- Email: [email protected]
Intensity of the geomagnetic field from recent Italian lavas using a new paleointensity method
Earth and Planetary Science Letters, 27 (1975) 314-320
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
INTENSITY OF THE GEOMAGNETIC F I E L D FROM RECENT ITALIAN LAVAS USING A NEW PALEOINTENSITY METHOD* J.C. TANGUY Laboratoire de GkomagnOtisme du Parc Saint-Maur, St. Maur-des-Foss~s (France)
Revised version received July 7, 1975
A new method has been tested on Etna historic lavas for determining the geomagnetic field intensity (F) using the thermoremanent magnetization of volcanic rocks. The procedure involves a number of very short duration heatings above the Curie point, to produce successive laboratory TRM. Thus, it is possible to check the variations in the TRMacquiring capacity of the samples with the time of heating (t). The curve J = f(t) is then extrapolated towards t = 0, leading to a virtual value of TRM without any laboratory heating, i.e., without the changes that currently occur when the lavas are heated to produce the TRM. Using such virtual values of TRM, satisfactory results of F had been derived from the majority of the samples studied. These results are consistent with Thellier's archeomagnetic data: they show a nearly constant intensity of the geomagnetic field during the last three centuries and, further back into the past, a significant increase of this.
1. Introduction Intensity values (F) of the ancient geomagnetic field are obtained from rocks or baked clays by comparing the natural thermoremanent magnetization (NRM) and a laboratory thermoremanent magnetization (TRM) produced in a known field. The major problem in such a method lies in the changes that occur when samples are heated to produce the TRM. Thellier and Thellier [1 ], who have studied mainly baked clays, detected these changes by numerous double-heatings at increasing temperatures until the Curie point was reached: when changes were found to occur (that is infrequent in baked clays), the sample was rejected. Unfortunately, most of the materials used in paleomagnetic work, especially volcanic rocks, are very sensitive to prolonged heating. Changes often occur even at relatively low temperatures, when the loss of NRM and the newly acquired TRM is too small to give the correct value of F. Here a new method is proposed for determining the ancient geomagnetic field strength using volcanic rocks, * Communication ($7-12) presented at the IAGA Symposium held in Kyoto, Japan, September 1973.
the validity of which is chiefly based on a very short time of heating to produce the TRM. The method has been tested during an archeomagnetic study of Mount Etna [2] and other recent Italian lavas. The advantage of this technique is also its simplicity and rapidity, and therefore the possibility to study a number of samples, as no special qualities o f these are required.
2. Method tested using historic lavas from Etna In a first attempt to determine the paleointensity from historic lava flows of Mount Etna, no consistent results could be obtained using Thellier and Thellier's method. Nevertheless, it was found during this study that the changes were enhanced by the long time necessary (in Thellier and Thellier progressive doublehearings) for reaching thermal equilibrium within the samples. On the other hand, it appeared that the changes were occurring slowly: thus, after ten or more heatings each lasting many hours, changes were still in progress. It was clear, therefore, that the difficulty should be greatly diminished by reducing the time of heating.
315 TABLE 1
1780
I
~-~
f~
Geomagnetic intensity from 1950 Etna lava Sample
Mo (NRM) Mt (TRMI) F=Mo/MI M2 (TRM2) (emu) (emu) × Flab. (emu) (Oe)
A3W A 12 W A 28 W A 30 W
0.201 0.191 0.215 0.265
0.250 0.200 0.224 0.277
0.372 0.442 0.443 0.441
F195o from the lavas (average)
0.424 Oe
F1954 in eastern Sicily (direct measures)
0.432 Oe
0.265 0.223 0.215 0.273
The new technique involves a very short time of heating: from 15 to 20 minutes only. As no thermal equilibrium is reached, the samples are directly heated to temperatures (about 700°C) higher than the Curie point. The short time of heating is due also: (1) to the small size of the samples (v = 2 5 - 3 0 cm 3 ; m = 5 0 - 7 0 g), (2) to their introduction into a pre-heated furnace, and (3) to their extraction and cooling in air out of the furnace. The magnetic field used is the geomagnetic field acting in the laboratory (Flab). Proceeding in this way, four cylinders of lava from the 1950 Etna flow have given good values o f F , as compared to the actual measurements in eastern Sicily (Table 1). It is important to control the possibility of changes during this short-time heating. Consequently, the samples were heated again as before. As it appears in Table 1 (last column), the observed variations for the TRM of the 1950 samples after this second heating are almost negligible. During further investigations on other recent historic flows of Mount Etna, it was found, however, that important changes can affect some samples even during the first heating. Possibly (1381 flow, Table 2, column 6), but not necessarily (1780 flow, ibid.), this phenomenon is revealed by a strong dispersion of the values of F obtained for the various samples from the same flow. For determining the paleointensity in such cases, it is necessary to know the evolution of TRM against the time of heating. Therefore (and this procedure was always used afterwards), several 6ther heatings were car-
1669
I~-t,3
10
i
0,8 .
~.o
,
a?
.
la.o
/
.~',,
~,0
80
120
t ,.n
Mo
1,5 1,3 L
I
/
~
"
0,$
~0
80
120
t ~.
Fig. 1. Variation in TRM-acquiring capacity of some samples against the time of heating. M = moments of successive laboratory TRM; Mo = moment of NRM; and r = time of heating. The diamond shows the ratio of the present geomagnetic fields acting in the laboratory and in Sicily.
ried Out under the same conditions as before, with the same constant magnetizing field (F acting in the laboratory: Flab). Thus, the curve J = f(t), t being the time of heating, could be established. In Fig. 1 are plotted the ratios M/Mo (11tlbeing the moments of successive TRM and Mo the moment of NRM) against the number of heatings, i.e. the time of heating, as all were of the same duration. It was ascertained on other specimens that the variation of TRM is related to the time of the heatings only and not to the number of heatings. No general law has been deduced from the evolution of the TRM of the successively heated samples (Fig. 1). The remanent moment sometimes increases and sometimes decreases, even for samples from the same flow unit. Occasionally, for a given sample the TRM may at first increase and then decrease, or vice-versa, and stabilization is hardly reached.
316 TABLE 2 Paleointensity from some historic Etna lavas Flow
Sample
Mo (NRM) (emu)
M 1 (TRM1) after storage in zero field 24 hours (emu)
8 days (emu)
F = M o / M 1 × Flab.
(Oe)
bextrap. (Oe)
1780
548 549 555 556
0.749 0.486 0.594 0.775
0.683 0.470 0.534 0.759
0.678 0.465 0.529 0.754
0.51 0.48 0.52 0.48
0.44 0.44 0.45 0.41
1669
312 313 315 316
0.195 0.273 0.303 0.290
0.230 0.295 0.336 0.330
0.229 0.294. 0.333 0.328
0.40 0.43 0.42 0.41
0.40 0.42 0.43 0.41
1381
353 356 360 361
0.317 0.195 0.453 0.305
0.296 0.231 0.314 0.317
0.295 0.228 0.313 0.315
0.50 0.40 0.67 0.45
0.51 0.53 0.55 0.47
(1)
(2)
(3)
(4)
(5)
(6)
(7)
It is possible, however, to extrapolate graphically the curve of each sample towards t = 0, i.e., to obtain a virtual value of TRM without any laboratory heating. In fact, reasonable and consistent results o f f have been deduced using such virtual values of TRM (Fextrap., Table 2, last column). These results will be discussed later. Other precautions were taken against parasitic magnetizations acquired in the field or even during the preparation of the samples in laboratory (sawing), by submitting several of them to alternating field (a.f.) demagnetizations. In order to prevent the occurrence of any viscous remanent magnetization (VRM) when producing TRM, the samples were stored in zero field immediately after cooling. The comparison between columns 4 and 5 of Table 2 shows that thanks to this precaution the VRM component of the TRM is negligible. Finally, the following experimental procedure was adopted: (1) The NRM of the sample (usually 3 X 3 X 3 cm) is measured after a storage of at least 8 days in zero field. (2) The sample is then a.f. demagnetized to ascertain
that no parasitic magnetization has occurred in field or laboratory. (3) The sample is given a TRM (=TRM1) by heating it for 15 minutes above its Curie temperature into a preheated (750°C) furnace. The temperature of the furnace usually falls down to about 700°C after introduction of the sample. Before removal of the sample, the electric current of heating is cut off to prevent any possible alternating field effect. Then, the sample is removed from the furnace and cooled on a support where the geomagnetic field has been previously determined. The time of cooling is about 4 0 - 4 5 minutes. (4) The sample is immediately stored for 24 hours in F = 0. Then, the newly created TRM is measured. (5) The same measurements are repeated after an 8day storage in F = 0. (6) Several further heatings are carried out as in (3) and (4), leading to TRM2, TRM3, etc. The TRM are produced with the Z component of the sample alternately up and down. Thus, by checking that the resultant TRM is in the expected direction, it can be ascertained that during each heating the Curie temperature has been reached in all parts of the sample.
317 (7) The curve M / M o = f ( t ) is constructed for each sample and then graphically extrapolated to deducing the virtual value of M (=My) when t = 0. The paleointensity Fo of the earth's magnetic field is derived from the equation: Fo -
Mo (NRM) M v (TRIM)
Foe
•
direct
•
lavas of Etna
measures
•
Arso d'lschla, Naples
0.50
X Flab.
Using the device shown in Fig. 1, four samples of size 3 X 3 X 3 cm can be heated and cooled together. As three heatings are easily performed during one day, the runs currently involve a total of twelve samples, The complete study of these twelve samples is performed in about one month, including the 8 days of storage in zero field before NRM and TRMI measurements (1 and 5). If necessary, a maximum of 24 samples can be studied within the same time. The measurements of remanences were made using the big spinner magnetometer at the Saint-Maur Laboratory [3].
3. Discussion of the results Experiments were carried out on the basaltic (tephritic basalts) lava flows from Mount Etna of the years 1950, 1910, 1865, 1843, 1780, 1669, 1610, 1536, 1381, and also on a part o f flow at Catania, presumed from the 122 B.C. eruption. A thick trachytic lava from the island of Ischia (west of Naples), that extruded in 1301, was likewise investigated. Except for the 1950 Etna samples, all the values of Fo were obtained by extrapolating the curve M / M o = f(t), as already mentioned.
o.3o. !
!
i
1000
1500
2000
~uAQ
Fig. 2. Variation of the geomagnetic paleointensity in southern Italy deduced from the lavas. Comparison with direct measures o f F [7].
Table 3 and Fig. 2 summarize the results derived from this work. The intensity of the geomagnetic field in southern Italy seems to have been nearly constant during the last three centuries and close to the present value of 0.43 Oe. In extending further back into the past, the values of Fo seem to increase appreciably. These results are consistent with Thellier's archeomagnetic data obtained from baked clays ([1,5] and personal communications). However, the comparison remains somewhat limited, as the lava flows older than the 13th century can hardly ever be recognized on the volcano [2]. Thus, the presumed samples of 122 B.C. have given a value o f F o = 0.47 Oe, which strongly
TABLE 3 Summary of the paleointensity results, with the 95% confidence limit on the mean (rn) using Student's t test. Flow
Sample
F = Mo/M 1 X Flab. (Oe)
Fextrap" (Oe)
1865 (Etna)
744-1 744-2 747 753-1 753-2 755-1 755-2
0.44 0.46 0.44 0.46 0.45 0.44 0.44
0.42 0.43 0.45 0.43 -
m
to
0.4325
1.9
TABLE 3 (continued) Flow
Sample
F = Mo[M 1 × Flab. (Oe)
Fextrap" (Oe)
1843
717 718-1 718-2 719-1 719-2 723 724 726
0.33 0.39 0.40 0.31 0.28 0.43 0.28 0.45
0,38 0.40 0.30* 0.41 0.29* 0.45
1792
376 539
0.47 0.43
0.44 0.44
1780
548 549 555 556
0,51 0.48 0.52 0.48
0.44 0,44 0.45 0.41
312 313 315 316
0.40 0.43 0.42 0.41
0.40 0.42 0.43 0.41
455 456 457 458
0.40 0.39 0.44 0.33
0.40 0.41 0.45 0.36
482 484 487 488
0.44 0.54 0.45 0,50
0,45 0.50 0,50 0.49
353 356 357 360 361 362 364
0.50 0.40 0.69 0.67 0.45 0.43 0.50
0.51 0.53 0.65* 0.55 0.47 0.47 0.53
1284 (?)
812 814
0.29 0.26
0.34 0.30
122 B.C. (?)
410 411 412 415
0.42 0.48 0.48 0.49
0.45 0.47 0.48 0.48
lschia (1301)
776 777 795 796
0.56 0.86 0.50 0.62
0.50 0.80* 0.49 0.54
1669
1610
1536
1381
* Discarded from the average (see text, section 4).
m
to
0.41
4.6
0.435
2.7
0.415
2.0
0.405
5,8
0,485
3.8
0.51
3.1
0.51
5.9
319
Fz
0 c 725
551 °
726
ec 723
71 g
Oc
= 554 °
724
~ 500 ° =
724
4
o
71g
2~o
4ao
e&
°C
Fig. 3. Thermomagnetic curves of samples from the 1843 Etna flow that give correct (723,726) and incorrect (719, 724) values of F. F z is the magnetic force on the Curie balance, in arbitrary units.
differs from that expected (about 0.75 Oe) from Thellier's data, so that the true age of this flow may be revised. Further investigations are nevertheless in progress on other approximately dated (by means of archeology) lavas of Etna and also on a flow from the island of Vulcanello, north of Sicily, that was probably erupted in 183 B.C. On the other hand, it is hoped that some new radioactive dating method [6] will soon be used for determining the age of lavas from the last several thousand years. This would make feasible more extended studies about the intensity of the earth's magnetic field; variation of intensity which appears to be much slower than that of direction so that less precision in dating is required.
4. Validity and limits of the method By means of the method described in this paper, more than 80% of the samples studied have given resuits in determining Fo. For the remaining samples, the values of Fo obtained were considerably lower or higher than those expected, and they must be discarded from the average. It seems unlikely that the discrepancy is due to a different opaque mineralogy. The recent lavas of Etna are very uniform in character, having similar ore minerals. Particularly, samples from the 1843 flow that gave both correct and incorrect values
of Fo have the same high Curie temperatures (Fig. 3), which are little affected by further hearings. In fact, the reasons of such a disagreement are not clear: (1) It may be the effect of some unchecked alteration in the TRM acquiring capacity during the first heating. (2) The NRM may not be entirely a TRM. It has been shown that during [4] or after the natural cooling of the lavas, mineralogical changes sometimes occur below the Curie temperature. The resultant NRM, therefore, is partly a CRM and partly a TRM. In this case, the comparison between NRM and laboratory TRM cannot be used for paleointensity studies. (3) The TRM (NRM) itself may have been altered. It is a fact that parasitic magnetizations (or demagnetizations), if nearly directed along the axis of the natural TRM, are difficult to detect by the usual means of magnetic cleaning. This difficulty is enhanced, in Etnean lavas, by their low resistance to a.f. demagnetization processes. The result is without real importance for the research of geomagnetic direction, but has a major effect on the intensity. Moreover, by the present method, no result can be derived from partly a.f. demagnetized NRM and corresponding laboratory TRM. It was found that considerable changes in a.f. demagnetization curves of TRM, before and after heating, can occur without any appreciable variation in the strengths of the two TRM's, or vice-versa. Thus, a disturbed NRM is irretrievably lost for this method. But how does one ascertain that NRM has remained unaltered? Because of the high intensity of magnetization of the lavas, that sometimes reaches 0.02 emu, there is also the possibility of selfdistortion of the field at the time of cooling [2]. The method can easily be applied to an extensive number of samples: it is recommended, therefore, to submit as varied a range of samples as possible for each paleointensity investigation. Concordant results from very different samples of the same deposit will be the most satisfactory guarantee of their validity.
Acknowledgments The author wishes to thank Prof. E. TheUier for his fruitful suggestions and Prof. R.L. Wilson who kindly corrected and improved the manuscript.
320
References 1 E. Thellier and O. Thellier, Sur l'intensit6 du champ magn~tique terrestre darts le pass6 historique et g~ologique, Ann. G6ophys. 15, 3 (1959) 285-376. 2 J.C. Tanguy, An archaeomagnetic study of Mount Etna: the magnetic direction recorded in lava flows subsequent to the twelfth century, Archaeometry 12, 2 (1970) 1 1 5 128. 3 E. Thellier, A big sample spinner magnetometer, in: Methods in Paleomagnetism(Elsevier, Amsterdam, 1967) 149-154.
4 C.S. Gromm~, T.L. Wright and D.L. Peck, Magnetic properties and oxidation of iron-titanium oxides minerals in Alae and Makaopuhi lava lakes, Hawaii, J. Geophys. Res. 74, 22 (1969) 5277-5293. 5 E. Thellier, Le champ magn~tique terrestre fossile, In: Nucleus 7, 1-2-3 (1966). 6 M. Condomines, Etude chronologique et g6ochimique du volcanisme r~cent du Costa-Rica ~ l'aide du d6s~quilibre radioactif 23°Th-238U, Th~se 3 e c., Univ. Paris VII, 1974. 7 M. Giorgi and F. Molina, Campo normale e variazione secolare media degli elementi magnetici in Sicilia, Ann. Geofis. VII (1954) 5 2 1 - 5 3 7 .
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
INTENSITY OF THE GEOMAGNETIC F I E L D FROM RECENT ITALIAN LAVAS USING A NEW PALEOINTENSITY METHOD* J.C. TANGUY Laboratoire de GkomagnOtisme du Parc Saint-Maur, St. Maur-des-Foss~s (France)
Revised version received July 7, 1975
A new method has been tested on Etna historic lavas for determining the geomagnetic field intensity (F) using the thermoremanent magnetization of volcanic rocks. The procedure involves a number of very short duration heatings above the Curie point, to produce successive laboratory TRM. Thus, it is possible to check the variations in the TRMacquiring capacity of the samples with the time of heating (t). The curve J = f(t) is then extrapolated towards t = 0, leading to a virtual value of TRM without any laboratory heating, i.e., without the changes that currently occur when the lavas are heated to produce the TRM. Using such virtual values of TRM, satisfactory results of F had been derived from the majority of the samples studied. These results are consistent with Thellier's archeomagnetic data: they show a nearly constant intensity of the geomagnetic field during the last three centuries and, further back into the past, a significant increase of this.
1. Introduction Intensity values (F) of the ancient geomagnetic field are obtained from rocks or baked clays by comparing the natural thermoremanent magnetization (NRM) and a laboratory thermoremanent magnetization (TRM) produced in a known field. The major problem in such a method lies in the changes that occur when samples are heated to produce the TRM. Thellier and Thellier [1 ], who have studied mainly baked clays, detected these changes by numerous double-heatings at increasing temperatures until the Curie point was reached: when changes were found to occur (that is infrequent in baked clays), the sample was rejected. Unfortunately, most of the materials used in paleomagnetic work, especially volcanic rocks, are very sensitive to prolonged heating. Changes often occur even at relatively low temperatures, when the loss of NRM and the newly acquired TRM is too small to give the correct value of F. Here a new method is proposed for determining the ancient geomagnetic field strength using volcanic rocks, * Communication ($7-12) presented at the IAGA Symposium held in Kyoto, Japan, September 1973.
the validity of which is chiefly based on a very short time of heating to produce the TRM. The method has been tested during an archeomagnetic study of Mount Etna [2] and other recent Italian lavas. The advantage of this technique is also its simplicity and rapidity, and therefore the possibility to study a number of samples, as no special qualities o f these are required.
2. Method tested using historic lavas from Etna In a first attempt to determine the paleointensity from historic lava flows of Mount Etna, no consistent results could be obtained using Thellier and Thellier's method. Nevertheless, it was found during this study that the changes were enhanced by the long time necessary (in Thellier and Thellier progressive doublehearings) for reaching thermal equilibrium within the samples. On the other hand, it appeared that the changes were occurring slowly: thus, after ten or more heatings each lasting many hours, changes were still in progress. It was clear, therefore, that the difficulty should be greatly diminished by reducing the time of heating.
315 TABLE 1
1780
I
~-~
f~
Geomagnetic intensity from 1950 Etna lava Sample
Mo (NRM) Mt (TRMI) F=Mo/MI M2 (TRM2) (emu) (emu) × Flab. (emu) (Oe)
A3W A 12 W A 28 W A 30 W
0.201 0.191 0.215 0.265
0.250 0.200 0.224 0.277
0.372 0.442 0.443 0.441
F195o from the lavas (average)
0.424 Oe
F1954 in eastern Sicily (direct measures)
0.432 Oe
0.265 0.223 0.215 0.273
The new technique involves a very short time of heating: from 15 to 20 minutes only. As no thermal equilibrium is reached, the samples are directly heated to temperatures (about 700°C) higher than the Curie point. The short time of heating is due also: (1) to the small size of the samples (v = 2 5 - 3 0 cm 3 ; m = 5 0 - 7 0 g), (2) to their introduction into a pre-heated furnace, and (3) to their extraction and cooling in air out of the furnace. The magnetic field used is the geomagnetic field acting in the laboratory (Flab). Proceeding in this way, four cylinders of lava from the 1950 Etna flow have given good values o f F , as compared to the actual measurements in eastern Sicily (Table 1). It is important to control the possibility of changes during this short-time heating. Consequently, the samples were heated again as before. As it appears in Table 1 (last column), the observed variations for the TRM of the 1950 samples after this second heating are almost negligible. During further investigations on other recent historic flows of Mount Etna, it was found, however, that important changes can affect some samples even during the first heating. Possibly (1381 flow, Table 2, column 6), but not necessarily (1780 flow, ibid.), this phenomenon is revealed by a strong dispersion of the values of F obtained for the various samples from the same flow. For determining the paleointensity in such cases, it is necessary to know the evolution of TRM against the time of heating. Therefore (and this procedure was always used afterwards), several 6ther heatings were car-
1669
I~-t,3
10
i
0,8 .
~.o
,
a?
.
la.o
/
.~',,
~,0
80
120
t ,.n
Mo
1,5 1,3 L
I
/
~
"
0,$
~0
80
120
t ~.
Fig. 1. Variation in TRM-acquiring capacity of some samples against the time of heating. M = moments of successive laboratory TRM; Mo = moment of NRM; and r = time of heating. The diamond shows the ratio of the present geomagnetic fields acting in the laboratory and in Sicily.
ried Out under the same conditions as before, with the same constant magnetizing field (F acting in the laboratory: Flab). Thus, the curve J = f(t), t being the time of heating, could be established. In Fig. 1 are plotted the ratios M/Mo (11tlbeing the moments of successive TRM and Mo the moment of NRM) against the number of heatings, i.e. the time of heating, as all were of the same duration. It was ascertained on other specimens that the variation of TRM is related to the time of the heatings only and not to the number of heatings. No general law has been deduced from the evolution of the TRM of the successively heated samples (Fig. 1). The remanent moment sometimes increases and sometimes decreases, even for samples from the same flow unit. Occasionally, for a given sample the TRM may at first increase and then decrease, or vice-versa, and stabilization is hardly reached.
316 TABLE 2 Paleointensity from some historic Etna lavas Flow
Sample
Mo (NRM) (emu)
M 1 (TRM1) after storage in zero field 24 hours (emu)
8 days (emu)
F = M o / M 1 × Flab.
(Oe)
bextrap. (Oe)
1780
548 549 555 556
0.749 0.486 0.594 0.775
0.683 0.470 0.534 0.759
0.678 0.465 0.529 0.754
0.51 0.48 0.52 0.48
0.44 0.44 0.45 0.41
1669
312 313 315 316
0.195 0.273 0.303 0.290
0.230 0.295 0.336 0.330
0.229 0.294. 0.333 0.328
0.40 0.43 0.42 0.41
0.40 0.42 0.43 0.41
1381
353 356 360 361
0.317 0.195 0.453 0.305
0.296 0.231 0.314 0.317
0.295 0.228 0.313 0.315
0.50 0.40 0.67 0.45
0.51 0.53 0.55 0.47
(1)
(2)
(3)
(4)
(5)
(6)
(7)
It is possible, however, to extrapolate graphically the curve of each sample towards t = 0, i.e., to obtain a virtual value of TRM without any laboratory heating. In fact, reasonable and consistent results o f f have been deduced using such virtual values of TRM (Fextrap., Table 2, last column). These results will be discussed later. Other precautions were taken against parasitic magnetizations acquired in the field or even during the preparation of the samples in laboratory (sawing), by submitting several of them to alternating field (a.f.) demagnetizations. In order to prevent the occurrence of any viscous remanent magnetization (VRM) when producing TRM, the samples were stored in zero field immediately after cooling. The comparison between columns 4 and 5 of Table 2 shows that thanks to this precaution the VRM component of the TRM is negligible. Finally, the following experimental procedure was adopted: (1) The NRM of the sample (usually 3 X 3 X 3 cm) is measured after a storage of at least 8 days in zero field. (2) The sample is then a.f. demagnetized to ascertain
that no parasitic magnetization has occurred in field or laboratory. (3) The sample is given a TRM (=TRM1) by heating it for 15 minutes above its Curie temperature into a preheated (750°C) furnace. The temperature of the furnace usually falls down to about 700°C after introduction of the sample. Before removal of the sample, the electric current of heating is cut off to prevent any possible alternating field effect. Then, the sample is removed from the furnace and cooled on a support where the geomagnetic field has been previously determined. The time of cooling is about 4 0 - 4 5 minutes. (4) The sample is immediately stored for 24 hours in F = 0. Then, the newly created TRM is measured. (5) The same measurements are repeated after an 8day storage in F = 0. (6) Several further heatings are carried out as in (3) and (4), leading to TRM2, TRM3, etc. The TRM are produced with the Z component of the sample alternately up and down. Thus, by checking that the resultant TRM is in the expected direction, it can be ascertained that during each heating the Curie temperature has been reached in all parts of the sample.
317 (7) The curve M / M o = f ( t ) is constructed for each sample and then graphically extrapolated to deducing the virtual value of M (=My) when t = 0. The paleointensity Fo of the earth's magnetic field is derived from the equation: Fo -
Mo (NRM) M v (TRIM)
Foe
•
direct
•
lavas of Etna
measures
•
Arso d'lschla, Naples
0.50
X Flab.
Using the device shown in Fig. 1, four samples of size 3 X 3 X 3 cm can be heated and cooled together. As three heatings are easily performed during one day, the runs currently involve a total of twelve samples, The complete study of these twelve samples is performed in about one month, including the 8 days of storage in zero field before NRM and TRMI measurements (1 and 5). If necessary, a maximum of 24 samples can be studied within the same time. The measurements of remanences were made using the big spinner magnetometer at the Saint-Maur Laboratory [3].
3. Discussion of the results Experiments were carried out on the basaltic (tephritic basalts) lava flows from Mount Etna of the years 1950, 1910, 1865, 1843, 1780, 1669, 1610, 1536, 1381, and also on a part o f flow at Catania, presumed from the 122 B.C. eruption. A thick trachytic lava from the island of Ischia (west of Naples), that extruded in 1301, was likewise investigated. Except for the 1950 Etna samples, all the values of Fo were obtained by extrapolating the curve M / M o = f(t), as already mentioned.
o.3o. !
!
i
1000
1500
2000
~uAQ
Fig. 2. Variation of the geomagnetic paleointensity in southern Italy deduced from the lavas. Comparison with direct measures o f F [7].
Table 3 and Fig. 2 summarize the results derived from this work. The intensity of the geomagnetic field in southern Italy seems to have been nearly constant during the last three centuries and close to the present value of 0.43 Oe. In extending further back into the past, the values of Fo seem to increase appreciably. These results are consistent with Thellier's archeomagnetic data obtained from baked clays ([1,5] and personal communications). However, the comparison remains somewhat limited, as the lava flows older than the 13th century can hardly ever be recognized on the volcano [2]. Thus, the presumed samples of 122 B.C. have given a value o f F o = 0.47 Oe, which strongly
TABLE 3 Summary of the paleointensity results, with the 95% confidence limit on the mean (rn) using Student's t test. Flow
Sample
F = Mo/M 1 X Flab. (Oe)
Fextrap" (Oe)
1865 (Etna)
744-1 744-2 747 753-1 753-2 755-1 755-2
0.44 0.46 0.44 0.46 0.45 0.44 0.44
0.42 0.43 0.45 0.43 -
m
to
0.4325
1.9
TABLE 3 (continued) Flow
Sample
F = Mo[M 1 × Flab. (Oe)
Fextrap" (Oe)
1843
717 718-1 718-2 719-1 719-2 723 724 726
0.33 0.39 0.40 0.31 0.28 0.43 0.28 0.45
0,38 0.40 0.30* 0.41 0.29* 0.45
1792
376 539
0.47 0.43
0.44 0.44
1780
548 549 555 556
0,51 0.48 0.52 0.48
0.44 0,44 0.45 0.41
312 313 315 316
0.40 0.43 0.42 0.41
0.40 0.42 0.43 0.41
455 456 457 458
0.40 0.39 0.44 0.33
0.40 0.41 0.45 0.36
482 484 487 488
0.44 0.54 0.45 0,50
0,45 0.50 0,50 0.49
353 356 357 360 361 362 364
0.50 0.40 0.69 0.67 0.45 0.43 0.50
0.51 0.53 0.65* 0.55 0.47 0.47 0.53
1284 (?)
812 814
0.29 0.26
0.34 0.30
122 B.C. (?)
410 411 412 415
0.42 0.48 0.48 0.49
0.45 0.47 0.48 0.48
lschia (1301)
776 777 795 796
0.56 0.86 0.50 0.62
0.50 0.80* 0.49 0.54
1669
1610
1536
1381
* Discarded from the average (see text, section 4).
m
to
0.41
4.6
0.435
2.7
0.415
2.0
0.405
5,8
0,485
3.8
0.51
3.1
0.51
5.9
319
Fz
0 c 725
551 °
726
ec 723
71 g
Oc
= 554 °
724
~ 500 ° =
724
4
o
71g
2~o
4ao
e&
°C
Fig. 3. Thermomagnetic curves of samples from the 1843 Etna flow that give correct (723,726) and incorrect (719, 724) values of F. F z is the magnetic force on the Curie balance, in arbitrary units.
differs from that expected (about 0.75 Oe) from Thellier's data, so that the true age of this flow may be revised. Further investigations are nevertheless in progress on other approximately dated (by means of archeology) lavas of Etna and also on a flow from the island of Vulcanello, north of Sicily, that was probably erupted in 183 B.C. On the other hand, it is hoped that some new radioactive dating method [6] will soon be used for determining the age of lavas from the last several thousand years. This would make feasible more extended studies about the intensity of the earth's magnetic field; variation of intensity which appears to be much slower than that of direction so that less precision in dating is required.
4. Validity and limits of the method By means of the method described in this paper, more than 80% of the samples studied have given resuits in determining Fo. For the remaining samples, the values of Fo obtained were considerably lower or higher than those expected, and they must be discarded from the average. It seems unlikely that the discrepancy is due to a different opaque mineralogy. The recent lavas of Etna are very uniform in character, having similar ore minerals. Particularly, samples from the 1843 flow that gave both correct and incorrect values
of Fo have the same high Curie temperatures (Fig. 3), which are little affected by further hearings. In fact, the reasons of such a disagreement are not clear: (1) It may be the effect of some unchecked alteration in the TRM acquiring capacity during the first heating. (2) The NRM may not be entirely a TRM. It has been shown that during [4] or after the natural cooling of the lavas, mineralogical changes sometimes occur below the Curie temperature. The resultant NRM, therefore, is partly a CRM and partly a TRM. In this case, the comparison between NRM and laboratory TRM cannot be used for paleointensity studies. (3) The TRM (NRM) itself may have been altered. It is a fact that parasitic magnetizations (or demagnetizations), if nearly directed along the axis of the natural TRM, are difficult to detect by the usual means of magnetic cleaning. This difficulty is enhanced, in Etnean lavas, by their low resistance to a.f. demagnetization processes. The result is without real importance for the research of geomagnetic direction, but has a major effect on the intensity. Moreover, by the present method, no result can be derived from partly a.f. demagnetized NRM and corresponding laboratory TRM. It was found that considerable changes in a.f. demagnetization curves of TRM, before and after heating, can occur without any appreciable variation in the strengths of the two TRM's, or vice-versa. Thus, a disturbed NRM is irretrievably lost for this method. But how does one ascertain that NRM has remained unaltered? Because of the high intensity of magnetization of the lavas, that sometimes reaches 0.02 emu, there is also the possibility of selfdistortion of the field at the time of cooling [2]. The method can easily be applied to an extensive number of samples: it is recommended, therefore, to submit as varied a range of samples as possible for each paleointensity investigation. Concordant results from very different samples of the same deposit will be the most satisfactory guarantee of their validity.
Acknowledgments The author wishes to thank Prof. E. TheUier for his fruitful suggestions and Prof. R.L. Wilson who kindly corrected and improved the manuscript.
320
References 1 E. Thellier and O. Thellier, Sur l'intensit6 du champ magn~tique terrestre darts le pass6 historique et g~ologique, Ann. G6ophys. 15, 3 (1959) 285-376. 2 J.C. Tanguy, An archaeomagnetic study of Mount Etna: the magnetic direction recorded in lava flows subsequent to the twelfth century, Archaeometry 12, 2 (1970) 1 1 5 128. 3 E. Thellier, A big sample spinner magnetometer, in: Methods in Paleomagnetism(Elsevier, Amsterdam, 1967) 149-154.
4 C.S. Gromm~, T.L. Wright and D.L. Peck, Magnetic properties and oxidation of iron-titanium oxides minerals in Alae and Makaopuhi lava lakes, Hawaii, J. Geophys. Res. 74, 22 (1969) 5277-5293. 5 E. Thellier, Le champ magn~tique terrestre fossile, In: Nucleus 7, 1-2-3 (1966). 6 M. Condomines, Etude chronologique et g6ochimique du volcanisme r~cent du Costa-Rica ~ l'aide du d6s~quilibre radioactif 23°Th-238U, Th~se 3 e c., Univ. Paris VII, 1974. 7 M. Giorgi and F. Molina, Campo normale e variazione secolare media degli elementi magnetici in Sicilia, Ann. Geofis. VII (1954) 5 2 1 - 5 3 7 .