Mixed domain states of magnetic grains in basalts and implications for palaeomagnetism

Fifty Thellier palaeointensity determinations were made from the Great Whin Sill (GWS) of northern England and the Midland Valley Sill (MVS) of central Scotland, which form the major part of a late Carboniferous quartz-dolerite complex. Palaeodirectional analysis confirms that the reversed polarity characteristic remanence carried by samples from both sills is consistent with the Permo-Carboniferous reversed superchron. Palaeomagnetic results suggest that the two sills may not be exactly contemporaneous. The MVS was intruded rapidly some time during the emplacement of the GWS, which was itself emplaced over a considerable time period. The MVS may well represent the northernmost expression of the instrusive activity. A mean palaeointensity of 22.9f2.6 pT was calculated for the Great Whin Sill, with a corresponding VDM value of 5.9 x loz2 A m2. The palaeointensity value calculated from the Midland Valley Sill was much lower, 13.0+0.5 pT, with a corresponding VDM value of 3.3 x 10" A m2. Detailed rock magnetic analyses and stepwise thermal demagnetization studies reveal that the remanence carrier in both sills is magnetite of mixed domain state. Multidomain grains are less important in the Midland Valley Sill and thus greater confidence is attached to palaeointensity results from this sill. VDM values from this and other studies of the Permo-Carboniferous reversed superchron (P-CRS) indicate that the dipole strength showed similar variations to that witnessed for the past 5 Ma. There is no conclusive support in favour of either the weak or strong field models that have been suggested for superchrons. More palaeointensity results are needed from the termination of the P-CRS and the late Permian/early Triassic in order to understand how the dipole field evolved during the P-CRS and its relationship to the so-called Mesozoic dipole low.

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“…Such behaviour is commonly witnessed in ancient igneous rocks (e.g. PrCvot et al 1990Perrin, PrCvot & Mankinen 1991Thomas 1993).…”

Section: Methodsmentioning

confidence: 96%

Palaeointensity results from the Permo-Carboniferous (Kiaman) reversed superchron: the Great Whin and Midland Valley sills of the northern United Kingdom

Thomas

Rolph

Shaw

1995

Geophysical Journal International

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“…Thus the B1 part, despite being more susceptible to thermal alteration, holds the highest fraction of SD and PSD particles and can be readily demagnetized using the microwave system. However, it has to be remembered that only a small fraction of the magnetic grains are the NRM carriers (Radhakrishnamurty 1989).…”

Section: Archaeointensity Analysismentioning

confidence: 99%

High-quality microwave archaeointensity determinations from an early 18th century ad English brick kiln

Casas

Shaw

Gich

et al. 2005

Geophysical Journal International

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SUMMARY Samples of bricks from the Dogmersfield Park post‐medieval brick kiln (Hampshire, UK), have been used to determine the intensity of the geomagnetic field during the early 18th century ad. Extensive magnetic mineralogical analyses have been carried out revealing weathered zones and multidomain dominated areas within the bricks. Likewise, statistical data treatment has been used to identify mineralogical heterogeneities. Once the most suitable parts for the study were selected, archaeointensity determinations were carried out applying Coe's double‐step variant of the Thellier method. The 14 GHz microwave system used in this study was developed at Liverpool University and the thermal demagnetizer is a commercial instrument. The former uses microwaves instead of heat to demagnetize the sample, the latter uses heat and was used for comparison. Data from the microwave system were of very high quality and uncertainties assigned to single measurements are smaller than the intrinsic variability of the archaeointensity stored within the bricks. Both pTRM and pTRM tail checks rarely fail, and even pass when the sample is almost fully demagnetized. Microwave intensities group around a central value (∼53 μT), slightly higher than the present‐day field intensity, and agree well with previous studies and geomagnetic field models. Thermal results exhibit higher and more scattered values; some samples show evidence of laboratory thermal alteration. The different behaviour of samples when subjected to microwave and thermal experiments appears to be linked with the magnetic mineralogy (high‐Ti titanomagnetite): easy microwave demagnetization without alteration is achieved whereas sample alteration is almost unavoidable during thermal demagnetization. High‐Ti titanomagnetite seems to be the ideal mineralogy for microwave experiments instead of pure magnetite, preferred in conventional Thellier experiments. The last part of this paper discusses the usefulness of conventional alteration tests applied to microwave experiments. It appears that for this technique negative alteration checks can originate from poor reproducibility of the absorbed microwave power.

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“…A temperature of 400°C is found to be sufficient to recover the characteristic component is these specimens. From the above laboratory demagnetizations and other magnetic properties it is inferred that titanium rich titanomagnetite is the magnetic carrier in these rocks as revealed by granulometric studies by Deutsch et al (1980) and Radhakrishnamurty (1990). As a result of laboratory demagnetization grouping of the remanent magnetic directions of the sites improved largely.…”

Section: Introductionmentioning

confidence: 84%

“…Independent studies on these traps have resulted in almost identical remanent magnetic vectors and on rockmagnetic count Radhakrishnamurty (1970Radhakrishnamurty ( ,1990 opined that these rocks possess only one component and hence need no laboratory demagnetization to unfold their characteristic remanent magnetism. A summary of all available results is given in Table 2 and mean direction and VGP for the Rajmahal Traps is evaluated by averaging out all these results.…”

Section: Introductionmentioning

confidence: 99%

Palaeomagnetism of the Rajmahal Traps of India: Implication to the Reversal in the Cretaceous Normal Superchron.

Rao

1996

J. geomagn. geoelec

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The Upper Gondwana Rajmahal Traps in northeast India were studied over the last fourdecades and these are referred to as normally magnetized rocks. Our study of these rocks from thirty-five sites has brought into light a palaeomagnetic record revealing the existence of a reverse magnetization event also. In the light ofthe 40Ar/39Ar age data of the Rajmahal Traps and the geomagnetic polarity time scale during the middle Cretaceous period, the observed magnetic reversal in the Rajmahal Traps has been interpreted to constitute a geomagnetic reversal in the Cretaceous Normal quiet magnetic interval (Cretaceous Normal Superchron, K-N). Similar short magnetic reversals have also been reported at other periods in this superchron from some other places.

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Mixed domain states of magnetic grains in basalts and implications for palaeomagnetism

Cited by 10 publications

References 26 publications

Palaeointensity results from the Permo-Carboniferous (Kiaman) reversed superchron: the Great Whin and Midland Valley sills of the northern United Kingdom

Palaeointensity results from the Permo-Carboniferous (Kiaman) reversed superchron: the Great Whin and Midland Valley sills of the northern United Kingdom

High-quality microwave archaeointensity determinations from an early 18th century ad English brick kiln

Palaeomagnetism of the Rajmahal Traps of India: Implication to the Reversal in the Cretaceous Normal Superchron.

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