Earth and Planetary Sciences ETDs

Publication Date

Summer 7-25-2021

Abstract

Tremolite is the calcium magnesium amphibole that widely occurs in igneous and metamorphic rocks of the crustal and upper mantle lithosphere, especially in the greenschist facies to amphibolite facies of ultramafic assemblages. The stability field of amphibole reaches 3-4 GPa and 1100-1200°C depending on water content and cation substitutions giving rise to amphibole to occur in highly variable compositions. This P-T condition, corresponding to the depth about 80-120 km, coincides with the depth range that a negative velocity gradient usually found within most stable cratonic lithospheres. This depth range is seismically called Mid-lithosphere discontinuity (MLD). The geologic processes that might generate the MLD include partial melting, elastically accommodated grain boundary sliding, radial and azimuthal anisotropy, and layering of hydrous minerals by metasomatism or subsequent accretion. In this study, we focus on the hypothesis of layered metasomatized hydrous minerals, in particular, tremolite. The elasticity and anisotropy parameters of single-crystal tremolite from 0 to 7 GPa are obtained through the Brillouin experiments, x-ray diffraction, and the Thermal EOS fit: Ks0= 86±6 GPa, G0= 59±4 GPa , Ks’=7.5±1.1, G’=1.9±1. The results from Voigt-Ruess-Hill averaging scheme indicted that the KVRH and GVRH increases with pressure, similar to previous studies. Our Ks0, G0, Ks’, and G’ are in reasonable agreement with previous studies. The anisotropy indices of tremolite in this study suggested that tremolite is more anisotropic than other major upper mantle minerals throughout the investigated pressure range. The limited data of tremolite here seems in support of the possible contribution of amphiboles to the seismic observations near MLD. However, the data presented in this study only include the velocity measurements from one crystallographic direction due to the spectral quality. Further investigations of amphibole at simultaneously high pressure-temperature conditions along different crystallographic directions are needed to evaluate its role in generating the MLD under cratons.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Asst. Prof. Jin Zhang

Second Committee Member

Prof. Adrian Brearley

Third Committee Member

Assoc. Prof. Brandon Schmandt

Keywords

tremolite, mineral physics, brillouin, amphibole, mantle, elasticity

Document Type

Thesis

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