Abstract The pattern of seafloor magnetic anomalies is a record for the self-reversals of the Earth magnetic field from the long past to the present. It has preserved crucial data for the formation and evolution of oceanic crusts and is one of the most important evidences for the theory of plate tectonics. However, the features and origins of magnetic carriers in the sheeted dikes of oceanic crusts have not been completely understood and are still in debate. In the present study, magnetic minerals in the core samples, which were drilled from the sheeted dikes at the DSDP/ODP 504B drill hole during Legs 83, 111, 137, 140, and 148, have been studied by using methods of rock magnetism and mineralogy with high-resolution petrographic tools (transmission electron microscopy, TEM). Our results indicate that the sheeted dike basalts have been subjected to different degrees of hydrothermal alterations, which are equivalent to greenschist facies to amphibolite facies metamorphism on the basis of the secondary mineral assemblages. The primary titanomagnetite in all the sheeted dike basalts has suffered high-temperature oxidation, exsolution, and hydrothermal alteration, and transformed into magnetite, which becomes the main magnetic mineral in the sheeted dikes. The lamellar widths of the secondary magnetite, as observed with electron microscopy, are consistent with the grain sizes inferred form the rock magnetic properties. The grain sizes of the magnetite are within the pseudo-single-domain field and increase with depths of the sheeted dikes. The consistent results of the whole-rock magnetic properties and the TEM observations have proved that the secondary magnetite and its textural features are representative of the features of magnetic mineral in the sheeted dikes. Therefore, on the basis of the formation model of the magnetite, it is inferred that the sheeted dike basalts obtained thermal chemical remanent magnetization (TCRM) at ~500¢XC (high-temperature oxidation, or exsolution), and then obtained chemical remanent magnetization (CRM) at ~350¢XC (hydrothermal alteration). The timing for the magnetization of the sheeted dike basalts thus lags slightly behind their formation. The primary titanomagnetite in the sheeted dikes has been completely transformed into pseudomorphs that consist of approximately half magnetite and half ilmenite or other phases. Thus, the natural remanent magnetization (NRM) of the sheeted dikes is only about half of that for the extrusive pillow basalts. However, the total thickness of the sheeted dikes is about three times of that for the pillow basalts. The sheeted dikes should have contributed to the seafloor magnetic anomalies to some extents.

This project examined the reduction and formation of cementite from hematite and titanomagnetite ores and cementite stability. The aim of the project was to develop further understanding of cementite stability under conditions relevant to direct ironmaking and the mechanism of cementite decomposition. The reduction of hematite and ironsand by hydrogen-methane-argon gas mixtures was investigated from 600??C to 1100??C. Iron oxides were reduced by hydrogen to metallic iron, which was carburised by methane to form cementite. The hematite ore was reduced more quickly than the ironsand. Preoxidation of the ironsand accelerated its reduction. Hematite was converted to cementite faster than preoxidised ironsand. The decomposition of cementite formed from hematite was investigated from 500??C to 900??C. This cementite was most stable at temperatures 750-770??C. The decomposition rate increased with decreasing temperature between 750??C and 600??C and with increasing temperature above 770??C. The stability of cementite formed from pre-oxidised titanomagnetite was studied from 300??C to 1100??C. This cementite was most stable in the temperature range 700-900??C. The rate of decomposition of cementite increased with decreasing temperature between 700??C and 400??C and with increasing temperature above 900??C. Cementite formed from ironsand was more stable than cementite formed from hematite

This project examined the reduction and formation of cementite from hematite and titanomagnetite ores and cementite stability. The aim of the project was to develop further understanding of cementite stability under conditions relevant to direct ironmaking and the mechanism of cementite decomposition. The reduction of hematite and ironsand by hydrogen-methane-argon gas mixtures was investigated from 600??C to 1100??C. Iron oxides were reduced by hydrogen to metallic iron, which was carburised by methane to form cementite. The hematite ore was reduced more quickly than the ironsand. Preoxidation of the ironsand accelerated its reduction. Hematite was converted to cementite faster than preoxidised ironsand. The decomposition of cementite formed from hematite was investigated from 500??C to 900??C. This cementite was most stable at temperatures 750-770??C. The decomposition rate increased with decreasing temperature between 750??C and 600??C and with increasing temperature above 770??C. The stability of cementite formed from pre-oxidised titanomagnetite was studied from 300??C to 1100??C. This cementite was most stable in the temperature range 700-900??C. The rate of decomposition of cementite increased with decreasing temperature between 700??C and 400??C and with increasing temperature above 900??C. Cementite formed from ironsand was more stable than cementite formed from hematite

Cette thèse concerne l’étude du message magnétique de la lithosphère océanique. Nous nous sommes, dans un premier temps, intéressés à l’évolution du signal magnétique à travers une section de croûte océanique complète et continue des basaltes jusqu’aux gabbros. Le but était de comprendre comment les propriétés magnétiques des roches peuvent nous renseigner sur les conditions d’altération dans la croûte océanique. Nous avons donc établi un jeu de données magnétiques (température de Curie, paramètres d’hystérésis, mesures magnétiques basse température) sur l’ensemble de la section de croûte océanique forée au site IODP 1256D, dans l’océan Pacifique. Ces données sont confrontées aux températures d’altération, établies par thermo barométrie et mettent en évidence une étroite relation entre l’altération des phases magnétiques et les températures d’altération. De plus, des analyses semi-quantitatives et des observations microscopiques (optique, MEB et MET) mettent en évidence un changement de structure cristalline, associée à une perte de titane, permettant la formation d’une phase secondaire, l’hydroschorlomite, dans un intervalle de forte altération des phases magnétiques (entre 670 et 1028 mbsf (meters below sea floor)). Dans un second temps, l’acquisition de profils d’anomalies magnétiques marines de surface et d’un profil d’anomalies de fond « deep tow » à travers le superchron du Crétacé (entre 83 et 120 Ma) nous a permis de tester la stabilité de polarité du champ géomagnétique durant cette période. Nous mettons en évidence la présence d’anomalies magnétiques : des anomalies de courtes longueurs d’onde ou « tiny-wiggles » à travers l’ensemble du superchron et des anomalies magnétiques de plus grande longueur d’onde, assimilables à de courts intervalles de polarité inverse. Nos mesures montrent que le comportement du champ magnétique durant le superchron n’est pas différent des périodes qui le précèdent (chrons M0-M1-M2) et le suivent (chrons 33n et 33r). La définition de superchron doit être remise en question. / So we, in a first part, studied the evolution of the magnetic signal through a section of a, complete and continuous, oceanic crust, from basalts to gabbros. In order to understand how the magnetic properties of rocks can tell us about the conditions of alteration in the oceanic lithosphere, we established a set of magnetic data (Curie temperature, hysteresis parameters, low temperature magnetic measurements) through the entire section of the oceanic crust, drilled at IODP Site 1256D, in the Equatorial Pacific Ocean. These magnetic data are compared to alteration temperatures, determined by thermobarometry (Alt et al., 2010) and show a close relationship between the alteration of the magnetic phases and the alteration temperatures, including the identification of an interval of strong alteration of the titanomagnetites (between 670 and 1028 mbsf (meters below sea floor). In addition, semi quantitative chemical analysis and microscopic observations (optical, SEM and TEM), performed on titanomagnetites, show a change in crystalline structure and a loss of titanium element (Ti4 +) in titanomagnetites to form a secondary phase rich in titanium, in this same interval of strong alteration. In a second part, the acquisition of numerous sea-surface magnetic profiles and a high resolution magnetic profile ("deep tow") through the Cretaceous Normal Superchron (83-120 Ma), allowed us to test the stability of the geomagnetic polarity of the superchron and to highlight the presence of numerous magnetic anomalies: anomalies of short wavelength or "tiny-wiggles” through the entire period and magnetic anomalies of greater length wave, similar to short intervals of reverse polarity. Our measurements show that the behavior of the magnetic field during the superchron is no different from previous periods (chrons M0-M1-M2) and the following magnetic period (chrons 33n and 33R) and the definition of ‘superchron’, long geomagnetic event without inversions, must be questioned