Single-crystal elasticity of majoritic garnets: stagnant slabs and thermal anomalies at the base of the transition zone

The elastic properties of two single crystals of majoritic garnet (Mg3.24Al1.53Si3.23O12 and Mg3.01Fe0.17Al1.68Si3.15O12), have been measured using simultaneously single-crystal X-ray diffraction and Brillouin spectroscopy in an externally heated diamond anvil cell with Ne as pressure transmitting medium at conditions up to 3c30 GPa and 3c600 K. This combination of techniques makes it possible to use the bulk modulus and unit-cell volume at each condition to calculate the absolute pressure, independently of secondary pressure calibrants. Substitution of the majorite component into pyrope garnet lowers both the bulk (Ks) and shear modulus (G). The substitution of Fe was found to cause a small but resolvable increase in Ks that was accompanied by a decrease in 02Ks/ 02P, the first pressure derivative of the bulk modulus. Fe substitution had no influence on either the shear modulus or its pressure derivative. The obtained elasticity data were used to derive a thermo-elastic model to describe Vs and Vp of complex garnet solid solutions. Using further elasticity data from the literature and thermodynamic models for mantle phase relations, velocities for mafic, harzburgitic and lherzolitic bulk compositions at the base of Earth's transition zone were calculated. The results show that Vs predicted by seismic reference models are faster than those calculated for all three types of lithologies along a typical mantle adiabat within the bottom 150 km of the transition zone. The anomalously fast seismic shear velocities might be explained if laterally extensive sections of subducted harzburgite-rich slabs pile up at the base of the transition zone and lower average mantle temperatures within this depth range

Thermodynamics of mixing in diopside-jadeite, CaMgSi2O6-NaAlSi2O6, solid solution from staticlattice energy calculations

Static lattice energy calculations (SLEC), based on empirical interatomic potentials, have beenperformed for a set of 800 different structures in a 2 2 4 supercell of C2/c diopside with compositionsbetween diopside and jadeite, and with different states of order of the exchangeable Na/Ca and Mg/Al cations. Excess static energies of these structures have been cluster expanded in a basis set of 37 pair-interaction parameters. These parameters have been used to constrain Monte Carlo simulations of temperature-dependent properties in the range of 273?2,023 K and to calculate a temperature?composition phase diagram. The simulations predict the order?disorder transition in omphacite at1,150 20C in good agreement with the experimental data of Carpenter (Mineral Petrol 78:433?440, 1981). The stronger ordering of Mg/Al within the M1 site than of Ca/Na in the M2 site is attributed to the shorter M1?M1 nearest-neighbor distance, and, consequently, the stronger ordering force. The comparison of the simulated relationship between the order parameters corresponding to M1 and M2 sites with the X-ray refinement data on natural omphacites (Boffa Ballaran et al. in Am Mineral83:419?433, 1998) suggests that the cation ordering becomes kinetically ineffective at about 600C

Elasticity of (K,Na)AlSi(3)O(8) hollandite from lattice dynamics calculations

International audienceWe compute the elastic constants tensor and the seismic properties of KAlSi(3)O(8) and K(0.8)Na(0.2)AlSi(3)O(8) up to the ferroelastic transition using density-functional theory and density-functional perturbation theory in the ABINIT implementation. We observe a softening of the tetragonal shear with pressure that precedes the ferroelastic transition. The Reuss shear moduli become negative at respectively 23 GPa and 13 GPa for the two compositions considered in here. The ferroelastic transition is associated with a strong decrease of the horizontal shear wave velocities and a corresponding increase of the seismic anisotropy. The presence of Na enhances these features. (C) 2010 Elsevier B.V. All rights reserved

High-pressure C2/c - P21/c phase transition along the LiAlSi2O6 - LiGaSi2O6 solid solution

Abstract 3.3

HIGH-PRESSURE BEHAVIOR, TRANSFORMATION AND CRYSTAL STRUCTURE OF THE SYNTHETIC IRON-FREE PIGEONITE

A single-crystal high pressure X-ray investigation was performed up to P = 6.5 GPa on a synthetic clinopyroxene of composition Ca0.15(1)Mg1.85(1)Si2.00(1)O6 [Di(15)En(85), unit-cell parameters at room pressure: a = 9.6525(6)Angstrom, b = 8.8461(2)Angstrom, c = 5.2036(5)Angstrom, beta = 108.370(5)degrees, V = 421.68(4)Angstrom(3)]. A first order P2(1)/c-C2/c displacive phase transition was found at P = 5.1 GPa; the transition was revealed by the disappearance of the b reflections (h + k = odd) and by sharp changes in the unit-cell parameters. Reversals through the transformation show that, if present, hysteresis is smaller than 0.1 GPa. The volume variation has been described by a third-order Birch-Mumaghan equation of state with V-0 = 421.68(8) Angstrom(3), K-T0 = 102(2) GPa, and K' = 8(1) for the low-symmetry phase (P2(1)/c) and with V-0 = 411.06(3) Angstrom(3) and K-T0 = 108(2) GPa for the high-symmetry phase (C2/c), with K' fixed to the value obtained for the low-symmetry form. The axial compressibility shows the following scheme: beta(b) > beta(A) = beta(c) > beta(asinbeta) for both phases. In comparison with pure clinoenstatite, Di(15)En(85) shows a similar step in unit-cell parameters at the transition, the disappearence of hysteresis and a decrease of transition pressure and of bulk modulus. Full intensity data sets were collected at room pressure, 2.6 and 4.5 GPa for the P2(1)/c phase and at P = 6.2 GPa for the C2/c phase. A slight increase of the intensity of h + k odd reflections and of the difference in the A and B chain kinking angles were observed. A comparison of the structural behavior of the P2(1)/c phase at high temperature and high pressure shows opposite behavior for M2-O bond lengths and O3-O3-O3 kinking angle