Please use this identifier to cite or link to this item: http://dx.doi.org/10.14279/depositonce-2435
Main Title: Water in coesite and wadsleyite: quantification, incorporation mechanism and effect on the phase stability
Translated Title: Wasser in Coesite und Wadsleyite: Quantifizierung, Einbau Mechanismus und Einfluss auf die Phase Stabilität
Author(s): Deon, Fiorenza
Advisor(s): Koch-Müller, Monika
Granting Institution: Technische Universität Berlin, Fakultät VI - Planen Bauen Umwelt
Type: Doctoral Thesis
Language: English
Language Code: en
Abstract: Das Hauptthema dieser Arbeit ist der Einbau von Wasserstoff (H) in drei wichtige nominell wasserefreie Minerale (NAMs) des oberen Erdmantels: Coesite (Cs), Olivine (Ol) und Wadsleyite (Wad). NAMs können H in die Struktur über Punktdefekte, wie z.B. Kationenleerstellen einbauen. Hauptziel dieser Arbeit ist eine vertrauenswürdige Quantifizierung von H in Cs, Wad und Ol und gleichzeitig eine genaue Beschreibung des Einflusses von H auf die Stabilitätsfelder dieser Mineralien. Das erste Kapitel bezieht sich auf den gekoppelten Einbau von H und B in Cs. Dieses Mineral kann H in die Struktur einbauen (i) via die Hydrogranatsubstitution und auch (ii) via die Substitution von Si durch Al und B. Die Ergebnisse belegen, dass Cs bei relativ hohen Drücken und Temperaturen H bevorzugt über den B-Defekt statt über die Hydrogranatsubstitution einbaut. Eine Volumenabnahme (zirka 20%) des Tetraeders begleitet den B-Defekt im Vergleich zur Hydrogranatsubstitution. Daher ist der Einbau von H in Coesite via den B-Defekt beim hohen Drücken energetisch günstiger (9-12 GPa). Das zweite Kapitel liefert die erste FTIR Kalibrierung zur Quantifizierung von H in Wadsleyite. Der molare Absorptionkoeffizient (εi,tot) wurde mit Hilfe von polarisierten FTIR Messungen in Kombination mit SIMS und RAMAN-Spektroskopie als 73000 (± 7000) (L mol -1H2O cm-2) bestimmt. Auf der Basis der Ergebnisse wurde ein Modell über den H-Einbau in Wad entwickelt: H sitzt entlang der O1•••O4 und/oder O3•••O4 Kanten eines leeres M3 Oktaeders. Das dritte Kapitel beschreibt den Einfluss von Wassers auf die P-T-x Koordinaten des Ol-Wad Phasenüberganges. Im System MgO-SiO2-H2O wird durch den bevorzgten H-Einbau in Wad im Vergleich zum Ol die Ol-Wad Phasengrenze um 0.6 GPa zu niedrigeren Drücken verschoben. Im MgO-FeO-SiO2-H2O System verbreitet der H-Einbau die Felder, in denen Wad-Ring und Ol-Wad koexstistierten in Bezug auf die Zusammensetzung. Insgesamt vergrößert sich das Wad Stabilitätsfeld durch den bevorzugten Einbau von H in Wad.
The current PhD thesis focuses on the incorporation of hydrogen in three important constituents of the Earth upper mantle: coesite (cs), olivine (ol) and wadsleyite (wad). They belong to the category of Nominally Anhydrous Minerals (NAMs), however they may incorporate water as hydrogen in their structure via point defects. Major aim of this work is to provide a reliable and proper quantification of hydrogen (expressed as water) of these minerals and to study the influence of water on their P-T stabilities. Coesite is a common constituent of high pressure metamorphic rocks. Due to the olivine-wadsleyite phase transition an important seismic-petrologic discontinuity in the Earth upper mantle occurs: the 410-km discontinuity. I will approach the topic by: 1) studying the coupled boron and hydrogen substitution in coesite and compare it with the hydrogarnet substitution; 2) analyzing the water storage capacity of Mg2SiO4 wadsleyite and the H incorporation mechanisms; 3) determining how water affects the phase transition of olivine to wadsleyite in the system MgO-SiO2-H2O and, to approach the nature, in the system MgO-FeO-SiO2-H2O. 1) Coesite can incorporate H via the hydrogarnet substitution, i.e. a vacant Si site with four coordinating OH groups instead of four oxygens for charge balancing and via Al or B based defects. In the latter substitution B3+ (or Al3+) enters the Si site and one of the tetrahedral oxygens forms a hydroxyl group for charge balancing. In this study coesite that stores H only via B based defects were synthesized for the first time at 9-12 GPa and 1000-2000°C with water in excess. The experimental methods used provide important information on the amount of B and H incorporated in the structure and on B location. All the results lead to the conclusion that the relatively high pressures and temperatures that were chosen for the experiments tend to favour the B based defect, instead of the hydrogarnet substitution, as the B-based defect is accompanied by a general decrease of the size of the tetrahedral site compared to the hydrogarnet substitution (approximately 20%). Thus, coesite with the B- based defect have a smaller volume than those with the hydrogarnet substitution and their formation is favoured at high pressure. 2) The second part of the current PhD thesis will provide a calibration to quantify H in wadsleyite by FTIR spectroscopy and will focus on its incorporation mechanism. Hydrous wadsleyite was synthesized at 13.3-13.5 GPa and 1150-1200°C. Raman Spectroscopy and SIMS water quantifications along with the results of FTIR polarized measurements on single oriented crystals give the first absorption coefficient εi,tot of 73000 (± 7000) (L mol -1H2O cm-2) for water in wadsleyite. Typically the oxygen site O1 of the M3 site is protonated via Mg vacancies. The single crystal X-ray refinements of hydrous wadsleyite suggest that the hydration of wadsleyite occurs along the O1•••O4 and/or O3•••O4 edges of a vacant M3 octahedron. H is bounded either on two O1, two O3, or on one O1 and one O3 sites of a vacant M3 site. 3) The final part will point out in which extent H affects the P-T-x coordinates of the 410-km discontinuity, e.g. the olivine-wadsleyite phase boundary. Two sets of experiments were performed one in the system MgO-SiO2±H2O and one in the system MgO-FeO-SiO2±H2O at 13-13.7 GPa and 1025-1300 °C and 11-12.7 GPa and 1200°C, respectively. In both systems wadsleyite incorporates a much higher amount of water than olivine. In the MgO-SiO2±H2O system the stronger fractionation of water in wadsleyite causes a shift of the olivine-wadsleyite phase boundary to lower P values (0.6 GPa). In the MgO-FeO-SiO2±H2O system H broadens unexpectedly the loops where respectively wadsleyite-ringwoodite (ring) and olivine-ringwoodite coexist. The stability field of hydrous wad enlarges in both directions, to lower (ol-wad loop) and higher (wad-ring loop) pressures. The presence of small concentrations of Fe3+ does not influence the phase boundaries. A new schematic phase diagram for hydrous conditions compared to dry ones will be presented.
URI: urn:nbn:de:kobv:83-opus-26255
http://depositonce.tu-berlin.de/handle/11303/2732
http://dx.doi.org/10.14279/depositonce-2435
Exam Date: 26-Mar-2010
Issue Date: 15-Apr-2010
Date Available: 15-Apr-2010
DDC Class: 550 Geowissenschaften
Subject(s): 410-km Discontinuität
B-Defekt
Coesite
Molare Absorptionkoeffizient
Wadsleyite
410-km discontinuity
Boron-based defect
Coesite
Molar absorption coefficient
Wadsleyite
Usage rights: Terms of German Copyright Law
Appears in Collections:Technische Universität Berlin » Fakultäten & Zentralinstitute » Fakultät 6 Planen Bauen Umwelt » Publications

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