The carbon speciation in the Earth’s interior as function of pressure, temperature and oxygen fugacity [Elektronische Ressource] / Vincenzo Stagno. Betreuer: David C. Rubie

Vincenzo Stagno

Informations

• Publié par: universitat_bayreuth
• Publié le: 01 janvier 2011
• Nombre de lectures: 43
• Langue: English
• Poids de l'ouvrage: 21 Mo

Extrait

The carbon speciation in the Earth’s
interior as function of pressure,
temperature and oxygen fugacity


Dissertation




Fakultät für Biologie, Chemie und Geowissenschaften
Universität Bayreuth







Vincenzo Stagno
(Diplom-Geologe)
aus Palermo (Italien)






Bayreuth, 2011


Die vorliegende Arbeit wurde von Oktober 2007 bis Februar 2011 am Bayerischen
Geoinstitut, Universität Bayreuth unter Leitung von Dr. D.J. Frost and Prof. D.C. Rubie
angefertigt.


Datum der Einreichung der Dissertation: 17 Februar, 2011

Datum des wissenschaftlichen Kolloquiums: 30 Mai, 2011





Prüfungssausschuß:
Prof. J. Senker, Universität Bayreuth (Vorsitzender)
Prof. D.C. Rubie, Universität Bayreuth (Erster Gutachter)
Prof. A. Woodland, Universität Frankfurt am Main (Zweiter Gutachter)
Prof. L. Dubrovinsky, Universität Bayreuth
Prof. L. Zöller, Universität Bayreuth Contents

Abstract p. 1
Zusammenfassung 3
1. Introduction 6
1.1 Carbon in the Earth’s mantle 6
1.2 Estimates of carbon abundance in the Earth’s mantle and the
global carbon content 10
1.3 The stability of carbonate minerals and melts and the solubility
of CO in natural magmas 13 2
1.4 Oxygen fugacity in the Earth’ s interior 20
1.5 The speciation of carbon as a function of redox state 26
1.6 Aims of this study 30

2. Methods 32
2.1 High pressure experiments 32
2.2 Analytical techniques 38
2.2.1 Electron microprobe analysis 38
2.2.2 Scanning electron microscopy 39
2.2.3 Raman spectroscopy 40
2.2.4 Mössbauer spectroscopy 41
2.2.5 Focused Ion beam 4
2.2.6 Electron Energy Loss Spectroscopy 46
2.3 Control of the oxygen fugacity during experiments 48


3. Carbon speciation in the asthenosphere: experimental measurements of the
redox conditions at which carbonate-bearing melts coexist with graphite or
diamond in peridotite assemblage 51
3.1 Introduction 51
3.2 Experimental techniques 54
3.3 Results 60
I 3.3.1 Phase relations and compositions 60
3.3.2 Determination of oxygen fugacity 63
3.3.3 Oxygen fugacity dependences 68
3.4 Discussion 72
3.4.1 Parameterisation of the carbon/carbonate-melts fo as a function of 2
P, T and CO melt content 72 2
3.4.2 Carbon speciation with respect to the mantle redox state and the onset of
redox melting 74
3.5 Conclusions 80

4. Carbon/ carbonate equilibrium in the transition zone and lower mantle 82
4.1 Introduction 82
4.2 Experimental methods 84
4.3 Results 88
4.4 Discussion and conclusions 96

3+5. Fe /Fe measurements of garnets equilibrated with carbon and carbonate tot
in a peridotite assemblage 101
5.1 Introduction 101
5.2 Experimental methods 103
5.3 Results 108
5.3.1 Phase compositions and attainment of divalent cation equilibrium 108
5.3.2 Oxygen fugacity measurements employing iridium as redox sensor 113
5.3.3 Ferric iron measurements and attainment of redox equilibrium 114
5.4 Oxygen-thermobarometry measurements on the experimental garnet
peridotite assemblages 119
5.5 Discussion 122
5.5.1 Parameterisation of logK as a function of pressure and temperature 122
5.5.2 The redox profile of the upper mantle revisited 125
5.6 Conclusions 128


II6. Carbon and carbonate equilibrium in eclogitic assemblage:
preliminary results 129
6.1 Introduction 129
6.2 Experimental methods 135
6.3 Results 137
6.4 Oxygen fugacity determination 140
6.5 Discussion 143
6.6 Conclusions 146

7. General conclusions 147
7.1 The oxidation of elemental carbon to carbonate beneath
mid-ocean ridges 147
7.2 Magnesite as a deep carbon source 150
7.3 Further work 151

Acknowledgements 153
References 154
Appendix 169












IIIAbstract

The redox state of the Earth’s interior will influence the speciation of volatile elements both in the
mantle and in mantle derived magmas. Carbon is one of the principal elements to be affected in this
way because under reducing conditions it forms graphite or diamond, and under oxidizing conditions
carbonate (or CO -bearing) minerals and melts. The cycling and residence time of carbon in the mantle 2
can be strongly effected by the oxygen fugacity because reduced phases such as diamond and graphite
are immobile and likely to remain within the mantle and potentially within subducting slabs, while at
more oxidizing conditions CO -rich fluids or melts can migrate and escape from the interior. The 2
carbon cycle in the Earth may therefore depend on the redox state of mantle rocks. Conversely, an
influx of CO -rich fluids or melts may act to oxidize the mantle as an additional aspect of 2
metasomatism.
In the first part of this study experiments were performed to measure the oxygen fugacity at which
carbon (graphite or diamond) oxidises to carbonate minerals or melts within mantle peridotite
assemblages between 2.5 and 11 GPa at 1100-1600 °C. The experiments were performed up to
temperatures where carbonate melts evolve towards more silicate-rich compositions. The dilution of
the carbonate melt component was found to lower the relative fo , expanding the melt stability field 2
with respect to reduced carbon. The results allow the fo of the diamond formation process to be 2
determined both as a function of pressure, temperature and melt CO concentration. These results also 2
have implications for the onset of melting within up welling mantle material. Several studies have
indicated that the mantle may become more reduced with depth. This means that the oxidation of
elemental carbon (graphite or diamond) may occur in up welling rocks where the oxidized product is a
carbonate bearing magma. When the experimental data are compared with current estimates for the fo 2
of mantle rocks the implication is that peridotitic mantle will remain in the diamond stability field up to
3+at least 100-150 km depth. Only at depths shallower than 150 km would Fe in mantle silicates react
with graphite to produce carbonate rich melts in a redox melting process. Redox melting should limit
the depth interval over which carbonate-rich melts can form beneath ridges.
Further experiments were performed to determine the fo at which diamond oxidises to carbonate in 2
the transition zone and lower mantle. Experiments at 45 GPa were performed using the MADONNA
D-DIA (1500 tons) apparatus with sintered diamond anvils installed at the Geodynamics Research
Centre, Ehime University in Japan. The measured oxygen fugacity was found to be approximately 3
1log units above the iron-wüstite oxygen buffer ( ΔIW+3). As the oxygen fugacity of the transition zone
and lower mantle is most likely at or below the IW buffer this confines the stability of solid carbonate
to the upper mantle or to unusually oxidized regions of the deeper mantle. The oxygen fugacity at
which magnesite and diamond coexist showed a slight decrease with pressure, however, implying the
possibility that magnesite may become the stable host for carbon at the very base of the lower mantle.
The oxygen fugacity at which mantle xenoliths equilibrated can be determined using oxy-
thermobarometry equilibria. For garnet-peridotite rocks the only calibrated and tested oxy-barometer
employs the equilibrium,

3+2Fe Fe Si O = 4Fe SiO + 2FeSiO + O 3 2 3 12 2 4 3 2
Garnet Olivine Orthopyroxene

3+In the final section of this thesis Fe / ΣFe ratios of garnets produced in a peridotite assemblage in
equilibrium with carbon and carbonate melts were measured between 3 and 7 GPa. The oxygen
fugacity in these experiments was also constrained, which allowed a test of this widely used oxy-
barometer to be made at pressures much higher than previously performed. The results indicate that the
pressure dependence of this oxy-barometer may be in error and a preliminary recalibration implies that
cratonic lithosphere may not be as reduced as previously considered.













2Zusammenfassung

Der Redoxzustand im Inneren der Erde beeinflusst das Auftreten unterschiedlicher chemischer Spezies
der volatilen Elemente sowohl im Erdmantel als auch in Magmen, die sich im Erdmantel bilden.
Kohlenstoff ist eines der wichtigsten Elemente, das auf diese Art beeinflusst wird, da es unter
reduzierenden Bedingu