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Holocene paleosecular variation from dated lava flows on Maui (Hawaii)
Physics of the Earth and Planetary Interiors 161 (2007) 267–280
Holocene paleosecular variation from dated lava flows on Maui (Hawaii) Emilio Herrero-Bervera a , Jean-Pierre Valet b,∗ b
a SOEST-Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Paleomagnetics and Petrofabrics Laboratory, 1608 East West Rd. Honolulu, HI 96822, USA Institut de Physique du Globe de Paris, Plaeomagnetism, UMR7154 CNRS-Univ. Denis-Diderot, 75252 Paris Cedex 05, France
Received 17 May 2006; received in revised form 10 October 2006; accepted 28 February 2007
Abstract Low inclination and low secular variation seem to be present for at least the past 5 Myear in central Pacific. The period covering the past 10 kyear is crucial to the study of the field variability and to fill the gap between historical field measurements and long-term paleomagnetic records. We have conducted a paleomagnetic study of 13 sites of basaltic lava flows from Maui island with 14 C ages between 10.3 and 0.2 ka. Two other sites dated at 45 and 730 ka were also sampled. Eight to 10 samples from each site were demagnetized using thermal treatment and companion specimens from the same samples were demagnetized by alternating fields (af). Thermomagnetic and hysteresis measurements indicated that magnetite (575 ◦ C) in fine grains was the dominant magnetic carrier, although in many cases we also observed a low-temperature phase which is likely carried by titanomagnetite with low titanium content. The existence of relatively high coercivities associated with these two mineralogical phases generated overlapping components which could not be properly isolated using af demagnetization. Successful results were obtained after thermal demagnetization for 13 sites with a mean inclination of 36.9 ± 4◦ . The mean inclination (inc. = 36.3◦ ) of the eleven sites younger than 10.5 ka is also very close to the value (37◦ ) of the geocentric axial dipole (GAD) at the site latitude, but the angular dispersion of the virtual geomagnetic poles (VGPs) about the spin axis (9◦ with N = 13 and 6.7◦ with N = 11) is significantly lower than the predictions of the models of paleosecular variation at this latitude. The inclination variations for the past 10 kyear are in excellent agreement with the very detailed dataset which has previously been obtained from the big island of Hawaii. The mean inclination of all volcanic records (N = 132) is 1.8◦ lower than expected, but this is likely caused by the lack of records between 5 and 7 ka B.P. We note also that the inclinations from Lake Waiau sediments (big island) are shallower than those of the volcanic records (Mauna Kea, Hawaii) and were thus probably affected by compaction. We infer that there is no striking evidence for an inclination anomaly under Hawaii during this period, being aware of the need for additional records covering at least the 5–7 ka B.P. time interval. The absence of a systematic deviation going beyond the dispersion of the inclinations obtained for the past millions of years neither pleads for a significant long-term persistent anomaly. However, all studies report a low dispersion of the VGPs which must reflect low secular variation. © 2007 Elsevier B.V. All rights reserved. Keywords: Earth’s magnetic field; Paleomagnetism; Paleosecular variation
1. Introduction
∗
Corresponding author. Tel.: +33 1 44273566; fax: +33 1 44277463. E-mail address: [email protected] (J.-P. Valet).
0031-9201/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2007.02.008
It is important for our understanding of the processes involved in the generation of the geomagnetic field to determine whether some regions of the world
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would be characterized by long-term persistent anomalies. Doell and Cox (1963) studied historic lavas from Hawaii and concluded that the low geomagnetic secular variation observed in the Pacific from magnetic observatory data extended at least as far as 1750 a.d. With the addition of new data (Doell and Cox, 1972; Coe et al., 1978; Herrero-Bervera et al., 1986) from Hawaii a consistent picture of anomalously low paleosecular variation referred to as the Pacific dipole low emerged progressively. Doell and Cox (1972) noticed that the Brunhes inclinations in Hawaii were 4◦ smaller than the value predicted by the GAD and is thus consistent with strong persistent non-dipole fields. More recently, McWilliams (1982) and Miki et al. (1998) observed a low angular dispersion of the virtual geomagnetic poles (VGP) during the past 800 kyear and suggested that this was caused by a weak non-dipole field. Despite the large dominance of data from the Hawaiian islands, the question of the Pacific non-dipole low has been widely debated (McElhinny and Merrill, 1975; Duncan, 1975; Coe et al., 1978; Hagstrum and Champion, 1995). During the past decade, the emergence of global models aimed at describing the time-averaged field (Johnson and Constable, 1995, 1997; McElhinny et al., 1996a; McElhinny and McFadden, 1997; Kono, 1997; Carlut and Courtillot, 1998; Kelly and Gubbins, 1997) brought up additional and new interest to the question. Apart from the large dipole dominance there is evidence for the presence of a small axial quadrupole component with a size varying between 2.5 and 8% of the axial dipole term. Several authors consider further departures from the axial dipole as reflecting actual geomagnetic features, while others (Quidelleur and Courtillot, 1996; Carlut and Courtillot, 1998; McElhinny et al., 1996b) see them as artifacts linked to quality as well as temporal and spatial distribution of data. The first ones consider that the Hawaiian area exhibits one of the largest long-term inclination anomalies whereas the others do not validate its existence and consider that many early studies of paleosecular variation (McElhinny et al., 1996b) should be revisited using demagnetization procedures which meet the present paleomagnetic standards. Recent studies (Yamamoto et al., 2002; Elmaleh et al., 2001; Herrero-Bervera and Valet, 2002, 2003; Gratton et al., 2004; Teanby et al., 2002; Laj et al., 2002) have focused on this area in order to constrain the time persistence and the geographical extension of the anomaly, but they involve relatively old periods over which the variability of the field could not be fully explored due to dating uncertainties. Because the cut-off between the time constants associated with the non-dipolar and dipolar components does not exceed
a few hundred years (Hulot and LeMou¨el, 1994) it is important to study the field variability over the past 10 kyears to detect persistent field components. The Hawaiian database is dominated by an impressive succession of records (Hagstrum and Champion, 1994, 1995, 2002; Mankinen and Champion, 1993a,b) from 14 C dated lava flows of the Kilauea and Mauna Loa lava flows (Big Island) which describe in detail the past 5 kyear. Records from different islands are needed to eliminate local effects and to constrain possible field anomalies generated by the volcanic edifices. Series of flows can also be affected by tilting by a few degrees which are not detectable on the field but large enough to create a persistent inclination anomaly. This is particularly true when most records come from the same side of one island which can be affected by subsidence (Riley et al., 1999) due to loading of the underlying Pacific plate by the growing volcanic edifices. For these reasons we considered that it was important to complete the 0–10 kyear database with new results from a different location. The young and recently dated volcanic series of the Maui island offered a unique opportunity to meet this goal. 2. Sites and sampling The East Maui volcano is the youngest and largest (see Fig. 1) of the two edifices of the island of Maui. During shield and post-shield phases of volcanism on east Maui, eruptions occurred from three principal rift zones radiating away from the summit region. Following the profound erosion that occurred after post-shield Kula volcanism (a period perhaps no more than about 200,000–300,000 years long) only two of East Maui’s rift zones (the east or Hana rift and the SW rift zone) were reactivated with rejuvenated volcanism (Bergmanis et al., 2000; Sinton et al., 1987; Sherrod et al., 2003). Hana volcanism is the only example of rejuvenated volcanism in Hawaii that follows former rift zone structures. It also follows the shortest quiet period which occurred after the earlier post-shield volcanism, suggesting that the conduits preferred by earlier eruptions were still opened to magma migration during rejuvenation. The shield stage of East Maui volcano is represented by the Honomanu Volcanics (see Fig. 1), a suite of lavas that vary from tholeiitic to alkalic basalts. Both post-shield Kula and rejuvenation stage Hana rocks are alkalic, although the latter tend to have lower SiO2 contents and to be less differentiated. Carbon dating and geological evidence suggest that there have been at least five eruptions along the SW rift zone in the last 900 years. The last one was about 1790 a.d. on the lower SW rift zone.
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Fig. 1. Schematic plot of the area covered by the Hana volcanics modified from Bergmanis et al. (2000) showing the location and names of the sampling sites. Colours indicate the different volcanic units. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
The sampling sites were selected exactly at the same locations as those of the previous geochronological (i.e. 14 C dating) studies (Bergmanis et al., 2000). A total of 166 samples were drilled from 15 widely separated lavas (Fig. 1 and Table 1) using a portable gasoline-powered drill. Sun orientation was used for the entire sample collection. The samples were spread out laterally within each flow and taken with inclination angles between 10◦ and 60◦ away from the horizontal plane. 3. Laboratory experiments 3.1. Thermomagnetic measurements Studies of magnetic mineralogy were performed on at least one sample from each lava flow. Low-field susceptibility versus temperature (k–T) experiments were conducted in order to determine the Curie temperature.
Fifteen specimens were progressively heated from room temperature up to 700 ◦ C and subsequently cooled down using the KLY2-CS3 apparatus (Hrouda et al., 1997) of the SOEST-HIGP Petrofabrics and Paleomagnetics Laboratory. In Fig. 2 are shown some typical diagrams of susceptibility versus temperature (k–T). A first common characteristic to all plots is that none of them displays perfectly reversible curves. We can roughly classify the diagrams in two different categories. In the upper two plots (Fig. 2) magnetite is identified as the unique mineral and the shape of the cooling curve remains identical to the heating curve. The little offsets between the high temperatures of the cooling and the heating curves in Fig. 2a were most likely caused by a drift of temperature measurements rather than by small mineralogical variations. The amplitude of the cooling curve is much larger than during heating. Given the absence of other visi-
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Table 1 Paleomagnetic results of thermal demagnetization for each sampled site of the East Maui Hana volcanics Site
Age (ka)
HKOL HGTA HHAN HWHL HNAE HLAA HMAK HKAM HWKW Hale HKEA HLLL KU
0.2 0.83 0.89 0.92 2.18 3.02 3.54 4.07 19 8.19 10.3 45.0 703
HPAE HKEO
8.83 ± 0.06 10.4 ± 0.06
± ± ± ± ± ± ± ± ± ± ± ± ±
0.015 0.06 0.17 0.07 0.06 0.065 0.04 0.09 0.06 0.06 0.05 1.8 13.0
Long. (◦ W), lat. (◦ N)
N/Ns
Dec.
Inc.
Kappa
␣95
VGP long. (◦ E), VGP lat. (◦ N)
156.42, 20.60 156.32, 20.61 156.38, 20.63 156.35, 20.72 156.28, 20.60 156.25, 20.62 156.36, 20.62 156.36, 20.70 156.39, 20.38 156.30, 20.6 156.40, 20.68 156.31, 20.61 156.34, 20.73
10/12 08/11 04/10 10/10 09/14 09/11 08/10 08/11 10/11 17/10 04/13 07/11 05/09
368.7 362.9 361.9 359.8 365.8 360.2 349.8 361.5 359.4 366.2 361.3 360.4 360.7
36.6 33.8 41 48.6 31.8 20.7 38.7 38.5 43.1 32.2 33.7 43.7 36.6
024 364 420 239 172 077 388 065 150 164 151 363 067
9.9 2.9 4.5 3.1 3.9 5.9 2.8 6.9 4 4.7 7.5 3.2 9.4
293.7, 81.8 327.9, 86.7 232.7, 86.4 202.6, 81.0 324.3, 83.6 022.6, 80.1 124.0, 80.4 250.8, 88.1 196.8, 85.5 318.8, 83.5 353.9, 87.5 207.9, 85.0 312.9, 89.3
156.39, 20.68 156.40, 20.66
0/11 0/12
Age (ka) = calibrated ages (Bergmanis et al., 2000), the ages of sites HWHL and KU were interpolated on the basis of stratigraphy; long. (◦ W), lat. (◦ N) are west longitude and north latitude of the sites; N gives the number of vectors used in the calculation of site mean directions after thermal demagnetisation; Ns is the total number of thermally demagnetized specimens (we do not report here the results of af demagnetization since they were not used in the final results, see text); dec. and inc. are declination and inclination of mean paleomagnetic directions; Kappa is concentration parameter; ␣95 is radius of 95% confidence; VGP long. and lat. are north latitude and east longitude of virtual geomagnetic pole. No reliable results were obtained for sites HPAE and HKEA.
ble Curie point, this indicates production of additional magnetite during heating. The lower two diagrams display more complex behavior due to the presence of a low Curie temperature, probably linked to titanomagnetite with a large titanium content (although we cannot rule out the possibility of titanomaghemite), and a second Curie point for magnetite. The amount of magnetite considerably increased after heating, which suggests that a portion of this phase was converted into magnetite. Being aware that these diagrams are not only indicative of the magnetic grains involved in the remanence, we can anticipate that these differences in mineralogy and more specifically the existence of two mineralogical phases could have some consequences on the behavior of the samples upon demagnetization. 3.2. Magnetic hysteresis Magnetic hysteresis parameters were performed on small chips of rocks with a variable field translation balance (VFTB). Saturation remanent magnetization (Mr ), saturation magnetization (Ms ), and coercive force (Hc ) were calculated after removal of the paramagnetic contribution. The coercivity of remanence (Hcr ) suggests that the NRM is rather carried by low-coercivity grains. The ratios of the hysteresis parameters plotted as a Day diagram (Day et al., 1977; Dunlop, 2002) in Fig. 3 show that most grain sizes are scattered within the pseudo-
single domain range with the exception of site HKAM which lie in the single domain range. There is no obvious relation with magnetic mineralogy, and the different k–T diagrams are characterized by similar grain-sizes. However, we shall see below that relatively high coercivities are present in the natural remanent magnetization (NRM) of the samples. 4. Natural remanent magnetization The remanent magnetization was measured with a JR-5 spinner and a 2G cryogenic magnetometer, both housed in the shielded room of the SOEST-HIGP Petrofabrics and Paleomagnetics Laboratory of the University of Hawaii. A minimum of 8 samples per flow were stepwise demagnetized by alternating fields (af) from 5 to 100 mT. Companion specimens from the same core were thermally demagnetized at 15 temperature steps. As a total, 300 specimens were af or thermally stepwise demagnetized. The magnetic behavior of the samples is not independent from their magnetic mineralogy. In Fig. 4, we compare several diagrams of thermal demagnetization from site HKAM with the thermomagnetic behavior of their companion samples. The magnetization of the first sample (07C) was removed after 250 ◦ C, and the remaining component displays erratic behavior upon demagnetization. The corresponding K–T curves indi-
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Fig. 2. Low-field susceptibility vs. temperature (K–T) curves for four specimens from the Hana volcanics. Arrows indicate the heating curve. The upper two diagrams are consistent with a single phase dominated by magnetite while the lower two plots indicate the existence of a high-titanium titanomagnetite phase.
Fig. 3. Hysteresis parameters—Mrs /Ms (ratio of remanent saturation moment Mrs , to saturation moment Ms ) against Hcr /Hc (ratio of remanent coercive force, Hcr , to coercive force, Hc ). Single domain (SD), pseudo-single domain (PSD), multidomain (MD).
cate that this first component is likely associated with titanomagnetite. In the second case (sample 01B) we observe the same situation, but with larger resistance to demagnetization. The comparison between the evolution of the magnetic moment and the thermomagnetic curve shows that this low-temperature component is again carried by titanomagnetite. Lastly, the lower sample in Fig. 5 shows dominance of magnetite over the full spectrum of temperature and its associated characteristic direction. Given the absence of additional constraints regarding the origin of the titanomagnetite, we did not validate the directions isolated below 250 ◦ C which belong to the viscosity domain. The first two columns in Fig. 5 show one thermal demagnetization diagram for each site. Except for a small component removed at very low-temperature, all diagrams show a univectorial behavior trending to the origin (a few cases like sample HKEO02AM show a small dispersion but no systematic deviation) and rather uniform resistance to demagnetization. The ChRM of all specimens was calculated using the principal component
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Fig. 4. Comparison between the evolution of the demagnetization diagrams, their normalized magnetic moment and the susceptibility upon heating for companion specimens of three samples with strikingly different magnetic characteristics. In spite of being performed on different specimens, the magnetic moment and the susceptibility curves show similar characteristics which indicate that the low-temperature component is carried by a mineralogical phase of low-titanium titanomagnetite.
analysis method for the demagnetization diagrams with a well-defined component trending to the origin. No bias or systematic departure from the origin was accepted. A total of 38 thermally demagnetized samples which were characterized by scatter of the directions or by uncoherent directions with respect to the rest of the site (mostly due to lightning effects that were not detected on the field) were rejected. This involved one or two samples per site. For sites HHAN, HKEA and KU, about half of the diagrams were characterized by erratic behavior or complex components. The less suitable results were obtained for site HPAE with only 3 determinations for 10 demagnetized samples. The six acceptable diagrams of thermal demagnetization from site HKEO are characterized by a unique component pointing south-east with an inclination between 10 and 20◦ .
The third column in Fig. 4 displays the af demagnetization diagrams for companions of the specimens shown in the second column. Despite apparently overall good quality, the best-fitting lines do not always go through the origin of the diagrams and in some cases a substantial part of magnetization remains unremoved at 100 mT. Thus the determination of the characteristic remanent magnetization (ChRM) can be affected by a persistent high coercivity component. Note also that the directions of some companion specimens do not always perfectly agree with each other. These differences were not linked to uncertainties in orientation, nor to errors in positioning or measuring the samples as the directions obtained after each demagnetization step are identical. A possibility is that they could be caused by inhomogeneities in the magnetization of the lava or be a direct
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Fig. 5. Left side column shows typical diagrams of thermal demagnetization (one per site). Middle and right side columns (respectively) show diagrams of thermal and alternating field demagnetization (respectively) of companion specimens from the same cores. The same demagnetization steps (5, 10 up to 100 mT in steps of 10 mT for af demagnetization and from 100 to 500 ◦ C in steps of 50 ◦ C and in steps of 25 ◦ C up to 575 ◦ C for thermal demagnetization) were used for all samples.
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consequence of field inhomogeneities (Baag et al., 1995; Valet and Soler, 1999). Following the results of thermal demagnetization, no coherent direction could be derived beyond 10 mT from site HKEO which was affected by lightning. 5. Results In the stereo plots of Fig. 6 the ChRM of all individual thermally demagnetized samples can be compared with the ChRM of the companion specimens that were af demagnetized (therefore not all specimens are plotted in this figure). It appears clearly that the af directions are much more scattered than those derived from thermal demagnetization. As indicated above, this is caused by incomplete removal of a high coercivity secondary component which overlaps the primary magnetization and, in some cases, even obliterates completely the initial magnetization. At this stage, it was obvious that we could not rely on the af data for the final interpretation of the results. We thus calculated the mean site directions using the thermal results only. We did not consider site HPAE which has only three acceptable demagnetization diagrams, and we ruled out site HKEO which was affected by lightning. The results of the 13 remaining sites are summarized in Table 1 which includes the location and the age of each site. The stereogram of all 13 sites mean directions is shown in Fig. 7a while the corresponding VGPs are plotted in Fig. 7b. The mean direction for all sites (dec. = 1.5◦ , inc. = 36.9◦ ) is in perfect agreement with the value of the geocentric axial dipole (GAD) at Maui (inc. = 37◦ ). These values do not change significantly if we restrain the calculation to the 12 sites younger than 45 ka (dec. = 1.6◦ , inc. = 37◦ ) or to the eleven sites younger than 10.5 ka (dec. = 1.7◦ , inc. = 36.3◦ ). In all cases (Table 1) the mean virtual geomagnetic pole is never distant by more than 1.5◦ of latitude from the north geographic pole. The dispersion of the VGPs of 6.7◦ for the 11 sites younger than 10.5 ka, which is lower by more than a factor of two than the predictions of the models of paleosecular variation at this latitude (McElhinny and McFadden, 1997) is reported here. Although unexpectedly large, this difference could be due to the relatively short time interval studied but this is then not compatible with the short period covered by the analyses of the historical field. We can now compare the present results with the impressive and high quality dataset which was obtained from radiocarbon dated lava flows of the Kilauea and Mauna Loa volcanoes (Mankinen and Champion, 1993a,b; Hagstrum and Champion, 1994, 2002). To these
results we added a few other reliable determinations from the same island (Coe et al., 1978; Tanaka and Kono, 1991). Depending on their location on the Big Island of Hawaii, the sites differ from 0.9◦ to 1.5◦ of latitude with respect to Maui. All inclinations were thus transferred at the latitude of Maui (we assumed that the deviations of the individual site mean inclinations from the GAD value which represent a few tens of degrees at most were not affected by this correction). The inclination variations of the present study shown in Fig. 8a match the pattern observed at the Big Island. The mean inclination derived from all sites for the past 10 kyear is 1.8◦ lower than the mean GAD value and the VGP dispersion of 11◦ remains lower than that predicted by global models of secular variation. The number of datapoints (N = 132) for this 10 kyear long interval may be large enough to provide a reasonable first-order pattern of the field changes during this period. However we note that the 5–7 ka time interval is almost depleted of data, which outlines the issue of the definition of an adequate data distribution to sample the secular variation. Interestingly, the deviation of the inclination from the GAD is only 1◦ for the past 2 kyears which is the best documented interval. Several inconsistencies could be caused by repeated sampling of the same geomagnetic field (McElhinny et al., 1996b) by the volcanics. Taking advantage of the existence of accurate dating for all sites, we averaged the successive directions of the lava flows (Fig. 8a) with overlapping age windows. The mean VGP dispersions of 8.4◦ and 9◦ obtained with windows either 500 years long or six times larger than the 1 sigma error on the age determination remain below the expected value. We infer that repeated samplings of the field, if any, had no significant consequence on the mean dispersion of the VGPs in the present case. Such low dispersion could also be caused by unsufficient sampling of the field. However, most previous studies from other locations exhibit normal dispersion despite much lower resolution. In their global data compilation, Korte et al. (2005) added the very detailed sedimentary record from Lake Waiau (Peng and King, 1992) to the volcanic Hawaiian database and used it in their geomagnetic model for the past 7 millennia (Korte and Constable, 2005). Questions regarding whether or not sediments and lava should be integrated in a unique database for analyses of paleosecular variation have been debated many times. The present situation offers a unique opportunity of comparing two detailed records obtained from sediments and volcanics (Fig. 8a) with apparent similar resolution. They display similar patterns but they also show significant differences. A first observation is that the volcanic record shows more fluctuations, specifically between 1.5 and
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Fig. 6. Stereographic projection of the site mean directions after thermal and alternating field demagnetization of the 14 sampled lavas. Note the dispersion of the results when derived from af demagnetization.
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Fig. 7. (a) Mean direction of 13 lava flows from the Hana volcanics (Maui). (b) Stereographic projection showing each site mean VGP and the position of the averaged mean VGP of all sites.
3 Ma. Another striking feature is that the sediment data lie mostly below the volcanics with mean inclinations of 29.7◦ ± 6.5◦ (at the Maui latitude) and 35.2◦ ± 7.6◦ for the lava flows. This difference is likely not due to sampling since polynomial fitting indicates also a 7◦ lower inclination in the sediments. Thus, it seems clear that the sediments were affected by inclination shallowing. A direct consequence is that, despite their similar features on a millennial scale, we do not think that the two records can be integrated together without introducing a bias and thus generating additional dispersion. For these reasons, we prefer to rely solely on the volcanic records.
Fig. 8. (a) Evolution of inclination obtained in the present study for the past 11 kyears compared with the results of the Kilauea lavas flows and with the sedimentary record from Lake Waiau. Full black circles are data from the USGS (Hagstrum and Champion, 1994, 1995, 2002; Mankinen and Champion, 1993a,b), closed orange circles are from Coe et al. (1978) and Tanaka and Kono (1991). Red closed circles indicate the results of this study. Blue circles represent the record from Lake Waiau (Peng and King, 1992). (b) Deviation of inclination from the expected value of the geocentric dipole at the latitude of Maui during the past 10 kyears. The green and red lines, respectively, represent a polynomial fit and a smoothing of the inclination changes. The blue line is the prediction of the CALSK7K model (Korte and Constable, 2005). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
We can attempt to evaluate the influence of irregular temporal sampling of the field by the lava flows by fitting the variations shown in Fig. 8a with a polynomial of order 9 or by a simple smoothing (Fig. 8b). In both cases, the deviation from the GAD inclination disappeared (delta inc. = 0.2◦ ± 6◦ and 0◦ ± 6.5◦ , respectively). If we restrain the analysis to the past 5 kyear which are much better documented, we obtain a small anomaly of 3.5◦ ± 5◦ . This suggests that series of data longer than a few thousand years with very good temporal resolution are needed to estimate the time-averaged field. In other words, what seems to be in cause is the
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length of the time interval over which biases can generate an inclination anomaly. It would be pertinent to do the same analysis using the CALS7K (Korte and Constable, 2005) model predictions. However due to the influence of the sedimentary data, the inclination variations derived from the model do not perfectly fit the volcanic results and the mean inclination of the model deviates by −6.3◦ ± 5.7◦ from the expected dipole value. From these considerations, we infer that there is no evidence for a significant persistent inclination anomaly in the present state of the database. However, it is clear that additional detailed records, at least between 5 and 7 ka B.P., would considerably reinforce this conclusion. 6. Discussion One may thus wonder whether there is a strong indication of a persistent anomaly for older periods. An exhaustive compilation of the hawaiian volcanic data has been presented by Love and Constable (2003) and the same database was used recently to study the paleosecular variation over the past 5 Myear from sites at ±20◦ latitude (Lawrence et al., 2006). In both cases, the authors conclude to a large inclination anomaly and note that the field in Hawaii is indeed unusual over timescales of 0–5 Myear. Given the temporal distribution of the records, it is also interesting to examine whether this inclination anomaly appears to be consistent between the individual records and over which time scale. If we consider all results published so far, we can separate the “precursory” Hawaiian records (many of them being revisited) which did not meet the standard criteria, specifically the existence of stepwise demagnetization, advocated for “modern” paleomagnetic studies from a second generation of recent data. Another important concern is the presence of dating which was also not present in many initial studies. We have updated our previous compilation (Herrero-Bervera and Valet, 2003) which is restrained to the “modern” records and plotted the deviation of their mean inclination from the GAD value in Fig. 9. The youngest detailed data were obtained from drilled cores. The SOH-1 and SOH-4 cores drilled on Kilauea volcano (Gratton et al., 2004; Teanby et al., 2002; Laj et al., 2002) document the past 100 kyears. We included 111 determinations of inclination from SOH-1 and limited the analysis of SOH-4 to the period between 40 and 100 ka to avoid the large number of “excursional” lava flows surrounding the past 40 kyears. The SOH-1 and SOH-4 cores indicate a negative deviation by 5◦ and 7◦ , respectively. However, the
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Fig. 9. Deviation of the mean inclination from the GAD value calculated for successions of lava flows older than 10 kyears in several hawaiian islands. N and R indicate the normal and reverse polarities of the sequences. Despite an apparent change of sign of the anomaly with field polarity, there is no large evidence for a stable and persistent inclination anomaly.
age control was restrained to a few radiometric determinations, which for SOH-1 are sometimes not fully consistent with each other. We note also that the mean deviation from the GAD value is 10◦ (±13◦ ) for the past 50 kyear, and 6◦ (±6/3◦ ) for the 0–18 ka period. Interestingly, the mean inclination found in SOH-1 which, on the basis of the interpolated chronology, appears to be younger than 10 kyear is less than 2◦ (±10◦ ). This small offset from the GAD value is likely not very significant in the absence of detailed chronological control. Another 941 m long drilled core (HSDP) of 190 lava flows (Holt et al., 1996) at Hilo provides a record of inclination for the past 400 kyear. The results are characterized by a lower deviation of 4.3◦ , but again in this case the lack of systematic dating does not rule out completely the possibility of serial correlation. Note that no estimate of the VGP dispersion can be obtained for these cores in the absence of declination. Lastly, the Honolulu volcanic series (HVS) from O’ahu were revisited using only dated flows (Herrero-Bervera and Valet, 2002) and are characterized by a deviation of 3.5◦ (±5.8◦ ) which becomes actually lower than 3◦ after correcting for the motion of the Pacific plate. Thus despite some uncertainties, all these data indicate a negative anomaly during the Brunhes period. It is interesting to test the persistence of the anomaly during older periods, particularly during Chrons with reverse polarity (Fig. 9). The dated lava flows from the island of Lanai (Herrero-Bervera et al., 2000) do not show any deviation from the expected field inclination during the Matuyama Chron after correction for the Pacific plate motion. In contrast, the older Koolau series (Herrero-Bervera and Valet, 2003) in O’ahu which are close to the onset of the Matuyama chron exhibit a
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deviation of 3.6◦ ± 9◦ which is not very different from the HVS series. Lastly, the sites of overlying lava flows from the Waianae (Herrero-Bervera and Valet, 1999, 2005) formation of O’ahu do not show any convincing evidence for a persistent anomaly. We observe a small positive anomaly of 4.3◦ for the reverse sequence between the Gilbert-Gauss and the lower Mammoth transitions, but there is no deviation during the previous interval with normal polarity. This difference is probably not a consequence of sampling as the number of flows (64 and 73, respectively) is in both cases very large and about the same. One can also probably exclude the presence of little normal overprint because all samples were thermally stepwise demagnetized. The short duration of these periods (120 and 50 kyear, respectively) could play some role, given the proximity of the reversal boundaries. Finally, we cannot exclude that some tilting of this part of the sequence could be linked to subsidence of the island. Thus, given all these uncertainties, we are rather inclined to conclude that there is no striking evidence in favor of a systematic and persistent inclination anomaly. The most significant observation may be the fact that the anomaly, when present, changes sign with field polarity, being consistently positive for reverse polarity flows and negative for normal polarity. However the results show also that the deviation from the GAD inclination varies considerably over short time scales. 7. Conclusion We measured new paleofield directions from 13 dated volcanic lava flows in the island of Maui with ages between 0.2 and 10.3 ka. The results confirm the directional changes reported at the Big Island for the past 10 kyear, particularly the existence of low inclinations about 2 kyear ago. A direct consequence is that dating was correct and that there were neither perturbations, nor local anomalies (volcanic edifice, terrain, island). These results emphasize also the importance of dating for PSV studies. The compilation of the volcanic records obtained for the past 10 kyear shows that the mean Hawaiian field is characterized by very low dispersion of the VGPs, which could be seen as a consequence of a low non-dipole field under Hawaii. The mean inclination is 1.8◦ lower than expected, but the depletion of data between 5 and 7 ka B.P. is most likely responsible for this small deviation. The volcanic records indicate also that shallow inclinations recorded at the nearby lake Waiau (Peng and King, 1992) for the same period likely result from compaction of the sediment. Thus, there would be no strong indication for a persistent inclina-
tion anomaly under Hawaii during the past 10 kyear. We also considered the possibility of a longer term persistent component after selecting the most reliable and long records within the Brunhes and the previous polarity chrons. The compilation shows some tendency for a deviation from the GAD inclination, but it is not systematic and never outside the variance. We also note that some mean inclinations are in perfect agreement with the expected value. Thus, we consider that the present database is not so consistent with the existence of a magnetic anomaly under Hawaii during the last million years. Acknowledgements We are very grateful to Mr. James Lau for his field and laboratory assistance. We greatly acknowledge Julie Carlut and Xavier Quidelleur for help during field work. We would like to thank Dr. Eric Bergmanis for his guidance, support and advice to us for the field work. Financial support to Emilio Herrero-Bervera was provided by SOEST-HIGP and the National Science Foundation grants EAR-9909206, EAR-INT-9906221, EAR-0207787, EAR-0213441, EAR-0510061. Data processing was performed using the paleomac program (Cogn´e, 2003). Financial support to Jean-Pierre Valet was provided by the INSU-CNRS Dyeti program. This is a SOEST contribution number 7100, HIGP contribution number 1486 and IPGP contribution number 2215. References Baag, C., Helsley, C.E., Xu, S.-Z., Lienert, B.R., 1995. Deflection of paleomagnetic directions due to magnetization of the underlying terrain. J. Geophys. Res. 100 (B7), 10013–10027. Bergmanis, E., Sinton, J.M., Trusdell, F.A., 2000. Rejuvenated volcanism along the southwest rift zone, East Maui, Hawaii. Bull. Volcanol. 62, 239–255. Carlut, J., Courtillot, V., 1998. How complex is the time-averaged geomagnetic field over the past 5 Myr? Geophys. J. Int. 134, 527– 544. Coe, R.S., Gromme, S., Mankinen, E.A., 1978. Geomagnetic intensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipole low. J. Geophys. Res. 83, 1740– 1756. Cogn´e, J.P., 2003. PaleoMac: a MacintoshTM application for treating paleomagnetic data and making plate reconstructions. Geochem. Geophys. Geosyst. 4 (1), doi:10.1029/2001GC000227, 1007. Day, R., Fuller, M.D., Schmidt, V., 1977. Hysteresis properties of titanomagnetites: Grain-size and compositional dependence. Phys. Earth Planet. Int. 13, 260–267. Doell, R.R., Cox, A.V., 1963. The accuracy of the paleomagnetic method as evaluated from historic Hawaiian lava flows. J. Geophys. Res. 68, 1997–2209.
E. Herrero-Bervera, J.-P. Valet / Physics of the Earth and Planetary Interiors 161 (2007) 267–280 Doell, R.R., Cox, A.V., 1972. The Pacific geomagnetic secular variation anomaly and the question of the lateral uniformity in the lower mantle. In: The Nature of the Solid Earth. McGraw-Hill, New York, pp. 245–284. Duncan, R.A., 1975. Paleosecular variation at the Society Islands, French Polynesia. Geophys. J. R. Astr. Soc. 41, 245–254. Dunlop, D., 2002. Theory and application of the Day plot (Mrs /Ms versus Hcr /Hc ) 1. Theoretical curves and tests using titanomagnetite data. J. Geophys. Res. 107, EM 4-1-EPM 4-22. Elmaleh, A., Valet, J.-P., Herrero-Bervera, E., 2001. A map of the Pacific anomaly during the Brunhes chron. Earth Planet. Sci. Lett. 193 (3–4), 315–332. Gratton, M., Shaw, J., Herrero-Bervera, E., 2004. An absolute palaeointensity record from SOH1 lava core Hawaii using the microwave technique. Phys. Earth Planet. Int. 148, 193–214. Hagstrum, J.T., Champion, D.E., 1994. Paleomagnetic correlation of late quaternary lava flows in the lower rift zone of Kilauea volcano, Hawaii. J. Geophys. Res. 99 (B11), 21679–21690. Hagstrum, J.T., Champion, D.E., 1995. Late Quaternary geomagnetic secular variation from historical and 14 C-dated lava flows on Hawaii. J. Geophys. Res. 100, 24393–24403. Hagstrum, J.T., Champion, D.E., 2002. A Holocene paleosecular variation record from 14 C dated volcanic rocks in western North America. J. Geophys. Res. 107 (B1), 10.1029. Herrero-Bervera, E., Urrutia-Fucugauchi, J., Martin Del Pozzo, A.L., Bohnel, H., Guerrero, J., 1986. Normal amplitude Brunhes paleosecular variation at low-latitudes; a paleomagnetic record from the Trans-Mexican Volcanic Belt. Geophys. Res. Lett. 13, 1442– 1445. Herrero-Bervera, E., Valet, J.P., 1999. Paleosecular variation during sequential geomagnetic reversals from Hawaii. Earth Planet. Sci. Lett. 171, 139–148. Herrero-Bervera, E., Vinuela, J.-M., Valet, J.P., 2000. Paleomagnetic study of lavas of the island of Lana’i, Hawaii. J. Vol. Geoth. Res. 104, 21–31. Herrero-Bervera, E., Valet, J.-P., 2002. Paleomagnetic secular variation of the Honolulu volcanic series (33–700 ka), O’ahu, Hawaii. Phys. Earth Planet. Int. 133, 83–97. Herrero-Bervera, E., Valet, J.-P., 2003. Persistent anomalous inclinations recorded in the Koolau volcanic series on the island of Oahu (Hawaii, USA) between 1.8 and 2.6 Ma. Earth Planet. Sci. Lett. 212, 443–456. Herrero-Bervera, E., Valet, J.P., 2005. Absolute paleointensity from the Waianae volcanics (Oahu, Hawaii) between the Gilbert-Gauss and the upper Mammoth reversals. Earth Planet. Sci. Lett. 234, 279–296. Holt, J.W., Kirschvink, J.L., Garnier, F., 1996. Paleomagnetic inclinations for the past 400 kyr from the 1 km core of the Hawaii Scientific Drilling Project. J. Geophys. Res. 101 (B5), 11663– 11665. Hrouda, F., Jelinek, V., Zapletal, K., 1997. Refined technique for susceptibility resolution into ferrimagnetic and paramagnetic components based on susceptibility temperature variation measurement. Geophys. J. Int. 129, 715–719. Hulot, G., LeMou¨el, J.L., 1994. A statistical approach to the Earth’s main magnetic field. Phys. Earth Planet. Interiors 82, 167–183. Johnson, C., Constable, C., 1995. The time-averaged field as recorded by lava flows over the last 5 Myrs. Geophys. J. Int. 122, 489– 519. Johnson, C.L., Constable, C.G., 1997. The time-averaged geomagnetic field: global and regional biases for 0–5 Ma. Geophys. J. Int. 131, 643–666.
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Kelly, J., Gubbins, D., 1997. The geomagnetic field over the past 5 million years. Geophys. J. Int. 128, 315–330. Kono, M., 1997. Paleosecular variation in field directions due to randomly varying Gauss coefficients. J. Geomag. Geoelectr. 49, 615–631. Korte, K.M., Genevey, A., Constable, C.G., Frank, U., Schnepp, E., 2005. Continuous geomagnetic models for the past 7 millennia II: CALS7K. Geochem. Geophys. Geosyst. 6 (2), doi:10.1029/2004GC00800, Q02H15. Korte, M., Constable, C.G., 2005. Continuous geomagnetic models for the past 7 millennia II: CALS7K. Geochem. Geophys. Geosyst. 6 (2), doi:10.1029/2004GC000801, Q02H16. Laj, C., Kissel, C., Scao, V., Beer, J., Thomas, D.M., Guillou, H., Muscheler, R., Wagner, G., 2002. Geomagnetic intensity and inclination variations at Hawaii for the past 98 kyr from core SOH-4 (Big island): a new study and a comparison with existing contemporary data. Phys. Earth Planet. Int., 205–243. Lawrence, K.P., Constable, C.G., Johnson, C.L., 2006. Paleosecular variation and the average geomagnetic field at ±20◦ latitude. Geochem. Geophys. Geosyst. 7, doi:10.1029/2005GC001181, QO7007. Love, J.J., Constable, C.G., 2003. Gaussian statistics for paleomagnetic vectors. Geophys. J. Int. 152, 515–565. Mankinen, E.A., Champion, E.D., 1993a. Broad trends in geomagnetic paleointensity on Hawaii during Holocen time. J. Geophys. Res. 98 (B5), 7959–7976. Mankinen, M.A., Champion, D.E., 1993b. Latest pleistocene and holocene geomagnetic paleointensity on Hawaii. Science 262, 412–416. McElhinny, M.W., Merrill, R.T., 1975. Geomagnetic secular variation over the past 5 m.y. Rev. Geophys. Space Phys. 13, 687–708. McElhinny, M.W., McFadden, P.L., Merrill, R.T., 1996a. The timeaveraged paleomagnetic field 0–5 Ma. J. Geophys. Res. 101, 25007–25027. McElhinny, M.W., McFadden, P.L., Merrill, R.T., 1996b. The myth of the Pacific dipole window. Earth Planet. Sci. Lett. 143, 13–22. McElhinny, M.W., McFadden, P.L., 1997. Palaeosecular variation over the past 5 Myr based on a new generalized database. Geophys. J. Int. 131, 240–252. McWilliams, M.O., Holcomb, R.T, Champion, D.E., 1982. Geomagnetic secular variation from 14 C-dated lava flows on Hawaii and the question of the Pacific non-dipole low. Philos. Trans., R. Soc. Lond. A 306, 211–222. Miki, Inokuchi, M.H., Yamaguchi, S., Matsuda, J.-I., Nagao, K., Isekaki, N., Yaskawa, K., 1998. Geomagnetic secular variation in Easter Island the southeast Pacific. Phys. Earth Planet. Int. 106, 93–101. Peng, L., King, J.W., 1992. A late quaternary geomagnetic secular variation record from Lake Waiau, Hawaii, and the question of the Pacific nondipole low. J. Geophys. Res. 97 (B4), 4407– 4424. Quidelleur, X., Courtillot, V., 1996. On low-degree spherical harmonics models of paleosecular variation. Phys. Earth Planet. Int. 95, 55–77. Riley, C.M., Diehl, J.F., Kirschvink, J.L., Ripperdan, R.L., 1999. Paleomagnetic constratints on fault motion in the Hilina fault system, south flank of Kilauea Volcano, Hawaii. J. Volcanol. Geotherm. Res. 94, 233–249. Sherrod, D.R.Y., Nishimitsu, T., Tagami, T., 2003. New-K-Ar ages and the geologic evidence against rejuvenated-stage volcanism at Haleakala, East Maui, a postshield-stage volcano of the Hawaii island chain. GSA Bull. 115, 683–694.
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Sinton, J.M., Diller, D.E., Chen, C.-Y., 1987. Geology and petrological evolution of West Maui volcano. Univ. Hawaii Inst. Geophys. Spec. Publ., 13–30. Tanaka, H., Kono, M., 1991. Preliminary results and reliability of paleointensity studies on historical and 14C dated hawaiian lavas. J. Geomag. Geoelectr. 43, 375–388. Teanby, T., Laj, C., Gubbins, D., Pringle, M., 2002. A detailed paleointensity and inclination record from drill core SOH1 on Hawaii. Phys. Earth Planet. 131, 101–140.
Valet, J.-P., Soler, V., 1999. Magnetic anomalies induced by lava flows and possible consequences for paleomagnetic records. Phys. Earth Planet. Int. 115, 109–118. Yamamoto, Y., Shimura, K., Tsunakawa, H., Kogiso, T., Uto, K., Barczus, H.G., Oda, H., Yamazaki, T., Kikawa, E., 2002. Geomagnetic paleosecular variation for the past 5 Ma in the Society islands, French Polynesia. Earth Planet. Sci. Lett., 797– 802.
Holocene paleosecular variation from dated lava flows on Maui (Hawaii) Emilio Herrero-Bervera a , Jean-Pierre Valet b,∗ b
a SOEST-Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Paleomagnetics and Petrofabrics Laboratory, 1608 East West Rd. Honolulu, HI 96822, USA Institut de Physique du Globe de Paris, Plaeomagnetism, UMR7154 CNRS-Univ. Denis-Diderot, 75252 Paris Cedex 05, France
Received 17 May 2006; received in revised form 10 October 2006; accepted 28 February 2007
Abstract Low inclination and low secular variation seem to be present for at least the past 5 Myear in central Pacific. The period covering the past 10 kyear is crucial to the study of the field variability and to fill the gap between historical field measurements and long-term paleomagnetic records. We have conducted a paleomagnetic study of 13 sites of basaltic lava flows from Maui island with 14 C ages between 10.3 and 0.2 ka. Two other sites dated at 45 and 730 ka were also sampled. Eight to 10 samples from each site were demagnetized using thermal treatment and companion specimens from the same samples were demagnetized by alternating fields (af). Thermomagnetic and hysteresis measurements indicated that magnetite (575 ◦ C) in fine grains was the dominant magnetic carrier, although in many cases we also observed a low-temperature phase which is likely carried by titanomagnetite with low titanium content. The existence of relatively high coercivities associated with these two mineralogical phases generated overlapping components which could not be properly isolated using af demagnetization. Successful results were obtained after thermal demagnetization for 13 sites with a mean inclination of 36.9 ± 4◦ . The mean inclination (inc. = 36.3◦ ) of the eleven sites younger than 10.5 ka is also very close to the value (37◦ ) of the geocentric axial dipole (GAD) at the site latitude, but the angular dispersion of the virtual geomagnetic poles (VGPs) about the spin axis (9◦ with N = 13 and 6.7◦ with N = 11) is significantly lower than the predictions of the models of paleosecular variation at this latitude. The inclination variations for the past 10 kyear are in excellent agreement with the very detailed dataset which has previously been obtained from the big island of Hawaii. The mean inclination of all volcanic records (N = 132) is 1.8◦ lower than expected, but this is likely caused by the lack of records between 5 and 7 ka B.P. We note also that the inclinations from Lake Waiau sediments (big island) are shallower than those of the volcanic records (Mauna Kea, Hawaii) and were thus probably affected by compaction. We infer that there is no striking evidence for an inclination anomaly under Hawaii during this period, being aware of the need for additional records covering at least the 5–7 ka B.P. time interval. The absence of a systematic deviation going beyond the dispersion of the inclinations obtained for the past millions of years neither pleads for a significant long-term persistent anomaly. However, all studies report a low dispersion of the VGPs which must reflect low secular variation. © 2007 Elsevier B.V. All rights reserved. Keywords: Earth’s magnetic field; Paleomagnetism; Paleosecular variation
1. Introduction
∗
Corresponding author. Tel.: +33 1 44273566; fax: +33 1 44277463. E-mail address: [email protected] (J.-P. Valet).
0031-9201/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.pepi.2007.02.008
It is important for our understanding of the processes involved in the generation of the geomagnetic field to determine whether some regions of the world
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would be characterized by long-term persistent anomalies. Doell and Cox (1963) studied historic lavas from Hawaii and concluded that the low geomagnetic secular variation observed in the Pacific from magnetic observatory data extended at least as far as 1750 a.d. With the addition of new data (Doell and Cox, 1972; Coe et al., 1978; Herrero-Bervera et al., 1986) from Hawaii a consistent picture of anomalously low paleosecular variation referred to as the Pacific dipole low emerged progressively. Doell and Cox (1972) noticed that the Brunhes inclinations in Hawaii were 4◦ smaller than the value predicted by the GAD and is thus consistent with strong persistent non-dipole fields. More recently, McWilliams (1982) and Miki et al. (1998) observed a low angular dispersion of the virtual geomagnetic poles (VGP) during the past 800 kyear and suggested that this was caused by a weak non-dipole field. Despite the large dominance of data from the Hawaiian islands, the question of the Pacific non-dipole low has been widely debated (McElhinny and Merrill, 1975; Duncan, 1975; Coe et al., 1978; Hagstrum and Champion, 1995). During the past decade, the emergence of global models aimed at describing the time-averaged field (Johnson and Constable, 1995, 1997; McElhinny et al., 1996a; McElhinny and McFadden, 1997; Kono, 1997; Carlut and Courtillot, 1998; Kelly and Gubbins, 1997) brought up additional and new interest to the question. Apart from the large dipole dominance there is evidence for the presence of a small axial quadrupole component with a size varying between 2.5 and 8% of the axial dipole term. Several authors consider further departures from the axial dipole as reflecting actual geomagnetic features, while others (Quidelleur and Courtillot, 1996; Carlut and Courtillot, 1998; McElhinny et al., 1996b) see them as artifacts linked to quality as well as temporal and spatial distribution of data. The first ones consider that the Hawaiian area exhibits one of the largest long-term inclination anomalies whereas the others do not validate its existence and consider that many early studies of paleosecular variation (McElhinny et al., 1996b) should be revisited using demagnetization procedures which meet the present paleomagnetic standards. Recent studies (Yamamoto et al., 2002; Elmaleh et al., 2001; Herrero-Bervera and Valet, 2002, 2003; Gratton et al., 2004; Teanby et al., 2002; Laj et al., 2002) have focused on this area in order to constrain the time persistence and the geographical extension of the anomaly, but they involve relatively old periods over which the variability of the field could not be fully explored due to dating uncertainties. Because the cut-off between the time constants associated with the non-dipolar and dipolar components does not exceed
a few hundred years (Hulot and LeMou¨el, 1994) it is important to study the field variability over the past 10 kyears to detect persistent field components. The Hawaiian database is dominated by an impressive succession of records (Hagstrum and Champion, 1994, 1995, 2002; Mankinen and Champion, 1993a,b) from 14 C dated lava flows of the Kilauea and Mauna Loa lava flows (Big Island) which describe in detail the past 5 kyear. Records from different islands are needed to eliminate local effects and to constrain possible field anomalies generated by the volcanic edifices. Series of flows can also be affected by tilting by a few degrees which are not detectable on the field but large enough to create a persistent inclination anomaly. This is particularly true when most records come from the same side of one island which can be affected by subsidence (Riley et al., 1999) due to loading of the underlying Pacific plate by the growing volcanic edifices. For these reasons we considered that it was important to complete the 0–10 kyear database with new results from a different location. The young and recently dated volcanic series of the Maui island offered a unique opportunity to meet this goal. 2. Sites and sampling The East Maui volcano is the youngest and largest (see Fig. 1) of the two edifices of the island of Maui. During shield and post-shield phases of volcanism on east Maui, eruptions occurred from three principal rift zones radiating away from the summit region. Following the profound erosion that occurred after post-shield Kula volcanism (a period perhaps no more than about 200,000–300,000 years long) only two of East Maui’s rift zones (the east or Hana rift and the SW rift zone) were reactivated with rejuvenated volcanism (Bergmanis et al., 2000; Sinton et al., 1987; Sherrod et al., 2003). Hana volcanism is the only example of rejuvenated volcanism in Hawaii that follows former rift zone structures. It also follows the shortest quiet period which occurred after the earlier post-shield volcanism, suggesting that the conduits preferred by earlier eruptions were still opened to magma migration during rejuvenation. The shield stage of East Maui volcano is represented by the Honomanu Volcanics (see Fig. 1), a suite of lavas that vary from tholeiitic to alkalic basalts. Both post-shield Kula and rejuvenation stage Hana rocks are alkalic, although the latter tend to have lower SiO2 contents and to be less differentiated. Carbon dating and geological evidence suggest that there have been at least five eruptions along the SW rift zone in the last 900 years. The last one was about 1790 a.d. on the lower SW rift zone.
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Fig. 1. Schematic plot of the area covered by the Hana volcanics modified from Bergmanis et al. (2000) showing the location and names of the sampling sites. Colours indicate the different volcanic units. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
The sampling sites were selected exactly at the same locations as those of the previous geochronological (i.e. 14 C dating) studies (Bergmanis et al., 2000). A total of 166 samples were drilled from 15 widely separated lavas (Fig. 1 and Table 1) using a portable gasoline-powered drill. Sun orientation was used for the entire sample collection. The samples were spread out laterally within each flow and taken with inclination angles between 10◦ and 60◦ away from the horizontal plane. 3. Laboratory experiments 3.1. Thermomagnetic measurements Studies of magnetic mineralogy were performed on at least one sample from each lava flow. Low-field susceptibility versus temperature (k–T) experiments were conducted in order to determine the Curie temperature.
Fifteen specimens were progressively heated from room temperature up to 700 ◦ C and subsequently cooled down using the KLY2-CS3 apparatus (Hrouda et al., 1997) of the SOEST-HIGP Petrofabrics and Paleomagnetics Laboratory. In Fig. 2 are shown some typical diagrams of susceptibility versus temperature (k–T). A first common characteristic to all plots is that none of them displays perfectly reversible curves. We can roughly classify the diagrams in two different categories. In the upper two plots (Fig. 2) magnetite is identified as the unique mineral and the shape of the cooling curve remains identical to the heating curve. The little offsets between the high temperatures of the cooling and the heating curves in Fig. 2a were most likely caused by a drift of temperature measurements rather than by small mineralogical variations. The amplitude of the cooling curve is much larger than during heating. Given the absence of other visi-
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Table 1 Paleomagnetic results of thermal demagnetization for each sampled site of the East Maui Hana volcanics Site
Age (ka)
HKOL HGTA HHAN HWHL HNAE HLAA HMAK HKAM HWKW Hale HKEA HLLL KU
0.2 0.83 0.89 0.92 2.18 3.02 3.54 4.07 19 8.19 10.3 45.0 703
HPAE HKEO
8.83 ± 0.06 10.4 ± 0.06
± ± ± ± ± ± ± ± ± ± ± ± ±
0.015 0.06 0.17 0.07 0.06 0.065 0.04 0.09 0.06 0.06 0.05 1.8 13.0
Long. (◦ W), lat. (◦ N)
N/Ns
Dec.
Inc.
Kappa
␣95
VGP long. (◦ E), VGP lat. (◦ N)
156.42, 20.60 156.32, 20.61 156.38, 20.63 156.35, 20.72 156.28, 20.60 156.25, 20.62 156.36, 20.62 156.36, 20.70 156.39, 20.38 156.30, 20.6 156.40, 20.68 156.31, 20.61 156.34, 20.73
10/12 08/11 04/10 10/10 09/14 09/11 08/10 08/11 10/11 17/10 04/13 07/11 05/09
368.7 362.9 361.9 359.8 365.8 360.2 349.8 361.5 359.4 366.2 361.3 360.4 360.7
36.6 33.8 41 48.6 31.8 20.7 38.7 38.5 43.1 32.2 33.7 43.7 36.6
024 364 420 239 172 077 388 065 150 164 151 363 067
9.9 2.9 4.5 3.1 3.9 5.9 2.8 6.9 4 4.7 7.5 3.2 9.4
293.7, 81.8 327.9, 86.7 232.7, 86.4 202.6, 81.0 324.3, 83.6 022.6, 80.1 124.0, 80.4 250.8, 88.1 196.8, 85.5 318.8, 83.5 353.9, 87.5 207.9, 85.0 312.9, 89.3
156.39, 20.68 156.40, 20.66
0/11 0/12
Age (ka) = calibrated ages (Bergmanis et al., 2000), the ages of sites HWHL and KU were interpolated on the basis of stratigraphy; long. (◦ W), lat. (◦ N) are west longitude and north latitude of the sites; N gives the number of vectors used in the calculation of site mean directions after thermal demagnetisation; Ns is the total number of thermally demagnetized specimens (we do not report here the results of af demagnetization since they were not used in the final results, see text); dec. and inc. are declination and inclination of mean paleomagnetic directions; Kappa is concentration parameter; ␣95 is radius of 95% confidence; VGP long. and lat. are north latitude and east longitude of virtual geomagnetic pole. No reliable results were obtained for sites HPAE and HKEA.
ble Curie point, this indicates production of additional magnetite during heating. The lower two diagrams display more complex behavior due to the presence of a low Curie temperature, probably linked to titanomagnetite with a large titanium content (although we cannot rule out the possibility of titanomaghemite), and a second Curie point for magnetite. The amount of magnetite considerably increased after heating, which suggests that a portion of this phase was converted into magnetite. Being aware that these diagrams are not only indicative of the magnetic grains involved in the remanence, we can anticipate that these differences in mineralogy and more specifically the existence of two mineralogical phases could have some consequences on the behavior of the samples upon demagnetization. 3.2. Magnetic hysteresis Magnetic hysteresis parameters were performed on small chips of rocks with a variable field translation balance (VFTB). Saturation remanent magnetization (Mr ), saturation magnetization (Ms ), and coercive force (Hc ) were calculated after removal of the paramagnetic contribution. The coercivity of remanence (Hcr ) suggests that the NRM is rather carried by low-coercivity grains. The ratios of the hysteresis parameters plotted as a Day diagram (Day et al., 1977; Dunlop, 2002) in Fig. 3 show that most grain sizes are scattered within the pseudo-
single domain range with the exception of site HKAM which lie in the single domain range. There is no obvious relation with magnetic mineralogy, and the different k–T diagrams are characterized by similar grain-sizes. However, we shall see below that relatively high coercivities are present in the natural remanent magnetization (NRM) of the samples. 4. Natural remanent magnetization The remanent magnetization was measured with a JR-5 spinner and a 2G cryogenic magnetometer, both housed in the shielded room of the SOEST-HIGP Petrofabrics and Paleomagnetics Laboratory of the University of Hawaii. A minimum of 8 samples per flow were stepwise demagnetized by alternating fields (af) from 5 to 100 mT. Companion specimens from the same core were thermally demagnetized at 15 temperature steps. As a total, 300 specimens were af or thermally stepwise demagnetized. The magnetic behavior of the samples is not independent from their magnetic mineralogy. In Fig. 4, we compare several diagrams of thermal demagnetization from site HKAM with the thermomagnetic behavior of their companion samples. The magnetization of the first sample (07C) was removed after 250 ◦ C, and the remaining component displays erratic behavior upon demagnetization. The corresponding K–T curves indi-
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Fig. 2. Low-field susceptibility vs. temperature (K–T) curves for four specimens from the Hana volcanics. Arrows indicate the heating curve. The upper two diagrams are consistent with a single phase dominated by magnetite while the lower two plots indicate the existence of a high-titanium titanomagnetite phase.
Fig. 3. Hysteresis parameters—Mrs /Ms (ratio of remanent saturation moment Mrs , to saturation moment Ms ) against Hcr /Hc (ratio of remanent coercive force, Hcr , to coercive force, Hc ). Single domain (SD), pseudo-single domain (PSD), multidomain (MD).
cate that this first component is likely associated with titanomagnetite. In the second case (sample 01B) we observe the same situation, but with larger resistance to demagnetization. The comparison between the evolution of the magnetic moment and the thermomagnetic curve shows that this low-temperature component is again carried by titanomagnetite. Lastly, the lower sample in Fig. 5 shows dominance of magnetite over the full spectrum of temperature and its associated characteristic direction. Given the absence of additional constraints regarding the origin of the titanomagnetite, we did not validate the directions isolated below 250 ◦ C which belong to the viscosity domain. The first two columns in Fig. 5 show one thermal demagnetization diagram for each site. Except for a small component removed at very low-temperature, all diagrams show a univectorial behavior trending to the origin (a few cases like sample HKEO02AM show a small dispersion but no systematic deviation) and rather uniform resistance to demagnetization. The ChRM of all specimens was calculated using the principal component
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Fig. 4. Comparison between the evolution of the demagnetization diagrams, their normalized magnetic moment and the susceptibility upon heating for companion specimens of three samples with strikingly different magnetic characteristics. In spite of being performed on different specimens, the magnetic moment and the susceptibility curves show similar characteristics which indicate that the low-temperature component is carried by a mineralogical phase of low-titanium titanomagnetite.
analysis method for the demagnetization diagrams with a well-defined component trending to the origin. No bias or systematic departure from the origin was accepted. A total of 38 thermally demagnetized samples which were characterized by scatter of the directions or by uncoherent directions with respect to the rest of the site (mostly due to lightning effects that were not detected on the field) were rejected. This involved one or two samples per site. For sites HHAN, HKEA and KU, about half of the diagrams were characterized by erratic behavior or complex components. The less suitable results were obtained for site HPAE with only 3 determinations for 10 demagnetized samples. The six acceptable diagrams of thermal demagnetization from site HKEO are characterized by a unique component pointing south-east with an inclination between 10 and 20◦ .
The third column in Fig. 4 displays the af demagnetization diagrams for companions of the specimens shown in the second column. Despite apparently overall good quality, the best-fitting lines do not always go through the origin of the diagrams and in some cases a substantial part of magnetization remains unremoved at 100 mT. Thus the determination of the characteristic remanent magnetization (ChRM) can be affected by a persistent high coercivity component. Note also that the directions of some companion specimens do not always perfectly agree with each other. These differences were not linked to uncertainties in orientation, nor to errors in positioning or measuring the samples as the directions obtained after each demagnetization step are identical. A possibility is that they could be caused by inhomogeneities in the magnetization of the lava or be a direct
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Fig. 5. Left side column shows typical diagrams of thermal demagnetization (one per site). Middle and right side columns (respectively) show diagrams of thermal and alternating field demagnetization (respectively) of companion specimens from the same cores. The same demagnetization steps (5, 10 up to 100 mT in steps of 10 mT for af demagnetization and from 100 to 500 ◦ C in steps of 50 ◦ C and in steps of 25 ◦ C up to 575 ◦ C for thermal demagnetization) were used for all samples.
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consequence of field inhomogeneities (Baag et al., 1995; Valet and Soler, 1999). Following the results of thermal demagnetization, no coherent direction could be derived beyond 10 mT from site HKEO which was affected by lightning. 5. Results In the stereo plots of Fig. 6 the ChRM of all individual thermally demagnetized samples can be compared with the ChRM of the companion specimens that were af demagnetized (therefore not all specimens are plotted in this figure). It appears clearly that the af directions are much more scattered than those derived from thermal demagnetization. As indicated above, this is caused by incomplete removal of a high coercivity secondary component which overlaps the primary magnetization and, in some cases, even obliterates completely the initial magnetization. At this stage, it was obvious that we could not rely on the af data for the final interpretation of the results. We thus calculated the mean site directions using the thermal results only. We did not consider site HPAE which has only three acceptable demagnetization diagrams, and we ruled out site HKEO which was affected by lightning. The results of the 13 remaining sites are summarized in Table 1 which includes the location and the age of each site. The stereogram of all 13 sites mean directions is shown in Fig. 7a while the corresponding VGPs are plotted in Fig. 7b. The mean direction for all sites (dec. = 1.5◦ , inc. = 36.9◦ ) is in perfect agreement with the value of the geocentric axial dipole (GAD) at Maui (inc. = 37◦ ). These values do not change significantly if we restrain the calculation to the 12 sites younger than 45 ka (dec. = 1.6◦ , inc. = 37◦ ) or to the eleven sites younger than 10.5 ka (dec. = 1.7◦ , inc. = 36.3◦ ). In all cases (Table 1) the mean virtual geomagnetic pole is never distant by more than 1.5◦ of latitude from the north geographic pole. The dispersion of the VGPs of 6.7◦ for the 11 sites younger than 10.5 ka, which is lower by more than a factor of two than the predictions of the models of paleosecular variation at this latitude (McElhinny and McFadden, 1997) is reported here. Although unexpectedly large, this difference could be due to the relatively short time interval studied but this is then not compatible with the short period covered by the analyses of the historical field. We can now compare the present results with the impressive and high quality dataset which was obtained from radiocarbon dated lava flows of the Kilauea and Mauna Loa volcanoes (Mankinen and Champion, 1993a,b; Hagstrum and Champion, 1994, 2002). To these
results we added a few other reliable determinations from the same island (Coe et al., 1978; Tanaka and Kono, 1991). Depending on their location on the Big Island of Hawaii, the sites differ from 0.9◦ to 1.5◦ of latitude with respect to Maui. All inclinations were thus transferred at the latitude of Maui (we assumed that the deviations of the individual site mean inclinations from the GAD value which represent a few tens of degrees at most were not affected by this correction). The inclination variations of the present study shown in Fig. 8a match the pattern observed at the Big Island. The mean inclination derived from all sites for the past 10 kyear is 1.8◦ lower than the mean GAD value and the VGP dispersion of 11◦ remains lower than that predicted by global models of secular variation. The number of datapoints (N = 132) for this 10 kyear long interval may be large enough to provide a reasonable first-order pattern of the field changes during this period. However we note that the 5–7 ka time interval is almost depleted of data, which outlines the issue of the definition of an adequate data distribution to sample the secular variation. Interestingly, the deviation of the inclination from the GAD is only 1◦ for the past 2 kyears which is the best documented interval. Several inconsistencies could be caused by repeated sampling of the same geomagnetic field (McElhinny et al., 1996b) by the volcanics. Taking advantage of the existence of accurate dating for all sites, we averaged the successive directions of the lava flows (Fig. 8a) with overlapping age windows. The mean VGP dispersions of 8.4◦ and 9◦ obtained with windows either 500 years long or six times larger than the 1 sigma error on the age determination remain below the expected value. We infer that repeated samplings of the field, if any, had no significant consequence on the mean dispersion of the VGPs in the present case. Such low dispersion could also be caused by unsufficient sampling of the field. However, most previous studies from other locations exhibit normal dispersion despite much lower resolution. In their global data compilation, Korte et al. (2005) added the very detailed sedimentary record from Lake Waiau (Peng and King, 1992) to the volcanic Hawaiian database and used it in their geomagnetic model for the past 7 millennia (Korte and Constable, 2005). Questions regarding whether or not sediments and lava should be integrated in a unique database for analyses of paleosecular variation have been debated many times. The present situation offers a unique opportunity of comparing two detailed records obtained from sediments and volcanics (Fig. 8a) with apparent similar resolution. They display similar patterns but they also show significant differences. A first observation is that the volcanic record shows more fluctuations, specifically between 1.5 and
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Fig. 6. Stereographic projection of the site mean directions after thermal and alternating field demagnetization of the 14 sampled lavas. Note the dispersion of the results when derived from af demagnetization.
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Fig. 7. (a) Mean direction of 13 lava flows from the Hana volcanics (Maui). (b) Stereographic projection showing each site mean VGP and the position of the averaged mean VGP of all sites.
3 Ma. Another striking feature is that the sediment data lie mostly below the volcanics with mean inclinations of 29.7◦ ± 6.5◦ (at the Maui latitude) and 35.2◦ ± 7.6◦ for the lava flows. This difference is likely not due to sampling since polynomial fitting indicates also a 7◦ lower inclination in the sediments. Thus, it seems clear that the sediments were affected by inclination shallowing. A direct consequence is that, despite their similar features on a millennial scale, we do not think that the two records can be integrated together without introducing a bias and thus generating additional dispersion. For these reasons, we prefer to rely solely on the volcanic records.
Fig. 8. (a) Evolution of inclination obtained in the present study for the past 11 kyears compared with the results of the Kilauea lavas flows and with the sedimentary record from Lake Waiau. Full black circles are data from the USGS (Hagstrum and Champion, 1994, 1995, 2002; Mankinen and Champion, 1993a,b), closed orange circles are from Coe et al. (1978) and Tanaka and Kono (1991). Red closed circles indicate the results of this study. Blue circles represent the record from Lake Waiau (Peng and King, 1992). (b) Deviation of inclination from the expected value of the geocentric dipole at the latitude of Maui during the past 10 kyears. The green and red lines, respectively, represent a polynomial fit and a smoothing of the inclination changes. The blue line is the prediction of the CALSK7K model (Korte and Constable, 2005). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
We can attempt to evaluate the influence of irregular temporal sampling of the field by the lava flows by fitting the variations shown in Fig. 8a with a polynomial of order 9 or by a simple smoothing (Fig. 8b). In both cases, the deviation from the GAD inclination disappeared (delta inc. = 0.2◦ ± 6◦ and 0◦ ± 6.5◦ , respectively). If we restrain the analysis to the past 5 kyear which are much better documented, we obtain a small anomaly of 3.5◦ ± 5◦ . This suggests that series of data longer than a few thousand years with very good temporal resolution are needed to estimate the time-averaged field. In other words, what seems to be in cause is the
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length of the time interval over which biases can generate an inclination anomaly. It would be pertinent to do the same analysis using the CALS7K (Korte and Constable, 2005) model predictions. However due to the influence of the sedimentary data, the inclination variations derived from the model do not perfectly fit the volcanic results and the mean inclination of the model deviates by −6.3◦ ± 5.7◦ from the expected dipole value. From these considerations, we infer that there is no evidence for a significant persistent inclination anomaly in the present state of the database. However, it is clear that additional detailed records, at least between 5 and 7 ka B.P., would considerably reinforce this conclusion. 6. Discussion One may thus wonder whether there is a strong indication of a persistent anomaly for older periods. An exhaustive compilation of the hawaiian volcanic data has been presented by Love and Constable (2003) and the same database was used recently to study the paleosecular variation over the past 5 Myear from sites at ±20◦ latitude (Lawrence et al., 2006). In both cases, the authors conclude to a large inclination anomaly and note that the field in Hawaii is indeed unusual over timescales of 0–5 Myear. Given the temporal distribution of the records, it is also interesting to examine whether this inclination anomaly appears to be consistent between the individual records and over which time scale. If we consider all results published so far, we can separate the “precursory” Hawaiian records (many of them being revisited) which did not meet the standard criteria, specifically the existence of stepwise demagnetization, advocated for “modern” paleomagnetic studies from a second generation of recent data. Another important concern is the presence of dating which was also not present in many initial studies. We have updated our previous compilation (Herrero-Bervera and Valet, 2003) which is restrained to the “modern” records and plotted the deviation of their mean inclination from the GAD value in Fig. 9. The youngest detailed data were obtained from drilled cores. The SOH-1 and SOH-4 cores drilled on Kilauea volcano (Gratton et al., 2004; Teanby et al., 2002; Laj et al., 2002) document the past 100 kyears. We included 111 determinations of inclination from SOH-1 and limited the analysis of SOH-4 to the period between 40 and 100 ka to avoid the large number of “excursional” lava flows surrounding the past 40 kyears. The SOH-1 and SOH-4 cores indicate a negative deviation by 5◦ and 7◦ , respectively. However, the
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Fig. 9. Deviation of the mean inclination from the GAD value calculated for successions of lava flows older than 10 kyears in several hawaiian islands. N and R indicate the normal and reverse polarities of the sequences. Despite an apparent change of sign of the anomaly with field polarity, there is no large evidence for a stable and persistent inclination anomaly.
age control was restrained to a few radiometric determinations, which for SOH-1 are sometimes not fully consistent with each other. We note also that the mean deviation from the GAD value is 10◦ (±13◦ ) for the past 50 kyear, and 6◦ (±6/3◦ ) for the 0–18 ka period. Interestingly, the mean inclination found in SOH-1 which, on the basis of the interpolated chronology, appears to be younger than 10 kyear is less than 2◦ (±10◦ ). This small offset from the GAD value is likely not very significant in the absence of detailed chronological control. Another 941 m long drilled core (HSDP) of 190 lava flows (Holt et al., 1996) at Hilo provides a record of inclination for the past 400 kyear. The results are characterized by a lower deviation of 4.3◦ , but again in this case the lack of systematic dating does not rule out completely the possibility of serial correlation. Note that no estimate of the VGP dispersion can be obtained for these cores in the absence of declination. Lastly, the Honolulu volcanic series (HVS) from O’ahu were revisited using only dated flows (Herrero-Bervera and Valet, 2002) and are characterized by a deviation of 3.5◦ (±5.8◦ ) which becomes actually lower than 3◦ after correcting for the motion of the Pacific plate. Thus despite some uncertainties, all these data indicate a negative anomaly during the Brunhes period. It is interesting to test the persistence of the anomaly during older periods, particularly during Chrons with reverse polarity (Fig. 9). The dated lava flows from the island of Lanai (Herrero-Bervera et al., 2000) do not show any deviation from the expected field inclination during the Matuyama Chron after correction for the Pacific plate motion. In contrast, the older Koolau series (Herrero-Bervera and Valet, 2003) in O’ahu which are close to the onset of the Matuyama chron exhibit a
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deviation of 3.6◦ ± 9◦ which is not very different from the HVS series. Lastly, the sites of overlying lava flows from the Waianae (Herrero-Bervera and Valet, 1999, 2005) formation of O’ahu do not show any convincing evidence for a persistent anomaly. We observe a small positive anomaly of 4.3◦ for the reverse sequence between the Gilbert-Gauss and the lower Mammoth transitions, but there is no deviation during the previous interval with normal polarity. This difference is probably not a consequence of sampling as the number of flows (64 and 73, respectively) is in both cases very large and about the same. One can also probably exclude the presence of little normal overprint because all samples were thermally stepwise demagnetized. The short duration of these periods (120 and 50 kyear, respectively) could play some role, given the proximity of the reversal boundaries. Finally, we cannot exclude that some tilting of this part of the sequence could be linked to subsidence of the island. Thus, given all these uncertainties, we are rather inclined to conclude that there is no striking evidence in favor of a systematic and persistent inclination anomaly. The most significant observation may be the fact that the anomaly, when present, changes sign with field polarity, being consistently positive for reverse polarity flows and negative for normal polarity. However the results show also that the deviation from the GAD inclination varies considerably over short time scales. 7. Conclusion We measured new paleofield directions from 13 dated volcanic lava flows in the island of Maui with ages between 0.2 and 10.3 ka. The results confirm the directional changes reported at the Big Island for the past 10 kyear, particularly the existence of low inclinations about 2 kyear ago. A direct consequence is that dating was correct and that there were neither perturbations, nor local anomalies (volcanic edifice, terrain, island). These results emphasize also the importance of dating for PSV studies. The compilation of the volcanic records obtained for the past 10 kyear shows that the mean Hawaiian field is characterized by very low dispersion of the VGPs, which could be seen as a consequence of a low non-dipole field under Hawaii. The mean inclination is 1.8◦ lower than expected, but the depletion of data between 5 and 7 ka B.P. is most likely responsible for this small deviation. The volcanic records indicate also that shallow inclinations recorded at the nearby lake Waiau (Peng and King, 1992) for the same period likely result from compaction of the sediment. Thus, there would be no strong indication for a persistent inclina-
tion anomaly under Hawaii during the past 10 kyear. We also considered the possibility of a longer term persistent component after selecting the most reliable and long records within the Brunhes and the previous polarity chrons. The compilation shows some tendency for a deviation from the GAD inclination, but it is not systematic and never outside the variance. We also note that some mean inclinations are in perfect agreement with the expected value. Thus, we consider that the present database is not so consistent with the existence of a magnetic anomaly under Hawaii during the last million years. Acknowledgements We are very grateful to Mr. James Lau for his field and laboratory assistance. We greatly acknowledge Julie Carlut and Xavier Quidelleur for help during field work. We would like to thank Dr. Eric Bergmanis for his guidance, support and advice to us for the field work. Financial support to Emilio Herrero-Bervera was provided by SOEST-HIGP and the National Science Foundation grants EAR-9909206, EAR-INT-9906221, EAR-0207787, EAR-0213441, EAR-0510061. Data processing was performed using the paleomac program (Cogn´e, 2003). Financial support to Jean-Pierre Valet was provided by the INSU-CNRS Dyeti program. This is a SOEST contribution number 7100, HIGP contribution number 1486 and IPGP contribution number 2215. References Baag, C., Helsley, C.E., Xu, S.-Z., Lienert, B.R., 1995. Deflection of paleomagnetic directions due to magnetization of the underlying terrain. J. Geophys. Res. 100 (B7), 10013–10027. Bergmanis, E., Sinton, J.M., Trusdell, F.A., 2000. Rejuvenated volcanism along the southwest rift zone, East Maui, Hawaii. Bull. Volcanol. 62, 239–255. Carlut, J., Courtillot, V., 1998. How complex is the time-averaged geomagnetic field over the past 5 Myr? Geophys. J. Int. 134, 527– 544. Coe, R.S., Gromme, S., Mankinen, E.A., 1978. Geomagnetic intensities from radiocarbon-dated lava flows on Hawaii and the question of the Pacific nondipole low. J. Geophys. Res. 83, 1740– 1756. Cogn´e, J.P., 2003. PaleoMac: a MacintoshTM application for treating paleomagnetic data and making plate reconstructions. Geochem. Geophys. Geosyst. 4 (1), doi:10.1029/2001GC000227, 1007. Day, R., Fuller, M.D., Schmidt, V., 1977. Hysteresis properties of titanomagnetites: Grain-size and compositional dependence. Phys. Earth Planet. Int. 13, 260–267. Doell, R.R., Cox, A.V., 1963. The accuracy of the paleomagnetic method as evaluated from historic Hawaiian lava flows. J. Geophys. Res. 68, 1997–2209.
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