Age of the Laschamp paleomagnetic excursion revisited

444

Earth and Planetary Science Letters, 42 (1979) 444-450

© Elsevier Scientific Publishing Company, Amsterdam

Printed in The Netherlands [61

AGE OF THE LASCHAMP PALEOMAGNETIC EXCURSION REVISITED P.Y. GILLOT, J. LABEYR1E, C. LAJ, G. V A L L A D A S , G. GUI~RIN, G. POUPEAU and G. DELIBRIAS Centre des Faibles Radioactivitds, Laboratoire mixte CNRS-CEA, Domaine du CNRS, 91190 Gif-sur- Yvette {France)

Received June 19, 1978 Revised vei'sior/received November 30, 1978

The reverse paleomagnetism of the lava flows of Laschamp and Olby, already discovered by Bonhommet and Babkine, is confirmed. The ages of these flows, measured by 14C, thermoluminescence and K-Ar dating are respectively 36,000 + 4000 and 42,000 + 5000 years. VGP's comparison of the "Laschamp event" with those of the 30,000-year Lake Mungo excursion does not show evidence of coincidence between these two events.

1. Introduction In 1967 Bonhommet and Babkine [1] discovered two lava flows, each with reversed remanent magnetism, at Laschamp and Olby in the recent volcanic province of the Chafne des Puys. Bonhommet and Z~ihringer [2] argued for the existence of reversed magnetic event (now called the Laschamp event), the end of which would have been between 20,000 and 8000 years ago. The limit o f 8000 years was given by 14C dating of a paleosoil underneath a domitic ash fall partially covering the Puy de Laschamp [3]. The limit of 20,00C years was deduced from a series of K-Ar measurements obtained on total rocks of the Laschamp and Olby flows. Later, Bonhommet [4] considering this x4C age and the youngest K-Ar age previously determined attributed a probable age of 12,500 + 4000 years to this magnetic event. He also remarked on the coincidence of this age and the age of the "end" o f the last Glacial period. Failure to find indisputable evidence of this reversed magnetic event in the same age range in other regions o f the world in spite of numerous investigations [5,6] raised our interest for new investigations o f both paleomagnetism and ages o f the Laschamp and Olby lava flows. The ages were ob-

tained using K-Ar and thermohiminescence (TL) on volcanic rocks and radiocarbon as well as TE alone on baked materials.

2. Sampling Fig. 1 gives a schematic outline of the Chafne des Puys, which is a north-south alignment of volcanoes, most of them of the Strombolian type. The Olby and Laschamp lava flows spread on the western and eastern sides respectively o f the axis o f the Chafne. The volcanic activity in this province seems to have started at least about 100,000 years ago (K-Ar dating of a basaltic flow near Egaules [7]) and extends up to 7600 years B.P. [8] (a basaltic lava flow issued from Puy de la Vache, a few kilometers south of Puy de Laschamp). The basaltic lapillis of the Strombolian cones have been emitted about 8000 years B.P., and also the trachytic ashfall covering these cones within 3 kin around the Play de D6me. The Laschamp flow is a relatively small outcrop o f trachyandesite (K-mugearite) mostly compact, finegrained and homogeneous, appearing as a cliff 200 m long and 2 - 7 m high. The trachytic ashes o f 8000 years cover its uppermost part. Our research for materials for 14C dating under

445 this flow was unsuccessful. A large block of about 25 kg of unaltered lava, extracted using a power shovel, was used for K-At and TL dating. This block was sampled inside of the flow at about 1 m from the surface. In addition, a granitic inclusion of about 10 cm included in the compact lava was sampled for TL dating. The Olby flow is a labradoritic basalt with olivine. It is the lowest o f a series o f overlapping lava flows coming down from the Puy de Barme. At the sampling site, the Olby flow (~3 m thick) appears as a cliff o f very homogeneous and compact lava along the northern bank o f the Sioule river. There, the flow lies on a paleosoil of clay and small quartzite, schist and quartz pebbles at least two meters thick. Downwards from the base o f the flow, one finds: (a) a dark brown and muddy level, very rich in organic material, about 5 cm thick, (b) a red layer of baked clays and pebbles, 20 cm thick, and (c) an apparently unmodified C soil (disaggregated schisteous mother rock). About 5 kg o f material was taken from layer (a) for 14C dating, as well as quartz pebbles for TL dating from layers (a) and (b). The distance between each of the pebbles and the b o t t o m o f the lava flow was measured to within an accuracy o f a few centimeters. Freshly broken blocks of about 25 kg were sampled about 1 m above the b o t t o m of the flow for K-Ar and TL dating.

3. Magnetic properties Samples for the study o f magnetic properties were drilled using an 1 inch (2.5 cm) standard diamond corer at five different sites: two on the Olby flow, three on the Laschamp flow. Samples were stepwise demagnetized in an alternating field up to about 300 Oe peak to peak, and measured after each demagnetization using a Digico spinner magnetometer. The results obtained are given in Table 1, which show that the five sites all have a reverse NRM. These results are similar to those already reported by Bonhommet, also reported in Table 1. The possibility o f self-reversal, which had been considered initially, was discarded on the basis o f thermal demagnetization, chemico-mineralogical analysis by neutron-activation and SEM-EDAX and M6ssbauer experiments, the results of which are given elsewhere [9].

4. Age results and comments 4.1. K-Ar ages The argon measurements were made on wholerock samples ground down to 0 . 5 - 2 ram, using about 7 g for each analysis. They were melted without pre-

TABLE 1 Declination (D) and inclination (/) of the remanent magnetic moment of Laschamp and Olby flows after AF cleaning a

(1) Laschamp: Oratoire (lava flow)

N

D (°)

12

184.2 (241.3)

I (°) -58.9 (-67.8)

K

a9s (°)

VGP

72

4.7

333.2°W, 83.4°S

(2) Laschamp: Sentier (probably an outcrop of the Laschamp flow, undated)

4

191

-80

57

9.3

191.2°W, 64°S

(3) Laschamp: Puy (near summit, undated)

11

248 (247)

-76.4 (-65)

63

5.3

220°W, 49°S

(4) OIby: Pont des Eaux (lava flow)

6

250 (231 b

-61.1 -68 b)

134

4.9

249°W, 41°S

(5) Olby: Oj (lava flow)

5

243

60.7

7.6

25 3° W, 45 ° S

77.5

N = number of independent cores per site (average number of samples 2 per core); VGP = virtual geomagnetic pole. a Bonhommet's [4] values are in brackets. b Mean value of the Olby flow.

446 TABLE 2 Potassium-argon data Sample

Experience number

Weight (g)

4 o Ar total (1012 at/g)

4 o Ar rad. (1010 at/g)

Age a (103 years)

M 663 M 664 M 665 M 706 M 791

6.710 7.479 7.621 7.430 8.140

4.696 4.751 5.967 4.533 5.375

8.3 7.1 8.1 8.3 8.5

43.9 38 43 44 45.3

+-+ +-+ +-

M 736 M 738 M 770 M 788 M 789

7.298 8.237 8.183 8.204 8.442

4.221 4.072 9.043 5.150 7.803

8.5 8.7 9.2 10.1 7.7

48.2 48.5 52 56 44

+- 7 +- 6.9 -+ 15.3 +- 8.7 -+ 15

Laschamp N 7421 K = 1.77 -+ 0.03%

7.4 b 7.6 9.5 7.0 8.6

Olby 30 F K = 1.67 -+ 0.03%

a he = 0.585 x 10 -1° yr - 1 , h 3= 4.72 x 10 -10 yr -1, 4°K/K = 1.19 x 10 -4 mole/mole. b The (+_) figures are calculated from the uncertainty of the correction of the atmospheric contamination based on 36Ar measurements. h e a t i n g to limit i s o t o p i c d i s c r i m i n a t i o n effects. The

analyses, t h e m a j o r u n c e r t a i n t y is due to 36At mea-

c o r r e c t i o n for a t m o s p h e r i c c o n t a m i n a t i o n and the

s u r e m e n t s , o w i n g to the p r e p o n d e r a n c e o f t h e argon

dosage o f the radiogenic 4°Ar were m a d e a c c o r d i n g to

c o n t a m i n a t i o n in such y o u n g rocks, giving an error o f

t h e original m e t h o d for K-Ar dating o f very y o u n g

a b o u t 20% o n the age value ( e r r o r r e p o r t e d for each analysis in Table 2). The p o t a s s i u m c o n t e n t s w e r e

rocks previously described [ 1 0 , 1 1 ] . In these argon TABLE 3 Therlnoluminescence data on quartz Mean radius (ram)

Lasehamp (1) quartz, granit enclave of Laschamp Olby (5) paleosoil pebbles

2.3 2.6 1.0 ~8 ~8 19 23 6

Weight (mg)

140 200 10 a

Paleodose (krad)

b

5.33 5.33 5.14 4.66 5.98

Annual dose rate (rad/yr)

b

c c c c c

0.145 0.145 0.133 0.129 0.147

d d d d d

Predicted errors in 103 years

TL age (103 years)

random

systematic

3 3.4 2.7

2.5

35.4 b 37.1 b 32.5 b

4

37 37 39 38 41

3 3 3 2 4

a Mean weight of a group of small quartz grains. Measurements made on the whole group (about 150 mg). b We have observed a non-linearity of the growth of the TL as a function of the dose. This effect, more pronounced near the surface of the crystal (3 + 3' irradiation) than at the center (3'), has been considered in the computer calculation of the ages. c Deduced from a second heating after artificial irradiation (see Valladas [17]). d Annual dose rate at the center of the grains.

447 measured by atomic absorption spectrophotometry, after hydrofluoric acid treatment. These values are accurate to +2%. The results o f the K-Ar measurements are reported in Table 2, they indicate an age of 43,000 + 5000 years for the Lascharap flow, and 50,000 + 7500 years for the Olby flow. Investigation of the possible origin of the 4°Ar measured leads us to consider these ages as maximum values.

TABLE 4 Thermoluminescence ages on plagioclase

Laschamp Olby

Paleodose (krad)

Estimated annual dose rate (rad/yr)

Age (103 years)

18.1 _+1.2 22.5 + 1.0

0.54 • 0.08 0.51 + 0.08

33.5 +-5 44.1 -+ 6.5

4.2. Thermoluminescence ages on quartz Laschamp. The Laschamp flow was dated by TL using quartz of the granitic inclusion. The annual irradiation dose rate for each crystal was determined from the distribution of the radioelements (U, Th, K) in the granite and in the surrounding lava using a Monte Carlo calculation [ 12]. The measurements of radioelements were done by 7-3' spectrometry. Several potassium analyses were made by atomic absorption spectrometry and by SEM-EDAX X-rays fluorescence. The ages obtained on different size quartz crystals (200 rag, 140 mg and a batch of several crystals each about 10 mg) gave an average value of 35,000 -+ 3000 years. Olby. The TL age of the Olby flow was measured on five quartz pebbles ( 1 - 5 cm in size) from the baked paleosoil. The annual dose rate was determined from the distribution of the radioelements in the lava and in the upper (organic layer) and the lower (red clay) levels of the paleosoil. The five quartz pebbles gave a mean value (Table 3) of 38,000 • 6000 years. This value was obtained by assuming a paleosoil constantly saturated with water, about 10% of the total weight (a 50% water distribution would give an age smaller by

10%).

within 5% above 530°C for Laschamp and above 580°C for Olby. The values are reported in Table 4 as well as the annual dose rate as deduced from U, Th, K concentrations. We have assumed that the uranium and thorium are uniformly distributed in the rock. The errors quoted include in both cases the systematic errors corresponding to this assumption and we estimate that these cannot exceed +15%. By the usual calculation [15], we obtain an age o f 33,500 (-+5000) years for Laschamp and 44,100 (e6500) years for Olby.

4.4. 14Cage The dark brown upper paleosoil at O1by, about 5 cm under the lava, contains a very high proportion of organic carbon (1.6% of carbon in the dryed soil). Qualitative analysis of the minerals shows grains of quartz, pyroxenes, feldspaths and also illite, kaolinite and montmorillonite. There was no evidence for charcoal in spite of the fact that this paleosoil could be a remnant o f ancient meadow or a shallow marsh with organic mud, baked at several hundred degrees Celsius by the lava flow, as suggested by the red color of the underlying clay. The upper layer remained dark probably because the heating by lava developed reducing

4.3. Thermoluminescence ages of plagioclases TABLE 5 Both Laschamp and Olby total rock were ground and the feldspar fraction 80 125/lm was submitted to high-temperature TL measurements, between 500 and 650°C. The plagioclase TL emission was measured with an original technique [12,13] in order to get rid of the anomalous fading below 450°C [14]. In this high-temperature range, we observe a plateau: the TL-deduced paleodose reaches a constant value

14C age of paleosoil under Olby flow a Fraction

pH of extraction

Age (103 years)

Humic acids (A 1) Humic acids (A2)

9.8 11.5-12

~>26.7 ~33.2

Humin

-

a Half-life = 5730 years.

~>36.0

448

I

I

I

I

I

I

2058

2053

G

A

.o°°-o.o.=

B

oo

D

45046

m 'o

m

:( w

°

Olby

J

Q' =

\

Barmes Laschamps

tQ~

O@ t

•

\< : *OO,o

J

°

45o42

Scale

1 :50,000

//

I

..t

\ I

t

~--'-"- I

I

I

Fig. 1. Map of the central part of the Chainc des Puys, Massif Central, France (from Camus et al. [ 19]). Legend: A = volcanic cones ("Puys") of basaltic lapilli; age: 5 dated, all about 8000 years B.P. [8] ; B = Approximate limit o f the trachyandesitic ashfall; age: 8100 years B.P. [3,8]; C = lava flows (mostly basaltic) (contours from successive flows); age: from > 100,000 [18] to 7650 years B.P. [8]; D = the lava flows with a reverse paleomagnetism are in black; age: about 36,000 (Laschamp) and 42,000 years (Olby) (this work); E = Montmeyre flow, reverse paleomagnetism [4]; age: 2.45 m.y. [18].

449 agents preventing the oxidation of iron to hematite. Extraction of pyrolysed material from this dark layer was attempted by organic solvent (C6H6,methylal and chloroform) but no coloration of these solvents was found. Although the paleosoil was protected by more than 2 m of massive lava and no rootlets were observed anywhere, it was, however, wetted by about 10% water, which implies a connection with the surface and therefore suggests a possible contamination by modern soluble organic material. Usually, such a contamination appears as humic acids, and is removed by dissolution in alkaline solutions. 100 g of the bulk soil were washed in 0.1N NHaOH to test the eventual removal of recent (free) humic acids, and no coloration was observed after one hour in this basic solution. Nevertheless, after a more complete extraction, two groups of very small amounts ofhumic acids were successively extracted from 150 g of the dark layer. The first group, extracted at pH 9.8, gave an age A l /> 26,700 years (Gif-4563); the second group, extracted at pH 11.5-12 gave an age A2 ~> 33,200 years (Gif-4564). Due to the small amounts of carbon

Site

recovered in these solutions, a special CO2 microcounter was used for the measurements and counting of very long duration were done (29,000 minutes for .42). The given values correspond to the limit ofdatation depending, in each case, on the amount of available organic carbon and on the duration of the measurement. The remnant part of the dark layer, after these two alkaline extractions, contained an important residual amount of organic material, designated as "humin". This part is considered as giving the most probable age of the Olby organic layer. The age obtained is ~>36,000 years (Table 5).

5. Conclusions (1) The age values obtained are between 33,50@ and 50,000 years (Fig. 2). The most probably age is 35,000 -+ 4000 years for Laschamp and 42,000 -+ 5000 for Olby. The age of this magnetic event appears to be much greater than the value estimated by Bonhommet (12,500) and Bonhommet and

ResuLts

Method

NNZ

K. A r

LASCHAHP FLOW

TL a n PLAGIOCLASES TL o n QUARTZ

K. Ar OLBY R0W

TLon PLAGI 0CLASES TL o n QUARTZ

lO

20

Fig. 2. Synoptic results.

3o

~

so

ogt (yrlo 3)

450

Z~_hringer (between 8000 and 20,000). * (2) Our measurements confirm that both the Laschamp and Olby flows have a reversed remanent magnetism, which is in agreement with the previous results of Bonhommet. We suggest that the flows of Laschamp and Olby were emitted during the same reverse magnetic excursion, despite of their slightly different apparent ages. Moreover, independent studies [9] show that the possibility of self-reversal is to be excluded: indeed the reversed magnetism of the two flows reflects a reversal of the geomagnetic field. Another lava flow at Montmeyre, in the vicinity of Laschamp and Olby (Fig. 1) with reverse remanent magnetism [4] has been dated by K-Ar [18] at 2.45 (+-0.05) m.y. Therefore, the Montmeyre flow appears to belong to the beginning of the so-called reverse Matuyama Period and not to the "Laschamp event". (3) The comparison of the virtual geomagnetic pole (VGP) calculated for the Laschamp and Olby flows with those given for the 30,000-year Lake Mungo excursion [16] does not show an evident coincidence. In both cases, the dispersion of the VGP's is quite great and can be interpreted in two ways: either the lava flows (or the fireplaces) have recorded discrete peculiar values of a sequence of directions of the magnetic reversal, as suggested for Lake Mungo [16], and thus the VGP's are not comparable, or the nondipole field was quite important during the transition and the VGP concept looses its meaning. We have no argument to choose between these two possibilities, and therefore no argument to identify the Lake Mungo and Laschamp events in spite of the age similarity.

3

4

5

6

7 8

9

10

11

12

13

14

References 1 N. Bonhommet and J. Babkine, Sur la pr6sence d'aimantations invers~es dans la Chafne des Puys, C.R. Acad. Sci. Paris, S~r. B, 264 (1967) 92. 2 N. Bonhommet and J. Z~ihringer, Paleomagnetism and

* While preparing the revised version of this paper, we note the recent publication of another Laschamp and Olby lava flow dating by Hall and York [20] which give weighed ages of 45,200 + 2500 years (K-Ar) and 48,400 +- 7900 years (39 Ar_4OAr). These ages are in excellent agreement with our K-At age determinations.

15

16

17 18 19

20

potassium argon age determinations of the Laschamp geomagnetic polarity event, Earth Planet. Sci. Lett. 6 (1969) 43. R, Broussc, G. Delibrias, J. Labeyrie and A. Rudel, Datation par la m6thode du carbone 14 d'une 6ruption domitique de la Chafne des Puys, C.R. Acad. Sci. Paris 263 (1966) 1812. N. Bonhommet, Sur la direction d'aimantation des laves de la Chafne des Puys et le comportement du champ terrestre en France au cours de l'6v~nement du Laschamp, Th~se d'Etat, Strasbourg (1972). C.R. Denham and A. Cox, Evidence that the Laschamp polarity event did not occur 13 0 0 0 - 3 0 400 years ago, Earth Planet. Sci. Lett. 13 (1971) 181. K.L. Verosub and S.K. Banerjee, Geomagnetic excursions and their paleomagnetic record, Rev. Geophys. Space Phys. 15 (1977) 145 and references therein. C. Cassignol and P.Y. Gillot, unpublished data. R. Brousse, G. Delibrias, J. Labeyrie and A. Rudel, E16ments de chronologie des ~ruptions de la Chafne des Puys, Bull. Soc. G~ol. Fr. 11 (1969) 770. D. Nordemann, C. Laj and J. Danon, Mineralogic magnetic properties of the Laschamp lava (Chaine des Puys, France), presented to the Joint IAGA-IAMAP Assembly, Seatlle, September 1977 (text in preparation); J. Whitney, H.P. Johnson, Saul Levi, and B.W. Evans, Investigations of some magnetic and mineralogic properties of the Laschamp and Olby flows, France, Quatern. Res. 1 (1971) 511. C. Cassignol, Y. Cornette, B. David and P.Y. Gillot, Technologic Potassium-Argon, Rapp. CEA No. R 4908 (1978). C. Cassignol, B. David and P.Y. Gillot, Contribution au dosage de l'Argon darts l'6chantillon de glauconite G.L.O., Geostandard Newslett. 1 (1977) 105. G. Valladas, Datation des roches par la thermoluminescence; application h une coul6c volcanique, Bull. Assoc. Fr. Etude Quatern. (in press). G. Valladas and H. Valladas, High temperature thermoluminescence, 18th Int. Symp. Archeometry and Archeological Prospection, Bonn, 1978. A. Wintle, Anomalous fading of thermolumineseence in mineral samples, Nature 245 (1973) 143. M.J. Aitken and S.J. Fleming, Thermoluminescent dosimetry in archeological dating, in: Topics in Radiation Dosimetry, F. Attix, ed. (Academic Press, 1972). M. Barbetti and M. McElhinny, Evidence of a geomagnetic excursion 30 000 years B.P., Nature 239 (1972) 327. G. Valladas, presented at the Symp. Thermoluminescence, Oxford, July, 1978. C. Cassignol and P.Y. Gillot, unpublished result. G. Camus et al., Volcanologie de la Chafne des Puys, Carte au 1/25 000 (Institut G6ographique National, Paris, 1975). C.M. Hall and D. York, K-Ar and 4°Ar-39Ar ages of the Laschamp geomagnetic polarity reversal, Nature 274 (1978) 462.