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

Pamato, MG; Kurnosov, A; Ballaran, TB; Frost, DJ; Ziberna, L; Giannini, M; Speziale, S; ... Prakapenka, VB; + view all (2016) Single crystal elasticity of majoritic garnets: Stagnant slabs and thermal anomalies at the base of the transition zone. Earth and Planetary Science Letters , 451 pp. 114-124. 10.1016/j.epsl.2016.07.019. Green open access

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Abstract

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 ∼30 GPa and ∼600 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 ∂Ks/∂P, 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.

Type: Article
Title: Single crystal elasticity of majoritic garnets: Stagnant slabs and thermal anomalies at the base of the transition zone
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.epsl.2016.07.019
Publisher version: https://doi.org/10.1016/j.epsl.2016.07.019
Language: English
Additional information: This version is the author accepted manuscript. For information on re-use, please refer to the publisher’s terms and conditions.
Keywords: Science & Technology, Physical Sciences, Geochemistry & Geophysics, Elasticity, Majoritic Garnet, Subduction, Transition Zone, Stagnant Slabs, Thermal Anomalies, 410-KM Seismic Discontinuity, Diamond-Anvil Cell, High-Pressure, Sound Velocities, Upper-Mantle, Phase-Transformations, Brillouin-Scattering, Wave Velocities, Solid-Solution, 26 GPA
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1521342
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