The University of Sydney Geodynamics of diamond deposits Craig O’Neill The University of Sydney

New Scientist, 21st September 2002

Formation of diamond deposits: the standard model Helmstaedt and Gurney, 1995

Southern Africa Shirey et al. 2002

Southern African geotherms Graphite Diamond Off-craton On-craton

Clifford’s Rule Helmstaedt and Gurney, 1995

Australia Argyle: Lamproite Hall’s Creek mobile belt (Proterozoic), adjacent to Kimberley block (possible Archean basement) Merlin: Kimberlite Central Australian mobile belts Australia’s diamond occurrences

Structure of the Australian Lithosphere

Cross-sections

Moresi et al., 2000; O’Neill et al., 2002 Numerical Models Moresi et al., 2000; O’Neill et al., 2002

Numerical Models Moresi et al., 2000 O’Neill et al., 2002 H F Off-craton On-craton H F Time in DSF

Percentage time geotherms spend in DSF

Distance to COB B. Archean cratons C. Distance from cratons D. Combined

Tomography cross-sections through Argyle and Merlin

Restriction to Archean terranes too stringent Conclusions Restriction to Archean terranes too stringent diamond formation may not always be related to formation of cratonic roots require PT conditions and C source Inclusion of many stable Proterozoic platforms Focus on where kimberlite volcanism is likely to occur Australia example: step changes in lithospheric thicknesses could be important