Osmium Isotope Constraints on Tectonic Evolution of the Lithosphere in the Southwestern United States

The Re-Os isotope system is increasingly providing new insight into continental dynamics due to the fact that it is the only radiogenic isotope system that provides information on melt depletion events (hence lithosphere formation) in the uppermost mantle. Here we review application of the ReOs isotopic system to dating melt depletion in peridotites. In particular, we highlight examples of how this system, when applied to mantle xenolith studies, provides information on the timing of lithospheric mantle formation and its replacement beneath the southwestern United States. Studies of xenoliths from the Sierra Nevada reveal a vertically stratified mantle lid at 8 Ma: cold, old (Proterozoic) lithosphere is preserved just beneath the Moho (~45 km) and is underlain by younger (Phanerozoic) lithosphere that reaches depths of 100 km. The deeper, younger lithosphere shows extensive evidence of cooling, whereas the overlying old, colder lithosphere shows evidence for heating. Collectively, these data suggest removal of the original Proterozoic lithosphere and its replacement by upwelling convecting mantle that cooled against the surrounding lithosphere. The timing of this replacement is not well constrained by Os systematics, but probably occurred during development of the Mesozoic arc. A second deep lithosphere removal event is suggested by the presence of fertile, hot peridotite xenoliths that erupted from the High Sierra in the Pliocene, which have Os isotopic compositions indistinguishable from convecting upper mantle. This later thinning event may be related to Basin and Range extension. Studies of mantle xenoliths from the Mojavia terrane (southern Basin and Range) and Colorado Plateau provide insights into the origin of the contrasting lithospheric strength between these two regions. The Mojavia terrane is underlain by anomalously fertile, Archean-aged mantle lithosphere. Its high density prohibited formation and stabilization of a thickened lithospheric lid, which, in combination with likely low viscosity, explains why this ancient lithosphere is thin and relatively weak compared to lithosphere from Archean cratons. In contrast, peridotite xenoliths from the Colorado Plateau are Proterozoic in age, but are refractory, and hence chemically buoyant. These properties led to development of a thick and strong lithosphere beneath the Colorado Plateau.