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High absolute paleointensity during a mid Miocene excursion of the Earth’s magnetic field
Earth and Planetary Science Letters 184 (2000) 141^154 www.elsevier.com/locate/epsl
High absolute paleointensity during a mid Miocene excursion of the Earth's magnetic ¢eld Roman Leonhardt a; *, Felix Hufenbecher a , Franz Heider b , Heinrich C. So¡el a b
a Institut fu«r Allgemeine und Angewandte Geophysik, Theresienstr. 41, University Munich, 80333 Munich, Germany Department of Geological Sciences, 230 Wallace Building, University of Manitoba, Winnipeg, Man., Canada R3T 2N2
Received 13 June 2000; received in revised form 28 August 2000; accepted 16 October 2000
Abstract A record of an excursion of the Earth's magnetic field was found in a V14.3 Myr old lava sequence of the shield basalts from Gran Canaria (Canary Islands). The thermal and alternating field demagnetization of the samples from 14 subsequent lava flows revealed mainly single component magnetizations and sometimes minor viscous overprints. Four consecutive flows showed intermediate paleodirections which represent an excursion of the virtual geomagnetic pole (VGP) from the South pole towards the equator in the central Atlantic. Rock magnetic and microscopic analysis revealed high temperature oxidized titanomagnetites as the dominant carrier of remanence. On the basis of reversibility of thermomagnetic curves and limited variation of susceptibility during thermal demagnetization, we selected suitable samples for three slightly different Thellier-type paleointensity methods. The results show a minimum in paleointensity after the directional excursion with intensity values of V9 WT. With the pole approaching the equator, values of V27 WT are reached. The intensity at the beginning and at the end of the section is V20 WT corresponding to a virtual dipole moment (VDM) of 6U1022 Am2 . The high intensities during the VGP excursion with a pole near the equator are explained by the same VDM. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: paleomagnetism; paleointensity; magnetic ¢eld
1. Introduction Since the second half of the 20th century the paleomagnetic community investigated variations of the Earth's magnetic ¢eld during reversals and excursions. These records of the variations in di-
* Corresponding author. Tel.: +49-89-2394-4231; Fax: +49-89-2394-4205; E-mail: [email protected]
rection and intensity are well known for representing important constraints for the geodynamo. So far the analysis of many di¡erent reversals sampled at di¡erent locations has shown that there are notable similarities in the directional data [1^3]. However, signi¢cant variations between reversals of di¡erent age observed at di¡erent locations as well as between the same reversal were also detected [4]. In comparison to the determination of paleodirections the extraction of absolute paleointensities is much more di¤cult. Despite this, there is more agreement among
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paleomagnetists regarding the variation of intensity during reversals and excursion. The available data from volcanic rocks, which are unfortunately few in number, point to a 75% reduction of the transitional ¢eld intensity compared to normal or reversed states [5]. In order to contribute to the ongoing debate we sampled a basaltic sequence in a V14.3 Myr old section of the shield basalts from Gran Canaria (Canary islands). Within a succession of 14 lava £ows, an excursion of the Earth's magnetic ¢eld was found. These samples were subjected to various paleo- and rock magnetic experiments in order to investigate the directional behavior and the associated paleointensities before, during and after the excursion. 2. Geology, sampling and methodology The sampled sequence is located on the west coast of Gran Canaria in the Barranco de Tasartico (Fig. 1) on a hill on the north side of the road to Playa del Asno. This sequence consists of ba-
saltic to more di¡erentiated lava £ows with a thickness from 1 to 4 m, belonging to the Guigui formation of the Gran Canaria shield basalts [6]. The lava £ows of the shield basalts were erupted within a time interval of 0.1^0.5 Myr [7]. Since the shield building phase ended between 13.95 and 14.1 Myr ago (40 Ar/39 Ar-ages [8]), the age range for the excursion is 14.0^14.6 Myr. The stratigraphy is shown in Fig. 2. The £ows are identi¢ed by the presence of basal and top scorias (AA-£ows) or by red lapillus (Pahoehoe-£ows). Between £ow C-TA2 and C-TA3 a debris accumulation is present (Fig. 2), which thins out in westward direction. The absence of paleosoils and/or erosional unconformaties suggests a relatively rapid extrusion of the 14 lava £ows. The £ows were sampled by taking oriented drill cores using a magnetic and a sun compass for orientation. Whenever possible the cores were drilled in the middle of the £ow with a spacing of one to two meters between di¡erent cores. Ninety-nine samples were stepwise demagnetized with at least 10 steps using alternating ¢eld (AF) and thermal demagnetization. Measure-
Fig. 1. Map of the site location. The sampled sequence is located near the west coast of Gran Canary in the Barranco de Tasartico at a elevation of 120 m. The black cross marks the exact location of the sequence.
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e¡ective in determining the characteristic remanent magnetization. Examples of demagnetization plots are shown in Fig. 3. Most of the specimens are characterized by a single component magnetization. Only few specimens (Fig. 3a) showed weak secondary components usually removed at demagnetization steps of 200³C or 20 mT. At least ¢ve samples from every £ow were used to calculate mean directions according to Fisher [9]. In Table 1 a summary of the directional data is shown. The K95 values are all less than 10³ associated with k values exceeding 50, which underlines the high quality of the directional data. By converting the mean directions to virtual geomagnetic poles (VGPs) the related pole of a dipolar ¢eld can be calculated. The VGP is found to move towards the equator in the central south Atlantic during the excursion, from reverse states of the Earth's magnetic ¢eld before and after this movement (Fig. 4). 3.2. Rock magnetic results
Fig. 2. Stratigraphy of the sequence. Between £ow C-TA2 and 3, a paleodebris accumulation is present.
ments of the remanence were conducted using a Molspin spinner magnetometer. In addition, a set of rock magnetic experiments was performed on at least one sample of every £ow. These measurements included thermomagnetic curves, hysteresis loops, and back¢eld curves using a variable ¢eld translation balance (VFTB). All Thellier-type experiments were conducted with a MMTD20 thermal demagnetizer.
Plotting the hysteresis parameters in a Day plot [10] results in a clustering of the samples within the pseudo single domain (PSD) grain size range. Thermomagnetic (Ms T) curves in a saturation ¢eld (V400 mT) revealed Curie temperatures between 500 and 570³C (Fig. 5). Additional intermediate Curie temperatures (200^400³C) were only detected in samples from £ow C-TA2 and C-TA5. Most of the samples showed reversible Ms T-curves for the heating and cooling cycle. This emphasizes their suitability for the determination of absolute paleointensities. Microscopy investigation was performed in order to identify the magnetic phases. Two samples from £ow 115 and 122 have been used for re£ected light microscopy studies. These samples show that the magnetic phase evolved from the primary Ti-magnetite by migration of titanium to ilmenite lamellae during high temperature oxidation.
3. Magnetic measurements
3.3. Paleointensity determination
3.1. Paleodirectional results
On the basis of reversible thermomagnetic curves and/or limited variation of susceptibility during thermal demagnetization only three £ows,
AF and thermal demagnetization were equally
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namely C-TA2, C-TA5, and C-TA9, seem unsuitable for paleointensity determination. The viscosity index [11] was determined for 20% of the collection and was in all cases less than 5%. For the experiments two di¡erent kinds of specimens were used, standard inch samples and small `mini samples' with a diameter of 0.5 cm. Both types of samples showed similar paleointensity results. Overall 26 mini samples and 23 standard 1-inch samples were used for the experiments. The bene¢t of using mini samples is that
the time for a heating cycle is reduced by a factor of 10. Heating was performed either in air or in an argon-rich atmosphere. We could not detect any di¡erences between the experiments in both atmospheres, even for the mini samples where the surface to volume ratio is much greater than for inch samples. 3.3.1. Methodologies For this study several di¡erent modi¢cations of the Thellier^Thellier technique [12] have been
Fig. 3. Typical Zijderveld plots of some samples during thermal and AF demagnetization. Most samples are characterized by a single component magnetization (b^d). Only in a few cases (a) weak secondary components have been observed.
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used to assess absolute paleointensities. At ¢rst, measurements of the absolute paleointensity were done according to the method of Coe [13], further referred to as MT1. However, there are two possible mechanisms that can cause failure to the experiment which cannot be deciphered using the MT1 method. These mechanisms are multidomain (MD) remanences and alteration products with higher blocking temperatures than the actual heating step. In order to test whether these mechanisms bias the paleointensity result we additionally used two slight modi¢cations for the paleointensity determination (MT2 and MT3). 3.3.1.1. MT1 technique The ¢rst method consists of stepwise demagnetization followed by pTRM-checks and pTRMsteps (Fig. 6). During most determinations, each heating step was followed by a pTRM-check. Thirty-four samples were subjected to this standard method. In order to quantify the grade of alteration during the experiment we calculated a relative error of the pTRM-checks at the temperature Ti : CKerror T i 100UMpTRM CK T i 3 pTRM T i M=TRM ab
Fig. 4. The directional excursion corresponds to a movement of the VGP from a reverse state towards the equator in the central South Atlantic.
with TRMab as the intersection of the best ¢tting line with the pTRM axis. 3.3.1.2. MT2 technique For the MT2 method a pTRM was induced at a speci¢c temperature before demagnetizing at the same temperature (Fig. 7). If none of the above mentioned mechanism is present the induced pTRM should be completely removed during
Table 1 Paleodirectional results Flow
Site number
Declination (³)
Inclination (³)
k
K95 (³)
Ntr /N
VGPlong (³)
VGPlat (³)
C-TA1 C-TA2 C-TA3 C-TA4 C-TA5 C-TA6 C-TA7 C-TA8 C-TA9 C-TA10 C-TA11 C-TA12 C-TA13 C-TA14
110 111 112 113 114 115 116 117 118 119 120 121 122 123
166.4 165.4 178.0 160.7 183.6 170.8 169.7 154.4 179.4 183.2 181.8 177.9 176.3 175.4
335.1 47.7 62.7 69.4 58.7 317.4 323.5 39.6 336.8 340.6 323.3 331.0 336.6 333.3
87 59 52 70 404 56 156 95 396 380 161 131 177 298
5.5 9.0 7.2 7.3 3.3 9.2 4.9 7.0 3.4 3.1 5.3 6.7 4.6 3.2
9/9 7/6 9/9 7/7 6/6 7/6 7/7 6/6 6/6 7/7 6/6 6/5 8/7 8/8
42.6 359.2 345.7 355.7 341.2 10.5 17.8 35.4 348.5 312.0 337.7 354.4 9.1 8.6
374.9 331.6 318.0 37.3 322.6 369.1 371.6 356.5 382.6 384.5 374.1 378.6 381.7 379.4
The mean inclinations and declinations, as well as the precision parameter k, the K95 , the success rate, which is the number of samples treated (Ntr ) versus the samples used (N) for calculating the mean direction, and the VGP longitude/latitude values.
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Fig. 5. Examples of typical thermomagnetic curves. Most samples show a single Curie temperature between 500 and 580³C and reversible curves.
heating in zero ¢eld to the same temperature. Since the laboratory ¢eld was aligned along the z-axis of the sample a movement of the inclination in core coordinates towards the direction of the applied ¢eld (I = þ 90³) is a clear sign that one of the mechanisms causes failure (Fig. 7a). Like in the MT1 experiment pTRM-checks were done at every temperature step. A disadvantage of this method is that, in contrast to MT1, no directional information can be obtained for the case of a failing experiment.
pTRM at a given temperature is additionally demagnetized at the same temperature (Fig. 8). This technique has been described by McClelland and Briden [14]. By comparing the results of two demagnetization experiments before and after the pTRM acquisition the in£uence of the two mechanisms can be veri¢ed. A directional change indicates the presence of alteration products with higher blocking temperatures (Fig. 8b). A change in intensity accompanied by stable directions is an indication of a MD tail.
3.3.1.3. MT3 technique The MT3 experiment was developed in order to check whether MD remanence or alteration products with high blocking temperatures a¡ect the paleointensity determination in addition to normal alteration identi¢ed by pTRM checks. This method is based on the MT1 experiment with checks every second heating step. At some temperatures (200, 400, and 550³C) the acquired
3.3.2. Additional rock magnetic experiments In addition to Thellier experiments we performed additional rock magnetic measurements. With these experiments we tried to scrutinize our Thellier results in order to verify the results or to identify undetected mechanisms causing errors during the paleointensity determination. Kosterov and Pre¨vot [15] have reported irreversible changes due to alteration at moderate tempera-
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Fig. 6. Arai plots of some determinations of absolute paleointensity using the modi¢ed Thellier technique. f is the fraction of the NRM, g is the gap factor, and q the quality factor [25]. w is the weighting factor according to Pre¨vot et al. [19]. Measurements of the examples were performed in argon.
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Fig. 7. Examples for the MT2 method. The pTRM is acquired before demagnetization at the same temperature. In sample 1114B the acquired pTRM could not be removed during thermal demagnetization at the same temperature. The inclination of the demagnetization steps (in core coordinates) moves towards the applied ¢eld direction during the pTRM steps. This indicates the presence of MD particles or alteration products with higher blocking temperatures.
tures by measuring hysteresis loops after di¡erent heating steps. Using standard checks during the Thellier experiment these changes normally can not be observed, simply because the NRM lost below 300³C is very weak. Therefore we performed heating step dependent rock magnetic investigations on our samples. In addition to hysteresis loops and back¢eld curves [15] we performed AF demagnetization of an ARM (150 mT AC, 50 WT DC) after several heating steps. In Fig. 9 the AF demagnetization of an ARM at room temperature is plotted versus the AF demagnetization of an ARM after 200, 400 and 550³C. This is a standard plot for checking alteration using the Shaw method for paleointensity determination [16]. If the ARM demagnetizations are linearly related with a slope of one, then no alteration occurred. At least one sample from every £ow was treated in this way. Most samples do not show any signi¢cant alteration until 550³C (Fig.
9b,d). Samples from £ow C-TA8 (Fig. 9c) show a linear relationship but not with a gradient of 1. Between room temperature and 200³C a signi¢cant alteration occurred. This feature is not detected during the paleointensity determination (Fig. 6d). Therefore samples from this £ow were rejected from further analysis. 3.3.3. Reliability criteria The following criteria were used to obtain absolute paleointensity values from the Arai plots [17]: 1. The temperature range used for the calculation of a linear ¢t must be related to the characteristic remanent magnetization of the sample. Therefore the MAD [18] has to be less than 15³. 2. The segments used for the linear ¢ts consist of at least ¢ve successive points. 3. The segments cover a fraction of the NRM (f) of at least 20%.
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4. No alteration has occurred. Therefore the CK_error must not exceed 5% before and within the linear segment. An additional criteria for MT2 and MT3 experiments is: 5. The direction in core coordinates should not move towards the direction of the applied ¢eld before and within the linear segment. For MT3 experiments: 6. The intensity change between NRM(Ti ) and NRMre (Ti ) must not exceed 5%. The linearity of the segments is veri¢ed by the standard deviation. Paleointensity determinations with standard deviations slightly above 10% of the intensity value were only accepted in six cases, where the results re£ect the £ow mean. At least two absolute paleointensity determinations of
149
every £ow were used to calculate a weighted mean intensity according to Pre¨vot et al. [19]. The results of all samples and the corresponding mean values are summarized in Table 2. 3.3.4. Paleointensity results The paleointensity pattern across the sequence shows a minimum after the directional excursion with intensity values of V9 WT (Fig. 10). During the excursion, when the pole is at low latitudes, values of V27 WT are reached. The intensity before the excursion is V20 WT and recovers to the same value at the end of the section. The £ows CTA1 and C-TA11-14 do not seem to be related to the transitional state of the ¢eld. Therefore we used these ¢ve £ows to calculate a mid Miocene value of the VDM. The mean VDM corresponds
Fig. 8. A Thellier-type experiment with pTRM-checks every second demagnetization step is the basis of the MT3 experiment. At 200, 400 and 550³C demagnetization was repeated after acquisition of the pTRM at the same temperature. A change of direction between the demagnetization before (NRM(Ti )) and after the pTRM-step (NRMre (Ti )) indicates a creation of chemical remanence (b), a decrease in intensity the presence of MD remanence (c) [14].
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Fig. 9. Plots of ARM demagnetizations (at 20, 40 mT) at room temperature versus demagnetizations after heating to 200, 400 and 550³C. Linear behavior with slope 1.0 indicates that no alteration occurred during the experiment. The example for £ow 117 shows signi¢cant changes between 20 and 200³C which were undetected during the paleointensity experiment (Fig. 6d).
to (5.9 þ 0.8)U1022 Am2 . A similar value (V6U1022 Am2 ) was obtained in submarine basaltic glasses of Miocene age by Jua¨rez et al. [20]. The mean VDM of the VGPs close to the equator (C-TA3 and 4) is (5.6 þ 0.4)U1022 Am2 , which is nearly the same as for the non-transitional lava £ows. 4. Discussion and conclusion Detailed rock magnetic investigations and several modi¢ed Thellier type paleointensity methods showed that good quality results for the absolute paleointensity could be obtained from a mid Mio-
cene geomagnetic excursion record. These results are not a¡ected by alteration during the experiment and/or the presence of MD particles. The usage of inert atmosphere during heating does not seem to have any advantage regarding the results. In the sampled basaltic sequence we found evidence for high paleointensities during intermediate states of the geomagnetic ¢eld. High intensities during excursions and reversals of the Earth's magnetic ¢eld have been observed previously. Shaw [21] reported strong geomagnetic ¢elds from an Western Icelandic polarity transition. Coe et al. [22] found high paleointensities during an excursion on Oahu, Hawaii. Relatively
EPSL 5664 8-12-00
0/3 2/4
3/4
0/1 0/1 2/3
2/2
3/3
5/5
C-TA5 C-TA6
C-TA7
C-TA8 C-TA9 C-TA10
C-TA11
C-TA12
C-TA13
mini mini mini inch mini inch mini inch mini mini inch mini inch mini inch mini mini
115-1 115-2uC 116-1 116-1D 116-3uB
119-1D 119-3 120-2C 120-3 121-1 121-3A 121-4uB 122-1B 122-2 122-2D 122-2uC 122-X
4/5
C-TA4
inch mini inch inch inch inch mini mini mini
112-1D 112-2 112-2D 112-4D 112-5D 113-1B 113-4 113-51A 113-51D
0/5 5/5
C-TA2 C-TA3
mini inch mini mini
4/4
C-TA1
Size
110-2 110-3B 110-5 110-51C
Success rate Sample
Flow
Table 2 Paleointensity results
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argon air argon air air
air argon air air air air argon air argon
argon air argon air
MT1 MT1 MT3 MT1 MT1 MT3 MT1 MT1 MT2 MT1 MT1 MT1
MT1 MT1 MT1 MT1 MT1
MT3 MT1 MT1 MT1 MT2 MT3 MT1 MT1 MT1
MT1 MT1 MT2 MT1
Atmosphere Type
200^550 100^600 100^450 200^500 300^500 100^450 300^524 200^550 350^500 200^440 200^524 200^435
300^500 300^524 200^500 20^490 20^553
300^450 20-470 100^520 20^520 100^435 20^450 20^435 20^524 20^475
200-470 200^520 200^475 300^524
9 12 8 7 6 8 7 9 5 5 8 5
6 7 7 8 10
5 8 7 9 6 9 7 9 8
6 8 6 7
Temperature range N
0.69 0.98 0.62 0.53 0.37 0.23 0.39 0.65 0.48 0.66 0.46 0.34
0.42 0.53 0.39 0.50 0.69
0.32 0.47 0.55 0.60 0.49 0.53 0.35 0.46 0.38
0.35 0.42 0.34 0.48
f
0.71 0.77 0.73 0.77 0.79 0.78 0.60 0.70 0.74 0.60 0.79 0.71
0.78 0.81 0.79 0.82 0.78
0.74 0.78 0.67 0.81 0.66 0.71 0.78 0.63 0.70
0.78 0.82 0.67 0.78
g
9.84 4.78 3.78 5.47 5.59
2.64 4.85 2.42 6.45 1.66 6.10 6.66 2.53 2.85
5.73 5.51 4.22 4.30
7.93 38.80 5.32 12.70 16.80 1.18 1.72 4.85 9.24 5.16 5.12 2.91
q
3.00 12.30 2.17 5.68 8.42 0.48 0.77 1.83 5.33 2.98 2.09 1.68
4.92 2.14 1.69 2.23 1.98
1.53 1.98 1.08 2.44 0.83 2.30 2.98 0.96 1.16
2.87 2.25 2.11 1.92
w
18.8 þ 1.17 11.8 þ 0.23 27.8 þ 2.37 19 þ 0.62 18.9 þ 0.32 18.7 þ 2.83 17.2 þ 2.32 23.5 þ 2.22 18.7 þ 0.71 15.8 þ 1.21 14.1 þ 1 17 þ 1.4
8.49 þ 0.28 9.21 þ 0.82 6.06 þ 0.49 10.9 þ 0.82 8.47 þ 0.81
23.7 þ 2.12 27.7 þ 2.08 33.7 þ 5.16 24.6 þ 1.84 33.4 þ 6.49 25.5 þ 1.56 28.3 þ 1.16 29.7 þ 3.42 25.9 þ 2.43
21.6 þ 1.02 25.7 þ 1.6 15.6 þ 0.83 19.2 þ 1.68
Hp þ S.D. (WT)
17.8 þ 3.6
18.8 þ 0.9
21.4 þ 6.2
13.2 þ 4.9
8.7 þ 2.4
8.7 þ 0.5
27.2 þ 2
27.4 þ 4.7
20.7 þ 4.2
Fp þ S.D. (WT)
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The £ows are listed in stratigraphic order from the bottom to the top. The success rate is number of successful determinations per £ow versus the number of used specimens. Size describes the type of the specimen, mini sample or inch sample. Atmosphere deciphers whether heating was performed in air or argon and type the used Thellier-type experiment. The temperature range speci¢es the interval used for paleointensity calculation and N the number of independent points in this interval. f, g, q, w were described in Fig. 6. Hp is the individual paleointensity estimate with standard deviation. Fp is the weighted mean paleointensity for the £ow using the weighting factor of Pre¨vot et al. [19].
22 þ 0.99 20.1 þ 2.4 19.3 þ 1.02 24.4 þ 1.86 0.80 0.69 0.76 0.74 0.31 0.38 0.31 0.24 7 10 7 6 300^520 100^500 200^500 200^480 MT1 MT3 MT1 MT1 air air argon air inch inch mini inch 4/4 C-TA14
123-2B 123-3C 123-4 123-4A
Success rate Sample Flow
Table 2 (continued)
Size
Atmosphere Type
Temperature range N
f
g
q
5.44 2.18 4.51 2.37
2.43 0.77 2.02 1.19
Hp þ S.D. (WT) w
21.4 þ 2.3
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152
high values are also reached during the Steens Mountain reversal [19]. In most of the other available transitional records of lava £ows the mean intensity appears to be reduced to 25% of its usual value [5]. At present it is still a matter of debate whether dipolar or non-dipolar components dominate the transitional ¢eld during excursions and reversals. Both theories will be discussed considering the characteristics of the observed excursion. On the one hand, the high paleointensities during the excursion could be explained by a dominantly dipolar ¢eld, where a signi¢cant proportion of the dipole energy is kept by equatorial dipole components (described by the Gauss coef¢cients g11 and h11 ). The relatively low values of the VDM of V2.5U1022 Am2 directly after the directional excursion argue against a simple rotating dipole without loss of dipole energy. The VGPs of the £ows with low VDMs plot near the pole. This gives strong evidence that the axial dipole component is dominating during this phase directly after the excursion. The relatively low ¢eld intensities may be explained by a dipole energy loss during the movement of the VGP from the equatorial position to the polar position. On the other hand the observation of the high VDMs during the intermediate state of the excursion raises the question whether it is possible for a non-dipole component to sustain high ¢eld intensities. Recent computer simulations of the Earth's dynamo [23] give further insight into the matter. Using a geodynamo model with uniform heat £ux at the core-mantle boundary the dipole moment is low during the transition. A di¡erent view is obtained by incorporating the thermal structure of the mantle. In this case, which considers heterogeneous boundary conditions from seismic tomography [24] very high intensity values are present during the transition despite that the non-dipole energy is high. Our record could have recorded such a local high ¢eld state with a strong vertical £ux producing VGP positions near the equator in the central Atlantic. Directly after the directional excursion the non-dipole energy is not dominating but still strong and no strong vertical £ux is present. This could explain the low paleointensity values. Then the dipolar
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Fig. 10. Plots of the VGP latitude and the paleointensity determinations across the sequence. Also shown are the corresponding VDMs across the sequence.
part of the ¢eld increases and the ¢eld strength recovers to the mid Miocene mean value of the VDM. Further records of the same excursion from dif-
ferent localities of the world are needed to test these hypotheses and to answer the question whether dipolar or non-dipolar components dominate the transitional Earth's magnetic ¢eld.
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Acknowledgements We thank H.-U. Schmincke for his support and suggestions of suitable sites during our ¢eld trips to Gran Canaria. B. Herr joined the ¢rst ¢eld trip and helped with the paleomagnetic ¢eld work. We pro¢ted from discussions with A. Muxworthy, N. Petersen and V. Bachtadse. Reviews by J. Shaw and one anonymous reviewer greatly improved the manuscript. The research was supported by a Grant from the Deutsche Forschungsgemeinschaft (He1814/9-1 and So72/67-2).[RV]
[12] [13] [14]
[15] [16]
References [1] K.A. Ho¡man, Dipolar reversal states of the geomagnetic ¢eld and core-mantle dynamics, Nature 359 (1992) 789^ 794. [2] K.A. Ho¡man, Transitional paleomagnetic ¢eld behavior: Preferred paths or patches?, Surv. Geophys. 17 (1996) 207^211. [3] J.J. Love, Statistical assessment of preferred transitional VGP longitudes on palaeo-magnetic lava data, Geophys. J. Int. 140 (2000) 211^221. [4] M. Pre¨vot, P. Camps, Absence of preferred longitude sectors for poles from volcanic records of geomagnetic reversals, Nature 366 (1993) 53^57. [5] R.T. Merrill, P.L. McFadden, Geomagnetic polarity transitions, Rev. Geophys. 37 (1999) 201^226. [6] H.-U. Schmincke, The Geology of Gran Canaria, in: G. Kunkel (Ed.), Biogeography and Ecology in the Canary Islands, Junk, the Hague, 1976, pp. 67^184. [7] I. McDougall, H.-U. Schmincke, Geochronology of Gran Canaria, Canary Islands: Age of shield building volcanism and other magmatic phases, Bull. Volcanol. 40 (1976) 57^77. [8] P. van den Bogaard, H.-U. Schmincke, Chronostratigraphy of Gran Canaria, in: P.P.E. Weaver, H.-U. Schmincke, J.V. Firth, W. Du¤eld (Eds.), Proc. ODP Sci. Results, College Station, TX (Ocean Drilling Program), volume 157, 1998, pp. 127^140. [9] R.A. Fisher, Dispersion on a sphere, Proc. R. Soc. Lond. A 217 (1953) 295^305. [10] R. Day, M.D. Fuller, V.A. Schmidt, Hysteresis properties of titanomagnetites: Grain size and composition dependence, Phys. Earth Planet. Int. 13 (1977) 260^266. [11] E. Thellier, O. Thellier, Recherches ge¨omagne¨tiques sur
[17] [18] [19]
[20] [21] [22] [23]
[24]
[25]
des coule¨es volcaniques d'Auvergne, Ann. Ge¨ophys. 1 (1944) 37^52. E. Thellier, O. Thellier, Sur l'intensite¨ du champ magne¨tique terrestre dans le passe¨e historique et ge¨ologique, Ann. Geophys. 15 (1959) 285^376. R.S. Coe, Palaeointensity of the Earth's magnetic ¢eld determined from tertiary and quaternary rocks, J. Geophys. Res. 72 (1967) 3247^3262. E. McClelland, J.C. Briden, An improved methodology for Thellier-type paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence, J. Geophys. Res. 101 (1996) 21995^22013. A.A. Kosterov, M. Pre¨vot, Possible mechanisms causing failure of Thellier palaeointensity experiments in some basalts, Geophys. J. Int. 134 (1998) 554^572. J. Shaw, A new method of determining the magnitude of the palaeomagnetic ¢eld: Application to ¢ve historic lavas and ¢ve archaeological samples, Geophys. J. R. Astron. Soc. 39 (1974) 133^141. Y. Arai, Secular variation in intensity of the past geomagnetic ¢eld, MSc Thesis, University Tokyo, Tokyo, 1963. J.L. Kirschvink, The least-squares line and plane and the analysis of paleomagnetic data, Geophys. J. R. Astron. Soc. 62 (1980) 699^718. M. Pre¨vot, E.A. Mankinen, R.S. Coe, S. Gromme¨, The Steens Mountain (Oregon) geomagnetic polarity transition 2. Field intensity variations and discussion of reversal models, J. Geophys. Res. 90 (1985) 10417^10448. M.T. Jua¨rez, L. Tauxe, J.S. Gee, T. Pick, The intensity of the Earth's magnetic ¢eld over the past 160 million years, Nature 394 (1998) 878^881. J. Shaw, Strong geomagnetic ¢elds during a single polarity transition, Geophys. J. R. Astr. Soc. 40 (1975) 345^ 350. R.S. Coe, S. Gromme¨, E.A. Mankinen, Geomagnetic paleointenities from excursion sequences in lavas on Oahu, Hawaii, J. Geophys. Res. 89 (B2) (1984) 1059^1069. G.A. Glatzmaier, R.S. Coe, L. Hongre, P.H. Roberts, The role of the Earth's mantle in controlling the frequency of geomagnetic reversals, Nature 401 (1999) 885^890. R.S. Coe, L. Hongre, G.A. Glatzmaier, An examination of simulated geomagnetic reversals from a palaeomagnetic perspective, Phil. Trans. R. Soc. Lond. 358 (2000) 1141^ 1170. R.S. Coe, S. Gromme¨, E.A. Mankinen, Geomagnetic paleointensities from radiocarbon-dated lava £ows on Hawaii and the question of the Paci¢c nondipole low, J. Geophys. Res. 83 (1978) 1740^1756.
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High absolute paleointensity during a mid Miocene excursion of the Earth's magnetic ¢eld Roman Leonhardt a; *, Felix Hufenbecher a , Franz Heider b , Heinrich C. So¡el a b
a Institut fu«r Allgemeine und Angewandte Geophysik, Theresienstr. 41, University Munich, 80333 Munich, Germany Department of Geological Sciences, 230 Wallace Building, University of Manitoba, Winnipeg, Man., Canada R3T 2N2
Received 13 June 2000; received in revised form 28 August 2000; accepted 16 October 2000
Abstract A record of an excursion of the Earth's magnetic field was found in a V14.3 Myr old lava sequence of the shield basalts from Gran Canaria (Canary Islands). The thermal and alternating field demagnetization of the samples from 14 subsequent lava flows revealed mainly single component magnetizations and sometimes minor viscous overprints. Four consecutive flows showed intermediate paleodirections which represent an excursion of the virtual geomagnetic pole (VGP) from the South pole towards the equator in the central Atlantic. Rock magnetic and microscopic analysis revealed high temperature oxidized titanomagnetites as the dominant carrier of remanence. On the basis of reversibility of thermomagnetic curves and limited variation of susceptibility during thermal demagnetization, we selected suitable samples for three slightly different Thellier-type paleointensity methods. The results show a minimum in paleointensity after the directional excursion with intensity values of V9 WT. With the pole approaching the equator, values of V27 WT are reached. The intensity at the beginning and at the end of the section is V20 WT corresponding to a virtual dipole moment (VDM) of 6U1022 Am2 . The high intensities during the VGP excursion with a pole near the equator are explained by the same VDM. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: paleomagnetism; paleointensity; magnetic ¢eld
1. Introduction Since the second half of the 20th century the paleomagnetic community investigated variations of the Earth's magnetic ¢eld during reversals and excursions. These records of the variations in di-
* Corresponding author. Tel.: +49-89-2394-4231; Fax: +49-89-2394-4205; E-mail: [email protected]
rection and intensity are well known for representing important constraints for the geodynamo. So far the analysis of many di¡erent reversals sampled at di¡erent locations has shown that there are notable similarities in the directional data [1^3]. However, signi¢cant variations between reversals of di¡erent age observed at di¡erent locations as well as between the same reversal were also detected [4]. In comparison to the determination of paleodirections the extraction of absolute paleointensities is much more di¤cult. Despite this, there is more agreement among
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 3 1 1 - 3
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paleomagnetists regarding the variation of intensity during reversals and excursion. The available data from volcanic rocks, which are unfortunately few in number, point to a 75% reduction of the transitional ¢eld intensity compared to normal or reversed states [5]. In order to contribute to the ongoing debate we sampled a basaltic sequence in a V14.3 Myr old section of the shield basalts from Gran Canaria (Canary islands). Within a succession of 14 lava £ows, an excursion of the Earth's magnetic ¢eld was found. These samples were subjected to various paleo- and rock magnetic experiments in order to investigate the directional behavior and the associated paleointensities before, during and after the excursion. 2. Geology, sampling and methodology The sampled sequence is located on the west coast of Gran Canaria in the Barranco de Tasartico (Fig. 1) on a hill on the north side of the road to Playa del Asno. This sequence consists of ba-
saltic to more di¡erentiated lava £ows with a thickness from 1 to 4 m, belonging to the Guigui formation of the Gran Canaria shield basalts [6]. The lava £ows of the shield basalts were erupted within a time interval of 0.1^0.5 Myr [7]. Since the shield building phase ended between 13.95 and 14.1 Myr ago (40 Ar/39 Ar-ages [8]), the age range for the excursion is 14.0^14.6 Myr. The stratigraphy is shown in Fig. 2. The £ows are identi¢ed by the presence of basal and top scorias (AA-£ows) or by red lapillus (Pahoehoe-£ows). Between £ow C-TA2 and C-TA3 a debris accumulation is present (Fig. 2), which thins out in westward direction. The absence of paleosoils and/or erosional unconformaties suggests a relatively rapid extrusion of the 14 lava £ows. The £ows were sampled by taking oriented drill cores using a magnetic and a sun compass for orientation. Whenever possible the cores were drilled in the middle of the £ow with a spacing of one to two meters between di¡erent cores. Ninety-nine samples were stepwise demagnetized with at least 10 steps using alternating ¢eld (AF) and thermal demagnetization. Measure-
Fig. 1. Map of the site location. The sampled sequence is located near the west coast of Gran Canary in the Barranco de Tasartico at a elevation of 120 m. The black cross marks the exact location of the sequence.
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143
e¡ective in determining the characteristic remanent magnetization. Examples of demagnetization plots are shown in Fig. 3. Most of the specimens are characterized by a single component magnetization. Only few specimens (Fig. 3a) showed weak secondary components usually removed at demagnetization steps of 200³C or 20 mT. At least ¢ve samples from every £ow were used to calculate mean directions according to Fisher [9]. In Table 1 a summary of the directional data is shown. The K95 values are all less than 10³ associated with k values exceeding 50, which underlines the high quality of the directional data. By converting the mean directions to virtual geomagnetic poles (VGPs) the related pole of a dipolar ¢eld can be calculated. The VGP is found to move towards the equator in the central south Atlantic during the excursion, from reverse states of the Earth's magnetic ¢eld before and after this movement (Fig. 4). 3.2. Rock magnetic results
Fig. 2. Stratigraphy of the sequence. Between £ow C-TA2 and 3, a paleodebris accumulation is present.
ments of the remanence were conducted using a Molspin spinner magnetometer. In addition, a set of rock magnetic experiments was performed on at least one sample of every £ow. These measurements included thermomagnetic curves, hysteresis loops, and back¢eld curves using a variable ¢eld translation balance (VFTB). All Thellier-type experiments were conducted with a MMTD20 thermal demagnetizer.
Plotting the hysteresis parameters in a Day plot [10] results in a clustering of the samples within the pseudo single domain (PSD) grain size range. Thermomagnetic (Ms T) curves in a saturation ¢eld (V400 mT) revealed Curie temperatures between 500 and 570³C (Fig. 5). Additional intermediate Curie temperatures (200^400³C) were only detected in samples from £ow C-TA2 and C-TA5. Most of the samples showed reversible Ms T-curves for the heating and cooling cycle. This emphasizes their suitability for the determination of absolute paleointensities. Microscopy investigation was performed in order to identify the magnetic phases. Two samples from £ow 115 and 122 have been used for re£ected light microscopy studies. These samples show that the magnetic phase evolved from the primary Ti-magnetite by migration of titanium to ilmenite lamellae during high temperature oxidation.
3. Magnetic measurements
3.3. Paleointensity determination
3.1. Paleodirectional results
On the basis of reversible thermomagnetic curves and/or limited variation of susceptibility during thermal demagnetization only three £ows,
AF and thermal demagnetization were equally
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namely C-TA2, C-TA5, and C-TA9, seem unsuitable for paleointensity determination. The viscosity index [11] was determined for 20% of the collection and was in all cases less than 5%. For the experiments two di¡erent kinds of specimens were used, standard inch samples and small `mini samples' with a diameter of 0.5 cm. Both types of samples showed similar paleointensity results. Overall 26 mini samples and 23 standard 1-inch samples were used for the experiments. The bene¢t of using mini samples is that
the time for a heating cycle is reduced by a factor of 10. Heating was performed either in air or in an argon-rich atmosphere. We could not detect any di¡erences between the experiments in both atmospheres, even for the mini samples where the surface to volume ratio is much greater than for inch samples. 3.3.1. Methodologies For this study several di¡erent modi¢cations of the Thellier^Thellier technique [12] have been
Fig. 3. Typical Zijderveld plots of some samples during thermal and AF demagnetization. Most samples are characterized by a single component magnetization (b^d). Only in a few cases (a) weak secondary components have been observed.
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used to assess absolute paleointensities. At ¢rst, measurements of the absolute paleointensity were done according to the method of Coe [13], further referred to as MT1. However, there are two possible mechanisms that can cause failure to the experiment which cannot be deciphered using the MT1 method. These mechanisms are multidomain (MD) remanences and alteration products with higher blocking temperatures than the actual heating step. In order to test whether these mechanisms bias the paleointensity result we additionally used two slight modi¢cations for the paleointensity determination (MT2 and MT3). 3.3.1.1. MT1 technique The ¢rst method consists of stepwise demagnetization followed by pTRM-checks and pTRMsteps (Fig. 6). During most determinations, each heating step was followed by a pTRM-check. Thirty-four samples were subjected to this standard method. In order to quantify the grade of alteration during the experiment we calculated a relative error of the pTRM-checks at the temperature Ti : CKerror T i 100UMpTRM CK T i 3 pTRM T i M=TRM ab
Fig. 4. The directional excursion corresponds to a movement of the VGP from a reverse state towards the equator in the central South Atlantic.
with TRMab as the intersection of the best ¢tting line with the pTRM axis. 3.3.1.2. MT2 technique For the MT2 method a pTRM was induced at a speci¢c temperature before demagnetizing at the same temperature (Fig. 7). If none of the above mentioned mechanism is present the induced pTRM should be completely removed during
Table 1 Paleodirectional results Flow
Site number
Declination (³)
Inclination (³)
k
K95 (³)
Ntr /N
VGPlong (³)
VGPlat (³)
C-TA1 C-TA2 C-TA3 C-TA4 C-TA5 C-TA6 C-TA7 C-TA8 C-TA9 C-TA10 C-TA11 C-TA12 C-TA13 C-TA14
110 111 112 113 114 115 116 117 118 119 120 121 122 123
166.4 165.4 178.0 160.7 183.6 170.8 169.7 154.4 179.4 183.2 181.8 177.9 176.3 175.4
335.1 47.7 62.7 69.4 58.7 317.4 323.5 39.6 336.8 340.6 323.3 331.0 336.6 333.3
87 59 52 70 404 56 156 95 396 380 161 131 177 298
5.5 9.0 7.2 7.3 3.3 9.2 4.9 7.0 3.4 3.1 5.3 6.7 4.6 3.2
9/9 7/6 9/9 7/7 6/6 7/6 7/7 6/6 6/6 7/7 6/6 6/5 8/7 8/8
42.6 359.2 345.7 355.7 341.2 10.5 17.8 35.4 348.5 312.0 337.7 354.4 9.1 8.6
374.9 331.6 318.0 37.3 322.6 369.1 371.6 356.5 382.6 384.5 374.1 378.6 381.7 379.4
The mean inclinations and declinations, as well as the precision parameter k, the K95 , the success rate, which is the number of samples treated (Ntr ) versus the samples used (N) for calculating the mean direction, and the VGP longitude/latitude values.
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Fig. 5. Examples of typical thermomagnetic curves. Most samples show a single Curie temperature between 500 and 580³C and reversible curves.
heating in zero ¢eld to the same temperature. Since the laboratory ¢eld was aligned along the z-axis of the sample a movement of the inclination in core coordinates towards the direction of the applied ¢eld (I = þ 90³) is a clear sign that one of the mechanisms causes failure (Fig. 7a). Like in the MT1 experiment pTRM-checks were done at every temperature step. A disadvantage of this method is that, in contrast to MT1, no directional information can be obtained for the case of a failing experiment.
pTRM at a given temperature is additionally demagnetized at the same temperature (Fig. 8). This technique has been described by McClelland and Briden [14]. By comparing the results of two demagnetization experiments before and after the pTRM acquisition the in£uence of the two mechanisms can be veri¢ed. A directional change indicates the presence of alteration products with higher blocking temperatures (Fig. 8b). A change in intensity accompanied by stable directions is an indication of a MD tail.
3.3.1.3. MT3 technique The MT3 experiment was developed in order to check whether MD remanence or alteration products with high blocking temperatures a¡ect the paleointensity determination in addition to normal alteration identi¢ed by pTRM checks. This method is based on the MT1 experiment with checks every second heating step. At some temperatures (200, 400, and 550³C) the acquired
3.3.2. Additional rock magnetic experiments In addition to Thellier experiments we performed additional rock magnetic measurements. With these experiments we tried to scrutinize our Thellier results in order to verify the results or to identify undetected mechanisms causing errors during the paleointensity determination. Kosterov and Pre¨vot [15] have reported irreversible changes due to alteration at moderate tempera-
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147
Fig. 6. Arai plots of some determinations of absolute paleointensity using the modi¢ed Thellier technique. f is the fraction of the NRM, g is the gap factor, and q the quality factor [25]. w is the weighting factor according to Pre¨vot et al. [19]. Measurements of the examples were performed in argon.
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Fig. 7. Examples for the MT2 method. The pTRM is acquired before demagnetization at the same temperature. In sample 1114B the acquired pTRM could not be removed during thermal demagnetization at the same temperature. The inclination of the demagnetization steps (in core coordinates) moves towards the applied ¢eld direction during the pTRM steps. This indicates the presence of MD particles or alteration products with higher blocking temperatures.
tures by measuring hysteresis loops after di¡erent heating steps. Using standard checks during the Thellier experiment these changes normally can not be observed, simply because the NRM lost below 300³C is very weak. Therefore we performed heating step dependent rock magnetic investigations on our samples. In addition to hysteresis loops and back¢eld curves [15] we performed AF demagnetization of an ARM (150 mT AC, 50 WT DC) after several heating steps. In Fig. 9 the AF demagnetization of an ARM at room temperature is plotted versus the AF demagnetization of an ARM after 200, 400 and 550³C. This is a standard plot for checking alteration using the Shaw method for paleointensity determination [16]. If the ARM demagnetizations are linearly related with a slope of one, then no alteration occurred. At least one sample from every £ow was treated in this way. Most samples do not show any signi¢cant alteration until 550³C (Fig.
9b,d). Samples from £ow C-TA8 (Fig. 9c) show a linear relationship but not with a gradient of 1. Between room temperature and 200³C a signi¢cant alteration occurred. This feature is not detected during the paleointensity determination (Fig. 6d). Therefore samples from this £ow were rejected from further analysis. 3.3.3. Reliability criteria The following criteria were used to obtain absolute paleointensity values from the Arai plots [17]: 1. The temperature range used for the calculation of a linear ¢t must be related to the characteristic remanent magnetization of the sample. Therefore the MAD [18] has to be less than 15³. 2. The segments used for the linear ¢ts consist of at least ¢ve successive points. 3. The segments cover a fraction of the NRM (f) of at least 20%.
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4. No alteration has occurred. Therefore the CK_error must not exceed 5% before and within the linear segment. An additional criteria for MT2 and MT3 experiments is: 5. The direction in core coordinates should not move towards the direction of the applied ¢eld before and within the linear segment. For MT3 experiments: 6. The intensity change between NRM(Ti ) and NRMre (Ti ) must not exceed 5%. The linearity of the segments is veri¢ed by the standard deviation. Paleointensity determinations with standard deviations slightly above 10% of the intensity value were only accepted in six cases, where the results re£ect the £ow mean. At least two absolute paleointensity determinations of
149
every £ow were used to calculate a weighted mean intensity according to Pre¨vot et al. [19]. The results of all samples and the corresponding mean values are summarized in Table 2. 3.3.4. Paleointensity results The paleointensity pattern across the sequence shows a minimum after the directional excursion with intensity values of V9 WT (Fig. 10). During the excursion, when the pole is at low latitudes, values of V27 WT are reached. The intensity before the excursion is V20 WT and recovers to the same value at the end of the section. The £ows CTA1 and C-TA11-14 do not seem to be related to the transitional state of the ¢eld. Therefore we used these ¢ve £ows to calculate a mid Miocene value of the VDM. The mean VDM corresponds
Fig. 8. A Thellier-type experiment with pTRM-checks every second demagnetization step is the basis of the MT3 experiment. At 200, 400 and 550³C demagnetization was repeated after acquisition of the pTRM at the same temperature. A change of direction between the demagnetization before (NRM(Ti )) and after the pTRM-step (NRMre (Ti )) indicates a creation of chemical remanence (b), a decrease in intensity the presence of MD remanence (c) [14].
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Fig. 9. Plots of ARM demagnetizations (at 20, 40 mT) at room temperature versus demagnetizations after heating to 200, 400 and 550³C. Linear behavior with slope 1.0 indicates that no alteration occurred during the experiment. The example for £ow 117 shows signi¢cant changes between 20 and 200³C which were undetected during the paleointensity experiment (Fig. 6d).
to (5.9 þ 0.8)U1022 Am2 . A similar value (V6U1022 Am2 ) was obtained in submarine basaltic glasses of Miocene age by Jua¨rez et al. [20]. The mean VDM of the VGPs close to the equator (C-TA3 and 4) is (5.6 þ 0.4)U1022 Am2 , which is nearly the same as for the non-transitional lava £ows. 4. Discussion and conclusion Detailed rock magnetic investigations and several modi¢ed Thellier type paleointensity methods showed that good quality results for the absolute paleointensity could be obtained from a mid Mio-
cene geomagnetic excursion record. These results are not a¡ected by alteration during the experiment and/or the presence of MD particles. The usage of inert atmosphere during heating does not seem to have any advantage regarding the results. In the sampled basaltic sequence we found evidence for high paleointensities during intermediate states of the geomagnetic ¢eld. High intensities during excursions and reversals of the Earth's magnetic ¢eld have been observed previously. Shaw [21] reported strong geomagnetic ¢elds from an Western Icelandic polarity transition. Coe et al. [22] found high paleointensities during an excursion on Oahu, Hawaii. Relatively
EPSL 5664 8-12-00
0/3 2/4
3/4
0/1 0/1 2/3
2/2
3/3
5/5
C-TA5 C-TA6
C-TA7
C-TA8 C-TA9 C-TA10
C-TA11
C-TA12
C-TA13
mini mini mini inch mini inch mini inch mini mini inch mini inch mini inch mini mini
115-1 115-2uC 116-1 116-1D 116-3uB
119-1D 119-3 120-2C 120-3 121-1 121-3A 121-4uB 122-1B 122-2 122-2D 122-2uC 122-X
4/5
C-TA4
inch mini inch inch inch inch mini mini mini
112-1D 112-2 112-2D 112-4D 112-5D 113-1B 113-4 113-51A 113-51D
0/5 5/5
C-TA2 C-TA3
mini inch mini mini
4/4
C-TA1
Size
110-2 110-3B 110-5 110-51C
Success rate Sample
Flow
Table 2 Paleointensity results
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argon air argon air air
air argon air air air air argon air argon
argon air argon air
MT1 MT1 MT3 MT1 MT1 MT3 MT1 MT1 MT2 MT1 MT1 MT1
MT1 MT1 MT1 MT1 MT1
MT3 MT1 MT1 MT1 MT2 MT3 MT1 MT1 MT1
MT1 MT1 MT2 MT1
Atmosphere Type
200^550 100^600 100^450 200^500 300^500 100^450 300^524 200^550 350^500 200^440 200^524 200^435
300^500 300^524 200^500 20^490 20^553
300^450 20-470 100^520 20^520 100^435 20^450 20^435 20^524 20^475
200-470 200^520 200^475 300^524
9 12 8 7 6 8 7 9 5 5 8 5
6 7 7 8 10
5 8 7 9 6 9 7 9 8
6 8 6 7
Temperature range N
0.69 0.98 0.62 0.53 0.37 0.23 0.39 0.65 0.48 0.66 0.46 0.34
0.42 0.53 0.39 0.50 0.69
0.32 0.47 0.55 0.60 0.49 0.53 0.35 0.46 0.38
0.35 0.42 0.34 0.48
f
0.71 0.77 0.73 0.77 0.79 0.78 0.60 0.70 0.74 0.60 0.79 0.71
0.78 0.81 0.79 0.82 0.78
0.74 0.78 0.67 0.81 0.66 0.71 0.78 0.63 0.70
0.78 0.82 0.67 0.78
g
9.84 4.78 3.78 5.47 5.59
2.64 4.85 2.42 6.45 1.66 6.10 6.66 2.53 2.85
5.73 5.51 4.22 4.30
7.93 38.80 5.32 12.70 16.80 1.18 1.72 4.85 9.24 5.16 5.12 2.91
q
3.00 12.30 2.17 5.68 8.42 0.48 0.77 1.83 5.33 2.98 2.09 1.68
4.92 2.14 1.69 2.23 1.98
1.53 1.98 1.08 2.44 0.83 2.30 2.98 0.96 1.16
2.87 2.25 2.11 1.92
w
18.8 þ 1.17 11.8 þ 0.23 27.8 þ 2.37 19 þ 0.62 18.9 þ 0.32 18.7 þ 2.83 17.2 þ 2.32 23.5 þ 2.22 18.7 þ 0.71 15.8 þ 1.21 14.1 þ 1 17 þ 1.4
8.49 þ 0.28 9.21 þ 0.82 6.06 þ 0.49 10.9 þ 0.82 8.47 þ 0.81
23.7 þ 2.12 27.7 þ 2.08 33.7 þ 5.16 24.6 þ 1.84 33.4 þ 6.49 25.5 þ 1.56 28.3 þ 1.16 29.7 þ 3.42 25.9 þ 2.43
21.6 þ 1.02 25.7 þ 1.6 15.6 þ 0.83 19.2 þ 1.68
Hp þ S.D. (WT)
17.8 þ 3.6
18.8 þ 0.9
21.4 þ 6.2
13.2 þ 4.9
8.7 þ 2.4
8.7 þ 0.5
27.2 þ 2
27.4 þ 4.7
20.7 þ 4.2
Fp þ S.D. (WT)
R. Leonhardt et al. / Earth and Planetary Science Letters 184 (2000) 141^154 151
The £ows are listed in stratigraphic order from the bottom to the top. The success rate is number of successful determinations per £ow versus the number of used specimens. Size describes the type of the specimen, mini sample or inch sample. Atmosphere deciphers whether heating was performed in air or argon and type the used Thellier-type experiment. The temperature range speci¢es the interval used for paleointensity calculation and N the number of independent points in this interval. f, g, q, w were described in Fig. 6. Hp is the individual paleointensity estimate with standard deviation. Fp is the weighted mean paleointensity for the £ow using the weighting factor of Pre¨vot et al. [19].
22 þ 0.99 20.1 þ 2.4 19.3 þ 1.02 24.4 þ 1.86 0.80 0.69 0.76 0.74 0.31 0.38 0.31 0.24 7 10 7 6 300^520 100^500 200^500 200^480 MT1 MT3 MT1 MT1 air air argon air inch inch mini inch 4/4 C-TA14
123-2B 123-3C 123-4 123-4A
Success rate Sample Flow
Table 2 (continued)
Size
Atmosphere Type
Temperature range N
f
g
q
5.44 2.18 4.51 2.37
2.43 0.77 2.02 1.19
Hp þ S.D. (WT) w
21.4 þ 2.3
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Fp þ S.D. (WT)
152
high values are also reached during the Steens Mountain reversal [19]. In most of the other available transitional records of lava £ows the mean intensity appears to be reduced to 25% of its usual value [5]. At present it is still a matter of debate whether dipolar or non-dipolar components dominate the transitional ¢eld during excursions and reversals. Both theories will be discussed considering the characteristics of the observed excursion. On the one hand, the high paleointensities during the excursion could be explained by a dominantly dipolar ¢eld, where a signi¢cant proportion of the dipole energy is kept by equatorial dipole components (described by the Gauss coef¢cients g11 and h11 ). The relatively low values of the VDM of V2.5U1022 Am2 directly after the directional excursion argue against a simple rotating dipole without loss of dipole energy. The VGPs of the £ows with low VDMs plot near the pole. This gives strong evidence that the axial dipole component is dominating during this phase directly after the excursion. The relatively low ¢eld intensities may be explained by a dipole energy loss during the movement of the VGP from the equatorial position to the polar position. On the other hand the observation of the high VDMs during the intermediate state of the excursion raises the question whether it is possible for a non-dipole component to sustain high ¢eld intensities. Recent computer simulations of the Earth's dynamo [23] give further insight into the matter. Using a geodynamo model with uniform heat £ux at the core-mantle boundary the dipole moment is low during the transition. A di¡erent view is obtained by incorporating the thermal structure of the mantle. In this case, which considers heterogeneous boundary conditions from seismic tomography [24] very high intensity values are present during the transition despite that the non-dipole energy is high. Our record could have recorded such a local high ¢eld state with a strong vertical £ux producing VGP positions near the equator in the central Atlantic. Directly after the directional excursion the non-dipole energy is not dominating but still strong and no strong vertical £ux is present. This could explain the low paleointensity values. Then the dipolar
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153
Fig. 10. Plots of the VGP latitude and the paleointensity determinations across the sequence. Also shown are the corresponding VDMs across the sequence.
part of the ¢eld increases and the ¢eld strength recovers to the mid Miocene mean value of the VDM. Further records of the same excursion from dif-
ferent localities of the world are needed to test these hypotheses and to answer the question whether dipolar or non-dipolar components dominate the transitional Earth's magnetic ¢eld.
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Acknowledgements We thank H.-U. Schmincke for his support and suggestions of suitable sites during our ¢eld trips to Gran Canaria. B. Herr joined the ¢rst ¢eld trip and helped with the paleomagnetic ¢eld work. We pro¢ted from discussions with A. Muxworthy, N. Petersen and V. Bachtadse. Reviews by J. Shaw and one anonymous reviewer greatly improved the manuscript. The research was supported by a Grant from the Deutsche Forschungsgemeinschaft (He1814/9-1 and So72/67-2).[RV]
[12] [13] [14]
[15] [16]
References [1] K.A. Ho¡man, Dipolar reversal states of the geomagnetic ¢eld and core-mantle dynamics, Nature 359 (1992) 789^ 794. [2] K.A. Ho¡man, Transitional paleomagnetic ¢eld behavior: Preferred paths or patches?, Surv. Geophys. 17 (1996) 207^211. [3] J.J. Love, Statistical assessment of preferred transitional VGP longitudes on palaeo-magnetic lava data, Geophys. J. Int. 140 (2000) 211^221. [4] M. Pre¨vot, P. Camps, Absence of preferred longitude sectors for poles from volcanic records of geomagnetic reversals, Nature 366 (1993) 53^57. [5] R.T. Merrill, P.L. McFadden, Geomagnetic polarity transitions, Rev. Geophys. 37 (1999) 201^226. [6] H.-U. Schmincke, The Geology of Gran Canaria, in: G. Kunkel (Ed.), Biogeography and Ecology in the Canary Islands, Junk, the Hague, 1976, pp. 67^184. [7] I. McDougall, H.-U. Schmincke, Geochronology of Gran Canaria, Canary Islands: Age of shield building volcanism and other magmatic phases, Bull. Volcanol. 40 (1976) 57^77. [8] P. van den Bogaard, H.-U. Schmincke, Chronostratigraphy of Gran Canaria, in: P.P.E. Weaver, H.-U. Schmincke, J.V. Firth, W. Du¤eld (Eds.), Proc. ODP Sci. Results, College Station, TX (Ocean Drilling Program), volume 157, 1998, pp. 127^140. [9] R.A. Fisher, Dispersion on a sphere, Proc. R. Soc. Lond. A 217 (1953) 295^305. [10] R. Day, M.D. Fuller, V.A. Schmidt, Hysteresis properties of titanomagnetites: Grain size and composition dependence, Phys. Earth Planet. Int. 13 (1977) 260^266. [11] E. Thellier, O. Thellier, Recherches ge¨omagne¨tiques sur
[17] [18] [19]
[20] [21] [22] [23]
[24]
[25]
des coule¨es volcaniques d'Auvergne, Ann. Ge¨ophys. 1 (1944) 37^52. E. Thellier, O. Thellier, Sur l'intensite¨ du champ magne¨tique terrestre dans le passe¨e historique et ge¨ologique, Ann. Geophys. 15 (1959) 285^376. R.S. Coe, Palaeointensity of the Earth's magnetic ¢eld determined from tertiary and quaternary rocks, J. Geophys. Res. 72 (1967) 3247^3262. E. McClelland, J.C. Briden, An improved methodology for Thellier-type paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence, J. Geophys. Res. 101 (1996) 21995^22013. A.A. Kosterov, M. Pre¨vot, Possible mechanisms causing failure of Thellier palaeointensity experiments in some basalts, Geophys. J. Int. 134 (1998) 554^572. J. Shaw, A new method of determining the magnitude of the palaeomagnetic ¢eld: Application to ¢ve historic lavas and ¢ve archaeological samples, Geophys. J. R. Astron. Soc. 39 (1974) 133^141. Y. Arai, Secular variation in intensity of the past geomagnetic ¢eld, MSc Thesis, University Tokyo, Tokyo, 1963. J.L. Kirschvink, The least-squares line and plane and the analysis of paleomagnetic data, Geophys. J. R. Astron. Soc. 62 (1980) 699^718. M. Pre¨vot, E.A. Mankinen, R.S. Coe, S. Gromme¨, The Steens Mountain (Oregon) geomagnetic polarity transition 2. Field intensity variations and discussion of reversal models, J. Geophys. Res. 90 (1985) 10417^10448. M.T. Jua¨rez, L. Tauxe, J.S. Gee, T. Pick, The intensity of the Earth's magnetic ¢eld over the past 160 million years, Nature 394 (1998) 878^881. J. Shaw, Strong geomagnetic ¢elds during a single polarity transition, Geophys. J. R. Astr. Soc. 40 (1975) 345^ 350. R.S. Coe, S. Gromme¨, E.A. Mankinen, Geomagnetic paleointenities from excursion sequences in lavas on Oahu, Hawaii, J. Geophys. Res. 89 (B2) (1984) 1059^1069. G.A. Glatzmaier, R.S. Coe, L. Hongre, P.H. Roberts, The role of the Earth's mantle in controlling the frequency of geomagnetic reversals, Nature 401 (1999) 885^890. R.S. Coe, L. Hongre, G.A. Glatzmaier, An examination of simulated geomagnetic reversals from a palaeomagnetic perspective, Phil. Trans. R. Soc. Lond. 358 (2000) 1141^ 1170. R.S. Coe, S. Gromme¨, E.A. Mankinen, Geomagnetic paleointensities from radiocarbon-dated lava £ows on Hawaii and the question of the Paci¢c nondipole low, J. Geophys. Res. 83 (1978) 1740^1756.
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