Z Wu

Publications (4)9.68 Total impact

• Article: Thermoelastic Properties of Olivine and Wadsleyite [Fe_x,Mg_(1-x)]_2SiO_4: Their Relationship to the 410 km Seismic Discontinuity

  M. Núñez Valdez, Z. Wu, Y. G. Yu, R. M. Wentzcovitch
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  ABSTRACT: We combine density functional theory (DFT) within the local density approximation (LDA), the quasiharmonic approximation (QHA), and a model of vibrational density of states (VDoS) to calculate elastic moduli and sound velocities of alpha- and beta-[Fe_x,Mg_(1-x)]_2SiO_4 (olivine and wadsleyite), the most abundant minerals of the Earth's upper mantle (UM) and upper transition zone (TZ). Comparison with experimental values at room-temperature and high pressure or ambient-pressure and high temperature show good agreement with our first-principles findings. Using our results, we investigate the discontinuities in elastic moduli and velocities associated with the alpha-to-beta-[Fe_x,Mg_(1-x)]_2SiO_4 transformation at pressures and temperatures relevant to the 410 km seismic discontinuity. We find the compressional velocity contrast to be smaller than the shear velocity contrast, in agreement with the preliminary reference Earth model (PREM).
  05/2012;
• Article: Thermoelastic Properties of Ringwoodite [Fe_x,Mg_(1-x)]_2SiO_4: Its Relationship to the 520 km Seismic Discontinuity

  M. Núñez Valdez, Z. Wu, Y. G. Yu, J. Revenaugh, R. M. Wentzcovitch
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  ABSTRACT: We combine density functional theory (DFT) within the local density approximation (LDA), the quasiharmonic approximation (QHA), and a model vibrational density of states (VDoS) to calculate elastic moduli and sound velocities of gamma-[Fe_x,Mg_(1-x)]_2SiO_4 (ringwoodite), the most abundant mineral of the lower Earth's transition zone (TZ). Comparison with experimental values at room-temperature and high pressure or ambient-pressure and high temperature shows good agreement with our first-principles findings. Then, we investigate the contrasts associated with the beta-to-gamma-[Fe_x,Mg_(1-x)]_2SiO_4 transformation at pressures and temperatures relevant to the TZ. This information offers clearly defined reference values to advance the understanding of the nature of the 520 km seismic discontinuity.
  05/2012;
• Source

  Available from: pnas.org

  Article: Anomalous compressibility of ferropericlase throughout the iron spin cross-over.

  R M Wentzcovitch, J F Justo, Z Wu, C R S da Silva, D A Yuen, D Kohlstedt
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  ABSTRACT: The thermoelastic properties of ferropericlase Mg(1-x)Fe(x)O (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 approximately 1,400-1,600 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.
  Proceedings of the National Academy of Sciences 06/2009; 106(21):8447-52. · 9.68 Impact Factor
• Article: Thermodynamics properties of ferropericlase

  Z. Wu, J. F. Justo, C. R. da Silva, S. de Gironcoli, R. M. Wentzcovitch
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  ABSTRACT: The thermodynamics properties of ferropericlase (Mg(1-x)FexO, xFe ~ 0.19), have been investigated by first principles using a combination of newly developed techniques designed to address materials of such complexity. The strongly correlated nature of ferrous iron has been successfully addressed previously in static calculations by using a first principles LDA+U approach. However, investigation of thermodynamics properties of the solid solution presents further challenges, particularly the inclusion of vibrational effects without which results are not predictive. We have developed a vibrational virtual crystal model (VVCM) to address this issue. The acoustic velocities of the VVCM are, by construction, precisely the same as those of the real solid solution. We present here unusual anomalies on the thermodynamics properties caused by the spin crossover transition.
  AGU Spring Meeting Abstracts. 04/2009; -1:04.