Self-Reversal of Remanent Magnetisation of Basalts - Origin, Mechanisms and Consequences

One of the main goals of current palaeomagnetic research is the attempt to acquire high-resolution data on palaeodirections and -intensities in order to obtain detailed information about the Earth's magnetic field in the geological past. The material best suited for such studies are basaltic rocks. For these high-quality directional investigations and especially for palaeointensity determinations, a profound knowledge about the stability, magnetomineralogical character and the domain state of the carriers of remanence is imperative. The emphasis of the present work was placed on the investigation of basalts exhibiting partial or complete self-reversal of natural remanent magnetisation (NRM). This phenomenon is not an exotic rarity but a widespread characteristic of many basaltic rocks. However it remains usually unnoticed by routine palaeomagnetic measurements as it requires special techniques for its detection. In this work samples from Olby and Laschamp (France) and Vogelsberg (Germany) showing the phenomenon were studied with rock magnetic, microscopic and microanalytical techniques in order to identify the carriers of self-reversed remanent magnetisation. Further aims were to determine the exact mechanism of self-reversal acting in basalts and to evaluate the consequences for the reliability of palaeomagnetic data. On the basis of the experimental work a numerical model was developed which shows that, from the physical point of view, the observed magnetomineralogy is capable of causing self-reversal. The present work provides the following new insights: The phenomenon is caused by two magnetic phases with different blocking temperatures which are magnetically coupled. The lower blocking temperature corresponds to the primary titanomagnetite (mother phase) crystallising from the basaltic magma. The remanence with higher blocking temperature is carried by titanomaghemite (daughter phase) evolving from the primary titanomagnetite by partial low-temperature oxidation. The daughter phase forms narrow bands along cracks in the otherwise unaffected mother phase particles. This yields a close side-by-side assemblage of titanomagnetite and titanomaghemite with markedly different magnetic properties in one and the same grain. By applying the various microscopic techniques on identical grains, it was possible to directly correlate magnetomineralogy with magnetic domain structure. Numerical calculations of remanence acquisition demonstrate that two-phase particles with the experimentally observed geometry and magnetic properties are able to acquire a partially or completely self-reversed remanent magnetisation. The calculations also prove that the two magnetic phases present in the studied samples are coupled by magnetostatic interaction. The experimental results indicate that the low-temperature oxidation process responsible for the formation of the second magnetic phase takes place at temperatures at or above the blocking temperature of this daughter phase during primary cooling. This titanomaghemite phase is thus carrying a stable remanence in direction of the ambient magnetic field. Although the original titanomagnetite as the mother phase is in a strict sense the primary magnetic mineral, it does not carry the primary magnetic remanence but is at least in part magnetostatically coupled to the titanomaghemite. Therefore, its remanence is - at least in part - antiparallel to the external field. MFM domain observations present evidence that the mother phase is in the magnetic multidomain range. Hence, its magnetic remanence is not stable and is replaced by a viscous overprint acquired at ambient temperatures. In contrast, the daughter phase has a higher coercivity due to oxidation induced stresses and an increased domain width. Regarding the samples from Olby, these magnetomineralogical investigations directly lead to new arguments in favour of the existence of the Laschamp geomagnetic event: As the high blocking temperature daughter phase carries a stable remanence in direction of the external magnetic field, the local geomagnetic field direction was indeed reversed at the time of emplacement. Due to their complex magnetomineralogy and remanence acquisition, samples exhibiting partial or complete self-reversal are not suitable for palaeointensity determinations. In order to identify and exclude such samples in the course of such experiments, a modification of the existing Thellier-Thellier method is proposed. Additionally, this new procedure is also able to detect remanence carried by multidomain (MD) particles. The method substantially improves the reliability and quality of palaeointensity estimates as multidomain behaviour is among the most common reasons for erroneous results in Thellier-type palaeointensity determinations.Die natürliche remanente Magnetisierung (NRM) von Gesteinen stellt die wichtigste Informationsquelle über das Verhalten des Erdmagnetfeldes in der geologischen Vergangenheit dar. Insbesondere Vulkanite bieten die Möglichkeit, hochgenaue Daten über die Richtung und vor allem auch die Intensität des Magnetfeldes zur Zeit ihrer Platznahme zu gewinnen. Allerdings bedarf eine derartige, verlässliche Rekonstruktion des Erdmagnetfeldes einer genauen Kenntnis der Minerale, welche die remanente Magnetisierung tragen. Besonders für die Bestimmung der Intensität des Erdmagnetfeldes gelten sehr spezifische Voraussetzungen bezüglich des Remanenzerwerbs, der thermischen Stabilität und des magnetischen Domänenzustands der Magnetominerale. Der Schwerpunkt der vorliegenden Arbeit wurde auf die Untersuchung von Basalten gelegt, die eine partielle oder vollständige Selbstumkehr der NRM aufweisen. Dieses Phänomen ist keine exotische Rarität, sondern tritt bei basaltischen Gesteinen durchaus häufig auf. Partielle wie vollständige Selbstumkehr lassen sich allerdings durch die Standarduntersuchungsmethoden der Paläomagnetik nicht identifizieren und bleiben daher üblicherweise verborgen. Proben aus Olby und Laschamp (Frankreich) und vom Vogelsberg (Deutschland), bei denen das Phänomen auftritt, wurden mittels gesteinsmagnetischer, mikroskopischer und mikroanalytischer Methoden untersucht, um die Träger der remanenten Magnetisierung zu identifizieren. Weitere Ziele der Arbeit waren die Bestimmung des Mechanismus, der zur Selbstumkehr in Basalten führt und die Untersuchung der Konsequenzen für die Zuverlässigkeit paläomagnetischer Daten. Auf der Basis dieser experimentellen Befunde wurde ein numerisches Modell entwickelt, welches bestätigt, daß die mikroskopisch beobachtete Struktur der Magnetominerale eine Selbstumkehr verursachen kann. Die folgenden Ergebnisse der vorliegenden Arbeit liefern einen wesentlichen Beitrag zum Verständnis der Selbstumkehr: Das Phänomen der Selbstumkehr wird in den untersuchten Proben durch zwei gekoppelte magnetische Phasen mit unterschiedlichen Blockungstemperaturen verursacht. Bei dem Mineral mit geringerer Blockungstemperatur handelt es sich um den primären, aus dem basaltischen Magma kristallisierenden Titanomagnetit (primäre Phase). Die höhere Blockungstemperatur entspricht einem Titanomaghemit (sekundäre Phase) der durch partielle Tieftemperatur-Oxidation des primären Titanomagnetits entsteht. Die sekundäre Mineralphase bildet schmale Streifen entlang von Rissen, die die ansonsten unveränderte primäre Phase durchziehen. Hierdurch entstehen Kristalle aus innig verwachsenem Titanomagnetit und Titanomaghemit. Es treten also ausgesprochen unterschiedliche magnetische Eigenschaften in ein und demselben Kristall auf. Indem die verschiedenen mikroskopischen Untersuchungsverfahren auf einzelne Erzkörner angewendet wurden, konnte eine direkte Korrelation zwischen Magnetomineralogie und magnetischen Eigenschaften hergestellt werden. Numerische Modellierungen des Remanenzerwerbs zeigen, daß die beobachtete räumliche Verteilung zweier magnetisch unterschiedlicher Phasen innerhalb eines Partikels und die magnetischen Eigenschaften der zwei Phasen dazu führen, daß sowohl partielle als auch vollständige Selbstumkehr auftritt. Durch die numerischen Simulationen und weitere experimentelle Ergebnisse konnte nachgewiesen werden, daß die beiden in den untersuchten Proben vorhandenen magnetischen Mineralphasen durch magnetostatische Wechselwirkung gekoppelt sind. Die Untersuchungen deuten darauf hin, daß der Prozess der Tieftemperatur-Oxidation und damit die Bildung der sekundären magnetischen Phase während des primären Abkühlens bei Temperaturen im Bereich der Blockungstemperatur dieser sekundären Phase oder darüber stattfindet. Diese Titanomaghemit-Phase ist daher der Träger der stabilen magnetischen Remanenz in Richtung des äußeren Erdmagnetfeldes. Obwohl Titanomagnetit die primäre Phase darstellt, trägt er nicht die primäre Remanenz, sondern ist zumindest zum Teil magnetostatisch an die sekundäre Mineralphase gekoppelt. Durch diesen Prozess der magnetischen Kopplung ist seine Remanenz - zumindest in Teilen - entgegengesetzt zum äußeren Feld gerichtet. Beobachtungen mittels magnetischer Kraftmikroskopie zeigen, daß sich die primäre Mineralphase im magnetischen Mehrbereichszustand befindet. Infolgedessen ist die von dieser Phase getragene Remanenz nicht stabil, sondern wird bei Raumtemperatur durch eine viskose Magnetisierung ersetzt. Im Vergleich dazu ist die Remanenz der sekundären Phase aufgrund ihrer höheren Koerzivität sehr viel stabiler. Für die Proben von Olby ergeben sich aus den magnetomineralogischen Untersuchungen neue Argumente, die für die Existenz des sogenannten Laschamp Events sprechen: Wenn die sekundäre Mineralphase mit hoher Blockungstemperatur eine stabile Remanenz in Richtung des äußeren Magnetfeldes trägt, besaß das lokale erdmagnetische Feld während der Eruption dieser Basalte tatsächlich inverse Polarität. Aufgrund ihrer komplexen Magnetomineralogie und des komplexen Remanenzerwerbs eignen sich Proben, die eine Selbstumkehr aufweisen, nicht für eine Bestimmung der Paläointensität des Erdmagnetfeldes. Um derartige Proben während solcher Experimente zu erkennen und von der weiteren Untersuchung ausschließen zu können, wird eine Erweiterung der bestehenden Thellier-Thellier Methode vorgeschlagen. Mit Hilfe dieser Modifikation können auch von Mehrbereichsteilchen getragene Remanenzen erkannt werden. Dies bedeutet eine entscheidende Verbesserung der Verlässlichkeit und damit Qualität der gewonnenen Ergebnisse, da Mehrbereichsteilchen einen der häufigsten Gründe für fehlerhafte Paläointensitätsbestimmungen darstellen

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