AGES OF CRATONIC DIAMOND AND LITHOSPHERE EVOLUTION: CONSTRAINTS ON PRECAMBRIAN TECTONICS AND DIAMOND EXPLORATION

Ages of syngenetic inclusions of peridotitic (P-type) and eclogitic (E-type) minerals in diamond show that episodes of diamond formation in the subcontinental lithospheric mantle correlate with important craton-forming events deduced from the surface geological record. The first major lithospheric diamond-forming episode at ca. 3.5 to 3.2 Ga closely followed formation of depleted roots under Paleoarchean continental nuclei and appears to have been a metasomatic event triggered by CO2-rich, probably subduction-derived fluids that produced P-type diamond associated with G-10 garnet-bearing harzburgite. The oldest E-type diamond-forming events are related to ca. 3.0 to 2.5 Ga accretion of Paleoarchean continental nuclei into composite Archean cratons involving subduction of intervening seafloor, partial melting of hydrated oceanic crust, and widespread formation of tonalite. Post-Archean P-type diamond has lherzolitic assemblages as mineral inclusions, and together with younger E-type diamond, records various Proterozoic modifications of the earlier cratonic roots. The E-type diamond source-rocks include progressively metamorphosed mafic rocks and garnet-clinopyroxene rocks representing the dense residue from partial melting of a hydrous mafic crust. Metamorphic diamond-bearing eclogite assemblages may contain coesite, demonstrating diamond formation in silica-saturated mafic rocks during ultrahigh-pressure (UHP) metamorphism in Archean and Proterozoic subduction zones. Archean diamond-formation events deduced from isotopic studies on mineral inclusions are corroborated by the occurrence on several cratons of detrital macrocrystals of diamond in Mesoarchean and Neoarchean sedimentary rocks and of rare macro-and microcrystals of diamond in Neoarchean non-kimberlitic igneous rocks. As the contribution of a harzburgitic P-type diamond budget remains crucial to the economic viability of most kimberlites, area selection for diamond exploration should continue to concentrate on identifying >3 Ga microcontinental nuclei within Archean or Proterozoic cratons. The tectonic history of these nuclei should be investigated to ascertain whether the diamond content of an older harzburgitic root may have been enhanced by the addition of later generations of E-type or lherzolitic P-type diamond (or both) prior to sampling by potential kimberlites.