The thermoelastic properties of ferropericlase Mg1؊xFexO (x ‫؍‬ 0.1875) throughout the iron high-to-low spin cross-over have been investigated by first principles at Earth's lower mantle conditions. This cross-over has important consequences for elasticity such as an anomalous bulk modulus (K S) reduction. At room temperature the anomaly is somewhat sharp in pressure but broadens with increasing temperature. Along a typical geotherm it occurs across most of the lower mantle with a more significant K S reduction at Ϸ1,400 -1,600 km depth. This anomaly might also cause a reduction in the effective activation energy for diffusion creep and lead to a viscosity minimum in the mid-lower mantle, in apparent agreement with results from inversion of data related with mantle convection and postglacial rebound.Earth's lower mantle ͉ viscosity ͉ thermodynamics ͉ thermal expansivity U nderstanding of the Earth's lower mantle relies on indirect lines of evidence. Comparison of elastic properties extracted from seismic models with computed or measured elastic properties of candidate minerals at mantle conditions is a fruitful line of enquiry. For instance, it has shed light on the lower mantle composition (1-3) and on the nature of the DЉ layer (4, 5). Such comparisons support the notion that the lower mantle consists primarily of ferrosilicate perovskite, Mg 1Ϫy Fe y SiO 3 , and ferropericlase, Mg 1Ϫx Fe x O (hereafter, Pv and Fp, respectively). In contrast, evidence based on solar and chondritic abundances suggests a deep lower mantle chemical transition into a pure Pv composition at Ϸ1,000 km depth (6). A chemical transition with wide topography, gentle, and diffuse changes in elasticity and density is also supported by geodynamic modeling (7). The discovery of the spin cross-over in Fp and Pv at lower mantle pressures (8, 9) introduces a new dramatic ingredient that demands a careful reexamination of these phases' elastic properties at appropriate conditions, the consequences for mantle elasticity, and reanalysis of lower mantle properties. This may, after all, support lower mantle models containing a chemical transition. Here, we show the effect of the spin cross-over on the bulk modulus and bulk velocity of Fp at high temperatures. We also show the effect it should have on the bulk modulus of a homogeneous lower mantle with pyrolite composition and confirm and justify the origin of anomalies in the elasticity of Fp recently demonstrated at room temperature (10). We point out that such an elastic anomaly might alter the activation energy for diffusion creep (11,12) in Fp, which might affect mantle viscosity. Results and DiscussionsThe high-spin (HS) to low-spin (LS) cross-over (13) in ferrous iron in Fp has been detected by several techniques at room temperature (8,10,(14)(15)(16)(17)(18) and recently up to 2,000 K (19). For typical mantle compositions the cross-over may start as low as Ϸ35 GPa (18) and end as high as 75 GPa (8) at room temperature. The observed variations in the pressure range of the transition seem to b...

Subduction is a major process of plate tectonics; however, its initiation is not understood. We used high-resolution (less than 1 kilometer) finite-element models based on rheological data of the lithosphere to investigate the role played by water on initiating subduction. A solid-fluid thermomechanical instability is needed to drive a cold, stiff, and negatively buoyant lithosphere into the mantle. This instability can be triggered slowly by sedimentary loading over a time span of 100 million years. Our results indicate that subduction can proceed by a double feedback mechanism (thermoelastic and thermal-rheological) promoted by lubrication due to water.

No abstract

No abstract