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Nature Geoscience. doi:10.1038/ngeo2270
Authors: Xuewei Bao, David W. Eaton & Bernard Guest
Continental plateaux, such as the Tibetan Plateau in Asia and the Altiplano–Puna Plateau in South America, are thought to form partly because upwelling, hot asthenospheric mantle replaces some of the denser, lower lithosphere, making the region more buoyant. The spatial and temporal scales of this process are debated, with proposed mechanisms ranging from delamination of fragments to that of the entire lithosphere. The Canadian Cordillera is an exhumed ancient plateau that abuts the North American Craton. The region experienced rapid uplift during the mid-to-late Eocene, followed by voluminous magmatism, a transition from a compressional to extensional tectonic regime and removal of mafic lower crust. Here we use Rayleigh-wave tomographic and thermochronological data to show that these features can be explained by delamination of the entire lithosphere beneath the Canadian Cordillera. We show that the transition from the North American Craton to the plateau is marked by an abrupt reduction in lithospheric thickness by more than 150 km and that asthenosphere directly underlies the crust beneath the plateau region. We identify a 250-km-wide seismic anomaly about 150–250 km beneath the plateau that we interpret as a block of intact, delaminated lithosphere. We suggest that mantle material upwelling along the sharp craton edge triggered large-scale delamination of the lithosphere about 55 million years ago, and caused the plateau to uplift.
Nature Geoscience. doi:10.1038/ngeo2272
Authors: Milena Marjanović, Suzanne M. Carbotte, Helene Carton, Mladen R. Nedimović, John C. Mutter & Juan Pablo Canales
Upper oceanic crust at fast- to intermediate-spreading mid-ocean ridges is thought to form from the intrusion and eruption of magma accumulated within a mid-crustal reservoir present beneath the ridge axis. However, the mechanisms for formation of the lower crust are debated. Observations from pieces of ancient oceanic crust exposed on land — ophiolites — imply that multiple small magma lenses exist throughout the lower crust at mid-ocean ridges and help form the crust, yet seismic data have imaged only a single lens beneath the innermost axial zones of various mid-ocean ridges. Here we use high-fidelity seismic data to image the crust beneath the East Pacific Rise. We identify a series of reflections below the axial magma lens that we interpret as magma lenses in the upper part of the lower crust. These reflections are present between 9° 20′ and 9° 57′ N and are located up to 1.5 km below the axial magma lens. From the geometry and amplitude of the reflections in a zone beneath a recent volcanic eruption, we infer that magma drained from a lower lens helped replenish the axial magma lens above and, perhaps, contributed to the eruption. Our data indicate that a multi-level complex of magma lenses is present beneath the East Pacific Rise and probably contributes to the formation of both the upper and lower crust.
Nature Geoscience. doi:10.1038/ngeo2271
Authors: Sami Mikhail & Dimitri A. Sverjensky
Volatile elements stored in the mantles of terrestrial planets escape through volcanic degassing, and thereby influence planetary atmospheric evolution and habitability. Compared with the atmospheres of Venus and Mars, Earth's atmosphere is nitrogen-rich relative to primordial noble gas concentrations. The compatibility of volatile elements in mantle minerals versus melts and fluids controls how readily these elements are degassed. However, the speciation of nitrogen in mantle fluids is not well constrained. Here we present thermodynamic calculations that establish the speciation of nitrogen in aqueous fluids under upper mantle conditions. We find that, under the relatively oxidized conditions of Earth's mantle wedges at convergent plate margins, nitrogen is expected to exist predominantly as N2 in fluids and, therefore, be degassed easily. In contrast, under more reducing conditions elsewhere in the Earth's upper mantle and in the mantles of Venus and Mars, nitrogen is expected predominantly in the form of ammonium (NH4+) in aqueous fluids. Ammonium is moderately compatible in upper mantle minerals and unconducive to nitrogen degassing. We conclude that Earth's oxidized mantle wedge conditions—a result of subduction and hence plate tectonics—favour the development of a nitrogen-enriched atmosphere, relative to the primordial noble gases, whereas the atmospheres of Venus and Mars have less nitrogen because they lack plate tectonics.