Paleointensity variation across the Matuyama Brunhes polarity transition: Observations from lavas at Punaruu Valley, Tahiti

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010jb008093, 2011 Paleointensity variation across the Matuyama Brunhes polarity transition: Observations from lavas at Punaruu Valley, Tahiti Nobutatsu Mochizuki, 1 Hirokuni Oda, 2 Osamu Ishizuka, 2 Toshitsugu Yamazaki, 2 and Hideo Tsunakawa 3 Received 7 November 2010; revised 18 January 2011; accepted 15 March 2011; published 10 June [1] We have conducted a paleointensity study of the Matuyama Brunhes (M B) polarity transition recorded in 34 successive lava flows of Punaruu Valley on Tahiti. A reversed polarity is obtained from the lower part of the record, major directional changes are derived from the middle part of the record, and a normal polarity is recorded in the upper part of the record. These paleomagnetic directions and five 40 Ar/ 39 Ar ages yielding a weighted mean of 771 ± 8 (1s) ka indicate that 30 lava flows recorded the geomagnetic field across the M B transition. The 215 specimens from 32 flows were subjected to the double heating technique of the Shaw method combined with low temperature demagnetization (LTD DHT Shaw method), yielding 73 successful results from 18 flows. For the reversed polarity period just prior to the major directional changes, paleointensity shows an oscillation like variation between 3 and 38 mt corresponding to virtual dipole moments (VDMs) between and Am 2. For the major directional changes, a weak paleointensity of 5 mt is obtained, which gives a VDM of Am 2. For the normal polarity period, paleointensities are mt, giving VDMs of Am 2. For the reversed polarity period just prior to the major directional changes, a linear relationship with a correlation coefficient of 0.96 is recognized on the diagram of VDM versus virtual geomagnetic pole latitude. This linear relationship may be a precursory feature of the geodynamo at the onset of the M B transition. Citation: Mochizuki, N., H. Oda, O. Ishizuka, T. Yamazaki, and H. Tsunakawa (2011), Paleointensity variation across the Matuyama Brunhes polarity transition: Observations from lavas at Punaruu Valley, Tahiti, J. Geophys. Res., 116,, doi: /2010jb Introduction [2] Precise paleomagnetic data of polarity reversals are crucial for understanding the geomagnetic field and have been expected to constrain geodynamo models in the deep interior of the Earth. Paleointensity studies on volcanic rocks have shown that the geomagnetic field intensity during polarity reversals is quite low, and is 1/10 1/3 of the nontransitional field intensity [e.g., Prévot et al., 1985; Quidelleur and Valet, 1996; Valet et al., 1999; Brown et al., 2009]. It is widely accepted that the field intensity drops prior to major directional changes and recovers after them [e.g., Merrill and McFadden, 1999; Brown et al., 2009]. In other words, the decrease of the geocentric axial dipole (GAD) occurs at the onset of polarity reversals, suggesting that the GAD intensity change is vital for the geodynamo at the onset of polarity reversals. Thus, precise absolute paleointensity data are required for clarifying the early stage 1 Priority Organization for Innovation and Excellence, Kumamoto University, Kumamoto, Japan. 2 Geological Survey of Japan, AIST, Tsukuba, Japan. 3 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan. Copyright 2011 by the American Geophysical Union /11/2010JB of a polarity reversal. For recording a polarity reversal, sequences of lava flows at hot spot volcanoes are one of the best materials because high eruption rate lava flows can provide temporal variations of paleointensity as well as paleodirection. [3] The last polarity reversal, the Matuyama Brunhes (M B) reversal, is the best documented one in terms of the number of paleomagnetic records. For the M B transition, paleomagnetic records of successive lava flows have been reported from six localities; Iceland [Kristjansson et al., 1988], Tahiti [Chauvin et al., 1990], Maui [Baksi et al., 1992; Coe et al., 2004], Chile [Brown et al., 1994, 2004], La Palma [e.g., Quidelleur and Valet, 1996; Valet et al., 1999], and La Guadeloupe Island [Carlut et al., 2000]. To date, successive paleointensity data were reported from two of these localities; La Palma [Quidelleur and Valet, 1996; Valet et al., 1999; Brown et al., 2009] and Chile [Gratton et al., 2007]. [4] Chauvin et al. [1990] reported the M B transition from lava flows in the southern side of Punaruu Valley on Tahiti. This exposure is one of the detailed volcanic records of the geomagnetic field from the late Matuyama Chron through the M B transition. Chauvin et al. [1990] reported two paleointensities of 3.4 (N = 2) and 3.6 mt (N = 3) for two of the transitionally magnetized flows and a paleointensity 1of17

2 Figure 1. (a) Map showing the location of Punaruu Valley on Tahiti. (b) Sampling sites of Punaruu Valley. (c) Enlarged map of sampling sites. (d) Stratigraphical relationship of the 34 lava flows. Shaded zone between flows A25 and B1 was not accessible and zone between flows A1 and A2 contains some flows which were not sampled. A thin weathered zone was recognized between flows A6 and A7. of 37.7 mt (N = 2) for a flow from the reversed polarity zone which is the sixth flow below the transitionally magnetized flows. However, no systematic paleointensity data was reported for the M B record from Punaruu Valley. Chauvin et al. [1990] applied the Thellier method [Thellier and Thellier, 1959] as modified by Coe [1967] to 48 selected samples recording the M B transition, the Jaramillo event, and others. Twenty two samples were rejected because of negative partial thermoremanent magnetization (ptrm) checks or concave up Arai diagrams. Based on this earlier work, we consider that paleointensity experiments on the Punaruu lavas are affected by alteration during laboratory heating and multidomain (MD) effects. [5] In this study, we collected 242 core samples and 13 block samples from 34 lava flows along the northern side of Punaruu Valley. We conducted a paleointensity study on the M B transition using the double heating technique of the Shaw method combined with low temperature demagnetization (LTD DHT Shaw method) [Tsunakawa and Shaw, 1994; Yamamoto et al., 2003; Mochizuki et al., 2004a]. In this method, a full thermoremanent magnetization (TRM) is imparted during cooling from high temperature ( 600 C), and is demagnetized by alternating fields (AFs) after LTD. Also, this method uses the AF demagnetization spectra of an anhysteretic remanent magnetization (ARM) to correct the AF demagnetization spectra of a laboratory TRM for any mineralogical alteration that occurred during heating. Hence, the LTD DHT Shaw method is likely suitable for Punaruu lavas from the information available from Chauvin et al. [1990]. In this paper, we report detailed temporal variations of the geomagnetic field at the M B transition. 2. Geology and Sampling [6] The Society Islands in the southern central Pacific comprises islands, atolls and seamounts, all of which are 2of17

3 Table 1. Paleodirections and Paleointensities from 34 Lava Flows in the Northern Side of Punaruu Valley a DG Flow Name Site Name TH Declination Curve N DIR (deg) Inclination (deg) a 95 (deg) k Latitude (deg) VGP Longitude (deg) N INT Intensity (mt) VDM (10 22 Am 2 ) 1 A1 TM35 L 9/ / ± ± A2 TM34 H 9/ / ± ± A3 TM10 H 13/ / ± ± A4 TM09 M 9/ /8 7.5 ± ± A5 TM08 H 7/ / ± ± A6 TM07 H 7/ / ± ± A7 TM06 H 8/ / ± ± A8 TM05 H 13/ / ± ± A9 TM04 H 8/ / ± ± A10 TM03 L 8/ / ± ± A11 TM02 L 7/ /5 12 A12 TM01 H 8/ / ± ± A13 TM11 H, M, L 14/ / ± ± A14 TM12 H, H 8/ /6 6.1 ± ± 0.49 A15 TM14 M 1/5 0/2 15 A16 TM13 L, L 6/ /5 16 A17 TM18 H, H 5/ /9 3.3 ± ± A18 TM19 L 17/ / ± ± 0.17 A19 TM16 H 2/5 A20 TM17 L 2/2 18 A21 TM33 L + Inv 6/ /5 19 A22 TM20 L, L 5/ /5 20 A23 TM21 M, M 7/ /4 21 A24 TM27 L + Inv 8/ /7 (22) A25 TM15 M 5/ /6 (22) A26 TM22 H 7/ /5 23 A27 TM23 L + Inv 5/ /4 24 A28 TM24 M 7/ /7 (25) A29 TM26 M 7/ /7 (22) B1 TM32 L 3/ /6 (25) B2 TM31 L 4/ /7 26 B3 TM30 H 9/ / ± ± B4 TM29 M 9/ / B5 TM28 L 9/ / ± ± 0.33 Grouped flows 22 A25, A26, B1 TM15, TM22, TM32 15/ A29, B2 TM26, TM31 11/ a TH curve, types of thermomagnetic curve, details of which are noted in text; N DIR and N INT, number of samples used for calculating the mean value/ number of samples measured; a 95, 95% confidence limit of circle; k, precision parameter. Uncertainties of paleointensity and VDM are noted by one standard deviation (1s). predominantly volcanic. The arrangement and age distribution of the islands are consistent with the absolute motion of the Pacific plate over a fixed hot spot during the last 5 Ma [Duncan and McDougall, 1976]. The paleopositions for the dated rocks have been estimated by assuming the absolute motion of the Pacific plate, which shows a narrow distribution of 100 km in length in the western part of the present active hot spot region; this suggests that the volcanism of each island might have persisted for about 1 Myr [Uto et al., 2007]. The island of Tahiti, which consists of two main volcanoes, Tahiti Nui and Iti (Figure 1a), lies near the southeastern end of the Society Islands. Tahiti Nui is a volcanic cone rising 6000 m above the surrounding seafloor, and is 2000 m above sea level. Punaruu Valley is located on the western side of Tahiti Nui. Chauvin et al. [1990] conducted a paleomagnetic study on a 700 m thick lava sequence exposed on Punaruu Valley, which was dated at 0.78 to 1.2 Ma using K Ar geochronology. K Ar ages indicated that 80% of the 700 m thick sequence was formed in Ma and the rest of the sequence in Ma [Duncan et al., 1994]. [7] In this study, we sampled 34 lava flows exposed on the northern side of Punaruu Valley (Figure 1). The altitudes of the 34 flows are m, which are similar to those of the M B record of the southern side of the valley (Figure 1b) [Chauvin et al., 1990]. A typical thickness of the flows is m, and particularly thick flows are noted in Figure 1d. Stratigraphic positions of flows A29 and B5 are not clearly recognized at each sampling site but they are probably underlain by flows A28 and B4, respectively. The stratigraphic positions of flows A19 and A20 are also not clearly recognized but they are estimated to be positioned between flows A18 and A21. The correlations between the B section and the upper part of the A section will be discussed later on the basis of paleomagnetic directions (section 4.2). [8] Six to ten cores were collected from each individual flow using a portable engine drill. For three flows (A18 A20), block samples were collected. These core and block samples were orientated with a magnetic compass. Some of the core samples were oriented with sun compass. These orientations from sun compass agree well with the orienta- 3of17

4 Figure 2. Typical themomagnetic curves. Samples were heated and cooled in helium gas flow. The vertical axis shows saturation magnetization (M S ) normalized to that at room temperature (M S0 ). Solid and dashed lines indicate heating and cooling curves, respectively. Obtained curves are classified into six types (H, H, M,L, L, and L + Inv). tions from magnetic compass. In the laboratory, 1 inch diameter cores were drilled from the oriented block samples. Cores were cut into specimens of about 1 inch in length for paleomagnetic measurements. Specimen name consists of site name, and core and specimen numbers (e.g., TM01 1 1). For specimens from block samples, specimen name comprises site name, block sample number, and core and specimen numbers (e.g., TM ). Flow name is given for each flow in the A and B sections indicating the stratigraphic order from the bottom to the top of each section (Table 1). Directional Group (DG) numbers are given to 28 units yielding mean paleomagnetic directions (see section 4.2). 3. Rock Magnetic Properties [9] Thermomagnetic analyses were performed using a vibrating sample magnetometer (VSM; Micro Mag 3900, Princeton Measurements Corp.). Samples were heated and cooled at a rate of 10 C/min in helium gas flow with a DC field of 0.5 T. Curie temperatures (T c ), estimated by using the differential method on heating curve [Tauxe, 1998], are at about C and/or C (Figure 2). These Curie temperatures indicate the existence of Ti poor and Ti rich titanomagnetite grains. The thermomagnetic curves during heating are categorized into six types (Figure 2 and Table 1). Samples with a single phase of high T c are classified into type H, and those with a single phase of low T c into type L. Curves showing both phases are classified as type M. The intermediate type between H and M is classified into type H, and that between L and M is into type L. Samples from three flows showed a dominant low T c phase and a minor phase at C associated with an increase above the temperature (type L + Inv), suggesting that some amount of titanomaghemite in the samples inverted to magnetite during heating. [10] Hysteresis properties were measured on a few chips from 1 to 3 cores from each flow using the VSM. On the Day plot [Day et al., 1977] (Figure 3), the data points are distributed in the region close to the single domain (SD) area and partly in the pseudo single domain (PSD) area. Most data points are distributed along the theoretical mixing lines of SD and MD grains by Dunlop [2002]. [11] These rock magnetic results indicate that magnetic carriers of the samples are explained by a mixture of Ti poor and Ti rich titanomagnetite grains. On the basis of the Day plot, the magnetic domains of the grains appear to be PSD or a mixture of SD and MD. Thus the samples from these lava flows may provide reliable paleointensity determinations except for those from three flows showing thermomagnetic curve of type L + Inv. 4. Paleodirections 4.1. Method [12] Paleomagnetic measurements were conducted at the Tokyo Institute of Technology and the Geological Survey of Japan, AIST. For the paleodirectional measurements, 302 specimens were subjected to either thermal demagnetization or AF demagnetization after LTD treatment. Thermal demagnetization was applied to 71 specimens, generally 4of17

5 Figure 3. Day plot showing hysteresis parameters (M s, saturation magnetization; M rs, saturation remanence; B c, coercivity; B cr, coercivity of remaence) of chip samples from individual lavas. Symbols denote six groups classified with respect to thermomagnetic curves. Closed symbols indicate sample used in thermomagnetic measurement. Theoretical SD + MD (or PSD/SD + MD) mixing lines by Dunlop [2002] are also shown. two specimens from each flow. Specimens were heated in air at C intervals up to 600 C with electric furnaces (Natsuhara Giken TDS 1). The hold time at top temperature was 15 min. Remanence was measured with spinner magnetometers (Natsuhara Giken SMD 88). [13] AF demagnetization up to mt was applied to 231 specimens after LTD, primarily as a part of the LTD DHT Shaw paleointensity procedure. For most specimens, remanence measurements and AF demagnetization were conducted using an automated spinner magnetometers with in line AF demagnetizer (Natsuhara Giken DSPIN 2) [Kono et al., 1984, 1997]. For weakly magnetized specimens, remanence was measured with a spinner magnetometer with a higher sensitivity (Natsuhara Giken ASPIN A), and demagnetization was conducted using a separate AF demagnetizer (Natsuhara Giken DEM 8601C). Intervals of AF demagnetization were 2 10 mt. Narrow intervals (2 or 2.5 mt) were usually set up to 20 or 30 mt because secondary components were progressively removed at these steps. Previous studies have shown AF demagnetization after LTD effectively removed secondary components from natural remanent magnetization (NRM) of transitionally magnetized basalts on New Zealand [Mochizuki et al., 2004b, 2006, 2007] and Maupiti, Tahaa, and Raiatea of the Society Islands [Yamamoto et al., 2007a] Results [14] Representative demagnetization results are shown in Figure 4. Directional data for individual flows are listed in Table 1, and mean directions are shown in Figure 5. Excluding secondary components, the characteristic remanent magnetization direction of each specimen is determined using principal component analyses [Kirschvink, 1980]. A mean direction for each flow is calculated from directions of three or more specimens. [15] For 13 flows of the lower part of the A section (flows A1 A13) and three flows of the upper part of the B section (flows B3 B5), natural remanent magnetizations (NRMs) are characterized by a stable component with a small secondary component in both thermal and AF demagnetizations (Figure 4). The secondary component was removed at a low temperature ( C) or low AF ( mt). Consistent primary directions were obtained from 7 or more specimens, yielding mean directions with 95% confidence limits (a 95 ) of less than 5. [16] For 16 flows from the upper part of the A section (flows A14 A29) and two flows from the lower part of the B section (flows B1 B2), NRMs typically consist of two components (Figure 4). The component with low blocking temperature/coercivity is regarded as secondary, whereas the component with high blocking temperature/coercivity as primary. For several specimens from these 18 flows, even the high coercivity/blocking temperature components appeared to be contaminated by secondary components. Thus directional results yielding a maximum angular deviation (MAD) of more than 30 in the principal component analyses were discarded since they were possibly affected by secondary components. Also, if a characteristic direction of a specimen was apparently inconsistent with those of other specimens within a flow and was out of the 95% confidence limit, it was not used for the average calculation. Mean directions were obtained for 15 flows with 95% confidence limits of The 95% confidence limits are larger than those of flows from the lower part of the A section (flows A1 A13) and the upper part of the B section (flows B3 B5). This observation is possibly related to the fact that NRM 30mT / ARM 30mT values are weak (section 7.1); the larger ratios of the secondary component to the primary one result in the larger dispersion of characteristic directions. For three flows (flows A15, A19, and A20), a mean direction was not calculated since only one or two reliable directions were obtained within each flow. [17] On the basis of the paleomagnetic directions, the flows of the B section are correlated with those of the A section. Five flows of the B section, which are underlain by flow A25, show a directional reversal from reversed to normal (Figure 5c). The reversed direction of flow B1 is indistinguishable from those of flows A25 and A26 at the 95% confidence limit (Figure 5b). The direction of flow B2 can be correlated with that of flow A29. Three flows at the top of the B section (flows B3 B5) record normal directions which are distinguished from those of flows of the A section, suggesting that these three flows were extruded above flow A29. Combining the directional results of correlated flows, 28 directional groups (DG) are assigned. Mean paleodirections in stratigraphic order are given in Table 1. [18] In summary, the lower part of the record (DGs 1 16) gives a reversed polarity, the middle part of the record (DGs 17 24) shows major directional changes including one reversal and two rebound like behaviors, and the upper part of the record (DGs 25 28) yields normal polarity (Figure 6a). Directional results in stratigraphic order are shown in Figure 7. 5of17

6 Figure 4. Examples of orthogonal vector plots for NRM of samples, which were obtained during thermal (TH) demagnetization and alternating field (AF) demagnetization after low temperature demagnetization (LTD). Solid and open symbols are projections onto horizontal and vertical planes, respectively. 6of17

7 Figure 6. (a) Equal area projection of mean paleomagnetic directions of 28 directional groups (DGs) from the northern side of Punaruu Valley (this study) after combining paleomagnetic data from the sections A and B. Crosses denote the geocentric axial dipole (GAD) directions. (b) Equal area projection of mean paleomagentic directions from flows in the southern side of the same valley [Chauvin et al., 1990] Comparison With the Data of Chauvin et al. [1990] [19] The directional changes obtained in this study (Figure 6a) are more complicated than those of Chauvin et al. [1990], who report a single directional reversal from Figure 5. Equal area projection of mean paleomagnetic directions for each lava flow. Mean paleomagnetic directions for lava flows from the (a) lower part of the A section (flows A1 A17), (b) upper part of the A section (flows A18 A29), and (c) B section (flows B1 B5). Solid (open) symbols denote a positive (negative) inclination. Gray ovals are 95% confidence limit (a 95 ). 7of17

8 Figure 7. Paleomagnetic results of 28 DGs from the flows in the northern side of Punaruu Valley. The vertical axis indicates DG number in the stratigraphic order. 40 Ar/ 39 Ar ages with uncertainty (1s) are also shown in the diagram for VGP latitude. Error bars in diagrams for declination and inclination denote 95% confidence limits, and those in diagram for paleointensity and NRM 30mT /ARM0 30mT denote 1 standard deviation. Dashed lines in the diagrams for declination and inclination indicate the GAD field directions, and that in the diagram for paleointensity is the past 5 Myr mean [Yamamoto and Tsunakawa, 2005]. This mean value is calculated from the paleointensities determined by the same method from volcanic rocks of the Society Islands, including Tahiti, which were formed at the same hot spot. the southern side of Punaruu Valley (Figure 6b). A similar oscillatory remanence change at the M B transition was reported from a lava flow sequence on Maui [Coe et al., 2004] and also from several sedimentary records [e.g., Tsunakawa et al., 1996, 1999; Yamazaki and Oda, 2001]. Complex directional behaviors were clearly observed in the highdeposition rate sediment drifts [Channell and Lehman, 1997]. It is also noted that the intermediate direction of DG 24 is similar to the cluster of intermediate directions reported by Chauvin et al. [1990]. [20] Distance between our sites and those of Chauvin et al. [1990] is 700 m (Figure 1b). Not all the lava flows from the volcanic center may have covered both areas. Also, exposed lavas may be not completely the same; the sites of Chauvin et al. [1990] were located at a section exposed along a steep fall into the valley while our sites were located at a section exposed along a road where the number of exposed lava flows may have been larger. These differences may explain why the paleomagnetic record of this study is different from that of Chauvin et al. [1990]. 5. Ar/Ar Dating 5.1. Method [21] The age of the basalt flows was determined using 40 Ar/ 39 Ar geochronology technique at the Geological Survey of Japan (GSJ), AIST. Details of the analytical procedure have been reported by Ishizuka et al. [2003, 2009]. Fresh groundmass of mg was analyzed using a stepwise heating procedure. The samples were treated ultrasonically in 3N HCl for 20 min and then 4N HNO 3 for 20 min to remove possible alteration products (clay and carbonates) prior to irradiation. Sample irradiation was done at the JRR3 and JRR4 reactors [Ishizuka, 1998]. Fast neutron flux in the Table 2. Summary of 40 Ar/ 39 Ar Dating Results of Lava Flows in the Northern Side of Punaruu Valley a DG Flow Name Sample Analysis Total Gas Age (ka) Weighted Average (ka) Inverse Isochron Age (ka) Plateau Age 40 Ar/ 36 Ar Intercept MSWD Fraction of 39 Ar (%) 4 A4 TAH9917 U ± ± ± ± A5 TAH9916 U ± ± ± ± A14 TAH9905 U ± ± ± ± A24 TM27 U ± ± ± ± A25 TAH9923 U ± ± ± ± B3 TM30 62 U ± ± ± ± a All the uncertainties are shown in ±1s. l b = yr 1, l e = yr 1, 40 K/K = % [Steiger and Jäger, 1977]. DG denotes directional group number. Total gas age is calculated using sum of the total gas released. MSWD is mean square of weighted deviates ((SUMS/(n 2))^0.5) of York [1968]. 8of17

9 Figure 8 9of17

10 reactor was about ncm 2 s 1 and n cm 2 s 1, respectively. Sanidine separated from the Fish Canyon Tuff (FC3) was used as a flux monitor. In the geochronology laboratory in GSJ, we adopted 27.5 Ma as an age for the Fish Canyon sanidine based on the calibration conducted in our laboratory; we have obtained 27.5 Ma for the Fish Canyon sanidine, which has been determined relative to a primary standard for our K Ar geochronology, Sori biotite, whose radiogenic 40 Ar content is determined to be ± ml STP/g by manometric method [Uchiumi and Shibata, 1980] and K Ar age is 91.2 Ma (calculated using K 2 O content of Matsumoto [1989] and the radiogenic 40 Ar content). This age for the Fish Canyon sanidine is consistent with the age (27.51 Ma) suggested by Lanphere and Baadsgaard [2001]. [22] The Fish Canyon sanidine has been widely used for a standard in 40 Ar/ 39 Ar geochronology and various efforts to determine a precise age of the Fish Canyon sanidine have been conducted. If the value of Ma [Kuiper et al., 2008] or Ma [Renne et al., 2010] is adopted for an age of the Fish Canyon sanidine, ages in this paper would shift older by 2.5 3%. [23] A continuous CO 2 laser (NEWWAVE MIR10 30) fitted with a faceted lens was used for sample heating. This heating system allows emission of a homogenized beam of 3.2 mm diameter to ensure the uniform heating of samples. Argon isotopes were measured on a VG Isotech VG3600 noble gas mass spectrometer fitted with a BALZERS electron multiplier. Correction for interfering isotopes was achieved by analyses of CaFeSi 2 O 6 and KFeSiO 4 glasses irradiated with the samples. The blank of the system including the mass spectrometer and the extraction line was ml STP for 36 Ar, ml STP for 37 Ar, ml STP for 38 Ar, ml STP for 39 Ar, and ml STP for 40 Ar. Blanks were measured after every four analyses. [24] All errors for 40 Ar/ 39 Ar results are reported at 1 standard deviation. Errors for age include analytical uncertainties for Ar isotope analysis, correction for interfering isotopes and J value estimation. An error of 0.5% was assigned to J values as a pooled estimate during the course of this study. Plateau ages were calculated as weighted means of ages of plateauforming steps, where each age was weighted by the inverse of its variance. The age plateaus were determined following the definition by Fleck et al. [1977]. Inverse isochrons were calculated using York s least squares fit, which accommodates errors in both ratios and correlations of errors [York, 1968] Results [25] Results of 40 Ar/ 39 Ar dating are summarized in Table 2. Ar isotope data and 40 Ar/ 39 Ar spectra are shown in Table S1 and Figure S1, respectively, in the auxiliary material. 1 An 40 Ar/ 39 Ar age of 845 ± 9 (1s) ka was obtained from flow A4 1 Auxiliary materials are available in the HTML. doi: / 2010JB in the lower part of the record. Five 40 Ar/ 39 Ar ages from flows A5, A14, A24, A25, and B3, which are located above flow A4, are 779 ± 17,777 ± 17,755 ± 15,768 ± 18, and 787 ± 24 ka. These five ages are indistinguishable at 1s level, yielding a weighted mean age of 771 ± 8 (1s) ka. These ages indicate that four flows (flows A1 A4) were erupted at 845 ± 9 ka or older and 30 other flows were extruded at 771 ± 8 ka. [26] Using the intercalibration factors between standards [Renne et al., 1998], and assuming the age of the Fish Canyon Tuff sanidine as 27.5 Ma, a weighted mean 40 Ar/ 39 Ar age from 10 results for three flows of the M B transition record on Tahiti, which were reported by Singer and Pringle [1996] and Singer et al. [2005], can be recalculated as 780 ± 3 ka. Therefore, the age estimate for the 30 flows in this study (771 ± 8 ka) is slightly younger but indistinguishable from the reported age for the M B transition at Tahiti. [27] The ages of the M B transition from different localities should be carefully discussed since the duration of the directional reversal depends on site location [Clement, 2004]. In addition, a precursor event, occurred at 18 kyr before the main reversal, has been suggested by Singer et al. [2005]. Further paleomagnetic and geochronological studies on volcanic rocks are required for this topic, but comparisons with the ages reported from other localities are given below. Our age estimate falls between the 40 Ar/ 39 Ar age for the record at Maui (a recalculated age of 762 ka [Singer et al., 2005]) and those for the records at Chile and La Palma (recalculated ages of 777 and 784 ka [Singer et al., 2005]). Also our age is consistent with the ages estimated by the directional reversal from high sedimentation rate sediments (773ka [Channell et al., 2010]) and the 10 Be flux anomaly with paleomagnetic behavior from sediments (770 ka [Suganuma et al., 2010]). [28] On the basis of the paleomagnetic data and 40 Ar/ 39 Ar ages, we conclude that 4 units of the reversed polarity were formed in the Matuyama Chron at 845 ka (DGs 1 4), and the subsequent 24 units recorded the geomagnetic field across the M B transition, which show the reversed polarity (DGs 5 16), the major directional changes (DGs 17 24), and the normal polarity (DGs 25 28). 6. Paleointensities 6.1. Method [29] The LTD DHT Shaw method [Tsunakawa and Shaw, 1994; Yamamoto et al., 2003; Mochizuki et al., 2004a] was applied to the samples for paleointensity determinations in this study. The reliability of this method as compared to Thellier style paleointensity technique has been checked by earlier workers using historical lavas in Hawaii and Japan [Yamamoto et al., 2003; Mochizuki et al., 2004a; Oishi et al., 2005; Yamamoto and Hoshi, 2008], and the method has been applied to older volcanic rocks for paleomagnetic studies [Yamamoto and Tsunakawa, 2005; Mochizuki et al., 2006; Yamamoto et al., 2007a, 2007b, 2010; Tsunakawa et al., 2009]. The details of the procedure are described in else- Figure 8. Examples of accepted paleointensity results. (a) TM from flow A12 and (b) TM from flow A18, which passed the selection criteria. NRM TRM1* diagram is used for the paleointensity estimation, and TRM1 TRM2* diagram is used for the validity check of the ARM correction for each sample. Closed circles are used for slope calculations. Units are 10 5 Am 2 /kg. 10 of 17

11 Figure 9 11 of 17

12 Figure 10. Diagram of mean paleointensity versus mean normalized NRM for each flow. Error bars denote 1 standard deviation. where [e.g., Yamamoto and Tsunakawa, 2005]. The selection criteria of the LTD DHT Shaw experiments were the same as those of Mochizuki et al. [2006]. The hold time at peak temperature in the first heating was min, and that in the second heating was 10 min longer than the first heating. Samples were heated and cooled in a vacuum of Pa. TRM was imparted in a 1 60 mt DC field, and ARM was given in a 50 mt DC field with the maximum AF available from each demagnetizer (140, 160, and 180 mt) Results [30] Two hundred fifteen specimens from 32 flows were subjected to the LTD DHT Shaw experiments. In total, 73 specimens from 18 flows passed the section criteria. All paleointensity results are listed in Table S2 in the auxiliary material, and the mean values are given in Table 1. Accepted and rejected examples are shown in Figures 8 and 9, respectively. [31] As noted above, the secondary components were generally removed by LTD and AF demagnetization up to mt and the resultant higher coercivity components can be used for paleointensity calculation in the NRM TRM1* diagram, where TRM1* is the TRM in the first heating after the ARM correction [Rolph and Shaw, 1985]. Several samples were almost fully demagnetized at the lower alternating field steps, and the remaining high coercivity components show noisy behavior around the origin of the NRM TRM1* and TRM1 TRM2* diagrams. In such case, slopes of the NRM TRM1* and TRM1 TRM2* diagrams were calculated by excluding the noisy part of the plots (Figure 8b). The most frequent reasons for rejection were insufficient linearity of NRM TRM1* diagrams (low correlation coefficient, r N < 0.995) as shown in Figure 9a and negative double heating check (nonunity of TRM1 TRM2* diagrams, slope T <0.95 or >1.05 [Tsunakawa and Shaw, 1994]) in Figures 9a and 9b. Fifty percent and 42% of the measured samples were rejected because of the former and later criteria, respectively. [32] The group of flows with the thermomagnetic curve types H and H yields a success rate of 60% which is three times higher than those of the groups of flows with the thermomagnetic types L and L, and M (these success rates are 17%). These results suggest that the samples containing a low Curie temperature phase tend to yield unsuccessful results possibly due to large alteration by heating (insufficient linearity of NRM TRM1* diagram and/or negative double heating check noted above). It should be noted that no correlation is observed between the paleointensity values and the types of themomagnetic curve. [33] In order to evaluate validity of a linear fit to NRM TRM* plots, the Akaike Information Criterion (AIC) [Akaike, 1980] is applied to the NRM TRM1* plots of the accepted data [Yamamoto and Tsunakawa, 2005]. A difference in the AIC values (DAIC) between the linear and quadratic fits for the accepted plots is included in Table S2. The previous LTD DHT Shaw experiments on the Society Islands samples suggest that DAIC exceeding 15 is possibly a sign of an undesirable paleointensity [Yamamoto and Tsunakawa, 2005]. Seventy four percent of the accepted results gave DAIC value less than 15, and the linear fits are generally acceptable. Following the criteria used in earlier studies using the LTD DHT Shaw method, we did not use DAIC as a selection criterion. [34] Multiple (two to six) paleointensity results were accepted for 17 flows, yielding a mean value for each flow, and also a paleointensity result was accepted for a flow (Table 1). Fifteen of the 17 mean paleointensities yield standard deviations smaller than 5 mt, 11 of which are smaller than 20% of the means, while two of them have standard deviations of about 10 mt (flows A3 and A8). 7. Discussion 7.1. Paleointensity Variation [35] As shown in Figure 7, 14 paleointensities from DGs 1 16 of the reversed polarity show a large variation between 3 and 41 mt, and a paleointensity of 5 mt (DG 17) is obtained for the beginning of the major directional changes. All paleointensity results of DGs recording the major directional changes were rejected according to the selection criteria. Finally, three paleointensities from DGs of the normal polarity range from 14 to 21 mt. [36] Mean paleointensity versus mean normalized NRM (NRM 30mT /ARM 30mT ) plots for each flow show a linear relation with the correlation coefficient of 0.86 (Figure 10). Figure 9. Examples of rejected paleointensity results. (a) TM from flow A11 is rejected since the NRM fraction and correlation coefficient of the NRM TRM1* diagram and the slope of the TRM1 TRM2* diagram do not fulfill the selection criteria. (b) TM from flow A12 is rejected since the slope of the TRM1 TRM2* diagram does not pass the selection criteria. 12 of 17

13 Figure 11. (a) VDMs versus VGP latitude plots for the M B transition from lava flows of Punaruu Valley, Tahiti (circles: this study; triangles: Chauvin et al. [1990]). Error bars indicate 1 standard deviation. (b) The same as Figure 11a which is enlarged for VGP latitude between 90 and 30. Solid data points are just prior to the major directional changes, which are used for calculating correlation coefficient except for an outlier (DG 8). (c) VDMs versus VGP latitude plots for the M B transition from lava flows on La Palma (solid diamond, ET ME section [Valet et al., 1999]; open diamond, ME section [Brown et al., 2009]; solid triangle and solid inverse triangle, LS and LL sections [Quidelleur and Valet, 1996]; open triangle and open inverse triangle, AN and AS sections [Brown et al., 2009]), Chile (right pointing triangles, QTW11 section [Gratton et al. [2007]), and La Guadeloupe Island (solid square, Carlut and Quidelleur [2000]; open square, Brown et al. [2009]). Each VDM is calculated from three or more paleointensity estimates. Solid and open symbols are on the basis of the Thellier and microwave Thellier methods, respectively. Data points obtained from Tahiti are shown by gray symbols. (d) The same as Figure 11c which is enlarged for VGP latitude between 90 and 30. Therefore, the mean normalized NRM can be used as relative paleointensity for the studied lava flows (Figure 7). The mean normalized NRMs for DGs are almost constant and very small. Hence, paleointensity during the major directional changes is expected to be comparable to that determined for DGs 14, 16, and 17 (3 6 mt). [37] The paleointensity data just prior to the major directional changes show an oscillation like variation between DG 5 and DG 16 (Figure 7). Our data do not show a simple decrease of paleointensity at the onset of the M B transition, but suggest a large variation in paleointensity at this period Relationship Between Paleointensity and Paleodirection [38] Paleointensity data of the M B transition from lavas on Tahiti are plotted in the diagram of VDM versus virtual geomagnetic pole (VGP) latitude (Figures 11a and 11b). As mentioned above, VDMs show an oscillation like variation for the reversed VGPs and subsequently decrease to Am 2 for the intermediate VGPs during the major directional changes. Finally, VDMs increase to Am 2 for the normal VGPs. The three data reported by Chauvin et al. [1990] are consistent with our data. [39] It should be notified that a linear relationship can be recognized for the revered polarity period (DGs 5 16) 13 of 17

14 Figure 12. Virtual geomagnetic poles (VGPs) of the M B transition reported from lavas. (a) VGPs calculated from paleodirections from lavas on Tahiti (circle, this study; triangle, Chauvin et al. [1990]). Directional group numbers are attached to the data of this study. (b) VGPs calculated from paleodirections from lavas on Maui (star, Coe et al. [2004]), Chile (solid and open triangles, QTW 10 and QTQ 11 sections [Brown et al., 2004]), Guadeloupe Island (square, Carlut et al. [2000]), and La Palma (pentagon, ME ET section [Valet et al., 1999]; solid and open diamonds, LS and LL sections [Quidelleur and Valet, 1996]; solid and open inverse triangles, TN and TS sections [Singer et al., 2002]; solid and open circles, AN and AS sections [Brown et al., 2009]). which are just prior to the major directional changes. The VDM VGP latitude plots with VGP latitudes of S show a linear trend with a correlation coefficient of 0.96 (Figure 11b). This linear trend exists even when the paleointensities of relatively large standard deviations and/or those with small number of data (N = 2) are excluded. [40] Paleointensity data of the M B transition from lavas on other localities, most of which are from lavas on La Palma, are shown in the diagram of VDM versus VGP latitude (Figures 11c and 11d). The overall movement in the diagram obtained in this study is consistent with data points from other localities (Figure 11c). However, it can be noticed that more than half of the La Palma data (diamond symbols in Figure 11d) are distributed above the linear trend observed at Tahiti. This may be related to difference of location, or to erroneously high Thellier paleointensities: several Thellier experiments on historical lavas showed that overestimated results which passed the general criteria were observed and the means were 10 40% higher than the expected intensities [Calvo et al., 2002;Yamamoto et al., 2003; Mochizuki et al., 2004a; Oishi et al., 2005]. [41] Oda et al. [2000] suggested a possible correlation between logarithm of relative paleointensity and VGP latitude for the M B transition on the basis of sedimentary records from the ODP Leg 124 in Celebes and Sulu seas, and the Boso Peninsula, Japan [Tsunakawa et al., 1996], and volcanic records from Tahiti [Chauvin et al., 1990] and La Palma [Quidelleur and Valet, 1996]. These data might support that the linear correlation in the VDM VGP latitude diagram at the beginning of the M B transition observed in this study. [42] Lu Lin et al. [1994] also suggested a linear correlation between logarithm of VDM and VGP latitude on the basis of paleointensity data from five polarity transitions and 14 of 17

15 geomagnetic excursions, which included the polarity transition reported from the Steens Mountain, Oregon [Prévot et al., 1985] and the M B transition from Tahiti [Chauvin et al., 1990]. These data may imply a linear correlation between VDM and VGP latitude is a character of reversals as well as the M B transition although detailed paleointensity data prior to major directional changes of other reversals are required Transitional VGP Movement [43] As shown in Figures 7 and 12, a transitional behavior of VGP was recorded by 10 lavas (DGs 16 25): swinging back and forth five times before the normal polarity. This VGP movement appears to be one of the most detailed behaviors of the M B transition obtained from lavas. If we assume a duration of 5 kyr for the directional transition [e.g., Clement, 2004], we can estimate an average duration for a swing of the VGP as 1 kyr. If we take stop and go character (discussed below) into account, the duration would be much shorter than 1 kyr, of the order of 100 yr. [44] Two possible stable locations on northern America can be recognized in the early part of the directional changes and two rebound like movements (four swings) are observed in the final part (Figure 12a). Note that a rebound like movement containing a cluster has been observed at lavas on Maui (Figure 12b). Also stop and go character with some swings has been observed in high deposition rate sediments [Channell and Lehman, 1997]. These behaviors have been observed in the high resolution volcanic record of the Steens Mountain reversal [Mankinen et al., 1985]. [45] The VGP movement from Tahiti (this study) has a similarity to that of Maui [Coe et al., 2004]. Because of their geographic proximity, the similarity of the VGP paths observed in Tahiti and Maui may be representative of a regional nonaxial dipole component of the geomagnetic field. [46] The transitional VGPs of this study are distributed in the eastern and western sides of the Pacific (Figure 12a), which have been recognized as the preferred VGP paths of reversals [Clement, 1991; Laj et al., 1991]. Also, it is noted that an intermediate VGP (DG 24) is located at the Australian VGP cluster of reversals [Hoffman, 1992; Hoffman and Singer, 2004], which includes the M B data from Tahiti [Chauvin et al., 1990] and Chile [Brown et al., 1994, 2004]. Thus the VGPs obtained in this study remind us of the biased VGPs during polarity reversals suggested in previous studies. 8. Conclusions [47] We have conducted paleodirectional and paleointensity measurements on 34 lava flows of the northern side of Punaruu Valley, Tahiti. On the basis of the paleomagnetic data and 40 Ar/ 39 Ar ages, we conclude that 4 units of the reversed polarity were formed in the Matuyama Chron at 845 ka (DGs 1 4), and the subsequent 24 units recorded the geomagnetic field across the M B transition, which show the reversed polarity (DGs 5 16), the major directional changes (DGs 17 24), and the normal polarity (DGs 25 28). For the reversed period (DGs 5 16) just prior to the major directional changes, the field intensity showed an oscillation like variation between 3 and 38 mt. For the major directional changes (DGs 17 24), the intensity, 5 mt, was very weak. For the normal period (DGs 25 28), the field intensity recovered to mt. For the reversed polarity just prior to the major directional changes, a linear relationship is recognized on the diagram of VDM versus VGP latitude, which has a correlation coefficient of This relationship may be a precursory character of the geodynamo at the onset of the M B transition. [48] Acknowledgments. We thank Kozo Uto for suggestions and help in the sampling. We also appreciate Masahiko Kurata for preliminary paleomagnetic measurements. We thank M. Narui and M. Yamazaki at the International Research Center for Nuclear Materials Science, Institute for Materials Research, Tohoku University, for providing opportunities of neutron irradiation of samples at the JRR3 reactor. We also appreciate Japan Atomic Energy Agency for providing irradiation opportunities at the JRR4 reactor. N. Mochizuki acknowledges the support by JSPS Research Fellowships for Young Scientists. We thank Joshua Feinberg, an anonymous referee, and the Associate Editor for helpful and careful review comments which improved the paper. References Akaike, H. (1980), Likelihood and the Bayes procedure, in Bayesian Statistics, edited by J. M. Bernardo et al., pp , Valencia Univ. Press, Valencia, Spain. Baksi, A. K., V. Hsu, M. O. McWilliams, and E. Farrar (1992), 40 Ar/ 39 Ar dating of the Brunhes Matuyama geomagnetic reversal, Science, 256, , doi: /science Brown, L., J. Pickens, and B. Singer (1994), Matuyama Brunhes transition recorded in lava flows of the Chilean Andes: Evidence for dipolar fields during reversals, Geology, 22, , doi: / (1994) 022<0299:MBTRIL>2.3.CO;2. Brown, L. L., B. S. Singer, J. C. Pickens, and B. R. Jicha (2004), Paleomagnetic directions and 40 Ar/ 39 Ar ages from the Tatara San Pedro volcanic complex, Chilean Andes: Lava record of a Matuyama Brunhes precursor?, J. Geophys. Res., 109, B12101, doi: /2004jb Brown, M. C., M. N. Gratton, J. Shaw, R. Holme, and V. Soler (2009), Microwave palaeointensity results from the Matuyama Brunhes geomagnetic field reversal, Phys. Earth Planet. Inter., 173, , doi: / j.pepi Calvo, M., M. Prevot, M. Perrin, and J. Riisager (2002), Investigating the reasons for the failure of palaeointensity experiments: A study on historic lava flows from Mt. Etna (Italy), Geophys. J. Int., 149, 44 63, doi: /j x x. Carlut, J., and X. Quidelleur (2000), Absolute paleointensities recorded during the Brunhes chron at La Guadeloupe Island, Phys. Earth Planet. Inter., 120, , doi: /s (99) Carlut, J., X. Quidelleur, V. Courtillot, and G. Boudon (2000), Paleomagnetic directions and K/Ar dating of 0 to 1 Ma lava flows from La Guadeloupe Island (French West Indies): Implications for time averaged field models, J. Geophys. Res., 105, , doi: /1999jb Channell, J. E. T., and B. Lehman (1997), The last two geomagnetic polarity reversals recorded in high deposition rate sediment drifts, Nature, 389, , doi: / Channell, J. E. T., D. A. Hodell, B. S. Singer, and C. Xuan (2010), Reconciling astrochronological and 40 Ar/ 39 Ar ages for the Matuyama Brunhes boundary and late Matuyama Chron, Geochem. Geophys. Geosyst., 11, Q0AA12, doi: /2010gc Chauvin, A., P. Roperch, and R. A. Duncan (1990), Records of geomagnetic reversals from volcanic islands of French Polynesia: 2. 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