MANTLE XENOLITHS MAY BE NON-REPRESENTATIVE OF THE CRATONIC MANTLE: GEOPHYSICAL PERSPECTIVE

I use three independent geophysical approaches to argue that xenolith data on densities and seismic velocities in the Archean-Proterozoic lithospheric mantle (LM) maybe non-representative of the intact (=unsampled by mantle-derived peridotite xenoliths) cratonic mantle.

The first example (Artemieva, 2007, Global Planet. Change, 58, 411-434) is based on buoyancy modeling for the East European Craton (EEC), that lacks surface relief despite huge amplitudes of topo­graphy at the top of the basement (20+ km), at the crustal base (ca. 30 km), and at the lithosphere-asthenosphere boundary (200+ km). The results indicate either a smaller density deficit (ca. 0.9 per cent) in the LM of the Archean-Paleo­pro­tero­zoic parts of the EEC than predicted by global data on mantle xenoliths (ca. 1.5 per cent) or the presence of a strong convective downwelling in the mantle beneath the craton interior.

The second example (Artemieva, 2009, Lithos, 109, 23-46) is based on the analysis of seismic S-velocity variations of a non-thermal origin in the continental LM (based on tomography models of S.Grand; N.Shapiro & M.Ritzwoller with introduced temperature-corrections). In agreement with xenolith data, strong positive velocity anomalies of non-thermal origin attributed to mantle depletion are determined for all of the cratons. However, in kimberlite provinces the amplitude of compositional velocity anomalies is much weaker than in adjacent "intact" cratonic mantle, implying that (pre-)kimberlite magmatism might have melt-metasomatised the cratonic LM.

The third example (Kaban et al., 2003, EPSL, 209, 53-69) is based on the results of global gravity modeling in which the effect of spatially differential thermal expansion has been eliminated from mantle residual gravity (density) anomalies. These results indicate a large scatter of density deficit in the cratonic lithosphere, uncorrelated with crustal differentiation ages.