments for cratonic lithosphere complexity often follow from difficulties in fitting seismic velocity profiles (taken from tomographic models beneath cratons) using peridotitic compositions. Some Rayleigh-wave inversions have also found difficulty fitting phase velocity dispersion curves without significant metasomatism, including models with up to 5wt% CO₂.

 

Recently developed methods of petrological inversion can relate geophysical and geological observations directly to the thermochemical structure of the lithosphere and asthenosphere. Here, we invert Rayleigh and Love surface wave phase velocities, elevation and heat flow data for temperature and composition at depth (Fullea et al. 2021) beneath a selection of cratons around the world and a global craton average. We aimed to assemble the most accurate surface-wave dispersion data, with broad period ranges and small errors. The models fit the data within 0.1-0.2% of the phase-velocity values. This accuracy is important in order to extract the information on the radial structure of the lithosphere from the dispersion data.

 

Our models use a harzburgitic (depleted peridotite) composition with major oxide weight percentages taken from prior global modelling (Fullea et al. 2021) and produce very close fits for the Rayleigh and Love dispersion curves averaged over cratons globally, as well as the Rayleigh and Love dispersion data measured in several cratons around the world. The cratonic lithospheric thicknesses range from 180 km (Guyana) to almost 300 km (Congo). We demonstrate that these new models can also be produced by careful regularisation of purely seismic inversions of the same data.

 

Our results do not rule out extensive metasomatism in the cratonic uppermost mantle but suggest that it is likely to be a rare anomaly in particular locations, rather than a common occurrence. Ubiquitous presence of substantial quantities of eclogite and diamond in cratonic lithosphere is not required by the data.

 

References:

 

Eeken, T., et al., 2018. Seismic evidence for depth-dependent metasomatism in cratons. Earth Planet. Sci. Lett. 491, 148-159.

 

Fullea, J., Lebedev, S., Martinec, Z. et al., 2021. WINTERC-G: mapping the upper mantle thermochemical heterogeneity from coupled geophysical–petrological inversion of seismic waveforms, heat flow, surface elevation and gravity satellite data. Geophys. J. Int. 226, 146-191.

 

Garber, J.M., et al., 2018. Multidisciplinary constraints on the abundance of diamond and eclogite in the cratonic lithosphere. Geochem., Geophys., Geosyst. 19, 2062-2086.