ARTICLE IN PRESS. Matuyama Brunhes reversal and Kamikatsura event on Maui: paleomagnetic directions, 40 Ar/ 39 Ar ages and implications $

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1 Earth and Planetary Science Letters xx (2004) xxx xxx Matuyama Brunhes reversal and Kamikatsura event on Maui: paleomagnetic directions, 40 Ar/ 39 Ar ages and implications $ Robert S. Coe a, *, Brad S. Singer b, Malcolm S. Pringle c, Xixi Zhao a a Earth Sciences Department, University of California, Santa Cruz, CA 95064, USA b Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA c Scottish Universities Research and Reactor Centre, East Kilbride, G75 OQF, Scotland, UK Received 12 June 2003; received in revised form 27 February 2004; accepted 1 March 2004 Abstract Eighty-nine basaltic lava flows from the northwest wall of Haleakala caldera preserve a concatenated paleomagnetic record of portions of the Matuyama Brunhes (M B) reversal and the preceding Kamikatsura event as well as secular variation of the full-polarity reversed and normal geomagnetic field. They provide the most detailed volcanic record to date of the M B transition. The 24 flows in the transition zone show for the first time transitional virtual geomagnetic poles (VGPs) that move from reverse to normal along the Americas, concluding with an oscillation in the Pacific Ocean to a cluster of VGPs east of New Zealand and back finally to stable polarity in the north polar region. All but one of the 16 Kamikatsura VGPs cluster in central South America. The full-polarity flows, with 40 Ar/ 39 Ar ages spanning a total of 680 kyr, pass a reversal test and give an average VGP insignificantly different from the rotation axis, with standard deviation consistent with that for other 0 5 Ma lava flows of similar latitude. Precise 40 Ar/ 39 Ar dating consisting of 31 incremental heating experiments on 12 transitional flows yields weighted mean ages of F 1.9 and F 4.7 ka for the M B and Kamikatsura transitional flows, respectively. This Matuyama Brunhes age is f 16 kyr younger than ages for M B flows from the Canary Islands, Tahiti and Chile that were dated using exactly the same techniques and standards, suggesting that this polarity transition may have taken considerably longer to complete and been more complex than is generally believed for reversals. D 2004 Published by Elsevier B.V. Keywords: Matuyama Brunhes; Kamikatsura event; 40 Ar/ 39 Ar ages 1. Introduction Polarity reversals are the most dramatic manifestation of the Earth s magnetic field. Paleomagnetic $ Supplementary data associated with this article can be found, in the online version at doi: /j.epsl * Corresponding author. addresses: rcoe@es.ucsc.edu (R.S. Coe), bsinger@geology.wisc.edu (B.S. Singer), xzhao@es.ucsc.edu (X. Zhao). records of reversals from lava flows, sedimentary rocks, and to a lesser extent igneous intrusions provide important insight into the geodynamo, not to mention lithospheric dynamics, geochronology and stratigraphy [1 4]. In addition, numerical simulations of the geodynamo [5,6] with different patterns of core mantle boundary heat flux have produced spontaneous reversals that open our eyes to the considerable range in duration and complexity that might occur in real reversals [7]. Thus, although they are X/$ - see front matter D 2004 Published by Elsevier B.V. doi: /j.epsl EPSL-07078; No of Pages 18

2 2 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx fragmentary or imperfectly recorded, these records comprise an important part of the observational data against which numerical simulations may be evaluated and improved. Lava flows are the most accurate paleomagnetic recorder, and they can often be dated precisely using 40 Ar/ 39 Ar dating [8]. Although even rapidly erupted sequences of lava flows provide only a series of snapshots of paleofield behavior, examining many lava sequences together that record a particular reversal has the potential to enrich our visualization of the reversal process [7,9 11]. Using a portable fluxgate magnetometer, one of us (RSC) discovered a reversed-to-normal polarity transition recorded in basalt flows along the Halemauu Trail into Haleakala caldera on the island of Maui, Hawaii (Fig. 1). Based on available K Ar ages [12], it seemed likely to be the Matuyama Brunhes (M B) transition. RSC and V. Hsu drilled 27 reconnaissance samples from nine of those flows and demonstrated stable reversed, transitional and normal directions. 40 Ar/ 39 Ar dating of four of the transitional flows by Baksi et al. [13] confirmed a M B age for three of them, but the lowest of the four gave an age of >850 ka, too old for the M B transition, that they dismissed as probably contaminated by excess argon [13]. Further paleomagnetic and geochronological study of a different, better-exposed section of the Haleakala caldera wall indicated that this flow and many others at the base of the transitional sequence likely record the Kamikatsura event of [14] at ca. 900 ka [10], one of a growing number of dipolar instabilities that are being documented in the Quaternary and hold great promise for long-range, high-resolution stratigraphic correlation [10,15]. In this paper, we present a set of paleomagnetic flow-mean directions for 89 successive basaltic units Fig. 1. Location of the sampled section along the northwestern rim of Haleakala caldera. Details from the Kilohana 7.5V U.S. Geological Survey topographic map. Contour labels are in meters. Flow unit numbers 1 89 correspond to Table 1.

3 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx 3 of this better-exposed section, expand the number of flows precisely dated by the 40 Ar/ 39 Ar from 9 to 23, and improve some of the earlier age determinations of Singer et al. [9,10]. Our objective is to document in as great detail as possible the paleomagnetic field behavior associated with the Kamikatsura event and the M B reversal as recorded at Haleakala and to place these recordings in as precise and accurate temporal a framework as possible. Comparing these data for the M B reversal record at Haleakala volcano with those obtained from three other lava sections distributed about the globe that have also been precisely dated as M B [11], it appears that this polarity transition took considerably longer than commonly thought for reversals. 2. Sample collection We carried out a thorough sampling of the section shown in Fig. 1. It spans 370 m in elevation, the entire caldera wall at that location, and is composed of basalt flows of the Kula Formation [16]. Reconnaissance work showed that the lower 125 m was of reversed polarity, the next 65 m was characterized by intermediate directions and mixed polarity, and the upper 190 m was of normal polarity. We drilled groups of standard paleomagnetic cores, each group consisting of samples typically spread laterally a distance of 5 15 m and confined to a single well-defined cooling unit, usually a flow but in one case an agglomeratic ashy deposit containing lava blocks (unit 19, Table 1). In the normal and reversed polarity zones we often skipped one or two flows between sample groups, whereas in the intermediate zone we sampled almost every cooling unit that could be drilled, sometimes in two or more separate places. We also took considerable care not to sample in places badly remagnetized by lightning, that is, in areas with extremely high remanent magnetization detected with a fluxgate gradiometer or by anomalous deflection of a compass needle. In total, we took 588 cores from 89 flows in stratigraphic succession, including 19 flows from the reversed zone, 30 from the normal zone, and 40 from the intermediate zone that lies between the full-polarity sequences (Table 1). All samples were oriented with respect to vertical and true north with an estimated uncertainty of about 1 2j using a paleomagnetic core-orienting stage equipped with inclinometer and a recess for holding a sighting compass. We measured azimuths for each core relative to local magnetic north and corrected them to true north by sighting the magnetic azimuth of the sun or of a distant landmark whose true bearing was known. The great majority of these corrections are between 0j and 5j, but some are larger, ranging up to a maximum of 87j for one sample from the uppermost flow. There were a few flows such as this one where lightning strikes were so common that we had trouble finding places to sample that were not strongly overprinted. 3. Paleomagnetic laboratory procedure We carried out progressive thermal or alternating field (AF) demagnetization on at least one specimen from each core. High unblocking temperatures generally less than 580 jc and median destructive alternating fields of 10 to 50 mt confirm that the remanence-carrying mineral is predominantly ironrich titanomagnetite. Lower-stability components removed by these methods appear to be a combination of viscous remanent magnetization (VRM) and lightning-induced isothermal remanent magnetization (IRM). These secondary components were relatively large in the transition zone, and substantial alteration of magnetic minerals frequently occurred during laboratory heating, foiling our attempts to obtain a record of absolute paleointensities of the reversing field. Nonetheless, we were able to obtain a good directional record throughout the section. In the full-polarity parts of the section, the secondary components were usually easy to eliminate. Fig. 2a shows the removal by AF demagnetization of an atypically large VRM to isolate the direction of characteristic remanent magnetization (ChRM) of a sample from the lowermost flow. We can be confident that this ChRM represents the primary direction acquired during original cooling because it agrees well with the ChRMs of the other five samples from the same flow. Compared to the mean NRM direction of the six samples from this flow, which is poorly defined (D/I = 146/ 13, a 95 = 41), the mean ChRM direction given in Table 1 is 10 times better determined (D/I = 161/ 31, a 95 = 4).

4 4 Table 1 Flow-by-flow paleomagnetic results, 40 Ar/ 39 Ar ages and stratigraphic position for Haleakala section Flow Field Height Age F 2r Directions VGPs No. No. Flow Field Height Age F 2r Directions VGPs No. No. unit no. (m) (ka) samples GC a unit no. (m) (ka) samples GC a Dec Inc a95 k Long. Lat. A95 K Dec Inc a95 k Long. Lat. A95 K F of of F of of F of A of of of F of B of of AV of of A of of F of of C of of B F of of A F of of of of D of of C of of B of of of of J L* F of of G I* of of E F* of of C D* of of B F of of A of F of F of of B F of of A of of agg of of F of of A of F of F of B F of AB of A F of of F of of D of of C of of B of of A of F of of of A F of of A of of F of of CCV of of CC of of BB of of AA of F of F of 6 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx ARTICLE IN PRESS a Number of remagnetization great circles used in the analysis. * Composites of two to three very thin pahoehoe cooling units, each 20 to 30 cm thick.

5 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx 5 Fig. 2. (a) Orthogonal vector diagram showing complete removal of an unusually large VRM by AF demagnetization to only10 mt, isolating the ChRM which then decays univectorially toward the origin between 10 and 100 mt. Pluses (diamonds) = horizontal (vertical) component. (b) Equal-area projection (lower hemisphere) showing successful application of remagnetization-circle technique. Arcuate lines = great circles bestfit to AF demagnetization results for two samples with deviant NRMs due to secondary IRM from lightning; crosses = NRMs; squares = ChRMs of samples with clustered NRMs; diamonds = ChRMs of samples with deviant NRMs. Secondary IRM due to lightning could also be completely removed by AF demagnetization to reveal the ChRM of most full-polarity samples. For one flow in the reversed zone and eight flows in the normal zone, however, some samples were too strongly remagnetized by lightning to recover the primary direction of magnetization. In such cases, we used a remagnetization-circle technique [17], as illustrated in Fig. 2b. Two of the six samples from flow 85 near the top of the section had NRM directions that deviated appreciably from the rest, and during demagnetization, they moved toward but failed to reach the clustered ChRM directions of the other four. Great circles fitted through the demagnetization directions do intersect the cluster, and thus these samples can be used to help define the flow mean. Twenty-two fullpolarity samples that otherwise would have been excluded from the mean, or would have significantly increased its uncertainty if they had been included, were recovered by this method. In the three flows at the very top of the section, many samples showed evidence of remagnetization in more than one direction by multiple lightning strikes, making application of the great-circle technique more difficult and less accurate (see Table 1). In the zone of intermediate directions, where the primary remanence is lower owing to the lower field intensity that prevailed during the polarity transition, directional deviations by secondary components are typically larger and characterisitic remanence more difficult to isolate than in the full-polarity zones. Nonetheless, AF demagnetization usually worked well for removing VRM, as illustrated in Fig. 3, where first AF and finally thermal demagnetization carried out on the same sample both yielded almost the same direction of ChRM. Thus, this direction very likely is the primary direction acquired during original cooling. Likewise, in the majority of cases both demagnetization techniques also yielded very similar directions of ChRM when they were carried out on sister specimens from the same core. On flows with directions of remanence very scattered by lightning, however, AF demagnetization clearly outperforms thermal demagnetization, as shown in Fig. 4. The magnified effects of lightning due to the weaker primary remanence in the intermediate zone

6 6 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx Fig. 3. Demagnetization of a transitional sample with a VRM overprint. AF treatment at 20 mt completely removes the VRM, as confirmed by subsequent thermal treatment from 350 to 530 jc, which continues the same trend toward the origin as the AF demagnetization and yields the same direction of ChRM. Orthogonal vector diagram: pluses (diamonds) = horizontal (vertical) component. required more use of the remagnetization-circle technique, and even this technique failed in some samples. This was especially true for flows 43 and 50, which we had to resample at different places to obtain useable results (Tables 1 and 2b). Generally, though, application of the remagnetization-circle technique was well worth the effort, enabling 29 samples with deviant or no ChRM to contribute usefully in defining flow-mean directions. The most extreme example was the lowest flow in the transition zone, which required remagnetization-circle analysis for all 13 of its samples (Table 1, flow 20). Although the mean is well defined (a 95 = 5.8j), the real accuracy could be considerably less because no stable-endpoint ChRMs contributed to the mean direction. 4. Paleomagnetic results In Table 1, we give the flow-mean data, stratigraphic positions and 40 Ar/ 39 Ar ages for every flow that was sampled. Of the 588 samples demagnetized, about half by AF and half by thermal demagnetization, 516 yielded useable results. The majority of the samples excluded, mainly because of severe remagnetization by lightning, were from only 5% of the flows. In retrospect, if AF demagnetization or hybrid (AF followed by thermal) demagnetization had been employed more frequently, the exclusion rate would have been lower. However, there were also a few samples rejected because they were unstable during AF demagnetization, their directions jumping erratically at each AF step. The clustering of sample directions within a flow was generally good: 50% of the flows had precision parameter k over 200 and 92% over 50. Moreover, the average k for transitional flows with intermediate directions was only 25% less than the average k of full-polarity flows, not a significant difference by the F test [18]. Because the primary objective of this study was to examine transitional field behavior, we typically kept the number of samples per flow to four

7 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx 7 Fig. 4. AF and thermal demagnetization of sister samples from the same core that carry an IRM overprint from lightning. (a) AF efficiently removes the IRM and recovers the direction of ChRM, as shown by the rapid initial drop in intensity and straight-line decay toward the origin on the orthogonal projection and clustering of points on the equal area projection from 20 to 70 mt. (b) Thermal demagnetization all the way to 575 jc clearly fails to the ChRM direction. or five so as to be able to collect essentially all of the units in the zone with intermediate directions. Additional samples were collected in later visits from flows that were important for determining the transitional behavior if their mean directions were not acceptably well defined. In the end, 82% of all the flows had 95% confidence limits less than 10j, with the median a 95 = 5.7j. Moreover, the considerable redundancy in successive flow directions adds confidence that the overall description of the field variation is robust. The means of the directions and virtual geomagnetic poles (VGPs) of the full-polarity flows from the normal and reversed sequences at the top and bottom of the section are given in Table 2a for two cases: (i) for all the flows and (ii) for those flows with a 95 <10j. In all cases, the normal and reversed means are within a few degrees of antipodal, deviations not significant at 95% confidence. For instance, the normal and reversed VGP means for the data sets with a 95 <10j deviate from antipodal by only 3.8j and have a probability of being different of only 61% by the F test [18]. This deviation would have to be 6.8j for the difference in directions from antipodal to be significant at 95% confidence; thus these data pass a class B reversal test of McFadden and McElhinny [19]. Moreover, these means, both separately and combined (flipping the polarity of the reversed and combining them with the normal VGPs), do not differ significantly from the rotation axis. The angular standard deviation S of 12j of the combined VGPs

8 8 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx Table 2a Mean directions, VGPs and statistics of full-polarity flows Polarity Directions VGPs Antipodal? a N Dec Inc a 95 k s R Longitude Latitude A 95 K S R Diff b Prob c Normal Reversed Both N and R N(a 95 < 10) R(a 95 < 10) Both (a 95 < 10) a How close the mean N and R VGPs are to antipodal. b Difference from antipodal (j). c Probability not antipodal (%). is slightly, but not significantly, lower than the average value for the latitude band containing Maui [20,21]. Five major swings of the field are recorded, shown in terms of both directions and VGPs in Figs. 5 and 6. Layers of ash, talus or soil or indications of gullying lie between the flows recording the largest jumps in direction; however, we also noted similar signs of possible temporal breaks between some flows with closely similar directions. A total of 29 flows comprising these swings have intermediate directions, defined here as having VGP latitudes less than 60j. Fig. 5. Plots of magnetic inclination, declination and latitude of the virtual geomagnetic pole (VGP) for the stratigraphic succession of 89 flow units at Haleakala volcano. Flows that were dated are shown in open symbols. 40 Ar/ 39 Ar isochron ages are in ka with F 2r uncertainties. The weighted mean ages are given for lavas thought to record the Kamikatsura event and Matuyama Brunhes reversal.

9 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx 9 Fig. 6. Virtual geomagnetic poles for Haleakala Caldera flow units from Table 1. Squares = flows in full-polarity zones: Matuyama reversed (1 19) and Brunhes normal (60 89). Pluses = Kamikatsura event (K, 20 35). Circles = Matuyama Brunhes transition (36 59). Star = sampling site. An important concern is how well the flows record the transitional field directions, when the weaker-thannormal field produced a smaller primary TRM. For instance, could there be a significant systematic error due to incomplete removal of VRM? To answer this question, we show the stable-endpoint and remagnetization-circle data for two of the most critical flows for constraining the duration of the M B transition, units 58 and 59 (Fig. 7). These flows are especially important because they are at the top of the transition zone and yielded exceptionally precise 40 Ar/ 39 Ar ages. One or two of the samples in these flows did not exhibit convincingly stable endpoints because of unusually stubborn VRM secondary overprints. The great-circle fits for these samples, however, are entirely consistent with the stable-endpoint directions of the other samples, both for thermal and AF demagnetization (Fig. 7). Thus, we do not think that unremoved secondary components are a serious source of systematic error in the flow-mean directions. Note, however, that the two site-mean directions for unit 58 are slightly (6.1j) different from each other (Fig. 7). This is most likely caused by differences in the local field direction due to magnetic anomalies, differences that are larger than usual because of the weaker transitional field, although other causes such as post-cooling movement of blocks could also have contributed. We sampled seven of the transitional flows at two or more locations to assess the magnitude of the directional deviations and found an average difference of 6.5j (Table 2b). The pairs of sites are typically about 15 m apart, but the four sites in unit 52 range in separation from 5 m up to 100 m. Two of the seven interflow directional differences are statistically significant, whereas five are not. Thus, there is no indication of a large error due to differences in the local magnetic anomaly (or other causes) at sites separated by up to 100 m. Nonetheless, it is still possible that a significant effect could arise from broader-scale anomalies distorting the ambient field direction away from that produced by the geodynamo for example, due to the magnetization of the whole volcanic edifice. To sum up, most of the flows yielded mean directions that probably represent the ancient field at the time they cooled within 10j or less, even for the

10 10 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx Fig. 7. Demagnetization results for the two flows at the top of the transition zone that provide the most important constraints on the duration of the M B reversal. The best-fit great circles to the demagnetization steps (circles and crosses) for five samples that did not yield stable endpoints in vector diagrams are consistent with the ChRM directions (squares) of samples that did. This is true for both AF (flow 58/site 1) and thermal (flow 58/site 2 and flow 59) demagnetization. Stars and surrounding circles give the combined stable-endpoint and remagnetization-circle mean directions and the 95% confidence limits for each site [17]. Thus, despite the unusually serious overprinting by VRM experienced by these flows, they give robust paleomagnetic directions. transition-zone flows with intermediate directions. A possible exception is unit 20, with a transitional direction unlike those in flows immediately above Table 2b Paleomagnetic results from sites in same flow Flow/site Dec Inc N k a 95 Ang diff Prob a 58/ / / / / / / / / / / / / / / / * 78.9 Average a Probability directions are different (%). * Average difference in direction (j) between all pairs of these four sites, which ranged from 5 to 100 m apart. and below that had to be estimated entirely by remagnetization circles. We encountered no evidence in the section of extraordinarily rapid change in field direction, such as pronounced smearing of paleomagnetic direction as a function of vertical position in a single flow like that found in the transition zone of the Steens Mountain reversal [22,23]. The transitional VGPs on the Americas (Fig. 6) fit the proposed ideas of VGP preferences for certain geographical areas on the globe [24,25], but the group of eight VGPs in the southern Pacific (units 52 59) do not Ar/ 39 Ar methods The ages of 23 of the lava flows were determined from 52 incremental heating experiments that were conducted at the University of Wisconsin-Madison, Scottish Universities Research and Reactor Centre, and the University of Geneva. Analyses in each lab employed virtually identical methods that are fully described in [9,10,15]. These experiments used a metal furnace to degas ca. 100 mg samples in 5 20 steps between 500 and 1400 jc. In the case of aphyric lavas, the samples were 5-mm-diameter

11 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx 11 cores drilled out of the 2.5-cm-diameter cores used for paleomagnetic analysis, whereas for the few olivine or clinopyroxene-phyric flows the holocrystalline groundmass was separated at the 200-Am size fraction and wrapped in copper foil. The 40 Ar/ 39 Ar ages are calculated relative to standard minerals including sanidine from the Ma Taylor Creek rhyolite (TCs) or Ma Alder Creek rhyolite (ACs) that have been calibrated against a common primary standard, the F 0.96 Ma GA-1550 biotite [26]. Ages for nine flows (18, 21, 22, 24, 28, 34, 35, 58 and 59) were originally reported relative to an earlier age of Ma for the TCs standard [10,15]; these and other ages from the literature have been recalculated where necessary so that they are comparable directly to the present results. We have measured new subsamples from flows 18, 35, 37, 58 and 59 that augment and improve the precision of the original ages given in [9] and [10]. Ages determined from flows 1, 9, 15, 61, 67, 85, 87, 88 and 89 are reported here for the first time. The samples were irradiated for 1 h adjacent to TCs or ACs monitors in evacuated quartz vials at the Oregon State University Triga reactor in the Cadmium-Lined In-Core Irradiation Tube (CLICIT). Corrections for undesirable nucleogenic reactions on 40 K and 40 Ca are [ 40 Ar/ 39 Ar]K = , [ 36 Ar/ 37 Ar] Ca = , [ 39 Ar/ 37 Ar]Ca = [27]. Inverse-variance weighted mean plateau ages and uncertainties are calculated according to [28]. Precision estimates for the neutron monitors based on six to seven measurements each suggest that the uncertainty in J, the neutron fluence parameter, was between 0.4% and 0.8% ( F 2r). This uncertainty was propagated into the final plateau and isochron age for each analysis, but contributes < 0.1% to the total uncertainty in these age estimates. Ages were calculated using the decay constants of Steiger and Jäger [29] and are reported with F 2r analytical and standard intercalibration uncertainties (see [26]). Criteria used to determine whether an incremental heating experiment gave meaningful results were (i) a plateau must be defined by at least three contiguous steps all concordant in age at the 95% confidence level and comprising >50% of the 39 Ar released, and (ii) a well-defined isochron, calculated using the algorithm of York [30] must exist for the plateau points as defined by the Mean Square Weighted Deviate (MSWD). The isochron ages are preferred over the weighted mean plateau ages because they combine estimates of analytical precision plus internal disturbance of the sample without making an assumption about the trapped argon component. To improve precision, multiple sub-samples from several lavas were measured. The resulting isochrons each calculated with its own J value and uncertainty in J were treated as independent from one another. Thus, the inverse-variance weighted mean [28] of the isochrons combines to give the best estimate of the age and uncertainty for these flows Ar/ 39 Ar results Given the aim of this study to resolve paleomagnetic field behavior recorded by the lava sequence in as precise and accurate a temporal framework as possible, we report isochron ages relative to a single primary 40 Ar/ 39 Ar dating standard with uncertainties that arise solely from the analytical procedures and intercalibration of our standards to the primary standard [26]. When comparing ages within the lava sequence this is the appropriate level of uncertainty, because the age of each sample was determined using an identical procedure and primary standard. Uncertainty in the age of the primary standard and 40 K decay constant may contribute additional uncertainty, perhaps up to 1.5% [26], to the ages reported here. However, this only becomes important should one wish to compare our ages to those obtained using a different 40 Ar/ 39 Ar standard or to chronometers that are independent of the 40 Ar/ 39 Ar system, including for example, U Pb, U Th/He, or astronomical methods. Forty-three of the 52 incremental heating experiments yielded age spectra with more than 75% of the gas defining the age plateau (Fig. 8 and Table 3). Table 1 in the online version of this paper gives the data for each heating step for all flows we have dated from Haleakala caldera: for flows 1, 9, 16, 18, 35, 37, 45, 50, 52, 58, 59, 60, 61, 67, 85, 87, 88 and 89 reported for the first time and for flows 21, 22, 24, 28 and 34 reported earlier [10]. Most of the small percentage of discordant steps yielded apparent ages only slightly lower or higher than the plateau ages,

12 12 ARTICLE IN PRESS R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx Fig Ar/ 39 Ar age spectra and isochron diagrams for eight of the dated lava flows from which new data are reported. Where multiple sub-samples were measured from a flow, the weighted mean isochron age is reported and gives the best estimate of time since eruption. The 40 Ar/ 36 Ar i values were obtained from regressing all the plateau points and verify that, with the exception of flow 35, no lavas contain evidence for an excess argon component (see text).

13 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx 13 suggesting that the effects of argon loss, or extraneous argon, were very minor and affected only a few of the samples (Fig. 8; Table 3). Four experiments, including the two from unit 21 and one each from 35 and 52 are discordant and have MSWD values larger than appropriate for the number of points regressed (Table 3). Nonetheless, considering the age uncertainties, the isochron ages calculated from selected portions of these experiments on the three flows are not inconsistent with their stratigraphic positions and the 40 Ar/ 39 Ar ages of the adjacent flows. Thus, we include these analyses in our evaluation of the overall temporal record of the lava sequence. Only one of these isochrons, from the discordant experiment on unit 35 in [10], yielded an 40 Ar/ 36 Ar value higher than 295.5, whereas the sub-sample from this flow analyzed at UW-Madison gave a concordant age spectrum and shows no evidence of excess argon (Table 3). Table 1 summarizes the ages obtained from the base to the top of the section together with the flowmean directions. From four lavas among the 19 flows that we sampled in the lowest, reversely magnetized part of the section we determined ages that range from F 23.4 to 915 F 10 ka, in stratigraphic order. Hence, these lavas erupted during the upper part of the Matuyama reversed chron (Fig. 5). Immediately above these reversed flows, we sampled 16 transitionally magnetized units (20 35), six of which gave ages between F 27.0 and F 12.0 ka. Although at face value, these six isochron ages are not in stratigraphic order, they are indistinguishable from one another at the 95% confidence level; thus the weighted mean of F 4.7 ka (MSWD = 0.95) gives the best age estimate for this period of transitional field behavior (Fig. 5). From the overlying sequence of 24 flows that preserve a sequence of reversed normal transitional paleomagnetic directions (units 36 59), six have yielded new 40 Ar/ 39 Ar ages that are based on 20 separate incremental heating experiments (Table 3). The ages of these six flows are between F 3.0 and F 8.0 ka, and although not in stratigraphic order, they are statistically indistinguishable from one another at the 95% confidence level (Fig. 8C H). The weighted mean age of F 1.9 ka (MSWD = 2.6) for the six isochrons is not significantly different from the F 7.6 ka age obtained earlier by Singer and Pringle [9] from the upper flow units 58 and 59, and gives our best estimate of the time since the transitional field behavior recorded by the flows. The new age of F 1.9 ka for this period of transitional behavior is a little younger and considerably more precise than the 795 F 16 ka age we recalculate (to a comparable value of the primary standard) from the results of Baksi et al. [13] for three transitionally magnetized flows 400 m to the northeast (Fig. 1). What is surprising is that this younger sequence of flows records a period of transitional field behavior that occurred 125 F 5 kyr later than that recorded by the almost directly underlying sequence of transitionally magnetized flows that we have dated at F 4.7 ka (Fig. 5). Only one flow, reversely magnetized unit 36 that was not suitable for dating, could possibly have erupted at a significantly different time between these two groups (Table 1). However, we favor the interpretation that it belongs with the younger group because there is no field evidence for erosion or a protracted period of time separating it from flow unit 37 just above. Moreover, in our field notes we recorded the presence of about 3 m of talus and ashy soil between it and the underlying unit 35 that, in retrospect, probably signals the missing time. Immediately above the younger sequence of transitionally magnetized flows, two normally magnetized flows 60 and 61 gave isochron ages of F 6.0 and F 18.0 ka, respectively (Table 3; Fig. 8A and B). Thus, these two flows with a weighted mean age of F 5.7 ka erupted near the base of the Brunhes Chron, ca. 20 kyr later than the underlying transitionally magnetized lavas that presumably record the M B reversal (Fig. 5). Further up section, normally magnetized flows 67, 85, 87, 88 and 89 yielded isochron ages of F 9.1, F 20.1, F10.8, F 3.8 and F 43.1 ka, respectively. 7. Discussion Before these flows were dated, we assumed that the entire sequence of 40 flows from unit 20 to 59 recorded transitional field behavior during the Matuyama Brunhes reversal. Now, the 40 Ar/ 39 Ar dating leads us instead to consider that two entirely unrelated transitional episodes of the geomagnetic

14 14 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx Table 3 Summary of 40 Ar/ 39 Ar data a from 52 incremental heating experiments on Haleakala basalt flows Sample Experiment K/Ca Total fusion Age spectrum Isochron analysis site no. (total) age (ka) Increments used (jc) 39 Ar (%) Age F 2r (ka) MSWD N MSWD 40 Ar/ 36 ArF 2r intercept Age F 2r (ka) 89 UW18H F F of F F52.4 UW18H F F of F F75.8 weighted mean isochron age from two experiments: 11 of F F UW18H F F of F F4.4 UW18H F F of F4.5 >147.2F6.4 weighted mean isochron age from two experiments: 9 of F F UW18H F F of F F UW18G F F of F F gec F F of F F12.0 MB5f F F of F F14.0 weighted mean isochron age from two experiments: 13 of F MB5f F F of F F26.0 MB5f F F of F F25.0 weighted mean isochron age from two experiments: 11 of F gec F F of F F6.4 MB5f F F of F F18.0 weighted mean isochron age from two experiments: 13 of F MB5f F F of F F23.0 MB5f F F of F F6.1 95GEC14 y F F of F F9.9 UW08M F F of F F8.6 UW08M F F of F F8.9 UW10G F F of F F8.4 weighted mean isochron age from six experiments: 50 of F MB5f F F of F F GEC20 y F F of F F GEC21 y F F of F F9.8 UW08M F F of F F8.5 UW10G F F of F F9.9 weighted mean isochron age from five experiments: 34 of F MB6f F F # 8 of # 293.4F F MB6f F F of F F MB6f F F of F F MB6f F F of F F4.7 UW08M F F of F F7.0 UW10F F F of F F7.4 UW10F F F of F F8.4 UW08M F F of F F12.1 UW08M F F of F F16.1 weighted mean isochron age from six experiments: 47 of F UW08M F F of F F12.0 MB6f F F # 11 of # 300.6F F26.0 weighted mean isochron age from six experiments: 17 of F MB6f F F of F F MB6f F F of F F MB6f F F of F F MB5f F F of F F40.0 MB5f F F of F F36.0 MB5f F F of F F gec F F of F F24.0 weighted mean isochron age from four experiments: 24 of F12.0

15 R.S. Coe et al. / Earth and Planetary Science Letters xx (2004) xxx xxx 15 Table 3 (continued) Sample Experiment K/Ca Total fusion Age spectrum Isochron analysis site no. (total) age Increments 39 Ar Age F 2r MSWD N MSWD 40 Ar/ 36 ArF (ka) used (jc) (%) (ka) 2r intercept Age F 2r (ka) 21 MBf F F of # 289.0F F gec F F of # 297.9F F50.0 weighted mean isochron age from four experiments: 17 of F gec F F of F F28.0 MB5f F F of F F14.0 UW09Q F F of F F31.7 UW08M99b F F of F F24.2 weighted mean isochron age from four experiments: 40 of F MB5f F F of F F159.0 MB5f F F of F F29.8 weighted mean isochron age from two experiments: 9 of F MB5f F F of F F25.6 MB5f F F of F F66.8 weighted mean isochron age from two experiments: 8 of F gec F F of F F23.4 a All ages calculated relative to sanidine from Ma Taylor Creek rhyolite, or Ma Alder Creek rhyolite [26]. y Data from Singer and Pringle [9] recalculated to revised age of Ma for Taylor Creek rhyolite sanidine standard used in this study. # MSWD suggests some geologic or experimental error beyond analytical precision (see text). field are juxtaposed in vertical section, an interpretation that demands careful evaluation. The earlier group of 16 transitional flows, numbers in Table 1, dated at F 4.7 ka appears to record the Kamikatsura event [14]. This brief episode of unstable field behavior variously termed an event, a cryptochron, or an excursion might represent an aborted reversal or simply abnormally large secular variation of the field. Our age from the six isochrons reported here accords well with the single 40 Ar/ 39 Ar age determined from a transitional flow on Tahiti [10]. Together, they give a revised age of F 4.6 ka (MSWD = 0.81) for the Kamikatsura event, which now stands as a well-established, high-resolution paleomagnetic stratigraphic marker in the late Matuyama Chron. The immediately overlying group, flows 36 59, records a reversed normal transitional sequence of directions that appears to have occurred during the later stages of the M B reversal. The weighted mean age F 1.9 ka of this group of 24 flows compares well with the astronomical age of the M B reversal determined from several orbitally tuned oxygen isotope records in marine sediment [32 34]. That the six lower reversed flows, units 36 41, do not belong to the fullpolarity Matuyama chron is evident for two reasons. (i) The intensity of primary TRM of these flows, as estimated by ChRM intensity after cleaning to 250 jc or 20 mt, is on average five to seven times lower than that of the full-polarity Brunhes and Matuyama flows and is not significantly different from that of the 18 overlying flows in the group (units 42 59). Although admittedly crude, the average ChRM intensity of sequences of basalt flows has proven useful in the absence of successful absolute paleointensity determinations as a qualitative indicator of relative paleointensity (e.g., [35]). (ii) Precisely dated sequences of M B transitional lava flows in several parts of the world are significantly older than the F 1.9 ka age of this one from Maui, as we discuss next. Sequences of lava flows thought to record the M B transition are known also from Iceland [36], La Guadeloupe Island [37], La Palma Island [9,15], Tahiti [38] and Chile [9,39]. Besides the results presented here, a total of 15 of these flows from Tahiti, La Palma and Chile have been dated using identical 40 Ar/ 39 Ar incremental heating methods and standards [9,11,15,39]. Nine isochrons from three flows on Tahiti yield an inverse-variance weighted mean age of F 5.6 ka (MSWD = 0.58); 19 isochrons from eight flows in Chile yield an age of F 3.0 ka (MSWD = 0.43); and 14 isochrons from three flows from La Palma, Canary Islands, yield