Testing the hypothesis of secular thinning and compositional evolution of the lithospheric mantle beneath cratons: Implications for kimberlite magmatism

Abstract

The sub-cratonic lithospheric mantle (SCLM) is known for its chemical buoyancy and mechanical strength, which have facilitated its preservation for billions of years. However, the thinning of lithospheric mantle beneath some cratons (e.g., North China, Sao Francisco) indicates that some cratons are not permanently stable. We have assembled a new compilation of SCLM thickness data based on previously published, robustly estimated palaeo-geotherms that are defined by mantle xenocrysts/xenoliths in kimberlites and lamproites globally. These data support a localised and episodic decrease in lithospheric thickness (e.g., Siberian craton) with thin lithosphere (< 200 km) only underlying some kimberlites/lamproites younger than 200 Ma. Some late-Phanerozoic thinning of the SCLM in areas of North America and southern Africa is supported by comparison of the xenocryst/xenolith-based estimates with present-day constraints from geophysical methods. To investigate the effect of lithospheric thinning on the compositional evolution of the SCLM and its implication for magmatism in cratonic settings, we have examined the compositions of olivine xenocrysts sampled by kimberlites and lamproites globally. The average composition of entrained lithospheric mantle olivine exhibits no correlation with kimberlite and lamproite eruption ages, nor with SCLM thickness, suggesting no simple relationship between lithospheric thinning and lithospheric composition over time. Our results further reveal a broad relationship between locally thinner cratonic lithosphere in the last 200 Ma and the frequency of kimberlite eruptions, which hints at more favourable conditions for the formation of kimberlite melts in the upper asthenosphere beneath thinned lithosphere (150–200 km) since the Mesozoic period.