Mimi J. Hill

Palaeomagnetic investigation of Tertiary lava from Barrington Tops, NSW, Australia, using thermal and microwave techniques

Abstract Tertiary lava (53 Ma) from Barrington Tops, New South Wales, Australia has been studied using conventional thermal and microwave techniques. Twenty-seven flows in two sections were sampled and their rock magnetic characteristics determined. On heating the majority of samples exhibited a single Curie temperature at around 200°C, indicating a titanium rich titanomagnetite. Some samples exhibited two magnetic phases and a few a single low titanium titanomagnetite phase. Thermal demagnetisation yielded a mean direction of D=189.5, I=63.6, α95=4.3, which corroborates the previous findings of Wellman et al. [Geophys. J. R. Astron. Soc. 18 (1969) 371–395]. A pilot conventional Thellier palaeointensity analysis was unsuccessful mainly due to the samples being highly susceptible to thermo-chemical alteration. Far greater success was achieved (58%) using the microwave palaeointensity technique, where heating of the bulk sample and hence alteration is vastly reduced. Palaeointensity estimates range from 3 to 28 μT (mean 11±5 μT), which, assuming that the remanence is a primary thermal remanent magnetisation, indicates a low field intensity (a fifth of the present day value) in the early Tertiary. This study demonstrates the applicability of the microwave palaeointensity technique to ancient lava.

Investigating the ancient Martian magnetic field using microwaves

Abstract The new microwave palaeointensity technique has been used to investigate samples from the Martian meteorite Nakhla. This technique is a promising new way to obtain absolute palaeointensity information regarding the ancient Martian magnetic field as recorded by the Martian meteorites. Assuming that a part of the magnetic remanence is of thermal origin and originating on Mars the two samples studied yield estimates of 4 μT for the Martian magnetic field at 1.35 Ga.

Magnetostratigraphic importance of secondary chemical remanent magnetizations carried by greigite (Fe3S4) in Miocene sediments, New Jersey shelf (IODP Expedition 313)

Paleomagnetic and mineral magnetic analyses were carried out on Miocene clays from upper unit II at Sites M0027 and M0028 recovered during Integrated Ocean Drilling Program Expedition 313 on the New Jersey shallow shelf. A zone of mixed polarity in the lower section of Hole M0028A and dual overlapping magnetization components in upper Hole M0027A indicate that the sediments may have been chemically remagnetized during one or several events. Mineral magnetic investigations reveal that the magnetization is carried by the ferrimagnetic iron-sulfide greigite (Fe3S4), possibly with traces of titano-magnetite. We find that several changes in polarity coincide with variations in magnetic mineral grain size and/or concentration. We interpret these variations as different stages of greigite growth, which were triggered by changes in pore-water chemistry and/or upward migration of methane.

Occurrence of greigite in the Pliocene sediments of Lake Qinghai, China, and its paleoenvironmental and paleomagnetic implications

Lake Qinghai in North China is the largest interior plateau lake in Central Asia and is climatically sensitive. An almost continuous 626 m long sediment core was drilled in an infilled part of the southern lake basin of Lake Qinghai. The magnetic susceptibility record reveals the presence of two distinct peaks within an interval of fine-grained lacustrine sediments of Lower Pliocene age. We selected a depth interval of approximately 40 m spanning the magnetic susceptibility peaks for detailed rock magnetic and geochemical analyses in order to identify the magnetic mineralogy responsible and to assess its possible paleoenvironmental and paleomagnetic implications. Rock magnetic, X-ray diffraction analysis, scanning electron microscopy (SEM), and energy dispersive analysis of X-ray (EDAX) analyses reveal that the main magnetic carrier is greigite (Fe3S4). The greigite is of early diagenetic origin and formed in an inerval of high lake level and inferred relatively warm, humid climate. The greigite-enriched zones are separated by an interval of relatively high total sulfur and organic carbon content, and we infer that in the adjacent greigite-bearing zones, the lower concentrations of sulfur and organic carbon, and high levels of reactive iron, arrested the process of pyritization resulting in the preservation of the greigite on a time scale of several million years. The greigite zones contain narrow intervals of normally magnetized sediments which may be previously unrecognized cryptochrons within the Gilbert Chron, or alternatively they may reflect the continued formation of greigite long after the age of deposition of the surrounding sediment matrix.

The use of the 'Kono perpendicular applied field method' in microwave palaeointensity experiments

For a complete understanding of the Earth’s magnetic field behaviour it is necessary to have information regarding variations in its strength as well as directional changes. In order to reduce the lengthy experimental time for palaeointensity experiments and potentially reduce the possibility of mineralogical alteration Kono suggested a single heating perpendicular applied field variant of the most widely used double heating palaeointensity method (the Thellier method). Whilst in the past this method has not been generally used, due mainly to technical difficulties, it is ideal for use with the microwave palaeointensity technique. Reproducing the amount of microwave energy absorbed by a sample is not as simple as reproducing the temperature in an oven so a palaeointensity method that only has a single application of microwave power at each step is an advantage. One of the uncertainties in the original Kono method lies in getting the natural remanent magnetisation of the sample aligned perpendicular to the applied laboratory field. With the microwave systems this uncertainty has been essentially removed. The microwave variant of the perpendicular applied field method which incorporates checks for perpendicularity, as well as the usual checks for alteration and non ideal behaviour is routinely applied as it is quick and easy to use, avoids any potential power reproducibility issues and provides a comparison with results obtained through other methods.

Coseismic magnetization of fault pseudotachylytes: 1. Thermal demagnetization experiments

Fault pseudotachylytes form by quenching of silicate liquids produced through coseismic frictional melting. Here we show that in natural pseudotachylytes the main carrier of magnetic remanence blocked in during cooling of the frictional melt is fine-grained magnetite. This confirms previous studies on friction melt experiments. Stoichiometric magnetite, produced during earthquakes by the breakdown of ferromagnesian silicates, records the ambient magnetic field during seismic slip. We find that most fault pseudotachylytes exposed in the Santa Rosa Mountains, southern California, a classic pseudotachylyte locality, acquired their natural remanent magnetization (NRM) upon cooling of the frictional melt through the range of magnetization blocking temperatures of the magnetite grains and this primarily constitutes a thermal remanent magnetization. NRM intensities typical of most pseudotachylyte veins range from 1 to 60·10−4 Am2/kg. A few specimens, however, contain magnetizations significantly higher than that caused by the Earths field as well as magnetization directions that are highly variable over short distances. Other magnetization processes, possibly related to coseismic electric currents, may be involved during the seismogenic process to control NRM acquisition.

Thermoremanent Behavior in Synthetic Samples Containing Natural Oxyexsolved Titanomagnetite

Understanding Earths geodynamo provides us a window into the evolution of the Earths core, which requires accurate data about how its strength varies with time. Classic Thellier-style paleointensity experiments assume that studied specimens contain only noninteracting single-domain (SD) magnetic particles. Interacting grains commonly occur in volcanic rocks but are generally assumed to behave like equivalently sized SD grains. Multidomain (MD) grains can cause erroneous PI estimates or cause Thellier-style experiments to fail entirely. Synthetic specimens containing naturally formed magnetite with MD grains and oxyexsolved titanomagnetite (closely packed SD grains) were subjected to various partial thermoremanent magnetization (pTRM) experiments, which tested nonideal behavior as a function of pTRM acquisition and loss inequality, thermal history, and repeated heating steps. For all grain sizes and domain states, pTRMc (heating and cooling in a nonzero field) gives larger values, compared to pTRMb (heating in a zero field and cooling in a nonzero field), by ∼5.5%. Oxyexsolved grains appear prone to the same concave-up, nonideal Arai plots commonly observed in MD specimens, which also has potential implications for the multiple-specimen, domain-state corrected protocol. Repeated heatings cause additive deviations from ideality with relatively small impacts on Arai plot curvature for both grain types. Experiments with higher initial demagnetization temperatures had lower curvatures, with the most SD-like behavior occurring in the uppermost 20 ° C of the (un)blocking temperature range. Samples containing mixtures of magnetic domain sizes are likely to behave less ideally at lower temperatures but become more ideal with increasing temperature as the nonideal grains unblock.

Short-Term Magnetic Field Variations From the Post-depositional Remanence of Lake Sediments

Palaeomagnetic records obtained from lake sediments provide important constraints on geomagnetic field behaviour. Secular variation recorded in sediments is used in global geomagnetic field models, particularly over longer timescales when archaeomagnetic data are sparse. In addition, by matching distinctive secular variation features, lake sediment palaeomagnetic records have proven useful for dating sediments on various time scales. If there is a delay between deposition of the sediment and acquisition of magnetic remanence (usually described as a post-depositional remanent magnetisation, pDRM) the magnetic signal is smoothed and offset in time. This so-called lock-in masks short-term field variations that are of key importance both for geomagnetic field reconstructions and in dating applications. Understanding the nature of lock-in is crucial if such models are to describe correctly the evolution of the field and for making meaningful correlations among records. An accurate age-depth model, accounting for changes in sedimentation rate, is a further prerequisite if high fidelity palaeomagnetic records are to be recovered. Here we present a new method, which takes advantage of the stratigraphic information of sedimentary data and existing geomagnetic field models, to account for both of these unknowns. We apply the new method to two sedimentary records from lakes Kalksjon and Gyltigesjon where 14C wiggle-match dating floating varve chronologies provide an independent test of the method. By using a reference magnetic field model built from thermoremanent magnetisation data, we are able to demonstrate clearly the effect of post-depositional lock-in and obtain an age-depth model consistent with other dating methods. The method has the potential to improve the resolution of sedimentary records of environmental proxies and to increase the fidelity of geomagnetic field models. Furthermore, it is an important step toward fully explaining the acquisition of post-depositional remanence, which is presently poorly understood.