Physical and chemical constraints on core-mantle differentiation in terrestrial planets [Elektronische Ressource] / Ute Mann

Ute Mann

Informations

• Publié par: universitat_bayreuth
• Publié le: 01 janvier 2008
• Nombre de lectures: 22
• Langue: Deutsch
• Poids de l'ouvrage: 6 Mo

Extrait

Physical and Chemical Constraints on
Core - Mantle Differentiation
in Terrestrial Planets




Dissertation

Fakultät für Biologie, Chemie und Geowissenschaften,
Universität Bayreuth








Ute Mann
(Diplom-Geologin)
aus Senden bei Neu-Ulm


Bayreuth, Oktober 2007



Die vorliegende Arbeit wurde von Oktober 2004 bis Oktober 2007 am Bayerischen
Geoinstitut, Universität Bayreuth unter Leitung von Prof. D:C: Rubie angefertigt.

Vollständiger Abdruck der von der Fakultät für Biologie, Chemie und Geowissenschaften
der Universität Bayreuth genehmigten Dissertation zur Erlangung des akademischen Grades
einer Doktorin der Naturwissenschaften (Dr. rer. nat.).








Datum der Einreichung der Dissertation: 23. Oktober 2007
Datum des wissenschaftlichen Kolloquiums: 13. Februar 2008







Prüfungssausschuß:
Prof. S. Peiffer, Universität Bayreuth (Vorsitzender)
Prof. D.C. Rubie, Universität Bayreuth (Erster Gutachter)
Prof. H. Keppler, Universität Bayreuth (Zweiter Gutachter)
Prof. K. Bitzer, Universität Bayreuth
Prof. J. Breu, Universität Bayreuth



Contents

Abstract 1
Zusammenfassung3
1. Introduction 7
1.1The Geochemical Signature of the Earth........................................................................ 8
1.2 Core Formation Models ................................................................................................... 10
1.3 Geochemical Constraints from Liquid Metal - Liquid Silicate Partitioning
Behaviour............................................................................................................................ 15
1.4 Mechanical Separation Processes of Core Forming Liquids...................................... 18
1.5 High Pressure - High Temperature Experimental Techniques ................................. 19
1.6 Aims of this Study .............................................................................................................25
2. The Wetting Ability of Si-bearing Liquid Fe-alloys in Solid Silicate
Mantle Matrix 27
2.1 Previous Work ...................................................................................................................27
2.2 Experimental Setup and Analytical Techniques........................................................... 28
2.3 Results ................................................................................................................................. 30
2.4 Discussion and Conclusions............................................................................................ 34
3. Liquid Metal - Liquid Silicate Partitioning of Nominally Lithophile and
Weakly Siderophile Elements 37
3.1 Introduction ....................................................................................................................... 37
3.2 Experimental Conditions ................................................................................................. 38
3.3 Run Products... 41
3.4 Analytical Techniques....................................................................................................... 43

I
3.5 Results ................................................................................................................................. 45
3.5.1 Calculation of the Oxygen Fugacity .................................................................... 45
3.5.2 Metal-Silicate Partition Coefficients - Dependence on Oxygen Fugacity..... 47
3.5.3 Compositional Effects and Influence of the Capsule Material....................... 51
3.5.4 Change of the Partitioning Behaviour with Pressure and Temperature ....... 56
3.6 Discussion and Implications for Existing Core Formation Models........................ 66
3.6.1 Testing Heterogeneous Low Pressure Core Formation Scenarios ................ 67
3.6.2 Constraints on High Pressure Core Formation Models .................................. 72
3.7 Conclusions ........................................................................................................................ 77
4. Liquid Metal - Liquid Silicate Partitioning of Highly Siderophile Elements
at High Pressures and High Temperatures 79
4.1 Introduction ....................................................................................................................... 79
4.2 HSE Glass Standards for Trace Element Microanalysis and
Analytical Techniques....................................................................................................... 82
4.2.1 Synthesis of the Glass Standards ......................................................................... 82
4.2.2 Quantitative Analysis of the Glasses and Evaluation of their Suitability
as Standards for HSE Microanalysis ................................................................... 84
4.3 Metal-Silicate Partitioning Experiments ........................................................................ 93
4.3.1 Starting Materials and Experimental Setup........................................................ 93
4.3.2 Run Products and Analytical Techniques .......................................................... 95
4.3.3 Results ....................................................................................................................102
4.4 Summary and Conclusions.............................................................................................114
5. Constraints on Planetary Core Formation Models -
Conclusions and Outlook 117
Acknowledgements 121
References 123
Appendix 133
II
Abstract
In this study a physical mechanism and geochemical parameters have been examined in high
pressure and high temperature experiments in order to place constraints on the conditions and
the manner by which core-mantle differentiation occurred on Earth and terrestrial planets.
The wetting characteristics of liquid Fe-Si alloys in a matrix of the respective predominating
stable silicate mantle mineral (forsterite or silicate perovskite) at pressures of 2 - 5 and 25 GPa and
temperatures of 1600 - 2000 °C were studied by determining the liquid metal-solid silicate contact
angles. The median angle values from texturally-equilibrated samples were found to be
independent of pressure, temperature, silicate mineralogy and the Si content in the metal fraction
and range between 130° and 140° which is far above the critical wetting boundary of 60°. This
shows that within the studied range of conditions dissolved Si does not lower the surface energies
between Fe-rich liquids and silicate mantle grains. As a consequence, under reducing conditions
the presence of Si in the metal phase of planetary bodies would not have induced or aided
percolative flow as the metal-silicate separation process.
Liquid metal - liquid silicate partitioning experiments for the elements Ta, Nb, V, Cr, Si, Mn,
Ga, In and Zn have been performed over a wide range of high-pressure and high-temperature
conditions of 2 - 24 GPa, 1750 - 2600°C and at low oxygen fugacities of -1.3 to -4.2 log units
below the iron wüstite buffer. The effects of pressure, temperature and oxygen fugacity on the
partitioning behaviour have been separated and the derived relationships have been applied to
test the respective element depletions in the mantle under various conditions suggested in core
formation models. These data indicate that Nb can serve as an important constraint on oxygen
fugacity and pressure for metal-silicate equilibration. Ta is less siderophile than Nb and its lack of
depletion in the mantle places a hard constraint on the minimum ƒo encountered during core 2
formation. Moreover, core formation must have occurred at conditions significantly greater than
20 GPa in order for Nb not to have been massively depleted under conditions necessary to
deplete the weakly siderophile element V. Moreover, our study shows that the volatile elements
Mn and Ga, would experience strong fractionations in any core-mantle equilibration scenario at
pressures below 60 GPa and temperatures at least as high as the peridotite liquidus, which is
inconsistent with their observed near-chondritic abundances in the mantle. The same observation
has been made for the elements Zn and In though to a more extreme extent such that pressures
over 80 GPa may be required to explain their near-chondritic ratio in the mantle. Based on these
observations we find strong support for the existence of a deep magma ocean during metal-
silicate separation, which is an essential component in current polybaric multi-stage core
formation models. Although these models succeed in reproducing the observed mantle
abundances of many siderophile elements, and can be constrained based on the partitioning
behaviour of elements such as Nb, the observed behaviour of the volatile elements Mn, Ga, Zn
and In may call for an additional process. Such a process may be the late accretion of volatiles in
1
material that did not undergo core-mantle separation or strong fractionation processes in the
condensing nebula that are r