No abstract

Continents, especially their Archean cores, are underlain by thick thermal boundary layers that have been largely isolated from the convecting mantle over billion-year timescales, far exceeding the life span of oceanic thermal boundary layers. This longevity is promoted by the fact that continents are underlain by highly melt-depleted peridotites, which result in a chemically distinct boundary layer that is intrinsically buoyant and strong (owing to dehydration). This chemical boundary layer counteracts the destabilizing effect of the cold thermal state of continents. The compositions of cratonic peridotites require formation at shallower depths than they currently reside, suggesting that the building blocks of continents formed in oceanic or arc environments and became “continental” after significant thickening or underthrusting. Continents are difficult to destroy, but refertilization and rehydration of continental mantle by the passage of melts can nullify the unique stabilizing composition of continents.

[1] Densities and elastic properties of solid natural spinel-and garnet-peridotite samples (n = 133) at standard temperature (T ) and pressure (P) (STP) conditions were calculated for compositions ranging from Mg# (100 Â Mg/(Mg + Fe)) of 86-94. The physical properties were used to investigate how natural compositional variations control density and seismic velocity. A corresponding set of compositional derivatives (d/dMg#) of density and seismic velocity is provided. Because the P and T derivatives of elastic moduli are very similar for different compositional end-members of peridotitic minerals, the variation of elastic moduli with Mg# at STP conditions holds at elevated P and T. Increased Mg# leads to a significant increase in V S because of the sensitivity of mineral shear moduli to this parameter (dV S /dMg# = 0.0143 ± 0.0009 km s À1). In contrast, the compressional wave velocity (V P ) is insensitive to Mg# and, instead, correlates weakly with increasing olivine abundance at STP conditions. The ratio V P /V S therefore exhibits a significant negative correlation with Mg# (d(V P /V S )/dMg# = À0.00407 ± 0.00038). Because the temperature dependency of V P /V S is small (<$0.04%/100°C) compared to the compositional dependency (1.7%/Mg# unit), the variation in V P /V S is a fairly robust measure of compositional variation even when temperature varies. Finally, a new density versus Mg# parameterization is derived. Combined with a compilation of bulk-rock Mg#s of peridotite xenoliths from cratonic lithospheric mantle, it is shown that the intrinsic density of cratonic mantle balances to within error its negative thermal buoyancy imposed by its cooler thermal state relative to upwelling asthenospheric mantle. INDEX TERMS:1025 Geochemistry: Composition of the mantle; 3909 Mineral Physics: Elasticity and anelasticity; 7207 Seismology: Core and mantle; 8120 Tectonophysics: Dynamics of lithosphere and mantle-general; KEYWORDS: xenolith, peridotite, seismic velocity, density, lithosphere, tectosphere Citation: Lee, C.-T. A., Compositional variation of density and seismic velocities in natural peridotites at STP conditions: Implications for seismic imaging of compositional heterogeneities in the upper mantle,

Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper's (Cu's) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

The Colorado plateau is a large, tectonically intact, physiographic province in the southwestern North American Cordillera that stands at ∼1,800-2,000 m elevation and has long been thought to be in isostatic equilibrium. The origin of these high elevations is unclear because unlike the surrounding provinces, which have undergone significant Cretaceous-Palaeogene compressional deformation followed by Neogene extensional deformation, the Colorado plateau is largely internally undeformed. Here we combine new seismic tomography and receiver function images to resolve a vertical high-seismic-velocity anomaly beneath the west-central plateau that extends more than 200 km in depth. The upper surface of this anomaly is seismically defined by a dipping interface extending from the lower crust to depths of 70-90 km. The base of the continental crust above the anomaly has a similar shape, with an elevated Moho. We interpret these seismic structures as a continuing regional, delamination-style foundering of lower crust and continental lithosphere. This implies that Pliocene (2.6-5.3 Myr ago) uplift of the plateau and the magmatism on its margins are intimately tied to continuing deep lithospheric processes. Petrologic and geochemical observations indicate that late Cretaceous-Palaeogene (∼90-40 Myr ago) low-angle subduction hydrated and probably weakened much of the Proterozoic tectospheric mantle beneath the Colorado plateau. We suggest that mid-Cenozoic (∼35-25 Myr ago) to Recent magmatic infiltration subsequently imparted negative compositional buoyancy to the base and sides of the Colorado plateau upper mantle, triggering downwelling. The patterns of magmatic activity suggest that previous such events have progressively removed the Colorado plateau lithosphere inward from its margins, and have driven uplift. Using Grand Canyon incision rates and Pliocene basaltic volcanism patterns, we suggest that this particular event has been active over the past ∼6 Myr.