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Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone
Author(s) -
Sinogeikin Stanislav V.,
Bass Jay D.
Publication year - 2002
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2001gl013937
Subject(s) - pyrope , transition zone , silicate perovskite , elasticity (physics) , mineralogy , geology , silicate , phase transition , elastic modulus , materials science , mantle (geology) , thermodynamics , chemistry , geochemistry , composite material , physics , organic chemistry
High seismic velocity gradients in the transition zone have been attributed to large pressure derivatives of the elastic moduli of transition zone minerals, such as majorite‐garnet. Here we present Brillouin scattering measurements of the elasticity of polycrystalline Mg‐majorite (Mj 100 , Mg 4 Si 4 O 12 ) and a Mj 50 Py 50 majorite ‐ pyrope solid solution to pressures similar to those at the top of the transition zone (15.1(1) GPa and 12.8(1) GPa, respectively). The pressure derivatives of the adiabatic bulk (K S ) and shear (μ) moduli of both Mj 50 Py 50 and Mj 100 are 4.2(3) and 1.4(2), respectively, and are equal to those of pyrope within the experimental uncertainties. We conclude that neither the adiabatic compression of majorite‐garnet nor any other likely transition zone phase is able to produce the high velocity gradients observed seismically. High velocity gradients are consistent with gradual transformations between minerals with very different elastic properties. The transformation of ∼25% or more of clinopyroxene into majorite above 520 km, and the formation of Ca silicate perovskite below 520 km could produce the observed seismic gradients.