Sound velocity study of natural orthopyroxenes with varying iron content

Determination of the acoustic wave velocities of orthopyroxene as a function of composition is essential to identify the underlying causes of seismic heterogeneity. Such laboratory measurements are essential to determine the structure, composition and temperature variability of the upper mantle. This study presents a systematic characterization of orthopyroxene elastic moduli as a function of composition in the enstatite-orthoferrosilite system. For each sample and composition, the sound velocities are measured by Brillouin scattering in three orthogonal crystallographic planes. Inverting sound velocity data allows determination of the complete elastic modulus tensor, as well as the adiabatic bulk (Ks) and shear (μs) moduli. Samples were naturally occurring with approximate compositions: En90, En79, En74, and En55. The results establish nearly linear trends of VP and VS as a function of mole fraction En between the end members enstatite (Jackson et al 2007) and orthoferrosilite (Bass and Weidner 1984). Analysis of previous studies on orthopyroxene and the present work confirms that aluminum has the effects of increasing Ks while also confirming no discernable effect from Ca substitution. Ks. is poorly correlated to Mg-Fe substitution whereas μs correlates in a more linear between the end-members. These findings will help better characterize chemical, namely Mg and Fe, heterogeneity in the upper mantle.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Geology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Picek, Stephen M

• orthopyroxene
• elasticity
• Brillouin scattering
• upper mantle heterogeneity

Statement dc:rights

• Copyright 2015 Stephen Picek

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/78756

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/78756