GP53C-1371
A new Automated Microwave Demagnetiser/Remagnetiser System for Palaeointensity Studies.
A new automated microwave system has been constructed for palaeointensity studies. The system operates at 14.2GHz and can demagnetise or remagnetise samples up to 5mm in diameter in any direction using programmable field coils. Magnetic moments are measured using a new small bore magnetometer (Tristan Technologies) and the whole system is controlled using an interactive computer interface. The new system has been tested on geological and archaeological samples and results compare favourably with conventional heating techniques. The instrument was funded by the UK NERC. http://www.liv.ac.uk/geomagnetism
GP53C-1372
Determining Archaeointensity From Anisotropic Ceramics
Archaeological ceramics provide a convenient source of information regarding the past magnetic field intensity, but they are often strongly anisotropic. The remanent magnetism is not parallel to the ancient field but lies towards the preferred direction of magnetization. If the laboratory field is applied parallel to the NRM, the TRM gained will not be in the direction of the NRM, and will have a different magnitude than the TRM that would have been gained if the laboratory field had been applied in the same direction as the ancient field. This can lead to erroneous values of archaeointensity. A correction can be made by determining the tensor of anisotropy of thermoremanence but if the laboratory field can always be applied in a direction that produces a TRM parallel to the NRM, no correction should be needed. At the University of Liverpool, the microwave system used for archaeointensity measurement employs an automated sample carriage and 3 axis programmable applied field coils that allow the applied field and the resulting sample moment to be determined with high precision, so that by careful control of the applied field direction we can produce a TRM nearly parallel to the NRM. If this is done no anisotropy correction is needed.
GP53C-1373
Development of the microwave LTD-DHT Shaw method for absolute paleointensity determination
In the last decade, the microwave demagnetization and remagnetization techniques (Walton et al., 1992, 1993) have been successfully incorporated into the Thellier-type method for absolute paleointensity determination (e.g. Shaw et al., 1996; Hill and Shaw, 1999). In contrast to the conventional heating by electric ovens for laboratory acquisition of thermal remanent magnetization (TRM), laboratory alteration is significantly suppressed when microwaves are used to demagnetize/remagnetize the sample. As for the Shaw-type method, similar efforts should be made and thus we have tried to incorporate the microwave techniques into the LTD-DHT Shaw method (Tsunakawa and Shaw, 1994; Yamamoto et al., 2003). A series of preliminary experiments have been conducted on (1) four samples with artificial microwave TRM (TmRM) given in a field of 15.0 microtesla, (2) 10 samples with artificial TRM in 25.0 microtesla and (3) 10 samples from the Kilauea 1970 lava. During the experiment, laboratory TmRMs (not TRMs) are imparted by the applications of 80 W of microwave power for 5-10 seconds using the 14 GHz microwave demagnetizing-remagnetizing system. The `microwave LTD-DHT Shaw method' results in 15.2 +/- 0.6 microtesla (N=4), 24.6 +/- 1.9 microtesla (N=5) and 39.1 +/- 1.0 microtesla (N=4) for the experiments of (1), (2) and (3), respectively. Although the result of (3) is statistically different from the expected field of 35.8 microtesla (DGRF 1970), there is a reasonable explanation for this. We conclude that the microwave LTD-DHT Shaw method has been successful.
GP53C-1374
A Comparison of Palaeointensity Results Obtained Using the Microwave Technique and LTD- DHT Shaw Method on Basalts From the Auckland Volcanic Field, New Zealand
Samples from five monogenetic volcanoes in the Quaternary Auckland volcanic field that record the same geomagnetic excursion have been investigated using the microwave palaeointensity technique. Both the perpendicular applied field method and Coe version of the Thellier technique were carried out, with sister sub samples being run in many cases to check for consistency. The experiments were performed using the Liverpool microwave systems operating at 14 GHz. A total of 58 samples gave results ranging from 1 to 24 μT with the mean palaeointensity per volcano ranging from 7 to 17 μT. Mochizuki et al (2006 Phys. Earth Planet. Int., 154, 168-179) previously studied samples from three of the five volcanoes using the LTD-DHT Shaw palaeointensity method. Their results from 17 samples showed greater consistency both between and within volcanoes. Possible reasons for the differences between results from the microwave method and LTD-DHT Shaw method will be discussed. Despite the differences, both overall mean palaeointensities are statistically indistinguishable and show that the geomagnetic dipole moment was reduced to about 2 x 1022 Am2 for this Auckland excursion.
GP53C-1375
Paleointensities From Cenozoic Lava Fflows Using the Multi-specimen Parallel Differential pTRM Method
Samples from tertiary and quaternary lava flows have been studied using the multi-specimen parallel differential pTRM method to obtain paleointensities. These samples come from the Transmexican Volcanic Belt, the San Quintin volcanic field in Baja California, and from the 1960 lava flow in Hawaii. In some cases results are also available from other PI methods, thus allowing to compare the different methods. We will report on the results obtained and the experiences applying the new method.
GP53C-1376
Evaluation of the Multispecimen Parallel Differential pTRM Method: A Test on Historical Lavas From Iceland and Mexico
Studies of the palaeointensity are of crucial importance as they provide information about the origin of the earth's magnetic field and the geodynamic processes within the earth's core, and are a central paleomagnetic research topic. The most commonly accepted technique for determining the palaeointensity is based on several modifications of the method of Thellier and Thellier (Thellier and Thellier, 1959), where the natural remanent magnetisation of a rock is being incrementally replaced with a replicated remanence in the laboratory through multiple heating cycles. However, this technique suffers from numerous drawbacks, both theoretically and experimentally, which has ultimately lead to high failure rates and thus, hampered the acquisition of larger bodies of palaeointensity data in the past. Dekkers and Boehnel (2006) proposed a new multispecimen approach (multispecimen parallel differential pTRM method), based on their study of historical samples from the Trans Mexcian Volcanic Belt. This new method appears to be more reliable and accurate than the Thellier-Thellier methods, because it is less dependent on magnetic domain state, requires no high-temperature, alteration-inducing heating steps and also is significantly faster to measure. They propose that their parallel differential pTRM method has the potential to supersede standard Thellier-Thellier- type techniques. So far this assumption is based on a limited number of samples from Mexico. In our presentation we will report the results from a more comprehensive and robust test of their multispecimen method on 12 historical lavas with well-defined rock magnetic characteristics and ages from 1729 to 2001, from both Mexico and Iceland.