The origin, evolution, and demise of continental lithospheric mantle: Perspectives from Re-Os isotopes, geochemistry, petrology, and modeling.

摘要

A variety of geophysical and geochemical studies show that continents are underlain by thick mantle keels, referred to here as continental lithospheric mantle (CLM). CLM seems to have the ability to be isolated from the convecting mantle for timescales of more than a hundred million years. Of concern here is its origin and evolution, and whether it is petrogenetically and dynamically linked to the overlying continental crust. It is shown here that the CLM is more refractory than fertile convecting mantle due to one or more partial melting events, which ultimately render it intrinsically less dense, and hence buoyant, with respect to the fertile convecting mantle. In general, the most refractory CLM lies beneath Archaean cratons, consistent with their long-term stability and extremely thick lithospheres. In contrast, post-Archaean regions tend to be more tectonized and appear to be underlain by fertile mantle, suggesting a correlation between bulk composition and lithospheric stability. A confirmation of this correlation is revealed by a case study of the CLM beneath the highly tectonized Mojavia block in southwestern USA. Here, Re-Os isotopic systematics on peridotite xenoliths indicate the CLM is late Archaean to early Proterozoic in age, and thus its tectonized nature is surprising given the general belief that Archaean cratons are more stable than post-Archaean regions. Instead, this study shows that the CLM is unusually fertile. Thus, as originally hypothesized by Jordan (1988), if intrinsic buoyancy plays a role in the stability of CLM, then more refractory mantle permits the equilibration of a thicker thermal boundary layer, ultimately yielding a stronger lithosphere. Using Re-Os isotopic systematics of peridotite xenoliths from southwestern USA to date the time of CLM formation, it is also shown that formation of the continental crust and the partial melting event, which gives rise to buoyant residual CLM, occurred simultaneously (to within error of the Re-Os isotopic dating system). Collectively, these observations suggest that the continental crust and CLM are dynamically and genetically linked.; To address the question of whether the CLM, once stabilized, can ever be destabilized, a detailed study was conducted of the deep lithosphere beneath the Sierra Nevada, California, where circumstantial evidence has led some to propose a recent delamination event. Re-Os isotopic systematics of Sierran peridotite xenoliths have predominantly asthenospheric Os isotopic compositions, implying recent derivation from the convecting mantle. Combined with thermobarometric evidence for cooling, it is concluded that most of the original Sierran CLM (Proterozoic in age) was removed during the Mesozoic. Finally, an analytical model for wholesale detachment of an eclogitic lower crust along with the underlying lithospheric mantle is presented. Detachment can occur on geologically reasonable timescales (100 Ma) if the middle crust is sufficiently weak (either hot or of wet granite rheology).