Self-reversal of remanent magnetisation of basalts [Elektronische Ressource] : origin, mechanisms and consequences / vorgelegt von David Krása

Krása, Laurent-Olivier David

Sujets

• Physik

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2003
• Nombre de lectures: 6
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

Self-Reversal of Remanent
Magnetisation of Basalts — Origin,
Mechanisms and Consequences
Inaugural-Dissertation
zur Erlangung des Doktorgrades
der Fakult¨ at fur¨ Geowissenschaften der
Ludwig-Maximilians-Universit¨ at Munchen¨
vorgelegt von
David Kr´asa
15. September 20031. Berichterstatter: Prof. Dr. N. Petersen
2. Bericht PD Dr. T. Kunzmann
Tag der mundlic¨ hen Prufung:¨ 17.11.2003Contents
Abbreviations and symbols 6
Preamble 9
Zusammenfassung 10
1 Introduction 14
1.1 Magnetomineralogy of basalts . . . . . . . . . . . . . . . . . . . . 15
1.2 Processes leading to self-reversal of the remanent magnetisation . 19
1.3 Scientificaims............................. 24
2 Rock magnetic properties 26
2.1 Samples................................ 26
2.2 Methods 27
2.3 Measurements of magnetic hysteresis and thermomagnetic curves 28
2.3.1 OlbyandLaschampsamples................. 28
2.3.2 Vogelsbergsamples...................... 32
2.4 Low-temperaturemeasurements................... 36
2.5 Conclusions.............................. 43
3 Microscopic observations 45
3.1 Reflectedlightmicroscopy...................... 45
3.2 Electronmicroprobeanalysis..................... 53
3.3 Magneticforcemicroscopy 60
3.3.1 Imagingtechnique . 60
3.3.2 Samplepreparation 61
1CONTENTS 2
3.3.3 Results and comparison to microprobe and light micro-
scopicobservations...................... 63
3.4 Conclusions.............................. 69
4 Self-reversal of TRM 71
4.1 Continuous thermal demagnetisation . . . . . . . . . . . . . . . . 72
4.2 NRMdemagnetisationresults.................... 75
4.3 Experiments with laboratory thermoremanent magnetisation . . . 85
4.4 The Laschamp event: Geomagnetic origin or self-reversal? . . . . 94
4.5 Conclusions.............................. 95
5 Numerical modelling 97
5.1 The analytical model of a spherical two-phase grain . . . . . . . . 98
5.2 Numerical model for arbitrary geometries . . . . . . . . . . . . . . 102
5.3 Conclusions11
6 Thellier experiments 112
6.1 Relevant properties of multidomain and partially self-reversed
thermoremanence...........................113
6.2 Sample description and experimental methods . . . . . . . . . . . 115
6.3 Results.................................120
6.4 Conclusions..............................125
7 Summary 127
Bibliography 130
Acknowledgements 142
Curriculum Vitae 144List of Figures
1.1 Temperature dependence of f inbasalts............. 17
O
2
2.1 M /M versus H of Olby/Laschamp samples . . . . . . . . . . 29RS S C
2.2 M (T)curvesofOlby/Laschampsamples.............. 30S
2.3 Rock magnetic properties of samples from Vogelsberg . . . . . . . 33
2.4 M /M versus H ofVogelsbergsamples............. 36RS S C
2.5 Typicallow-temperaturehysteresisloops.............. 38
2.6 Temperature dependence of M (Olby/Laschamp)......... 39S
2.7 Temperature dependence of M (Vogelsberg)............ 40S
2.8 SIRM measurements on sample V23-1 . . . . . . . . . . . . . . . . 41
2.9 Temperature dependence of coercive force (Olby/Laschamp) . . . 42
2.10 Temperature dependence of coercive force (Vogelsberg) . . . . . . 43
3.1 Polished section of sample L-1A covered with magnetic colloid . . 46
3.2 PolishedsectionofsampleO-1E................... 47
3.3 MicroscopicdetailofsampleV25-5................. 48
3.4 Titanomagnetite grain from sample O-1C . . . . . . . . . . . . . . 49
3.5 T grain from sample O-1G . . . . . . . . . . . . . . 51
3.6 T grain from sample V28-5 . . . . . . . . . . . . . 52
3.7 Element distribution of a grain from sample O-1G . . . . . . . . . 55
3.8 Schematic view of the proposed model for partial oxidation . . . . 57
3.9 Element distribution of a grain with unfilled crack . . . . . . . . . 59
3.10 Magnetic force microscope at the IRM in Minneapolis . . . . . . . 60
3.11 Composite MFM micrograph of grain 2 of sample V27 . . . . . . 64
3.12MicrographofsampleO-1C..................... 65
3LIST OF FIGURES 4
3.13 Grain from sample O-1G represented by MFM and microprobe . . 66
3.14 Detail of sample O-1E viewed with different microscopic techniques 67
3.15 Detail of sample V27 viewed with different m techniques 68
4.1 Continuous thermal demagnetisation . . . . . . . . . . . . . . . . 73
4.2 Results of single phase samples L-1A and O-2C . . . . . . . . . . 76
4.3 ResultsofsampleO-1G ....................... 7
4.4 ResultsofsampleV27-0. 78
4.5 ResultsofsampleV25-5. 79
4.6 ResultsofsampleO-1R........................ 80
4.7 Results of sample O-1C displaying total self-reversal . . . . . . . . 83
4.8 ResultsofsampleO-1E. 84
4.9 Continuous demagnetisation of TRM acquired in varying H .. 86ext
◦ ◦
4.10 Continuous d of pTRM(500 C,220 C,25 µT) . . . 87
◦ ◦4.11 Continuous d of pTRM(600 C,210 C,25 µT) . . . 89
◦ ◦4.12 Continuous demagnetisation of p C,210 C,25 µT) for
samples with high T (1)....................... 91C
◦ ◦
4.13 Alternating field demagnetisation of pTRM(600 C,210 C,25 µT) 92
5.1 Spherical two-phase model . . . . . . . . . . . . . . . . . . . . . . 99
5.2 Resultsforsphericalmodel10
5.3 Results for spherical model with blocking condition for phase B . 102
5.4 Results of numerical calculations for spherical model . . . . . . . 105
5.5 Magnetisation structure for the spherical grain . . . . . . . . . . . 106
5.6 Result for spherical model with phase A consisting of one cell . . 107
5.7 Rectangular two-phase model grain . . . . . . . . . . . . . . . . . 108
5.8 Results of numerical calculations for rectangular model . . . . . . 109
5.9 Magnetisation structure for the rectangular grain . . . . . . . . . 110
6.1 Low-temperature SIRM warming curves of the synthetic samples . 117
6.2 Viscous decay of SIRM for sample W1 . . . . . . . . . . . . . . . 118
6.3 Scheme of the proposed additivity check . . . . . . . . . . . . . . 119
6.4 Results of the Thellier experiments . . . . . . . . . . . . . . . . . 121
6.4 (continued)..............................12List of Tables
2.1 Rock magnetic parameters of Olby and Laschamp samples . . . . 31
2.2 Rock magnetic of Vogelsberg samples . . . . . . . . . 34
3.1 Microprobe analysis: Elements, spectrometers, analysis crystals . 54
◦ ◦4.1 Results of pTRM(600 C,210 C,25 µT) demagnetisation . . . . . 90
6.1 Rock magnetic parameters of synthetic magnetite samples . . . . 116
6.2 Resultsofthepalaeointensityexperiment..............123
5Abbreviations and symbols
AC ...........................................................additivitycheck
AFalternatingfield
CRM ........................................chemicalremanentmagnetisation
FC ................................................................fieldcooled
FFT ...............................................fastFouriertransformation
Hem .................................................................hematite
HI ...............................................................hemoilmenite
Ilm ...................................................................ilmenite
IRM .......................................isothermalremanentmagnetisation
LT ...........................................................low-temperature
MD .............................................................multi-domain
MFM ...............................................magneticforcemicroscopy
MOKE ............................................magneto-opticalKerreffect
MPMS .............................MagneticPropertiesMeasurementSystem
Mt ..................................................................magnetite
NRM .........................................naturalremanentmagnetisation
OP-Samorphoussilica polishing liquid
PSD .....................................................pseudosingle-domain
pTRM .................................partialthermoremanentmagnetisation
QFM ..........................................quartz-fayalite-magnetitebuffer
RGB ..............................................red-green-blue colour space
RT .........................................................roomtemperature
SD ..............................................................single-domain
SEM .............................................scanningelectronmicroscopy
6ABBREVIATIONS AND SYMBOLS 7
SIRM ...........................saturationisothermalremanentmagnetisation
SP .........................................................superparamagnetic
TD ...............................................................two-domain
TM ...........................................................titanomagnetite
TCRM ...............................thermochemicalremanentmagnetisation
TRM ..........................................thermoremanentmagnetisation
Usp .................................................................ulvospinel
VFTB .....................................VariableFieldTranslationBalance
VSM .........................................VibratingSampleMagnetometer
ZFC ..........................................................zerofieldcooled
Symbols and constants
˚a [A] ........................................................latticeparameter0
f [Pa] .......................................................oxygenfugacity
O
2
H [T] ...........................................................magneticfield
H [T] .................