Formation of magnetic minerals at hydrocarbon-generation conditions

In this paper, we report the pyrolysis and formation of magnetic minerals in three source rock samples from the Wessex Basin in Dorset, southern England. The experimental conditions in the laboratory recreated the catagenesis environment of oil source rocks. Magnetic analysis of both the heated and the unheated samples at room temperature and at very low-temperatures (5 K), coupled with transmission electron-microscopy imaging and X-ray analysis, revealed the formation of nanometre-sized (<10 nm), magnetic particles that varied across the rock samples analysed, but more importantly across the pyrolysis temperature range. Magnetic measurements demonstrated the formation of these magnetic minerals peaked at 250 °C for all rock samples and then decreased at 300 °C before rising again at 320 °C. The newly formed magnetic minerals are suggested to be primarily pyrrhotite, though magnetite and greigite are also thought to be present. The sizes of the magnetic minerals formed suggest a propensity to migrate together with oil potentially explaining the magnetic anomalies observed above and within oil fields

The meaning of maxima and minima in first order reversal curves: Determining the interaction between species in a sample

First-order reversal curves (FORCs) are a characterization technique for magnetic materials used in a wide range of research fields. Since their first application in the Earth Sciences two decades ago, their importance in science has been continuously growing and new experimental techniques have been subsequently designed based on the original idea of FORCs. Nonetheless, very recent experimental works on very well designed and simple magnetic structures demonstrate that even for the most simple cases the interpretation of FORC data lacks understanding. In this work, we address this problem analytically, explaining the meaning of maxima, minima and noisy tails and set a strategy to extract the interaction field between magnetic structures. The origin of this interaction field is often the magnetostatic energy, however, we propose that this strategy could be applied for estimating exchange interactions too

Comparison of methods for determining leaf area in tree rows

Accepted versio

First-order reversal curve (FORC) diagrams

Accepted versio

Low-temperature viscous magnetization of multidomain magnetite: evidence for disaccommodation contribution

Low-temperature viscous acquisition and decay measurements above and below the Verwey transition have been carried out for a selection of natural and synthetic multidomain magnetite samples. A strong correlation between the viscosity spectra and published disaccommodation spectra was found, where disaccommodation reflects electron mobility. Assuming the viscosity is controlled by identical mechanisms as disaccommodation, the reduction in electron mobility below the Verwey transition is found to significantly increase viscous acquisition and decay rates over the time scales measured (1–3000 s). Although strongly affecting the viscosity, disaccommodation processes do not appear to control the rate of change of viscosity with time, i.e. the viscosity curvature. It is suggested that the curvature is controlled by the shape of relaxation-time distributions, which is approximately the same for all the magnetite samples studied. In addition, the acquisition and decay curvature parameters mirror each other when plotted as a function of temperature, inferring that at any given temperature, the acquisition and decay processes are identical

Observations of viscous magnetization in multidomain magnetite

The viscous behavior of multidomain magnetite has been directly observed in both natural and synthetic samples using Bitter pattern imaging. A computer-controlled fully automated microscope fitted with a heating stage, field coils, and digital camera was used to record viscous acquisition and decay sequences as a function of time, temperature, and field. Domain walls (DW) were observed to move continuously through a series of quasi-static states over many hours rather than instantaneously. Viscosity was observed only on grains oriented near to the {111} surface. Generally, DWs moved perpendicular to their surface in the direction of the applied field; however, because observed domains respond primarily to the movement of main domains underneath the surface, occasionally, DWs moved in the opposite direction to the applied field. Small variations in temperature were found to strongly influence the viscosity, supporting the idea that viscosity is thermally activated. Viscous and nonviscous domain structures were examined using magnetic force microscopy. These images revealed that the domains displaying viscous behavior tended to be narrow (~2 ?m in width). Larger domains on grains oriented near the {111} surface did not display viscosity, reflecting the greater energy required to move larger domain structures. This may explain why no viscosity was observed on the {110} surfaces, as the domains were wider, that is, ~6–10 ?m. A complex spiraling vortex-like magnetic domain structure was imaged. Etch pit analysis found a corresponding dislocation pit at the same location. It is suggested that this corresponds to the microstructure around a screw dislocation line

Low-temperature cycling of isothermal and anhysteretic remanence: microcoercivity and magnetic memory

Accepted versio

The effect of cooling rate on the intensity of thermoremanent magnetization (TRM) acquired by assemblages of pseudo-single domain, multidomain and interacting single-domain grains

Experiments designed to measure the absolute palaeointensity of the geomagnetic field generally do so by comparing the ancient thermoremanent magnetization (TRM) retained by an igneous rock with a new TRM imparted in the laboratory. One problem with this procedure is that the relative magnitudes of the ancient and laboratory TRMs may be influenced, not only by the external field intensities at the time the two coolings took place, but also by the rate at which the coolings themselves occurred. Here, we present new measurements of this ‘cooling rate effect’ obtained from treatments in the laboratory differing in cooling rate by a factor of ∼200. Synthetic samples containing sized ferrimagnetic grains were used in the experiments. Theoretical considerations and previous experiments have indicated the cooling rate effect to be dependent on domain state. Increases in TRM magnitude of more than 7 per cent per order of magnitude decrease in cooling rate have been reported for assemblages of non-interacting single-domain (SD) grains. Here, we focus on magnetite grains in the less well-studied pseudo-single domain (PSD) and multidomain (MD) states using a range of applied field intensities to impart the TRMs. For the first time, we also measure the cooling rate effect in grains of titanomagnetite that have been oxyexsolved so that they contain strongly interacting SD lamellae. In all cases, the cooling rate effect measured was in the same sense as already observed in ideal magnetically non-interacting SD grains but was considerably weaker. On average, the effect did not exceed ∼3 per cent increase in TRM per order of magnitude decrease in cooling rate and did not show any systematic dependence on applied field intensity. In some samples containing coarser grains, the cooling rate effect was not distinguishable from zero. The sense and magnitude of the cooling rate effect remain uncertain in truly MD grains as different studies have produced discrepant results. For the more practically relevant case of PSD and interacting SD grains, which commonly dominate the TRM in igneous rocks, however, it appears that we can be more confident in our assertions. The cooling rate effect in such materials is in the same sense as in non-interacting SD grains but smaller: a consequence of long-range ordering. In lavas and small intrusions containing these, it is unlikely to exceed 10 per cent. Although a correction should always be attempted, the results of palaeointensity studies based upon such samples will generally not be severely biased