Major element diffusion in garnet and the exsolution of majoritic garnet from aluminous enstatite in Earth's Upper Mantle
type: CreativeWork
description: Majorite is a high pressure polymorph of enstatite with the garnet structure. The amount of enstatite that can be dissolved in garnet as a majorite component increases significantly with pressure, and therefore, majoritic garnet is thought to be a major constituent of the Earth's transition zone. The transport properties of majoritic garnet are, however, not well constrained at the moment. The magnitude of the diffusivity of the majorite component in garnet influences our understanding of the homogenization time scale of Earth's mantle. This is important in subduction zone settings, where the subducting oceanic crust will form a majorite inhomogeneity in the transition zone because of its higher aluminium content. Reaction kinetics in the dry transition zone are diffusion controlled and therefore an improved dataset on the diffusivity of the majorite component in garnet will enable us to better understand the of role of disequilibrium in subduction zones. This dissertation therefore reports the results of diffusion experiments on garnet. Diffusion experiments have been conducted with diffusion couples of majoritic garnet – Dora Maira pyrope, Dora Maira pyrope and Ötztal almandine and Ötztal almandine and majoritic garnet in a multi-anvil press between 1400 – 1900 °C and 12 – 20 GPa. The diffusion experiments with the majoritic garnet – Dora Maira pyrope garnet couples show that the diffusion of the majorite component in garnet is very slow, comparable to the diffusivity of silicon in wadsleyite and ringwoodite. The activation energy, activation volume and the pre-exponential for diffusion of the majorite component in garnet were determined to be 241 ± 54 kJ mol-1, 3.3 ± 0.1 cm3 mol-1 and 2.3 x 10-7 cm2 s-1, respectively. The diffusivity of the majorite component in garnet was determined to be 2-3 orders of magnitude slower than the self-diffusivity of Mg, Fe and Ca in garnet at the same conditions. Also Fe – Mg interdiffusion appeared to be significantly faster in m…
abstract: Majorite is a high pressure polymorph of enstatite with the garnet structure. The amount of enstatite that can be dissolved in garnet as a majorite component increases significantly with pressure, and therefore, majoritic garnet is thought to be a major constituent of the Earth's transition zone. The transport properties of majoritic garnet are, however, not well constrained at the moment. The magnitude of the diffusivity of the majorite component in garnet influences our understanding of the homogenization time scale of Earth's mantle. This is important in subduction zone settings, where the subducting oceanic crust will form a majorite inhomogeneity in the transition zone because of its higher aluminium content. Reaction kinetics in the dry transition zone are diffusion controlled and therefore an improved dataset on the diffusivity of the majorite component in garnet will enable us to better understand the of role of disequilibrium in subduction zones. This dissertation therefore reports the results of diffusion experiments on garnet. Diffusion experiments have been conducted with diffusion couples of majoritic garnet – Dora Maira pyrope, Dora Maira pyrope and Ötztal almandine and Ötztal almandine and majoritic garnet in a multi-anvil press between 1400 – 1900 °C and 12 – 20 GPa. The diffusion experiments with the majoritic garnet – Dora Maira pyrope garnet couples show that the diffusion of the majorite component in garnet is very slow, comparable to the diffusivity of silicon in wadsleyite and ringwoodite. The activation energy, activation volume and the pre-exponential for diffusion of the majorite component in garnet were determined to be 241 ± 54 kJ mol-1, 3.3 ± 0.1 cm3 mol-1 and 2.3 x 10-7 cm2 s-1, respectively. The diffusivity of the majorite component in garnet was determined to be 2-3 orders of magnitude slower than the self-diffusivity of Mg, Fe and Ca in garnet at the same conditions. Also Fe – Mg interdiffusion appeared to be significantly faster in m…
author: Mierlo, Willem L. van
date published: 2011-07-26
date modified: 2026-07-27T18:49:09Z
provider: Universität Bayreuth
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: Universität Bayreuth