geodynamics of diamond deposits craig o’neill the university of sydney

Geodynamics of diamond deposits

Craig O’NeillThe 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

On-craton

Off-craton

Graphite

Diamond

Clifford’s Rule

Helmstaedt and Gurney, 1995

AustraliaArgyle: Lamproite

Hall’s Creek mobile belt (Proterozoic), adjacent to Kimberley block (possible Archean basement)

Australia’s diamond occurrences

Merlin: Kimberlite

Central Australian mobile belts

Structure of the Australian Lithosphere

Cross-sections

Numerical Models

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

Numerical Models

Moresi et al., 2000O’Neill et al., 2002

Time in DSF

On-craton

Off-craton

H F

Percentage timegeotherms spend in

DSF

A. Distance to COB B. Archean cratons

C. Distance from cratons D. Combined

Tomographycross-sectionsthrough Argyle

and Merlin

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