HOW LONG CAN DIAMONDS REMAIN STABLE IN THE CONTINENTAL LITHOSPHERE?

Abstract

Do Archean ages obtained for diamonds from many of the world’s cratons constitute a strong constraint on the thermal state of the Archean continental lithosphere? The apparent longevity of diamonds obtained from cratonic kimberlites [Boyd et al., Nature 315 (1985) 387–389; Richardson et al., Nature 310 (1984) 198–202] has been used to infer the physical and chemical isolation of cratonic roots from the convecting mantle since 3 Ga. This would also provide an extremely strong constraint on the thermal history of the lithospheric mantle – requiring low temperatures at depth for its entire history. Recent evidence suggests, however, that the published ‘diamond’ ages may not represent the ages of the diamonds themselves, but significantly pre-date them [Shimizu and Sobolev, Nature 375 (1995) 308–311; Spetsius et al., Earth Planet. Sci. Lett. 199 (2002) 111–126]. We use a particle-in-cell finite element code to model the thermal stability of the continental lithosphere in a convecting mantle. The continental crust modulates the thermal conditions of the underlying mantle lithosphere, increasing the depth of the thermal boundary layer beneath the continent and providing a mechanism for stabilizing the sub-continental thermal field. If diamonds have survived in cratonic roots since the Archean, the conditions necessary for diamond stability must have existed in the Archean continental lithosphere, and those conditions must have remained relatively unperturbed for ∼3 Gyr [Boyd et al., Nature 315 (1985) 387–389]. Here, the longevity of the diamond stability field is explored for systems with chemically distinct continental crust and a strongly temperature-dependent mantle viscosity. Such models frequently produce the temperature conditions needed to form diamonds within the Archean lithosphere, but the temperature fluctuations experienced within the modeled mantle lithosphere are generally able to destroy these diamonds within 1 Gyr. Increasing the distance to active margins has only a marginal effect on the longevity of the diamond stability field. Convectively stable cratonic roots extend the lifetime of the diamond stability field in those regions. However, while the residence time of diamonds approaches the order of magnitude required (284–852 Myr), extremely fortuitous mantle conditions are required to explain Archean diamonds.