Magnetic Properties of Lunar Samples
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The magnetic properties of samples of rock, fines, and magnetic separate from the fines from Apollo 11 have been measured. Native iron, or possibly nickel-iron, of submicroscopic particle size is the most important constituent, with minor contributions from ilmenite, paramagnetic iron minerals, and other iron-titanium oxides. The remanent magnetization of a sample of the micro-breccia rapidly acquires a viscous magnetization and does not appear to have a significant stable remanence. The crystalline sample has a weak natural remanence showing some stability.Keywords:
Ilmenite
Rock magnetism
Natural remanent magnetization
Breccia
Magnetism
We have investigated basic properties of transition remanent magnetization of natural magnetite in granite samples collected from the Minnesota River Valley, North America. Transition remanence was imparted during cooling and/or warming through the Verwey transition around 120 K. Depending on magnetic field conditions during cooling and warming, three types of transition remanences have been categorized: (1) TrRM, acquired during a cycle of field cooling and field warming; (2) TrWRM, acquired during zero-field cooling and field warming and (3) TrCRM, imparted during field cooling and zero-field warming. These remanences fulfil basic laws of remanent magnetization: (1) directions of the transition remanences are parallel to direction of the applied field (the law of parallelism), (2) intensities of the transition remanences are proportional to the applied field intensity (the law of proportionality) and (3) sum of the partial transition remanences is equal to the total transitional remanence, that is, TrRM = TrWRM + TrCRM. In addition, the ratio of TrRM to TRMLTD (the demagnetized component of thermoremanent magnetization by low-temperature demagnetization) shows a nearly constant value of ∼0.34. This relationship might reflect differences in equilibrium magnetic domain state at low and high temperature.
Natural remanent magnetization
Thermoremanent magnetization
Rock magnetism
Stoner–Wohlfarth model
Charge ordering
Single domain
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Natural remanent magnetization and self-demagnetization on high-susceptibility bodies are two important factors affecting magnetic data inversion. We propose a framework for the inversion and interpretation of magnetic anomalies, in which significant remanent magnetization and self-demagnetization are present simultaneously. The framework is based on the assumptions that the external applied field and internal self-demagnetization field are uniform and the deflection of self-demagnetization in the total magnetization direction is negligible. First, the magnetization vector distributions are obtained from magnetic data by estimating the magnetization direction, then inverting for the magnetization intensity distribution, using the inferred magnetization direction as a constraint. Based on a priori information about the Koenigsberger ratio derived from petrophysical measurements, the direction and intensity of the remanent magnetization are obtained. The self-demagnetization factor is then computed using the finite volume method. Finally, the true-susceptibility distribution is achieved by correcting for the self-demagnetization effect. The method is first applied to synthetic magnetic data produced by a prism-shaped source model that has significant remanent magnetization and high susceptibility. In a case study of the Daye iron-ore deposit, Hubei province, China, the true susceptibility and remanent magnetization are reconstructed. The remanence direction information reveals that local geological activities such as synclines and faults lead to changes in the remanence directions at different local deposits.
Natural remanent magnetization
Rock magnetism
Stoner–Wohlfarth model
Single domain
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Natural remanent magnetization
Rock magnetism
Magnetism
Magnetostratigraphy
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Geological processes, such as burial, can lead to remagnetization in rocks due to neoformed magnetic minerals that have passed a critical volume, called blocking volume. In this study, we designed a heating experiment for claystones obtained from the Paris Basin (France), in the 50–130°C temperature range, in order to simulate <4 km burial remagnetization. At a given temperature, remanence increased rapidly within a couple of days and stabilized afterward. There was a positive relation between the experimental temperature and the obtained remanence. Remanence was determined to be carried equally by stable chemical remanent magnetization and unstable thermo‐viscous remanent magnetization. By assuming that magnetite formed during the experiment, we interpreted the increase of chemical remanent magnetization and the increase of thermo‐viscous remanent magnetization as the continuous growth of the >20 nm and ∼20 nm minerals respectively. This result led us to propose a conceptual model of nucleation‐and‐growth process of magnetite during low grade burial from ∼2 to ∼4 km depth. Ultrafine magnetite (≤20 nm) was predominant over single domain magnetite (>20 nm) for <4 km depth. Transposed to natural conditions, our heating steps experiment suggested that claystone‐type rocks are remagnetized during burial. For temperatures higher than 200°C, the extrapolation of our results indicated that burial remagnetization, due to the chemical remanent magnetization, might be larger than the natural remanent magnetization.
Natural remanent magnetization
Rock magnetism
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The characteristics of the remanent magnetization of red sediments are first considered, followed by a brief account of present ideas on the origin of these rocks. Possible processes by which sediments in general might acquire a remanent magnetization are postulated, with reference to chemical and depositional remanent magnetization, and these hypotheses are then discussed both from a geological standpoint and in connection with the observed characteristics of the remanence. The experimental work done on the magnetic properties of red rocks is summarized, and this is followed by a discussion of all the available evidence on the origin of the remanent magnetization of red beds, with particular reference to whether it is of a chemical or depositional nature. The importance or otherwise of the red iron oxide in the rocks in contributing to the remanence is also discussed. From the evidence available at the present time no general process of magnetization can be adduced for red sandstones, nor can the NRM be attributed to either the red or black iron oxide present. Chemical magnetization is favoured as the most likely process in many formations.
Natural remanent magnetization
Rock magnetism
Red beds
Magnetostratigraphy
Stoner–Wohlfarth model
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Citations (32)
Abstract Development in instrumentation and technology now allows for mapping of magnetic anomalies, caused by spatial variations in magnetization in the source material, from the mineral to the crustal anomalies scale. High‐resolution magnetic mapping techniques allow for accurate investigation of the magnetization in natural rock samples and particularly of their remanence carriers, which can record geologically meaningful information. Multidomain magnetic grains are expected to retain a remanence that is susceptible to change by exposure to magnetic fields or by changes in temperature. Although this makes multidomain grains less reliable remanence carriers for paleomagnetic studies, their magnetization contributes to rocks bulk magnetization and to induced anomalies in the Earth crust. Here, we investigate the fine‐scale magnetization of a sample that exhibits a multidomain behavior. We used a scanning magnetic microscope, equipped with a room temperature magnetic tunnel junction sensor, to map the magnetic field over a petrographic thin section of the sample containing large magnetite grains (>100 μm) surrounded by serpentine and carbonate. We modeled the fine‐scale remanent magnetization of the magnetite by inverting the magnetic scans data acquired in near‐field‐free conditions. We applied a multistep inversion, a priori tested on a synthetic model, with a controlled range on the intensity of the magnetization. Modeling results on the study sample suggest homogenously magnetized regions within the magnetite grains with variable remanent magnetization intensities and directions coherent with the multidomain behavior inferred from bulk measurements.
Natural remanent magnetization
Rock magnetism
Stoner–Wohlfarth model
Single domain
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