arXiv subjects

Mireille Perrin

Publications and source records attributed to Mireille Perrin.

3 recordsLinked to original sources

arXiv2019

Cooling rate effect on thermoremanent magnetization in archaeological baked clays: an experimental study on modern bricks

The influence of cooling rate on the intensity of thermoremanent magnetization (TRM) and the necessity to correct archaeo/palaeointensities for this effect have long been recognized. However the reliability of the correction is still questioned. We studied 35 bricks baked in two modern kilns (SK and BK) in known experimental conditions and with measurements of the direction and intensity of the geomagnetic field at the site. The smallest kiln (SK, 0.2 m 3) cooled in around 12 hours and the biggest kiln (BK, 8 m 3) in around 40 hours. Thermomagnetic, hysteresis and backfield curves indicated that the main magnetic carriers were Ti-poor titanomagnetites and Tipoortitanohematites. The fraction of the TRM carried by Ti-poor titanohematites is the maindifference between the two sets of bricks. This fraction is around 5-10% in bricks from BK kilnand up to 40% in those from SK kiln. Intensities of the Earth's magnetic field were determinedusing the original Thellier-Thellier protocol with correction of TRM anisotropy. The averageintensities overestimate the expected field intensity by 5% (SK) and 6% (BK). This resultemphasizes the necessity of the cooling rate correction. In order to have a detailed evaluation ofthe cooling rate effect, we used several slow cooling rates: 0.8, 0.4, 0.2 and 0.1{\textdegree}C/min. Thecorrection factors obtained with the 0.8{\textdegree}C/min cooling ranged between -2% and 21% and wereproportional to the TRM fraction carried by Ti-poor titanohematite. The higher proportion ofthese grains in bricks from SK kiln led to an overestimate of the correction factor and anunderestimate of the intensity by 7%. However, the expected intensity is recovered whentemperature steps higher than 580{\textdegree}C (i.e. in the range of Ti-poor titanohematites unblockingtemperatures) were excluded from the calculation of archaeointensity and cooling rate correction.In the case of the BK kiln bricks, for which Ti-poor titanohematites does not contributesignificantly to the TRM, all tested cooling rates give average intensities close to the expectedvalue. Incorrectly estimating the duration of the archaeological cooling has therefore a lowimpact on the accuracy of the archaeointensity data on these kinds of material.

cond-mat.mtrl-sci↗

arXiv2003

Revisiting the Jurassic Geomagnetic Reversal recorded in the Lesotho Basalt (Southern Africa)

We carried out a detailed and continuous paleomagnetic sampling of the reversed to normal geomagnetic transition recorded by some 60 consecutive flow units near the base of the Lesotho Basalt (183  1 Ma). After alternating field or thermal cleaning the directions of remanence are generally well clustered within flow units. In contrast, the thermal instability of the samples did not allow to obtain reliable paleointensity determinations. The geomagnetic transition is incompletely recorded due to a gap in volcanic activity attested both by eolian deposits and a large angular distance between the field directions of the flows underlying or overlying these deposits. The transition path is noticeably different from that reported in the pioneer work of van Zijl et al. (1962). The most transitional Virtual Geomagnetic Poles are observed after the volcanic hiatus. Once continents are replaced in their relative position 180 Ma ago, the post-hiatus VGP cluster over Russia. However, two successive rebounds from that cluster are found, with VGP reaching repeatedly Eastern Asia coast. Thus, the VGP path is not narrowly constrained in paleolongitude. The decrease in intensity of magnetization as the field deviates from the normal or reversed direction suggests that the decrease in field magnitude during the reversal reached 80-90%. We conclude that although the reversal is of a dipole of much weaker moment than that which existed on average during Cenozoic time, the characteristics of the reversing geodynamo seem to be basically similar.

physics.geo-ph↗

arXiv2003

Absolute palaeointensity of Oligocene (28-30 Ma) lava flows from the Kerguelen Archipelago (southern Indian Ocean)

We report palaeointensity estimates obtained from three Oligocene volcanic sections from the Kerguelen Archipelago (Mont des Ruches, Mont des Tempetes, and Mont Rabouillere). Of 402 available samples, 102 were suitable for a palaeofield strength determination after a preliminary selection, among which 49 provide a reliable estimate. Application of strict a posteriori criteria make us confident about the quality of the 12 new mean-flow determinations, which are the first reliable data available for the Kerguelen Archipelago. The Virtual Dipole Moments (VDM) calculated for these flows vary from 2.78 to 9.47 10e22 Am2 with an arithmetic mean value of 6.15+-2.1 10e22 Am2. Compilation of these results with a selection of the 2002 updated IAGA palaeointensity database lead to a higher (5.4+-2.3 10e22 Am2) Oligocene mean VDM than previously reported, identical to the 5.5+-2.4 10e22 Am2 mean VDM obtained for the 0.3-5 Ma time window. However, these Kerguelen palaeointensity estimates represent half of the reliable Oligocene determinations and thus a bias toward higher values. Nonetheless, the new estimates reported here strengthen the conclusion that the recent geomagnetic field strength is anomalously high compared to that older than 0.3 Ma.

physics.geo-ph↗