Absolute paleointensity and reversal records from the Waianae sequence (Oahu, Hawaii, USA)

Earth and Planetary Science Letters 234 (2005) 279 – 296 www.elsevier.com/locate/epsl

Absolute paleointensity and reversal records from the Waianae sequence (Oahu, Hawaii, USA) Emilio Herrero-Berveraa, Jean-Pierre Valetb,T a

Paleomagnetic and Petrofabric Laboratory, SOEST-Hawaii, Institute of Geophysics and Planetology, 1608 East West Rd. Honolulu, HI 96822, University of Hawaii at Manoa, USA b Laboratoire de Ge´omagne´tisme et Pale´omagne´tisme, (UMR 7577) Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France Received 9 September 2004; received in revised form 18 February 2005; accepted 19 February 2005 Available online 25 April 2005 Editor: R.D. van der Hilst

Abstract Paleointensity experiments were performed on samples from four parallel sections of the Waianae volcanics of Oahu (Hawaii) that document 0.4 Myr of geomagnetic field variations. The records of the three successive Gilbert–Gauss, Lower and Upper Mammoth reversals confirm that large oscillations of directions precede or follow the reversals, which are similar to waveforms generated by paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole. Determinations of absolute paleointensity were attempted on 546 samples. We only selected data which were obtained from segments covering a large part of the remanent magnetization (more than 70% on the average) and without concave-up Arai diagrams. This procedure limited the success rate to 14% but provided consistent and reliable paleointensities. In addition to other time intervals, the results document the field variations surrounding the lower Mammoth transition. A weak field period dominated before the reversal, then the transition was initiated by a transit from normal to reverse polarity followed by a short restoration of the field intensity in reverse polarity. A second episode of a very weak field was accompanied by a return to positive inclinations before definitely reaching the reverse polarity. A strong and apparent rapid recovery of the dipole following the completion of the reversal culminated at a value of 16  1022 Am2 similar to field intensities reported for the other detailed volcanic records of reversals studied so far. The asymmetry between the pre- and the post-reversal phases indicates the importance of field regeneration to initiate a new stable polarity interval. Published by Elsevier B.V. Keywords: reversals; paleointensity; geomagnetic field; paleomagnetism

1. Introduction T Corresponding author. Tel.: +33 14427 3566; fax: +33 14427 7463. E-mail addresses: [email protected] (E. Herrero-Bervera), [email protected] (J.-P. Valet). 0012-821X/$ - see front matter. Published by Elsevier B.V. doi:10.1016/j.epsl.2005.02.032

One of the first observations derived from paleomagnetic studies related to the field behavior was that reversals were always accompanied by a large decrease

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in field intensity. This led to the suggestion that nondipolar components [1–3] had become dominant during the transitional period. A few years after the publication of the first reversal records from volcanics, the development of cryogenic magnetometers favored the acquisition of a large number of data from sedimentary sequences, which, in principle, provided continuous descriptions of the field variations. However the relatively low deposition rates combined with some smoothing introduced over the stratigraphic thickness of the samples were incompatible with the rapid field variations of the non-dipole components. In addition, the low field intensity prevailing during reversals may not provide enough magnetic energy to align a large number of magnetic particles [4]. Thus, volcanic records of reversals appear as being more appropriate to decipher the mechanisms of reversals. After more than 30 yrs of exploration by the paleomagnetic community, only a few reversals could be studied from sequences with many overlying lava flows. This could be seen as an indirect indication that the field reverses over quite a short period of time, probably in the same range as the secular variation [5,6] does. Another observation is that no successive reversals were studied so far from a unique pile of superimposed lavas at the same location. A direct consequence is the lack of information about the field variations and especially the changes in the field intensity that prevail during a complete period of polarity separating two transitions, as well as during the critical periods which precede and follow reversals. This opportunity is offered by the sequences of the Waianae volcano on the island of O’ahu (Hawaii) which contain successive reversals, but also duplicate records of the same events in parallel sequences. The sections are sufficiently distant from each other being associated with different eruptions, and thus recorded the successive events with a different resolution. This approach allows us to reduce uncertainties inherent to the irregular character of the volcanic eruptions. In previous papers [7,8], we reported on the directional changes across two successive reversals recorded in four parallel volcanic sequences of flows. In this paper, we add the results obtained for the third (upper Mammoth) reversal and present the determinations of absolute paleointensity that were obtained

from a total of 546 experiments performed on a selection of lavas flows spanning the entire sequence.

2. Sampling and correlations between sequences The Wai’anae volcano (Fig. 1a) in the western part of the island of O’ahu has a total exposed thickness of about 2000 m and is divided into the Wai’anae and the Kolekole volcanics [9–12]. The Wai’anae volcanics include lavas with Potassium–argon ages between 3.8 Ma and 2.4 Ma [13–16] which have been formally subdivided into the Lualualei member (3 J–3.5 Ma), the Kamai’leunu member (3.5–3.2 Ma) and the youngest member, the Palehua member. All sections are deep successions of very well-exposed lava flows which are uncovered by vegetation and thus relatively accessible. Given the impressive number of flows we have concentrated the very detailed samplings to the flows within or at the proximity of reversals. Thus the resolution of the sampling depends on the location. On the average we sampled one every other three lava flows and all flows associated with the transitions. At least 8 distinct samples 10 to 15 cm long were taken from each flow using a portable drill. All samples were sun oriented. Surveys using a magnetic gradiometer were systematically performed prior to drilling in order to detect and avoid zones that were affected by lightning. The various samplings can be described from the oldest to the most recent section. Sampling of the Lualualei member was concentrated in the Pu’u Heleakala section. Thirty six different units were sampled in this section which contains a record of the Gilbert–Gauss (3.58 Ma) transition. Nineteen reverse directions precede 8 successive transitional directions with latitudes of the virtual geomagnetic poles (VGPs) lower than 508. Four of these directions with VGP latitudes between 248 and 348N do not markedly differ from each other and may thus reflect a period of faster eruption rate. The next two sections Pu’u Paheehee and Pu’u Kamai’leunu respectively belong to the Kamai’leunu member and incorporate 29 sampled lava flows. The largest sampling (98 flows) was done at Pu’u Kea’au and includes more flows than the two other sections in both its lower and upper parts. There is no overlap with Pu’u Heleakala. Very few directions were

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281

a)

b) 1000

214m

Meters

380m

130m

800

138

323m

196

209 Upper Mammoth

133 Pu'u Kaulakauila

141

Lower Mammoth

91 Pu'u Paheehee 51 Pu'u Kea'au

600 84 Pu'u Kamai'leunu 447m 400

36 29 Gauss-Gilbert

200

0

1

Pu'u Heleakala Fig. 1. a) Schematic map with the location of the sections (after 8 and 9) and stratigraphic column of the Wai’anae range. The stratigraphy is shown with respect to the geomagnetic time scale ([10,11] and Sinton, 2004, personal communication). b) Polarity columns of each section according to the results published in [7,8]. The results of Pu’u Kaulakauila are described in the present paper. The composite sequence was constructed after correlating the paleomagnetic directions for each section. The thickness of the sampled intervals is shown in the upper left side of each column. The numbers on the right side of the columns indicate the flow numbers within the composite sequence. Succeeding numbers indicate the succession of the sampled flows but they do not necessarily represent overlying flows (depending on the resolution of the sampling).

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recorded above the Gilbert–Gauss transition in Pu’u Heleakala and they are different from those measured in the lower part of Pu’u Kea’au. The lowest part of the Pu’u Paheehee section was previously sampled by [15] who performed preliminary paleointensity experiments using the Thellier technique. They sampled also the nearby Pu’u Mailiilii sequence which is considered as equivalent to Pu’u Paheehee. However, due to severe problems of alteration, very few estimates of absolute paleointensity could be obtained from this study. The detailed stratigraphy and correlations between the five sections were established on the basis of the successive paleomagnetic directions recorded in each of them. After such correlation was done, all units were numbered as a function of their occurrence in the stratigraphic column. Each section is represented in Fig. 1b by its polarity column. Using the correlation between all sections and the thickness of the successive lava flows we composed the compounded thickness of the entire sampled sequence. The cumulative thickness of the flows varies linearly with the correlation number, and each section is characterized by the same slope with the exception of Pu’u Heleakala and the lower part of Pu’u Kamai’leunu. In these two parts the samplings were indeed performed with lower resolution while the flows were given successive numbers in the final correlation. Additional sampling is being done to improve further the correlations with very high resolution and to fill in the gap between the top of Heleakala and the base of Pu’u Kamaileunu.

3. Transitional directions The Lower Mammoth reversal was recorded in detail in the three sections but with different successions of transitional directions. The first transitional episode is a rapid and well documented transit from normal to steep negative inclinations while the declination remains to the north and then moves abruptly to south. The next phase is a return to positive inclinations with reverse declinations still pointing south, and followed by a final switch of inclination to full reverse polarity. Thanks to these large directional changes, the three sections were relatively easily correlated. As expected from the

number of lava flows and the stratigraphic thickness, the Pu’u Kea’au sequence covers a much longer period than the other two. Based on this correlation and on the mean eruption rate, we estimate that the time missing between the lower part of the samplings at Pu’u Kea’au and the upper part of Pu’u Heleakala is of the order of 0.1 Ma. Forty-five flows were recently sampled in the youngest section called Pu’u Kaulakauila which contains a record of the Upper Mammoth transition. The correlation of Pu’u Kaulakauila with the upper part of Pu’u Kea’au is much less straightforward. One of their common characteristics is the presence of large inclination swings. We consider that they are not linked to complex processes affecting the magnetization, as there are no significant changes in the rock magnetic parameters and also because these swings are present in the two sections with a different resolution. They probably reflect large secular variation. The record of the Upper Mammoth transition in the Pu’u Kaulakauila section (Table 1) has not been published before. Similarly to the other two reversals the transition seems to have occurred in at least two distinct phases. The polarity change was preceded by an excursion with southwestern declination values and low inclinations. The reversal is completed by a rapid switch of the declination while the inclination remained low for a certain period and finally moved to positive values. It is not clear whether the very end of the transition had been recorded. Except for the excursional period, the VGP path is not detailed enough to define a trajectory and thus does not support any comparison with other paths. Interestingly, the three reversals shown in Fig. 2 are characterized by two clusters of their transitional VGPs. The first one lies over northwest Australia and could thus be associated with the frequent occurrence of VGPs close to Australia [17] reported in the litterature. The second one lies over northwestern Africa and was also reported before [18]. Note however that the clusters do not always occupy exactly the same location. Following a previous suggestion [18] we noted that these longitudes corresponded with the distribution of maximum inclination anomalies (and thus larger inclinations which bring the VGPs close to these longitudes) predicted by time-averaged field models. Additional

E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296 N

Table 1 Flow mean directions and poles from the new Pu’u Kaulakauila section which recorded the upper Mammoth polarity transition Unit Thickness Dec. 134 135 136 139 140 142 144 147 149 151 153 154 156 158 160 162 164 167 171 174 179 181 182 184 186 187 189 190 191 193 194 195 197 198 199 200 201 202 203 204 205 206 207 208 209

10 13 17 22 26 31 36 43 48 53 59 64 68 75 85 91 106 113 123 128 130 133 137 141 144 148 151 153 155 157 158 159 161 163 164 167 169 174 176 186 192 197 202 209 214

191.2 196.4 201.4 181.6 180.4 178 135.1 184.9 200.1 197.6 184.1 189 185.7 185.4 189.8 170 169 166.8 174.6 179.3 211.9 208 188.1 173.1 172.7 189.3 169.5 249.6 255.7 251.3 233.9 248.8 196.5 202.7 4.9 15.7 7.7 8.4 5.8 8 8.6 11.9 10.7 9.7 6.8

Inc. 31.5 27 46 21.2 19.1 23 42.2 29.6 21.2 34.1 37.9 15.5 12.3 03.2 26.3 27.7 27.6 11.5 14.9 45.3 51.3 41.6 6.8 11.3 14.9 36.5 53.1 12.2 7.7 11.8 5.1 10.4 7 20.4 4.8 7.9 9 4.9 10 24.5 28.7 29.4 30.4 26.9 50.5

kappa alpha95 VGPLat VGPLong 309 259 119 292 358 173 256 278 380 298 118 252 214 66 260 172 41 89 343 14 265 164 236 80 92 164 103 276 180 308 363 114 121 470 134 92 230 272 118 104 20 123 183 116 597

4.4 7.7 11.4 5.4 4.9 7.1 4.8 5.5 6.3 5.3 2.1 4.8 8.4 10 5.7 1.9 19.6 9.8 5 21.6 4.7 6 6 8.7 9.7 6 9.1 7.4 5.7 4.4 6.5 7.2 7 11.5 5.8 9.7 5.1 4.6 7.1 6.9 16.8 8.3 9.2 11.5 5.1

78.6 72.9 69.7 79.5 78.42 80.4 48.7 82.77 68.1 73.26 86.18 73.94 73.85 66.4 78.02 78.38 77.5 69.84 74.69 84.56 59.74 64.08 70.36 72.9 74.49 81.24 74.63 16.47 11.82 15.01 32.12 17.57 60.31 51.06 70.41 66.84 71.55 69.37 72.7 78.53 79.82 77.38 78.69 78.32 78.43

132.41 133.7 90.6 193.3 199.94 213.7 304.95 161.14 137.15 118.6 113.13 167.83 181.11 188.31 149.16 258.4 260.87 243.13 222.61 15.3 84.72 100.91 177.16 226.26 229.99 117.42 347 125.56 12.88 124.64 138.51 125.3 166.99 164.77 7.15 338.55 356.94 357.43 2.11 64.9 76.59 87.23 316.6 328.14 232.23

Unit, corrleation number of the successive units in the entire sequence; Thickness, relative position of the lava flows sampled from the base; N, total number of specimens demagnetized, Dec and Inc, Flow mean declinations and inclinations; kappa, Fisher precision parameter; alpha95, Fisher circles of 95% confidence, VGP Lat and Long, Latitude and Longitude of the Virtual Geomagnetic Poles, in degrees.

283

3.22Ma Upper Mammoth R-N

Site

N

3.33Ma Lower Mammoth N-R

N

3.6Ma Gilbert-Gauss R-N

Site

Site

1

Fig. 2. Paths of the virtual geomagnetic poles obtained from the most detailed sections that recorded each successive transition. Note the existence of clusters in the vicinity of Indonesia–northern Australia and northern Africa.

data are evidently needed before drawing any firm conclusion on this matter. Due to uncertainties in the correlation between the Pu’u Kea’au and the Pu’u Kaulakauila sections, the paleointensity study was focused on Pu’u Kaulakauila.

4. Methods and techniques for determination of absolute paleointensity Paleointensity experiments were conducted using the Coe version of the Thellier and Thellier [19] experiments but with a different protocol. Instead of measuring the NRM first, we preferred to apply a TRM before heating the sample in zero field [20,21]. In this case magnetomineralogical transformations occur in presence of the field resulting in a CRM (chemical remanent magnetization) component which is detected by a deviation of the NRM toward the

E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296

direction of the field in the oven. The opposite situation (demagnetization in zero field first) does not allow one to detect remagnetization components with unblocking temperatures higher than the last temperature step Ti [22] because the remagnetized grains keep a zero magnetization. They will be involved in the magnetization acquired during the following step (Ti+1), but remain undetectable and ultimately yield incorrect paleofield determinations. We have performed pTRM checks regularly at (Ti 1) after each heating step. It is obvious that this procedure is lengthy and also very time-consuming. However this is a very efficient way to detect changes in mineralogy or grain size that affect the determination of absolute paleointensity. The experiments were conducted in a Pyrox oven with a capacity of 80 samples. Heating regulation was driven by three external thermocouples and accurate temperature control monitored by three additional thermocouples located close to the samples. Cooling was performed by sliding the heating chamber away from the hot specimens. Measurements were done using a JR-S fluxgate magnetometer in the shielded room of the SOEST-HIGP Petrofabrics and Paleomagnetics laboratory. Each series of experiments included between 40 and 50 samples which were positioned a few millimeters away from each other within the oven. The magnetization level of the samples was too low to expect significant interactions between adjacent samples within the oven. Indeed, no remagnetization was observed in the demagnetization diagrams of the NRM, except those caused by chemical remagnetization in presence of the field.

curves indicate also whether magnetite with a very small titanium content or no titanium dominates the magnetization. Given the large number of samples and lava flows that were measured in the present study, we did not perform systematic thermomagnetic experiments. The diagrams in Fig. 3 indicate typical cases with reversible curves for which successful determinations of paleointensity were obtained using a twin

a)

Upper Mammoth Pu'u Kaulakauila 0.7 0.6

SD

0.5

Mr/Ms

284

0.4

PSD

0.3 0.2 0.1 0

MD 0

1

2

3

4

5

Hcr/Hc

b)

Heleakala 0.6 SD 0.5

5. Selection of data and calculation of absolute paleointensity Various techniques have been proposed to select appropriate data for paleointensity measurements. Thermomagnetic experiments performed in low or high fields are certainly the most efficient approach. The presence of reversible heating and cooling curves tells us that no mineralogical changes have happened during heating and that the samples are likely to be appropriate. In contrast, the opposite situation is incompatible with paleointensity experiments. These

Mr/Ms

0.4 PSD 0.3

0.2

0.1

MD 0

0

1

2

3

4

5

6

Hcr/Hc Fig. 3. Typical thermomagnetic curves showing that magnetite carries the natural remanence of the samples.

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Susceptibility (arbitrary units)

120

UM083P2 100 80 60 40 20 0 0

100

200

300

400

500

600

700

Temperature (°C) Susceptibility (arbitrary units)

250 UM017A

200

150

100

50

0 0

100

200

300

400

500

600

700

Temperature (°C) 400

Susceptibility (arbitrary units)

specimen from the same core. All samples were characterized by almost pure magnetite. Magnetic grain sizes can also be indicative of the behavior of the samples upon demagnetization and thus of the stability of magnetization. The Day plots shown in Fig. 4 reveal single (SD) and pseudo-single domain (PSD) grain sizes. However, a similar distribution can be caused by a mixture of single and multidomain grains, although not excluding the presence of stable carriers of magnetization. Most recent papers dealing with absolute paleointensity discussed new criteria to obtain reliable determinations. This yielded different critical values of the parameters that were initially defined by Coe et al. [23]. There is no doubt that stringent criteria for these parameters improve the quality of the determinations. However, there are also other basic requirements which should be considered with great attention, which is not always the case in the selection procedures. Since paleointensity determinations rely on a vector component, the directional aspects cannot be neglected. First of all, it is essential that the demagnetization of the vector be anchored to the origin. Various criteria [24–26] have been discussed to avoid acceptance of diagrams that lie to far away from the origin. Any deviation from the origin introduces a significant uncertainty in the determination of paleointensity as it reflects failure to isolate the primary TRM. When plotted in sample coordinates, the evolution of the NRM direction also gives crucial information about possible deviations caused by CRM with unblocking temperatures higher than the last heating step. Samples that exhibited remagnetization during heating in the presence of the field are characterized by large directional swings of the remaining NRM towards the direction of the applied field. In the present study this was the case for almost 50% of the samples. This is probably also a consequence of our severe protocol with TRM acquisition preceding demagnetization in zero field. The demagnetization diagrams are evidently important to indicate the temperature range over which the primary TRM has been isolated. Samples with a stable thermoremanent magnetization carried by single domain grains of magnetite are not characterized by a large viscous component, which does not represent more than 30– 40% of the initial TRM (except for a few transitional

285

PA153P1 350 300 250 200 150 100 50 0 0

100

200

300

400

500

600

700

Temperature (°C) Fig. 4. Grain sizes approximations using hysteresis parameters are consistent with single (SD) and pseudo-single domain (PSD) grains. However they could also represent a mixing of single domain and multidomain (MD) particles.

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HE008A

M/Mmax Mmax = 5.14e 0 A/m

1.0

W Up

5 NRM (x1e 0 A/m)

286

0.5 Scale: 1e 0 A/m S 0.0

N

100 200 300 400 500 600

4 3 2 1 0

E Down

0

1 2 3 4 TRM (x1e 0 A/m)

5

M/Mmax 2

Mmax = 1.82e 0 A/m 1.0 W Up S

N

0.5

E Down

NRM (x1e 0 A/m)

HE086A

1

Scale: 1e 0 A/m 0.0

0

100 200 300 400 500 600

1 TRM (x1e 0 A/m)

2

M/Mmax Mmax = 1.18e 0 A/m

UM064C

1.0

S

N

0.5

NRM (x1e 0 A/m)

W Up 1

Scale: 1e 0 A/m E Down 0.0 100 200 300 400 500 600

0

1

2

3

4

5

6

TRM (x1e 0 A/m)

MA081C1

M/Mmax W Up

Mmax = 5.06e 0 A/m

5 S

N Scale: 1e 0 A/m

0.5

0.0 100 200 300 400 500 600

E Down

NRM (x1e 0 A/m)

1.0

4 3 2 1 0 TRM (x1e+1 A/m)

1

7

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287

M/Mmax Mmax = 1.59e 0 A/m 1.0

W Up 0.5 S

N E Down

NRM (x1e 0 A/m)

UM176D

1

Scale: 1e 0 A/m 0.0

0

100 200 300 400 500 600 MA501B

M/Mmax W Up

N NRM (x1e 0 A/m)

S

0.5

E Down 0.0

3

2

Mmax = 4.35e 0 A/m 1.0

1 2 TRM (x1e 0 A/m)

1

Scale: 1e 0 A/m

0

100 200 300 400 500 600

1 TRM (x1e 0 A/m)

2

PA204A1

M/Mmax Mmax = 6.47e 0 A/m

7

W Up

1.0

S

0.5

NRM (x1e 0 A/m)

6

N

5 4 3 2 1

0.0

0

100 200 300 400 500 600

E Down

M/Mmax

PA200B2-IS Mmax = 5.08e 0 A/m

Scale: 1e 0 A/m

0

1

2 3 4 5 TRM (x1e 0 A/m)

0

1

2 3 4 5 6 TRM (x1e 0 A/m)

6

W Up 5

0.5 S

N

NRM (x1e 0 A/m)

1.0

4 3 2 1

0.0

0 100 200 300 400 500 600

Scale: 1e 0 A/m E Down

Fig. 5. From left to right: NRM intensity upon thermal demagnetization, demagnetization diagrams, and plots of NRM loss versus TRM gained during paleointensity experiments for typical samples with different polarities from different sections. After removal of a viscous component which is mostly observed in samples with reverse or intermediate polarity, the selected diagrams are characterized by an univectorial component. The segment of NRM used in the paleointensity calculation represents more than 50% of the initial NRM.

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or close by) samples with very weak magnetization intensity but directions still pointing north or south (e.g. UM 064C). Thus it seems reasonable that at least 50% of the total NRM intensity should be involved in the segment used for the determination of absolute paleointensity. Another direct consequence linked to the importance of a large NRM segment is to discard concave up diagrams that are usually typical of multidomain grains. We have retained only Arai plots with a single linear slope within the temperature range that lie above the viscous domain. Different options have been proposed to determine the limit of acceptance of pTRM checks (pTRM checks were considered to be positive when they did not deviate by more than 5% from the initial TRM). We believe that this question is not fundamental when pTRM checks are performed almost systematically after every heating step. Indeed, acceptance of a small deviation of the pTRM checks from the initial TRM can be caused by an actual change in mineralogy or accidentally by experimental uncertainties. In the first case, the following step will almost be systematically accompanied by a larger deviation, which confirms that the sample must be rejected. We performed a total of 546 complete experiments of absolute paleointensity. In Fig. 5 we show typical diagrams selected for the determination of absolute paleointensity. For each sample we plot the evolution of NRM intensity with temperature, the demagnetization diagram and the Arai plot. A common feature to almost all diagrams is that they display very similar, if not identical evolution of NRM with temperature. This shows that the same characteristics involving almost pure magnetite in fine grains and very little viscosity was a basic condition for successful experiments.

linearity of the NRM–TRM diagrams. Due to our severe selection rules, the averaged number of points used for slope determination amounts to 9 and is never smaller than 6, and the f parameters ( f = 72 F 12%) are always greater than 40%. Note however, that this parameter is meaningless when there is a strong viscous low temperature component with an opposite direction to the characteristic remanence, which is the case for a few transitional samples, and when the average quality factor (12 F 7) lies much beyond the limits of 1 or 2 that are usually required. The mean value of the b ratio (b = .06 F .4) measuring the relative uncertainty on the definition of the best-fit line is almost twice weaker than the acceptable limit of 0.1 [25]. The paleofield intensities fall between 6 and 84 AT for individual samples. Also given in Table 1 are the site mean values derived for each flow and the correlation number which was assigned to the successive sites in the construction of the composite sequence. Most flows are characterized by several determinations of paleointensity. The mean intra-flow dispersion of paleointensity estimates of 12 F 6.6% is much lower than the requirement of 25% [25]. One exception is for flow 19 which exceeds this value by 3% due to a sample which we do not have any reason to rule out. Note that the limit of 25% implicitly assumes that a dispersion as large as one-fourth the field value is acceptable and common in studies of paleointensity. Should we restrain this limit down to 10 or 15%, then a large number of data would probably be removed from many studies. This is not due to technical difficulties but to the intrinsic complex magnetization of volcanic rocks and probably also to the inhomogeneities of the field caused by the morphology of the lava flow [27,28]. In any case this dispersion is too large for extracting other features than those related to the dipole field.

6. Results The results listed in Table 2 summarize the ancient paleofield, the standard deviation and the f, g, q, and w parameters for each sample. Altogether 76 samples provided successful determinations of paleointensity, which corresponds to only a 14% success rate. Rejection of the samples was caused by the acquisition of CRM, presence of multidomain grains, negative pTRM checks and the absence of full

6.1. Absolute paleointensity across successive reversals and discussion In Figs. 6 and 7 we plot the field determinations of all specimens as a function of their occurrence numbers in the composite sequence. In the same figure are reported also the variations of inclination and declination up to the Upper Mammoth, but no field determination could be obtained across this reversal.

E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296

289

Table 2 Sample ID: name of the samples—initials indicate the section (HE=Pu’u Heleakala, MA= Pu’u Kea’au, PA= Pu’u Paheehee and UM = Pu’u Kaulakauila), Flow No.: flow number in the stratigraphic sequence after correlation of the sections, Tre inter.: temperature interval used to calculate the slope of the NRM–TRM diagrams Sample ID

Flow no.

Tre inter.

Paleofield

std

q

f

w

g

N points

Mean Paleo

std (F)

Std error

HE 008Aa HE 020Ca HE 019A HE 037Ba HE 086Aa HE 089B HE135Aa MA 496C MA 498B1 MA 499B1 MA 501B1 MA 502B1 MA 504B2 MA 506B2 MA 510 MA 034B MA 035D MA 036B MA 067C MA 066D MA 068E MA 069A MA 071B2 MA 074D2 MA 077D1 MA 081C1 MA 076C PA 006C PA 038A PA 041A PA 240A1 PA 241C2 PA 241D PA 242C1 PA 244A1 PA 246C1 PA 243Da PA 245A PA 247B PA 147A PA 152B PA 153B PA 154A PA 167Ba PA 174Da PA 186B UM 114Aa UM 111Da UM 111C PA 199B1

1 3 3 5 12 12 15 67 67 67 67 68 68 68 68 74 74 74 77 77 77 77 77 79 79 79 79 91 95 95 114 114 114 114 114 114 114 114 114 120 121 121 121 129 129 131 135 135 135 137

020–530 300–560 120–500 020–500 020–560 020–500 250–460 020–560 020–460 020–530 300–560 020–560 250–560 250–560 350–530 350–560 200–560 200–560 020–560 020–560 020–530 020–560 020–560 020–500 020–560 200–560 250–560 420–560 120–560 120–560 020–460 020–460 020–460 020–460 020–460 020–460 120–420 020–560 120–560 120–560 200–560 300–530 020–420 300–530 300–530 120–560 250–530 200–530 120–530 300–530

69.3 63.7 63.2 45.7 40 34 54 32.8 33.8 26.5 25.5 20.6 20.2 24.8 29 10 9.5 11.3 8.7 11.6 11 10.6 14 10.6 11 15.5 11.4 7.7 4.4 3.9 54 54 48.6 53.1 56 59.5 61 56 54.3 13.8 26 15.9 16.4 34 37 18 84.4 76.6 63.5 75

1.2 7.4 6 3.7 1.5 1 2.7 1 1.7 0.8 1.2 1 1.5 2.4 2.5 0.3 0.4 0.4 0.4 0.7 0.2 0.5 0.5 0.8 0.5 1 0.6 0.3 0.3 0.3 2.2 1.4 1 1.7 1.9 1.8 2.1 4.8 3.2 1.6 1.2 1.7 1.8 3.2 3.2 3.3 7.9 7.1 3.2 8.1

3.7 4.2 6 5 18 19 16 17 8.5 14 12 15 8.7 6.5 5.7 17 13 19 12 6 17 9.4 10 4.8 8 9 10 15 4.2 3.8 16 26 30 20 17 20 14 9 13 4.9 6.9 5.2 4.8 5.8 5.6 3.4 6.1 7.3 12 6

79 66.7 50 50 84 72.5 96.5 64 53 52 82 90 83 80 59 73 80 87.5 76 70 67 74 72 74 73 79 72 89 34 43 84 82 80 81 73 74 72 99 95 84 75 72 70 70 60 80 69 79 68 78

13 1.7 2.3 1.9 6 6.8 6.2 6.4 3.3 5.8 5 4.6 3.3 2.5 2.6 7.7 4.6 6.7 3.7 2 5.6 3 3.1 1.5 2.7 3.6 3.8 0.9 1.5 1.3 6.2 10 11 7.6 6.5 7.4 7 3.2 5.2 2.8 6.9 2.6 2.4 2.6 2.5 1.5 2.5 2.8 4.1 2.7

0.83 0.73 0.79 0.84 0.84 0.82 0.84 0.8 0.8 0.81 0.7 0.78 0.79 0.79 0.82 0.66 0.72 0.81 0.65 0.56 0.53 0.6 0.53 0.51 0.55 0.7 0.77 0.65 0.75 0.63 0.81 0.8 0.79 0.81 0.8 0.81 0.7 0.78 0.8 0.7 0.71 0.77 0.76 0.78 0.79 0.8 0.83 0.87 0.85 0.8

10 8 9 9 11 10 9 9 9 9 8 12 9 9 7 7 10 10 12 12 10 12 12 12 11 8 9 5 10 10 9 9 10 9 9 9 6 12 8 10 9 6 6 7 7 7 8 9 10 7

69.3 63.4

0.3

0.25

45.7 37

4.2

3

54 29.6

4.2

2.1

23.6

4.1

10

VADM

std

15.1 13.8

0.065

9.97 8.07

0.92

11.8 6.46

0.92

2

5.15

0.89

0.9

0.53

2.18

0.20

11.2

1.9

0.85

2.44

0.41

12.1

2.2

1.1

2.64

0.48

7.7 4.1

0.28

0.2

1.7 0.9

0.07 0.06

55.15

3.6

1.2

12.0

0.79

13.8 19.4

5.6

3.3

3.0 4.23

0.35 1.2

35.5

2.1

1.5

7.75

0.46

18 75

10.5

71

3.4

6

3.9 16.4

0.7 2.3

2

15.5

0.74

(continued on next page)

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E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296

Table 2 (continued) Sample ID

Flow no.

Tre inter.

Paleofield

std

q

f

w

g

N points

PA 200B2 PA 204A1 UM 137Aa UM 136B UM 176Da UM 178Ba UM 179B UM 177D UM 216B UM 219A UM 218B UM 217Aa UM 015D UM 017C UM 018B UM 016B UM 019A UM 026A UM 027B1 UM 064C UM 065Ba UM 063Da UM 071Ba UM 070C UM 072B UM 083B

137 137 144 144 156 156 156 156 171 171 171 171 179 179 179 179 181 182 182 197 197 197 198 198 198 200

350–530 300–530 120–530 Corrected 020–560 200–560 200–560 020–560 300–560 350–560 200–560 200–560 120–530 120–530 200–530 200–560 200–530 020–530 300–530 200–560 200–560 350–560 020–530 300–530 250–530 020–380

69 69 41 45 20.5 22.1 17.7 22.4 17.8 13.1 16 18.2 60 57 38.5 49 50 18.3 19.6 7.5 6.1 7.7 10.9 16 15.8 12.2

5.2 7 1.9 2.5 0.6 0.6 0.3 1 0.5 0.7 0.6 1.7 2.2 2.8 1.3 5.7 5.1 0.7 2.9 0.2 0.2 0.5 0.6 1.8 0.7 1.2

5.5 5 15 7.2 21 22 32 12 22 10 17 6.8 15 8.1 3.3 6 5 15 3.8 21 13 8 10 5 8.3 5.1

64 65 85 53 79 74 76.5 70 81.6 70 82 75 64 50 53 85 60 78 71 80 68 77 71 73 50 62

4 2.5 5.3 3.6 6.6 7.9 11 4 9.1 4.7 6.1 2.4 5.2 3 4.7 2.2 2 4.9 1.7 7.3 4.7 3.6 3.3 2 3.7 2.3

0.7 0.8 0.82 7.5 0.84 0.83 0.8 0.75 0.82 0.81 0.83 0.83 0.83 0.79 0.79 0.81 0.86 0.8 0.8 0.76 0.72 0.7 0.76 0.79 0.8 0.83

6 6 10 6 12 10 10 12 8 7 10 10 10 10 9 10 9 12 7 10 10 7 11 8 7 7

Mean Paleo

std (F)

Std error

VADM

std

43

2.8

2

9.38

0.61

20.6

2.15

1

4.50

0.47

16.2

2.3

1.1

3.54

0.50

51

9.6

4.8

11.1

2.1

50 19

0.9

0.65

10.9 4.15

1.1 0.20

7.1

0.8

0.5

1.55

0.17

14.2

2.8

1.6

3.10

0.61

12.2

2.66

The paleofield estimated for each sample is reported in AT with the number of data points (N points) involved in the calculation of the slope and their dispersion (sd). Also indicated are the quality factor q, the NRM fraction, f, and the gap factor g. Mean Paleo and std. (F): arithmetic mean of absolute paleointensity obtained for each lava flow with its standard deviation. VADM: corresponding value of the virtual axial dipole moment with its standard deviation (std).

Lastly the results were converted in terms of the virtual axial dipole moments (VADMs) and plotted in Fig. 8 with the latitudes of the VGPs. These results represent our best determination of absolute paleointensity for the 360-kyr-long period covered by the sampled sections. Unfortunately, no field determination was obtained for many flows but despite these partial results, some significant characteristics emerge from the data. The best documented interval is by far the period surrounding the Lower Mammoth (LM) transition, which is also the most detailed reversal record in terms of directional changes. The successive episodes of the reversal cannot be separated from the succession of the directional changes and must be interpreted jointly. Following standard criteria, the directions can be considered as being transitional when the VGP latitude is lower than 608. A period of low field intensity preceded the LM transition but we have no estimate of the time span before the first directional

changes. Following the transit of the inclination to steep negative values and the sudden motion of the declination to the south, there was a small recovery of field intensity which coincides with a short period of apparent full polarity. The next step displays a return to positive inclinations while the dipole decreased again, indicating that the field failed to reverse completely and reached a stable polarity. This succession bears similarities with the Reunion event recorded in Ethiopia [29]. Finally, the reversal was completed after an ultimate transit to negative inclinations expected at the site latitude. This stage was immediately followed by an apparently sudden and strong recovery of the dipole moment to almost twice a larger value than the present-day field (16  1022 Am2). Given the absence of a direct correlation, it is very delicate to attempt any comparison between the samplings that were performed by Laj et al. [15] in the Pu’u Paheehee and Pu’u Mailiilii sections with

E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296

a)

291

450

Declination (°)

400 350 300 250 200 150 100 0

b)

20

40

60

80

100

120

140

160

180

200

220

80 60

Inclination (°)

Gilbert-Gauss (3.58Ma)

Upper Mammoth (3.22Ma)

40 Lower Mammoth (3.33Ma)

20 0 -20 -40 -60 -80

0

c)

20

40

60

80

100

Gilbert-Gauss

120

140

160

180

200

220

Upper Mammoth

Lower Mammoth

Paleointensity (µT)

100 80 60

? Present-day field

40 20 0

0

20

40

60

80

100

120

140

160

180

200

220

Flow Number Fig. 6. a) and b) Declination and inclination variations as a function of flow number in the composite sequence constructed after correlating the individual sections. Each data point represents the mean direction of a single lava flow obtained from a minimum of eight individual samples. The sections are shown by different colors (Pu’u Heleakala in green, Pu’u Kea’au in black, Pu’u Kamai’leunu in blue, Pu’u Paheehee in red and Pu’u Kaulakauila in orange by order of appearance in stratigraphic succession). c) Composite record of absolute paleointensity with respect to the succession of polarity intervals (black/white = normal/reverse; grey = intermediate). The data points correspond to measurements of individual samples.

the present ones. The two determinations published in [15] at Pu’u Pahehee are characterized by declinations (1648 and 1718) and inclinations ( 398 and 348) which are similar to the directions of the

units numbered between 132 and 135. Unfortunately, we did not obtain any reliable determination of paleointensity from the corresponding flows. Note that the Arai diagram of the sample shown in [15] is

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E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296

a)

Lower Mammoth

100 80 60 40

Present-day field

20 0 80

90

100

110

120

130

140

150

160

Unit Number

b)

80 Microwave experiments

70 60 50 40 30 20 10 0

Thellier experiments

0

10

20

30

40

50

60

70

80

Paleointensity (µT)

Fig. 7. a) Comparison between the paleointensity variations derived from the microwave experiments (opened symbols) [30] and the present dataset (closed symbols) as a function of unit numbers. Each value represents the average paleointensity for one individual lava flow. b) Paleointensity values derived from the microwave experiments [30] as a function of the present determinations for the same flows. The agreement is better for low field intensities than for large values.

characterized by two slopes, the high temperature interval being used for calculation. Three determinations were published for the upper part of the Mailiilii section with directions that are similar to those from our unit 129. The averaged field value of 29 F 9 AT is not incompatible with our estimate of 35 F 2 AT. Interestingly, another study was recently performed on a succession of samples from 29 flows from the Pu’u Paheehee section [30], limited to the interval between flows 91 and 140. The major characteristic of this work was to propose determinations of absolute paleointensity using microwave experiments. The success rate of 61% is considerably larger than in the present case. The results of the microwave experiments are compared to the present ones in Fig. 7. With the exception of the upper part of their common interval, the two data sets are coherent

although the values remain mostly different. In fact, the lowest field intensities are similar and the largest discrepancies are observed for strong values. The most striking difference is for the interval encompassing units 18 and 26 which opposes very high Thellier paleointensities to low values derived from the microwave experiments. A possible explanation could be that the specimens used for the microwave specimens were taken from the upper part of the cores and they may not be exempt of alterations (e.g. chemical). We are very confident in the present determinations, since similar results were obtained using samples from two different sections. We observe that diagrams with double slopes were incorporated in the microwave results while they were rejected in our case, which may also explain the difference between success rates. Further comparisons between the two techniques will help clarify these aspects.

E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296

a)

293

90

VGP Latitude

60 30

Upper Mammoth (3.22Ma) Gilbert-Gauss (3.58Ma)

0

Lower Mammoth (3.33Ma)

-30 -60 -90

0

20

40

60

80

100

120

140

160

180

200

220

b)

Virtual axial dipole moment (1022Am2)

Flow number Gilbert-Gauss

Lower Mammoth

Upper Mammoth

20

15

Present GAD value

10

5 0

0

20

40

60

80

100

120

140

160

180

200

220

c)

Virtual axial dipole moment (1022Am2)

Flow Number Gilbert-Gauss

Lower Mammoth

20

15

10 Present GAD value

5

0 0

100

200

300

400

500

600

700

800

900

1000

Composite thickness (m) Fig. 8. a) VGP latitude as a function of the flow numbers in the composite sequence. b) Virtual axial dipole moments (VADM) derived from the averaged field value of each lava flow. The flow numbers are plotted as a function of the stratigraphic numbers and thus not linearly related to time. In c) theVADMs are shown as a function of the composite thickness which is (almost) linearly related to time (see text).

As discussed before, the lower Mammoth transition seems to display a two-stages process characterized by an Texcursionr followed by an apparent very rapid transition. Although the mean extrusion rate was constant over the entire time interval covered by the sampling of the sequence, the detailed

succession of the excursional and transitional directions is constrained by the timing of the eruptions. Since the two phases of the reversal were recorded with a different resolution in each section, the alternation of similar directions with rapid changes reflects periods of activity and quiescence of the

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E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296

volcano. Despite the absence of records during the first phase, the low field intensities recorded prior to the transition probably prevailed during the switch from north to south. This first phase was followed by a significant recovery of the intensity while the directions pointed south. The location of the VGP positions at very high latitudes and the paleointensity values indicate that the field restored a dipolar state during this very short period. Indeed, the intensity decreased again yielding the second Texcursionr phase, which is mostly observed on the inclination values. We interpret this phase as enhanced secular variation in presence of a low-dipole field. The end of the record is characterized by large recovery while the field stabilized in the reversed polarity. Large and sudden field recovery immediately following transitions has been reported in all detailed (i.e. transitions defined by at least 8 intermediate directions [18] recorded from a unique section of overlying flows) reversal records from volcanic sequences with a sufficient number of determinations of absolute paleointensity as well as in many records of relative paleointensity from sedimentary sequences [22]. Going back from the most recent to the oldest transitions studied so far from volcanic sequences, the VADMs following the Brunhes–Matuyama transition recorded in the Canaries [31], the 4 Ma reverse to normal (R–N) transition at Kauai [32] and the 15.3 Ma (R–N) reversal at Steens Mountain [33] are characterized by similar values between 15 and 16  1022 Am2. The 3.6 Ma record from Georgia [34] and the oldest record from Greenland [35] has a slightly lower field (11–12  1022 Am2). The overall presence of strong fields immediately accompanying the completion of the transition rules out the hypothesis of a coincidence that may be linked to the irregular temporal succession of the lava flows. In contrast, their presence in all sections is a strong indication that this phase was long enough to be recorded by at least one eruption. This observation agrees with classical schemes of the reversing field in which the reversal is completed, or in other words a new polarity is initiated when the core is entirely dominated by the new reverse flux. In contrast with the post-transitional period, the time intervals preceding and accompanying the Lower and the Upper Mammoth provide additional evidence that the field was very low before the directional

changes took place, as frequently suggested. This does not appear to be the case for the Gilbert–Gauss transition. However it is important to keep in mind that this period was not documented with the same resolution. We must refer to the plot of the field variations as a function of the stratigraphic thickness of the sequence (Fig. 8c) which is linearly related to the age. The long time interval preceding the transition is poorly documented by five data points and there is no result close to the transition. In the absence of detailed resolution for this period, we cannot draw any conclusion. Unfortunately, the resolution of the present record does not provide any indication about the detailed pattern of field intensity changes during the polarity intervals. The time interval between the Lower and the Upper Mammoth transition shows fluctuations that seem to be superimposed on an overall decreasing trend, not inconsistent with features observed in some sedimentary records [36]. However there is no linear relation between the flow number and the timing of the successive events so that any interpretation must be taken with great caution [37]. It is clear that additional results are also needed to constrain further the exact pattern of field intensity between the strong field associated with the beginning of a new polarity interval and the very low field prevailing before the following reversal.

7. Conclusion The present results confirm the fact that studies of absolute paleointensity of exposed lava flows can be characterized by very poor success rate when their determinations rely on stringent criteria. As a consequence, it is difficult to extract the pattern of the field variations during a complete period of stable polarity or during field reversals with appropriate resolution. This objective is also difficult to meet because of the irregular temporal succession of the volcanic eruptions. In the present case, we could combine the directional changes recorded in nearby piles of lava flows that were built up during the same period. This improved the resolution of the record and provided a better description of the successive episodes that prevailed during the Lower Mammoth transition. We observe that the evolution

E. Herrero-Bervera, J.-P. Valet / Earth and Planetary Science Letters 234 (2005) 279–296

of the directional changes during the transitional period is linked to the changes in dipole intensity. The reversal is characterized by a rapid switch of polarity, a subsequent very short restoration of the dipole and a new degradation of the field that yielded enhanced secular variation. We cannot exclude the possibility that the field could have reversed again during this second phase, then yielding a very short event. Another characteristic is the presence of a strong dipole strength culminating at 16  1022 Am2 immediately after the reversal, similarly to the determinations found for all other reversal records from piles of lava flows. This reinforces the suggestion that a strong field, implying that the entire core is dominated by the new reverse flux, is necessary to initiate a new stable polarity interval. The results are also consistent with the existence of weak field intensity prior to reversals. These observations appear to be in accordance with classical reversal schemes which depict the progressive degradation of the new polarity as being linked to the propagation of zones with opposite flux within the core. A recent compilation of the sedimentary records and analyses of the database for absolute paleointensity [24,38] indicate that the polarity intervals are characterized by large fluctuations and that this degradation would be initiated only about 80 kyr before the transition [36].

Acknowledgements We are grateful to Mr. James Lau for his field and laboratory assistance and help with the laboratory measurements. We also give special thanks to the people of Hawaii particularly those living in the Wai’anae Volcano areas for allowing us to do the field work during all these years. Data treatment and processing were performed using the Paleomac software [39]. Financial support to E. H-B was provided by SOEST-HIGP and by the National Science Foundation grants EAR-9909206, EARINT-9906221, EAR-0207787 and EAR-0213441. Financial support to J-P Valet was provided by the INSU-CNRS Interieur de la Terre program. This is a SOEST 6537, HIGP 1369 and an IPGP 2042 contribution. Tables are available electronically upon simple request at [email protected].

295

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