Brunhes' research revisited: Magnetization of volcanic flows and baked clays

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Brunhes’ research revisited: Magnetization of volcanic

flows and baked clays

Carlo Laj, Catherine Kissel, Herve Guillou

To cite this version:

Carlo Laj, Catherine Kissel, Herve Guillou. Brunhes’ research revisited: Magnetization of volcanic

flows and baked clays. Eos, Transactions American Geophysical Union, American Geophysical Union

(AGU), 2002, 83 (35), pp.381. �10.1029/2002EO000277�. �hal-03242540�

Eos,Vol. 83, No. 3 5 , 2 7 August 2002

1900 1920 1940 1960 Year

1980 2000

Fig. 4. Bottom panel shows the velocity of the NMP Triangles denote the years of the surveys. Circles denote the average velocity between the surveys. Top panel shows first dif­ ferences of the annual mean values of horizon­ tal intensity (H) at Resolute Bay. Straight-line segments have been drawn to emphasize the abrupt change in slope. Vertical lines indicate the times of well-established magnetic jerks.

logistical support and the Foly-Arctic Project for financial support for all three campaigns. We also thank the following individuals and organizations for their support and encour­ agement: C. J. Allegre, J. PGratier, and V Cour-tillot from the French Ministry of Education, Research and Technology; the Bureau de Recherches Geologiques et Minieres; B. Mesqui of Groupe Taitbout; and S. Husainy of the French Embassy in Canada. The following individuals participated in one or more of the campaigns: Mary Angatookaluk, Andrei Bee-lenki, Nicolas Simon, Baruch Spiro, and Mari-annick Orgeval.Thanks also to Anne Papillault and Jean-Francois Dars of CNRS-Images for making a wonderful documentary of the 1998 campaign that showed the joys and sorrows of Arctic field work.

Authors

L. R. Newitt, M. Mandea, L.A. McKee,

and J.-J. Orgeval

For additional information, contact L. R. Newitt, Natural Resources Canada, Geological Survey of Canada, 7 Observatory Crescent, Ottawa, Canada, Kl A 0Y3; E-mail: newitt@ geolab.nrcan.gc.ca.

References

Haines, G.V, Spherical c a p h a r m o n i c analysis,

J. Geophys. Res., 90,2583-2591,1985.

Haines, G.V, a n d L. R. Newitt,The C a n a d i a n Geomag­ netic Reference Field 1 9 9 5 , i Geomag. Geoelectr., 4 9 , 3 1 7 - 3 3 6 , 1 9 9 7 .

Huy, M. L., M. Alexandrescu, G. Hulot, a n d

J.-L. Le Mouel, On the characteristics of successive g e o m a g n e t i c jerks, Earth, Planets, Space, 50, 7 2 3 - 7 3 2 , 1 9 9 8 .

Jackson,A.,A. R. Jonkers, a n d M. R.Walker, Four c e n ­ turies of g e o m a g n e t i c s e c u l a r variation from his­ torical records, Phil. Trans. R. Soc. bond., A, 358, 9 5 7 - 9 9 0 , 2 0 0 0 .

Macmillan,S.,and J.M.Quinn,The derivation of World Magnetic Model 2 0 0 0 , Brit. Geol. Surv.Tech.

Rep.WM/00/17R,2000.

Mandea, M.,S.et al., International g e o m a g n e t i c reference field - 2 0 0 0 , P/zys/cs of the Earth and

Planetary Interiors, 120,39-42,2000.

Newitt, L. R., and C. E. Barton,The position of the north magnetic dip pole in 1 9 9 4 , i Geomag.

Geoelectr.,48,221-232,1996.

Newitt, L. R., a n d E. Dawson, Secular variation in North A m e r i c a during historical times, Geophys.

J.RAstr.Soc, 7 5 , 2 7 7 - 2 8 9 , 1 9 8 4 .

Ross, J. C , On the position of the north m a g n e t i c

po\e, Phil. Trans. R. Soc. Lond., 124,47-51,1834.

Brunhes' Research Revisited:

Magnetization of Volcanic Flows

and Baked Clays

PAGES 381,386-387

On 21 April 1906, at a meeting of the Societe Francaise de Physique, Bernard Brunhes (Fig­ ure 1) addressed his colleagues on an impor­ tant discovery concerning the magnetization of several formations in the volcanic Massif Central.The talk was published in the Journal

de Physique in November of the same year

under the title "Recherches sur la direction de l'aimantation des roches volcaniques" [Brun­

hes, 1906].

This title only partially reflects the contents of his paper, which focused mostly on the magnetization of baked clays underlying lava flows, rather than on the flows themselves. This subject had stimulated Brunhes' interest since his appointment as professor at the Universite de Clermont-Ferrand and director of the Observatoire du Puy de Dome on 1 November 1900. His first investigations on the subject, with Pierre David, date back to 1901— 1902, almost exactly 100 years ago.

Brunhes had been very impressed with the work of Giuseppe Folgheraiter [Folgheraiter, 1894],who had shown that ordinary bricks and pottery carried a particularly strong and stable remanent magnetization that is aligned

with the direction of the magnetic field in which they were baked. Following Melloni, Folgheraiter also suggested that lava flows from Vesuvius would have recorded the direc­ tion of the magnetic field in which they cooled. Neither of these pioneers, however, performed systematic measurements on sam­ ples taken at different points in a flow to ascertain whether the direction of magnetiza­ tion was uniform or well-defined.

Brunhes was the first to realize that, given the exceptional stability of the magnetization of baked clays,"if the direction of the magneti­ zation in beds of natural baked clay is well defined and different from that of the present field, then we are entitled to admit that the direction of magnetization is that of the geo­ magnetic field existing when the volcanic flow baked the clays" ("Si Ton a, dans les bancs d'argile naturelle, une direction d'aimantation bien definie et qui differe de la direction du champ terrestre actuel,on est done fonde a admettre que la direction d'aimantation est bien celle du champ terrestre a l'epoque oil la coulee volcanique a transforme en brique l'argile") [Brunhes, 1906].With this idea in mind,he sampled sev­ eral sites of baked clay. His first sites were at

Fig. 1. This photograph of Bernard Brunhes was taken in 1899 (enlarged from a group photograph).

Beaumont, where the clays were baked by lavas of the Montjoli volcano. He then sampled near the village of Boissejour, where the clays are beneath the lavas of the Gravenoire volcano, and then at Royat. Finally, following an indica­ tion by Mr.M.Vinayan engineer from the Ponts et Chaussees (road works

4 5 ° 0 2 N

2 ° 5 0 E 3 ° 0 5 E

4 4 ° 5 5 N

Fig. 2. The location ofPontfarein is shown in the neighborhood of Saint-Flour.

24CPC W-21c N R M , , » 2.74 10' Aim Dec ^ \4T inc * -783^ 53§oC M A D - L4° W / U p E/Dowa baked clays 34CTC • W / U p S

Jfa

/

f/g. 3. Paleomagnetic results from baked clays and the lava flow at Pontfarein are shown, (a)

Seen here is a representative orthogonal projection of thermal demagnetization of sample PF-27c from the baked clays (an enlarged view of the high temperature sector is presented on the right hand side). NRM0 is the intensity of the NRM before demagnetization (b) Stereographic

projection of data from baked clay samples (open symbols). The mean values (full symbols), and confidence angles (a9) were obtained from Fisher statistics. Triangles refer to Brunhes'

results, circles to authors'present study; (c) same as (a) for sample PF-04ffrom the lava flow; (d) same as (b) for samples from the lava flow obtained with thermal demagnetization.

Pontfarein some 20 km west of the town of Saint-Flour (Figure 2).

Magnetization of the Baked Clays at Pontfarein

Brunhes was particularly interested in this outcrop because it extends for over 100 m along the road, which allows for sampling at different points. He took several oriented samples, which were partially cut and oriented before detachment from the outcrop.The final size was obtained by re-cutting in the labora­ tory. Due to the softness of the clays, Brunhes had some difficulties obtaining perfectly shaped and well-oriented samples. We realize how lucky we were in 2001 to be able to col­ lect over 15 oriented cores with our electric drill equipped with a water-cooled diamond edge cutting bit in just a few hours!

In his 1906 article, Brunhes reported the direction of magnetization of three samples from Pontfarein; respectively, (D = 174°; I = -74°) (D = 135°; I = -78°) (D = 135°]; I = -69°); and noted that the directions obtained from several other samples were in between these extreme values. Brunhes remarked that, given the low value of the horizontal component of magnetization, the declination was very diffi­ cult to estimate. His final conclusion was "that at a certain moment of the Miocene epoch, in the neighborhood of Saint-Flour, the North Pole was directed upward: it was the South Pole which was the closest to central France." ("qu'en un moment de l'epoque miocene, aux environs de Saint-Flour, le pole Nord etait dirige vers le haut: c'est le pole Sud de la Terre qui etait le plus voisin de la France cen-trale.") This is the first suggestion ever reported that the Earth's magnetic field had reversed itself in the geological past.

Brunhes' results reflect the direction of the natural remanent magnetization (NRM), as demagnetization techniques were unknown at this time. We have, of course, demagnetized our samples to retrieve the direction of the characteristic remanent magnetization. In Fig­ ure 3a, we report an orthogonal projection of thermal demagnetization data obtained for one of our baked clay samples.There is hardly any secondary component, and the estimated mean direction [Fisher, 1953] is virtually iden­ tical to that given by Brunhes (Figure 3b).

Directions of Magnetization of the Baked Clays and Lava Flow Agree

Having completed the measurements on the baked clays, Brunhes studied the magnetiza­ tion of the overlying lava flow. He wrote,

"If the direction of the magnetization is the same in the basalt which has baked the clays

[as in the clays themselves], this will be pre­ cious experimental evidence in support of the hypothesis that it is in cooling and hardening that the lava became magnetized, and that this magnetization has not been modified since." (Si la direction de l'aimantation est la meme dans la couche de basalte qui a cuit l'argile, ce sera une verification experimentale precieuse de l'hypothese suivant laquelle

Eos,Vol. 83, No. 35, 27 August 2002

Sample K Weight 4 0 ^ * 4 0A r* A g e A g e

(wt.%) molten ( (lfj-12 ( ± 2 a ) M a mean value ® moies/g)

Pontfarein 0 . 6 7 9 1 0 . 0 0 7 0 . 9 7 5 1 0 2 0 . 4 8 4 7 . 1 2 0 6 . 0 4 ± 0 . 1 2

1 . 0 5 3 7 8 2 5 . 2 9 8 7 . 4 1 8 6 . 2 9 + 0 . 1 3 6 . 1 6 + 0 . 0 8

Table 2. Paleointensity results from the baked clays at Pontfarein,

Sample f g q r Fe ± l a CO (fiT) P F b r 4 0.9 0.9 39 0.997 120-640 36.2 ± 0.7 P F br 5 0.6 0.8 9 0.991 120-520 34.4 ± 1.6 P F b r 6 0.5 0.8 13 0.994 20-580 28.5 ± 0.9 P F b r 7 0.4 0.8 8 0.992 120-580 35.3 ± 1.4 P F b r 8 0.4 0.8 7 0.989 120-580 36.4 ± 1.7 P F b r 9 0.9 0.7 31 0.996 20-610 36.7 ± 0.8 PFbr 10 0J 0.6 21 0.998 120-520 46.3 ± 0.9 P F br 11 0.7 0.9 30 0.997 20-610 33.5 ± 0 . 6 P F br 12 0.5 0.9 27 0.998 20-610 29.9 ± 0.4 M e a n ( N = 8 ) = 33.8 ± 3.0 juT

Sample PFbr 10 has not been considered in the final average. Table 1. K-Ar age o f the Pontfarein flow.

c'est en se refroidissant et se solidifiant que la lave s'est aimantee, pour ne plus se modifier une fois refroidie").

Brunhes was able to obtain only two oriented samples from the lava flow at Pontfarein, which he called "un maigre resultat"—a disap­ pointing result. Sampling was indeed difficult: Brunhes described how some of the chisels he used to obtain his samples broke against the very hard basalt. In addition, and most unfortunately, one of the samples had a verti­ cal magnetization about 20 times larger than the other. Brunhes recognized that it had been sampled at a "point distinct," that is, a point struck by lightning—a phenomenon already recognized by Folgheraiter [1894]. But the sec­ ond sample yielded the paleomagnetic direc­ tion he was hoping to find: (D = 154°; I = -76°), in perfect agreement with the results from the baked clays.The first baked contact test in the history of paleomagnetism was positive!

We ourselves also sampled the lava flow at different spots over a distance of about 25 m. Again, we realized how much simpler it was for us to obtain oriented samples with a gaso­ line-powered drill, compared to Brunhes' chisels!

Our samples were demagnetized using ther­ mal methods. Depending on the precise sam­ pling location, some samples showed the presence of a large overprint. In two cases, this overprint was large enough to yield very shallow inclinations of the NRM—one slightly

positive, one slightly negative—while other samples had virtually no overprint. The lava sample studied by Brunhes presumably was taken at one of these latter locations. During thermal demagnetization, all of the samples lost 50% or more of their magnetization at low temperatures (< 240°C) (Figure 3c). The mean obtained by Fisher statistics from all samples is shown in Figure 3d. As was the case for the baked clays, the mean is indistin­ guishable from Brunhes' result.

Can we say that Brunhes was fortunate to sample the flow at a spot with, by chance, a negligible secondary component of magneti­ zation? Maybe he was fortunate, but certainly he was well aware of the possibility that the natural magnetization might not reflect the direction of the ancient geomagnetic field because of superimposed secondary compo­ nents.

Folgheraiter had already suggested, as quoted by Brunhes, that "in general one must consider an effect more or less intense of the slow and prolonged action of the Earth's magnetic field on the rocks already cold and solid" ("en general il faut attibuer un effet plus ou moins intense a Taction lente et prolongee de Tin-duction terrestre sur les roches deja solidifiees et refroidies"). [Brunhes, 1906]. Brunhes argued that "if certain rocks have a coercive force sufficiently small—which does not appear to be the case for fired clays—for the prolonged action of the Earth's magnetic

field in the last few centuries to induce at room temperature an additional magnetiza­ tion in the rocks distorting the initial magneti­ zation, this action could only result in transforming rocks with a up-seeking North Pole into rocks with down-seeking North Pole, and not the other way around" ("Si certaines roches ont une force coercitive assez faible -ce qui ne paraTt nullement etre le cas de la brique - pour que,comme l'indique Folgheraiter, Taction prolongee du champ terrestre leur ait communique a froid une cer-taine aimantation qui ait pu alterer Taimanta-tion primitive, cette acTaimanta-tion de la terre depuis plusieurs siecles n'aurait pu agir que pour transformer des roches ayant leur pole Nord en haut en roches ayant leur pole Nord en bas, et nullement pour operer la transforma­ tion inverse.")[Brunhes, 1906].In other words, the action of the modern normal field can only mask, not create, pristine reverse magne­ tizations. This observation is a precursor to the modern reversal test for magnetic stability!

A Precise K/Ar Age Determination for the Lava Flow at Pontfarein

Brunhes supposed that the Pontfarein flow had the same age as the St. Flour "Planeze" basalts, estimated to be Miocene in age. The Pontfarein flow is the 8th and top flow of a sequence composing the basal part of the Saint-Flour western "Planeze" basalts.There is no age determination from the flow itself, but an age of about 4.8 Ma can be estimated from conventional whole rock K/Ar dates on lavas of the same sequence outcropping elsewhere. (For example, see http://www.brgm.fr/volcan, then pontfarin).This would place the Pontfarein flow in the early Pliocene rather than in the Miocene, as Brunhes thought.

We decided to scrutinize this point more thoroughly because of frequent and systematic errors in K/Ar whole-rock age determinations of young mafic volcanic rocks [e.g., Guillou et

al, 2001]. A 5-cm diameter, 20-cm-long sample

drilled at a location chosen for its freshness was crushed and sieved to a 0.25-0.125 mm size fraction. K and Ar were then measured on the microcrystalline ground mass, after removal of phenocrysts and xenocrysts using heavy liquids of appropriate density, and mag­ netic separation. K content was determined with Inductive Coupled Plasma Atomic Emis­ sion Spectrometry (ICP-AES) by J. Cotten at the University of Brest. Ar content was deter­ mined from 1 g of ground mass using the unspiked technique developed by Cassignol and Gillot [1982],the new calibration protocol of Charbit et al. [1998],and the decay and abundance constants of Steiger and Jager

[1977].The results from two replicate measurements (Table 1) yield a date of 6.16 ± 0.08 Ma, suggesting that the Pontfarein flow probably cooled in reverse chron C3An.lr

[Berggren et al, 1995], in the late Miocene.

The successful baked contact test indicates that the age of the magnetization acquisition of the baked clays is that of the lava flow.

a)

NRM/NRMo

PF

-04C

F = 3 6 . 2 ± 0.7

jtT

f = 0.92

= 0 . 8 6

q = 3 8 . 9

r = 0.997

550°C

5 8 0 ° C

610°C

6 4 0 ° C

1 TRM/NRMo

Fig. 4. (a) This stereographic projection shows the progressive alignment of the NRM of lava sam­ ples along the laboratory field (indicated by the square symbols) during the Thellier experiments; (b) NRM/TRM (Arai) plot for sample PF-04c from the baked clay (dots refer to double heatings, triangles to pTRM checks).

Intensity of the Geomagnetic Field at Pontfarein

To complete the description of the Earth's magnetic field at Brunhes' original site, we carried out paleointensity measurements using the Thellier-Thellier technique in air for the baked clays, and in argon for the basaltic

samples. For the lava flow (Figure 4a), the direction of the remanent magnetization of the samples moved toward the applied field in the very first low temperature steps of the experiment. No reliable determination could therefore be obtained. On the other hand, very precise and reliable results were obtained from the baked clay; the paleointen­

sity diagrams obtained from the nine samples studied are all perfectly linear over a large fraction of the initial magnetization (f factors are all > 0.4 with 2 > 0.9, the best example being shown in Figure 4b). All sam­ ples yielded very consistent results around the mean (F= 33.8 ± 3.0 uT;Table 2), after removal of a single outlier.

Averaging the directions from the flow and the baked clays and using the intensity results from the baked clays, we can conclude that "en un moment de l'epoque Miocene, precise-ment pendant le chron C3An.lr,aux environs de Saint-Flour" the geomagnetic vector field was defined by (D = 149.8 ± 10.7°; I = -77.6 ± 2.3°; F = 33.8 ± 3.0 uT).

Acknowledgments

We thank Brigitte Colineau for providing the photograph of Bernard Brunhes, her grandfa­ ther. We thank Jo Cotten for measuring the K content of the lava sample, Jim Channell for polishing the English of the manuscript, and Renato Funiciello for providing copies of Fol-gheraiter's publications.The Eos GP editor, John Geissman, and the technical staff of Eos are also thanked for their constructive remarks.

Authors

Carlo Laj, Catherine Kissel, and Herve Guillou

For additional information, contact Carlo Laj, Laboratoire des Sciences du Climat et de l'En-vironnement, Unite Mixte CEA-CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France; E-mail: laj@lsce.cnrs-gif.fr

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