Crustal Noble Gases in Jwaneng Diamonds With Links to Seismic Tomography

Recent seismic tomography studies of the Kaapvaal-Zimbabwe craton of southern Africa reveal distinct seismic velocity profiles at 150 km depth within the diamond stability field, that appear to correlate with differences in diamond paragenesis. Diamond mines with predominantly eclogitic diamond inclusions (e.g. Jwaneng, Orapa, Premier) overlie lithospheric mantle with relatively slow P-wave velocities, whereas localities with predominantly peridotitic diamond inclusions (e.g. Kimberley, Finsch) are associated with faster P-wave velocities at 150 km depth in the mantle. This distinction in P-wave velocities between the two groups can be interpreted in terms of different chemical compositions in the lithospheric mantle (Shirey, S. B. et al., Science 297, 1683-1686, 2002). Thus, the region with slower P-wave velocities could correlate with an oceanic lithospheric component and/or metasomatising fluids introduced by ancient subduction-related processes. In contrast, the region with faster P-wave velocities may reflect mid-Archean mantle depletion events initiated by craton keel formation. As the mantle beneath the Jwaneng mine is characterized by slower P-wave velocities at 150 km depth, our finding of crustal noble gases in Jwaneng diamonds (gem-quality diamond aggregates, this work; and framesites, Honda, M. et al., Chemical Geology 203, 347-358, 2004) appears to be consistent with the tomographic observations. It is noteworthy that early helium work on diamonds from the Orapa mine also showed radiogenic He-enriched 3He/4He ratios, as low as 0.16 R/Ra (Kurz, M. et al., Earth Planet. Sci. Lett. 86, 57-68, 1987), which could indicate the involvement of crustal helium; consistent with our findings from the Jwaneng diamonds. Thus, it is postulated that diamonds from eclogitic mines could clarify whether or not material subducted into the deep mantle retained crustal and atmospheric noble gases, and could quantify the influence of subducted material through time. In contrast, diamonds from the peridotitic mines could contain pristine ancient mantle noble gas compositions at the time of their formation. Verification of our hypothesis will be significant in providing a basis for the reconciliation of geophysical and geochemical observations in terms of mantle structure and mantle geodynamics.