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

Hill, Mimi J.; Gratton, M. N.; Shaw, John

doi:10.1016/s0012-821x(02)00534-4

Cited by 27 publications

(28 citation statements)

References 26 publications

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“…Hill et al, 2002bHill et al, , 2005Hill et al, , 2006Gratton et al, 2005a). Both methods have advantages and disadvantages and consistency within the methods provides added confidence in the palaeointensity results.…”

Section: The Microwave 'Kono Perpendicular Applied Field Method'mentioning

confidence: 99%

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

Hill

Shaw

2007

Earth Planet Sp

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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.

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Section: The Microwave 'Kono Perpendicular Applied Field Method'mentioning

confidence: 99%

“…Casas et al, 2005) to geological (e.g. Hill et al, 2002b;Thomas et al, 2004;Halls et al, 2004;Gratton et al, 2005b;Hill et al, 2005).…”

Section: Microwave Palaeointensity Systemsmentioning

confidence: 99%

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

Hill

Shaw

2007

Earth Planet Sp

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“…The most recent technology utilizes microwaves to directly excite magnons within the magnetic material of a sample (Walton et al 1992, 1993, 1996) so that, in microwave Thellier experiments, the sample is heated to much lower temperatures than it would be in a conventional oven. This significantly reduces sample alteration and has been demonstrated to yield success rates equivalent to, or better than, conventional Thellier analyses on historic and ancient igneous rocks (Hill & Shaw 1999, 2000; Hill et al 2002). The most impressive indication of the potential of the new microwave Thellier technique to date recorded a 58 per cent success rate from sister samples of Tertiary basaltic material that yielded no conventional Thellier results (Hill et al 2002).…”

Section: The Global Palaeointensity Recordmentioning

confidence: 99%

Analysis of long-term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics

Biggin¹,

Thomas²

2003

Geophysical Journal International

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SUMMARY The 1167 published cooling unit (CU) palaeointensity estimates contained in the 400–10 Ma portion of the PINT global database were rigorously filtered according to accurate age determinations, palaeodirectional reliability, recognition of polarity and the method of palaeointensity acquisition. The remaining 865 estimates (group 1) were further filtered to ensure self‐consistency, reducing the data set to 425 estimates (group 2). Group 1 and 2 data were clustered into temporally and/or spatially distinct rock suites (RS) enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation (PSV). The record was segmented according to the distribution of the data, rather than using arbitrary time windows, to ensure quasi‐consistent behaviour within each segment. Differences between these segments clearly indicate that a significant long‐timescale (107 and 108 yr) variation of the mean geomagnetic poloidal field intensity (GPFI) occurred during the 400–10 Ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field. Both the mean dipole moment and its standard deviation appear to be a function of the range of values each CU may adopt at one particular time. This range is itself controlled by the variation of the maximum limit of dipole moment, while the value of the minimum limit remains relatively constant. Tentative support is provided for the recent suggestion that PSV may have been reduced during the Cretaceous normal superchron (CNS), though more data are needed in the range 120–60 Ma to confirm this. No conclusive evidence was found to support the suggestion that the GPFI record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt pTRM checks. Indeed, offsets caused by unreliable data in well‐represented parts of the record are likely to be random and cancel one another out. When GPFI variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency (RF), it is not possible to confirm whether the two parameters are anticorrelated, decoupled or related in some more complex way. However, it is clear that GPFI and RF are definitely not positively correlated as has been previously suggested. The present database documents sharp increases in GPFI around the onset times of the two recognized superchrons, itself implying an anticorrelation. The implications, for geodynamo and mantle modelling, of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly. A generic geodynamic model is proposed to explain the relationship between observed long‐term changes in GPFI and global geodynamic processes. The model predicts that changes in GPFI result from a chain of geodynamic processes extending from crust to core, beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection. Supercontinents...

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“…Six samples underwent investigation; three samples of the 1960 Hawaiian lava flow (Hill & Shaw 2000) and three samples from different flows of Tertiary Australian lava (Hill et al 2002). Standard sized samples (25 mm diameter) were heated slowly in the oven to 700°C and held at this temperature for 30 min with a known field applied to impart a full TRM.…”

Section: Demagnetisation Experimentsmentioning

confidence: 99%

“…The inability to isolate the primary component of magnetisation satisfactorily has also been found to sometimes occur when using samples containing a natural TRM (e.g. Hill et al 2002).…”

Section: Demagnetisation Experimentsmentioning

confidence: 99%

A comparison of thermal and microwave palaeomagnetic techniques using lava containing laboratory induced remanence

Hill¹,

Gratton²,

Shaw³

2002

Geophysical Journal International

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Summary A series of laboratory experiments has been carried out using lava from Hawaii and Australia containing laboratory induced remanence to compare thermal and microwave palaeomagnetic techniques. Three samples of Hawaiian lava containing a known laboratory induced thermal remanence (TRM) were subjected to microwave palaeointensity experiments (using the Liverpool 8.2 GHz microwave system). Conventional modified Thellier palaeointensity experiments were carried out on the same three lava samples, this time containing a known microwave induced thermoremanent magnetisation (TMRM). Both palaeointensity techniques produced the correct intensity value, demonstrating the equivalence of TRM and TMRM. No indications of alteration during the experiment were evident during the microwave experiments, whereas alteration was pronounced after heating to 350°C in the Thellier experiments demonstrating that alteration during microwave palaeointensity analysis is reduced when compared to thermal techniques. In a second set of experiments, six samples (three Hawaiian and three Australian lava samples) were given a laboratory induced full TRM and a secondary perpendicular partial TRM (pTRM) (at lower temperatures). Sister samples were then demagnetised using thermal and microwave methods. Analysis of the orthogonal vector plots (OVPs) revealed little difference in separating the two components for five out of the six samples. For one sample, microwave demagnetisation did not clearly distinguish between the two components with the primary component not being resolved until 74 per cent of the moment had been demagnetised compared to just 4 per cent with thermal demagnetisation. For this one sample, it appears that the microwave unblocking spectra is not equivalent to the thermal unblocking spectra, probably due to a combination of microwave demagnetisation processes occurring. Comparison of the intensity of magnetisation plots during demagnetisation shows that the relationship between temperature and microwave power appears more logarithmic than linear. This study demonstrates the validity of the microwave palaeointensity technique and, as long as the primary component of magnetisation can be isolated from multicomponent remanence, then the microwave palaeointensity technique can be applied.

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

Cited by 27 publications

References 26 publications

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

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

Analysis of long-term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics

A comparison of thermal and microwave palaeomagnetic techniques using lava containing laboratory induced remanence

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