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Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for... 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 (Mj100, Mg4Si4O12) and a Mj50Py50 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 (KS) and shear (μ) moduli of both Mj50Py50 and Mj100 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geophysical Research Letters Wiley

Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

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References (34)

Publisher
Wiley
Copyright
Copyright © 2002 by the American Geophysical Union.
ISSN
0094-8276
eISSN
1944-8007
DOI
10.1029/2001GL013937
Publisher site
See Article on Publisher Site

Abstract

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 (Mj100, Mg4Si4O12) and a Mj50Py50 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 (KS) and shear (μ) moduli of both Mj50Py50 and Mj100 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.

Journal

Geophysical Research LettersWiley

Published: Jan 1, 2002

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