Cooling-rate dependence of thermoremanent magnetisation

Fox, Judith M.; Aitken, M. J.

doi:10.1038/283462a0

Cited by 180 publications

(94 citation statements)

References 4 publications

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“…This effect consists of a progressive increase of TRM intensity with the decrease of the cooling rate. For laboratory cooling times this effect is significant in potteries (e.g., Fox and Aitken, 1980;Chauvin et al, 2000) and occasionally for rock samples as well (Morales et al, 2006). In order to quantify the cooling rate effect, we carried out three in-field heatings after the paleointensity measurements: (a) fast-cooling (∼1 h), (b) slow-cooling (∼20 h) and (c) fast-cooling (∼1 h) again.…”

Section: Methodsmentioning

confidence: 99%

First archeointensity results from Portuguese potteries (1550-1750 AD)

et al. 2009

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Geomagnetic field variations at archeomagnetic timescales can be obtained from well-dated heated structures and archeological potsherds. Here, we present the first archeointensity results obtained on Portuguese ceramics (1550 to 1750 AD) collected at Brazilian archeological sites. The results are compared to those obtained from Western Europe and currently available geomagnetic field models. Continuous thermomagnetic and IRM acquisitions curves indicate that Ti-poor titanomagnetite is responsible for the remanence in these ceramic fragments. Five fragments (24 samples) out of twelve analyzed yielded reliable intensity estimates. The row archeointensity data were corrected for TRM anisotropy and cooling rate effect. The mean dipole moments are obtained for three different age intervals: 1550±30 AD, 1600±30 AD and 1750±50 AD. Mean intensities vary from 37.9±4.2 μT to 54.8±7.6 μT in agreement with the previously reported data for 1550 AD and 1750 AD. Relatively weaker, but still highly dispersed, values were obtained for 1600 AD ceramics.

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Section: Methodsmentioning

confidence: 99%

First archeointensity results from Portuguese potteries (1550-1750 AD)

et al. 2009

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“…Another factor affecting the Ba is the cooling rate (Fox and Aitken, 1980;Dodson and McClelland-Brown, 1980). Our laboratory cooling time (-1 hour) during the Thellier run is two orders of magnitude shorter than the cooling time occuring in antiquity (-2 days for pottery and r.7 days for bricks; Leino and Pesonen, 1994) and hence there will normally be an overestimation of Ba unless corrections are done (Pesonen and Halls, 1983).…”

Section: Cooling Ratementioning

confidence: 99%

Archaeomagnetic Intensity in Finland during the Last 6400 Years: Evidence for a Latitude-Dependent Nondipole Field at -AD 500.

Pesonen

Leino

Nevanlinna

1995

J. geomagn. geoelec

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Archaeomagnetic field intensity in Finland (Lat. 61°N, Long. 22°E) has been determined for the past 6400 years with the Thellier technique of bricks, potsherds and baked clays. The normalized archaeointensity curve for Finland, based on 23 mean values, shows an increase from 4360 BC to the maximum at AD 500, after which it decreases to the present value. The high intensity at AD 500 as found in Finland, but not in Bulgaria and Japan, is not a consequence of the applied Thellier technique since we are able to reproduce the known field intensities in the laboratory, and some of the dated bricks yield values which are in broad agreement with Nurmijarvi Magnetic Observatory data. We also show that variations in grain size of the magnetic carriers, cooling rate or magnetic refraction are unlikely to cause systematic errors in intensity larger than a few percent. Based on results of self-made bricks we show that the archaeointensity, however, depends on the angle between the laboratory and ancient field directions. If these are not aligned errors up to ±10% in archaeointensity can occur and are probably caused by anisotropy of remanence or by other factors not yet understood.The high intensity at AD 500 in Finland can be modelled by a radial nondipole field source at the core-mantle boundary producing enhanced field at higher latitudes (Finland) and relatively weaker fields at lower latitudes (e.g. Bulgaria and Japan). Extrapolation of the present field (IGRF 1990) back in time shows similar field behaviour.

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“…This should correlate to the same amount of bias in paleointensity estimates. This prediction was directly tested in control experiments by changing the cooling time between a few minutes to tens of hours, and the prediction was generally supported, but with a large sample-to-sample variation of a factor of 3-5 (Fox and Aitken, 1980;Chauvin et al, 2000;Genevey and Gallet, 2002;Leonhardt et al, 2006;Ferk et al, 2010;Yu, 2011). For plutonic rocks, the theory predicts that the paleointensity estimate will overestimate the true field by ∼30-60%, and it has become common to use these values as correction factors (e.g., Tarduno et al, 2007;Selkin et al, 2008;Donadini et al, 2011).…”

Section: Introductionmentioning

confidence: 99%

Paleointensity estimates from oceanic gabbros: Effects of hydrothermal alteration and cooling rate

Usui

2013

Earth Planet Sp

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Gabbros recovered by previous ocean drillings were investigated in paleointensity and rock magnetic experiments. The young ages of the samples (ca. 0.78-1.3 Ma) enable a direct comparison between the plutonic paleointensity and volcanic data. Microscopic observations revealed two kinds of magnetite: needle-shaped exsolution in plagioclase and aggregate associated with the hydrothermal alteration of olivine. In Shaw paleointensity experiments, some samples revealed reasonable estimates, while some others showed an anomalously low ratio of natural remanent magnetization (NRM) versus thermoremanent magnetization (TRM). First-order reversal curve (FORC) diagrams indicated that the reasonable NRM/TRM were from non-interacting single domain magnetite exsolved in plagioclase, while the anomalously low NRM/TRM were from secondary magnetite associated with olivine. From the paleointensity results, the mean virtual axial dipole moment (VADM) was calculated to be 8.2 ± 2.1 10 22 Am 2 . Volcanic records in the PINT database for 0.78-1.3 Ma revealed a mean virtual dipole moment (VDM) of 6.3 ± 0.73 10 22 Am 2 . The difference between them is consistent with the theoretical prediction of the cooling rate effect on paleointensity. These results indicate that oceanic gabbros are reliable paleointensity recorders.

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Cooling-rate dependence of thermoremanent magnetisation

Cited by 180 publications

References 4 publications

First archeointensity results from Portuguese potteries (1550-1750 AD)

First archeointensity results from Portuguese potteries (1550-1750 AD)

Archaeomagnetic Intensity in Finland during the Last 6400 Years: Evidence for a Latitude-Dependent Nondipole Field at -AD 500.

Paleointensity estimates from oceanic gabbros: Effects of hydrothermal alteration and cooling rate

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