Iron oxidation state in (Mg,Fe)0 [Elektronische Ressource] : calibration of the Flank method on synthetic samples and applications on natural inclusions from lower mantle diamonds / vorgelegt von Micaela Longo

Longo.Micaela

Sujets

• Wissenschaft

Informations

• Publié par: universitat_bayreuth
• Publié le: 01 janvier 2009
• Nombre de lectures: 44
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

IRON OXIDATION STATE IN (MG,FE)O: CALIBRATION OF THE FLANK
METHOD ON SYNTHETIC SAMPLES AND APPLICATIONS ON NATURAL
INCLUSIONS FROM LOWER MANTLE DIAMONDS


Von der Fakultät für Chemie und Geowissenschaften
der Universität Bayreuth


zur Erlangung der Würde eines Doktors der Naturwissenschaften

- Dr. rer. nat. -

Dissertation


vorgelegt von
Micaela Longo
aus Rom (Italien)


Bayreuth, July 2009 This doctoral thesis was prepared at the Bayerisches Geoinstitut, University of Bayreuth
between April 2006 and July 2009. It was supervised by Dr. Catherine McCammon.




This is a full reprint of the dissertation submitted to attain the academic degree of Doctor of
Natural Sciences (Dr. rer. nat.) and approved by the Faculty of Biology, Chemistry and Geosciences
of the University of Bayreuth.







Date of submission: July 29, 2009
Date of defence (disputation): December 4, 2009




Doctoral Committee:

Prof. Falko Langerhornst Chairman
stLeonid Dubrovinsky 1 reviewer
ndProf Gerhard Brey 2 reviewer
Prof. Hans Keppler
Prof. Friederich Seifert
Prof. Jürgen Senker


Acknowledgments
I would like to thank the European Commission to provide the funding for the present Ph.D
thproject under the Marie Curie Action Stage Training of Researchers (6 Framework Programme,
contract number MEST-CT-2005-019700).
I would like to thank my dissertation supervisor Dr. Catherine McCammon for her assistance
during my Ph.D work, with her patience in teaching me and a huge number of stimulating
discussions and feedbacks.
My two advisors, Tiziana Boffa Ballaran and Dan Frost, for providing stimulating discussions,
exchange of ideas and support in the labs.
Detlef Krau βe, for his help during the long time spent at the electron microprobe. I would like
to thank him for his patience teaching me how to use the instrument and for his constant
constructive help during the calibration developments.
Steven Jacobsen for showing interest in the project and providing samples; Galina Bulanova,
Felix Kaminsky and Ralf Tappert for providing precious natural inclusions from lower mantle
diamonds. Kazuhiko Otsuka and Vincenzo Stagno for providing synthetic samples from their Ph.D
work.
Thanks to Niko Walte for his help in the laboratory and his help with translating documents in
German for me. Thanks to Gudmundur Gudfinnsson for his precious assistance in the multi anvil
laboratory.
Thanks to Hubert Schultze, Uwe Dittman, Heinz Fisher and Stefan Übelhack for sharing their
skills in technology and in the sample preparation. Gerti Gollner, Anke Potzel, Sven Linhardt, Kurt
Klasinsky are also warmely thanked for their assistance and support in the labs and help for
technical problems.
Thanks to Lydia Kison-Herzing, Petra Buchert and Stefan Keyssner to make Bayerisches
Geoinstitut different from every other place in the world! Thanks for making our life easier many
many times. Special thanks to all my colleagues for stimulating discussions at any time, and for
their support, at any time. Special thanks to all my friends - almost a family - Olga Narygina,
Coralie Weigel, Polina Gavrilenko, Shantanu Keshav, Martha Pamato and Davide Novella for their
friendship and support. Thanks to Fabrizio Nestola for his patience in answering to all my numerous
questions, to always share with me his own experience and enthusiasm for science.
And finally I would like to thank my family in Rome for their comprehension and support even
from far: Francesca, Fiorentino, Eleonora and Dario, always in my thoughts.
Table of Contents
Summary I

Zusammenfassung i

1. Introduction 1
1.1 MgO-FeO solid solution 1
- MgO Periclase 2
- FeO Wüstite 3
- (Mg,Fe)O Ferropericlase 4
3+ - Fe incorporation and point defects in (Mg ,Fe )O 7x 1-x
1.2 Earth’s interior structure and mineral composition 8
1.3 Diamonds from the lower mantle 14
1.4 Ferropericlase as a diamond inclusion 18
1.5 Oxygen fugacity in lower mantle diamonds and the determination of the 19
3+Fe / ∑Fe ratio
1.6 The “flank method”: state of the art 22
1.7 Aim of the project 25
2. Experimental Methods 26
2.1 Synthesis of (Mg,Fe)O crystals 26
2.2 Gas-Mixing furnace 26
2.3 Multi Anvil Apparatus 28
2.4 Mössbauer Spectroscopy 29
2.4.1 The basic principles 30
2.4.2 Conventional source and point source 33
2.5 Powder X-Ray Diffraction 35
2.6 Electron MicroProbe Analysis (EMPA) 36
- Basic principle 36
- X-ray emission spectra and electronic transitions 37
- X-rays: intensity and absorption effects 39
- Heat production 40
- Wave Lentgh Dispersive Spectrometers 40
2.6.1 Major elements analysis plus qualitative analysis 42
- Flank Method procedure: spectrometer calibration 42
44 - Flank Method measurements
- Major element analysis combined with flank method measurements 44
3. Results (1): Flank Method Calibration 46
3.1 Flank Method Results: Determination of the L α and L β flank method 46
measuring positions
3.2 Flank Method calibration for natural garnets on the Jeol JXA-8200 @ BGI 49
3.3 Flank Method calibration for synthetic (Mg,Fe)O ferropericlase: present 56
study
- Attempt no 1 61
- Attempt no 2 62
- Attempt no 3 62
3.3.1 A new calibration for (Mg,Fe)O after spectrometer adjustments 69
4. Results (2): Flank Method Applications 72
4.1 Synthetic (Mg,Fe)O from a different study 72
4.1.1 (Mg,Fe)O containing secondary mineral phase 72
4.1.2 (Mg,Fe)O from High Pressure High Temperature diffusion 77
experiments
4.1.3 Flank Method applied to synthetic (Mg,Fe)O at 24 GPa 90
4.2 Natural (Mg,Fe)O diamond inclusions 89
- Juina Area, Mato Grosso (Brazil) 90
- Machado River (Brazil) 91
- Eurelia and Springfield Basin, Orooro (Australia) 93
4.2.1 Sample preparation 93
4.2.2 Flank Method and Major element analysis results for natural 94
(Mg,Fe)O
5. Discussion and Future Perspectives 100
5.1 Overview of the present study and research goals achieved 100
5.2 Compositional variation determined by flank method 102
5.2.1 Detection of extra phase(s) other than primary (Mg,Fe)O 1023+ 5.2.2 Fe variation along diffusion profiles 103
5.3 Oxygen fugacity in lower mantle (Mg,Fe)O ferropericlase 105
5.3.1 Implication for diamond formation 107
5.4 Future perspectives for flank method applications and lower mantle studies 112
6. Concluding statements and further work 113
7. References 116
Summary
(Mg,Fe)O ferropericlase is the most common mineral found in diamonds originating in the
3+lower mantle (more than 50% of occurrences). It is well known that the Fe concentration in
(Mg,Fe)O is sensitive to oxygen fugacity, even at high pressures. Therefore, the determination of
3+Fe / ∑Fe in such inclusions provides a direct method for investigating lower mantle redox
conditions during diamond formation. The goal of the present research is to calibrate the “flank
method” by electron microprobe using synthetic (Mg,Fe)O, and then apply the method to
3+determine in situ Fe / ΣFe in ferropericlase inclusions from lower mantle diamonds. Up to now a
calibration of the flank method is available only for garnets.
Initially, the flank method was calibrated for garnets to test the reproducibility of the method
on the Jeol XA-8200 electron microprobe in use at Bayerisches Geoinstitut. Results showed that
for garnets a new calibration curve needs to be established at each working session.
Then the flank method was calibrated for the Jeol XA-8200 electron microprobe in use at
Bayerisches Geoinstitut for a homogeneous set of (Mg,Fe)O ferropericlase crystals over a wide
3+range of composition (x = 2 to 60 at.%) and Fe / ΣFe (1 to 15%). Samples were obtained by Fe
performing high pressure high temperature experiments in a multi anvil apparatus. In order to
avoid compositional effects on flank method measurements, the high sample homogeneity was
3+essential. Moreover, the determination of the Fe / ΣFe ratio needed to be extremely accurate. For
this purpose, a more accurate procedure for fitting the Mössbauer spectra of the final set of
synthetic (Mg,Fe)O was adopted.
2+ The calibration curve determined is Fe = 46.238 + 8.161 * ln ( ∑Fe) - 137.01 * (L β/L α) +
285.57 * (L β/L α) , for a Fe compositional range between 3 and 47 wt. %. A comparison of
3+Fe / ΣFe determined by flank method and values determined earlier by Mössbauer spectroscopy
shows that results are