Matuyama–Brunhes reversal and Kamikatsura event on Maui: paleomagnetic directions, 40Ar/ 39Ar ages...

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Earth and Planetary Science Letters 222 (2004) 667–684

Matuyama–Brunhes reversal and Kamikatsura event on Maui:

paleomagnetic directions, 40Ar/39Ar ages and implications$

Robert S. Coea,*, Brad S. Singerb, Malcolm S. Pringlec, Xixi Zhaoa

aEarth Sciences Department, University of California, Santa Cruz, CA 95064, USAbDepartment of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USAcScottish 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 40Ar/39Ar 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 40Ar/39Ar dating consisting of 31 incremental

heating experiments on 12 transitional flows yields weighted mean ages of 775.6F 1.9 and 900.3F 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 Elsevier B.V. All rights reserved.

Keywords: Matuyama–Brunhes; Kamikatsura event; 40Ar/39Ar ages

1. Introduction

Polarity reversals are the most dramatic manifesta-

tion of the Earth’s magnetic field. Paleomagnetic

0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.epsl.2004.03.003

$ Supplementary data associated with this article can be found,

in the online version at doi: 10.1016/j.epsl.2004.03.003.

* Corresponding author.

E-mail addresses: [email protected] (R.S. Coe),

[email protected] (B.S. Singer), [email protected]

(X. Zhao).

records of reversals from lava flows, sedimentary

rocks, and to a lesser extent igneous intrusions pro-

vide 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 spon-

taneous reversals that open our eyes to the consider-

able range in duration and complexity that might

occur in real reversals [7]. Thus, although they are

R.S. Coe et al. / Earth and Planetary Science Letters 222 (2004) 667–684668

fragmentary or imperfectly recorded, these records

comprise an important part of the observational data

against which numerical simulations may be evaluat-

ed and improved. Lava flows are the most accurate

paleomagnetic recorder, and they can often be dated

precisely using 40Ar/39Ar dating [8]. Although even

rapidly erupted sequences of lava flows provide only

a series of ‘‘snapshots’’ of paleofield behavior, exam-

ining 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 tran-

sition 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

Fig. 1. Location of the sampled section along the northwestern rim of Halea

topographic map. Contour labels are in meters. Flow unit numbers 1–89

samples from nine of those flows and demonstrated

stable reversed, transitional and normal directions.40Ar/39Ar 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]. Fur-

ther 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

kala caldera. Details from the Kilohana 7.5V U.S. Geological Surveycorrespond to Table 1.

R.S. Coe et al. / Earth and Planetary Science Letters 222 (2004) 667–684 669

of this better-exposed section, expand the number of

flows precisely dated by the 40Ar/39Ar 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 vol-

cano 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 interme-

diate 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 consider-

able care not to sample in places badly remagnetized

by lightning, that is, in areas with extremely high

remanent magnetization detected with a fluxgate gra-

diometer 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-polar-

ity 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 gen-

erally less than 580 jC and median destructive

alternating fields of 10 to 50 mT confirm that the

remanence-carrying mineral is predominantly iron-

rich titanomagnetite. Lower-stability components re-

moved by these methods appear to be a combination

of viscous remanent magnetization (VRM) and light-

ning-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 second-

ary 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, a95 = 41), the mean ChRM

direction given in Table 1 is 10 times better deter-

mined (D/I= 161/� 31, a95 = 4).

Table 1

Flow-by-flow paleomagnetic results, 40Ar/39Ar ages and stratigraphic position for Haleakala section

Flow Field Height AgeF 2r Directions VGPs No. No. Flow Field Height AgeF 2r Directions VGPs No. No.

unit no. (m) (ka)Dec Inc a95 k Long. Lat. A95 K

samples GC a unit no. (m) (ka)Dec Inc a95 k Long. Lat. A95 K

samples GC a

89 56 368.0 136.5F 43.1 8.9 8.6 21.8 7.4 354.9 71.4 15.6 13.4 9 of 9 8 44 24 166.5 26.5 56.0 3.7 335.3 253.4 62.0 4.2 254.6 6 of 6

88 55 364.0 146.4F 3.8 0.5 16.6 18.4 16.0 21.5 77.7 13.5 28.1 6 of 6 3 43 23 162.1 41.7 57.7 6.2 32.5 255.7 49.3 7.8 20.8 18 of 39

87 54 359.0 509.5F 10.8 6.9 50.4 12.1 143.1 233.3 77.9 10.9 169.0 3 of 5 1 42 22A 159.8 134.0 � 50.9 2.6 406.3 318.5 � 47.7 3.2 266.8 9 of 9

86 53 354.0 353.3 44.9 4.7 215.6 157.9 81.6 4.7 215.6 6 of 6 1 41 22 157.6 192.3 � 53.8 3.8 252.1 59.6 � 72.3 4.8 156.6 7 of 7

85 52 351.0 530.6F 20.1 358.3 43.9 2.4 843.1 186.4 84.9 2.4 843.1 6 of 6 2 40 21B 154.6 189.1 � 58.4 5.3 94.8 43.3 � 69.9 6.8 58.7 9 of 11

84 51 343.0 28.2 8.8 6.6 195.6 320.8 58.1 4.7 383.1 4 of 5 39 21AV 153.9 180.7 � 52.9 8.5 573.5 26.5 � 77.3 9.8 416.9 3 of 3 2

83 50 331.7 10.1 22.1 5.8 281.9 336.1 76.6 4.5 417.9 4 of 4 38 21A 153.2 150.2 � 41.4 5.3 208.2 305.6 � 62.2 6.1 160.9 5 of 9

82 49 322.4 14.6 24.5 10.1 83.0 321.2 74.1 8.7 113.3 4 of 4 37 21 151.6 773.0F 3.0 188.6 0.1 4.9 62.5 180.9 � 67.5 3.5 120.2 15 of 16

81 48 309.0 357.4 37.8 10.1 153.8 123.6 87.5 9.2 180.6 4 of 4 3 36 20C 148.7 174.5 � 26.8 4.2 491.1 243.3 � 81.6 2.5 1305.7 4 of 4

80 47 303.8 1.0 33.0 7.6 146.6 4.1 87.1 5.7 256.4 4 of 4 35 20B 147.9 902.0F 11.0 113.0 � 28.8 6.5 108.3 301.3 � 26.4 5.2 165.0 6 of 6

79 46 299.7 359.0 33.9 3.8 578.8 46.9 87.6 2.8 1045.9 4 of 4 34 20A 146.1 896.0F 8.0 106.8 � 26.1 4.2 172.5 301.4 � 20.3 3.4 274.1 8 of 8

78 45 295.6 12.3 47.3 3.6 734.6 257.0 76.5 3.8 642.6 4 of 6 1 33 20 144.9 108.2 � 26.5 5.9 105.3 301.2 � 21.6 5.6 117.1 7 of 7

77 44 291.5 10.8 45.1 7.1 166.8 260.0 78.5 6.7 191.3 4 of 4 32 19D 143.5 102.3 � 27.8 10.5 77.8 303.9 � 16.3 9.8 89.5 4 of 4

76 43 286.3 19.1 41.8 7.4 155.9 279.0 72.2 6.3 216.2 4 of 4 31 19C 142.8 104.4 � 27.9 5.5 277.1 303.2 � 18.4 3.8 594.4 4 of 4

75 42 279.1 4.1 47.3 9.2 100.1 226.5 81.2 8.5 117.5 4 of 4 30 19B 139.8 109.2 � 30.4 27.4 85.1 303.4 � 23.1 24.7 104.0 2 of 2

74 41 276.0 359.4 42.0 6.8 184.4 192.5 86.4 7.1 167.4 4 of 4 29 19 137.5 101.3 � 27.2 16.1 59.5 303.7 � 15.4 12.9 92.4 3 of 4

73 40 273.5 358.6 40.7 10.4 79.4 174.1 86.8 9.4 96.6 4 of 4 28 17J – L* 135.2 898.7F 10.0 97.7 � 38.0 15.1 67.7 311.6 � 14.3 16.6 56.5 3 of 3

72 39 272.9 12.0 46.0 9.3 98.9 259.1 77.0 7.9 134.7 4 of 4 27 17G – I* 133.8 107.5 � 30.1 4.7 377.7 303.6 � 21.7 3.7 634.4 4 of 4

71 38 269.3 8.7 42.9 8.3 122.7 264.2 81.1 10.0 85.4 4 of 4 26 17E – F* 132.4 102.3 � 25.8 3.8 1067.1 302.6 � 16.1 2.3 2919.0 3 of 3

70 37 263.7 0.6 48.2 5.1 331.5 207.6 81.5 5.7 264.0 4 of 4 25 17C –D* 130.1 95.7 � 30.3 9.8 89.3 307.3 � 10.9 10.3 80.9 4 of 4

69 36 257.5 2.5 46.0 5.8 249.4 222.5 82.9 5.5 282.7 4 of 4 24 17B 128.7 900.7F 16.0 99.3 � 23.6 5.7 471.2 302.3 � 12.9 4.7 677.3 3 of 4

68 35 254.4 2.1 45.2 4.0 534.9 221.2 83.7 3.4 710.7 4 of 4 23 17A 126.4 101.0 � 33.4 5.8 1856.7 307.6 � 16.3 5.1 2354.6 2 of 2

67 34 249.2 577.0F 9.1 6.5 43.9 1.7 2042.6 253.5 82.3 1.8 1736.6 5 of 5 22 17 125.0 911.0F 12.0 104.2 � 27.0 5.3 301.0 302.7 � 18.0 3.7 618.3 4 of 4

66 33 242.0 359.0 44.8 5.8 253.2 194.7 84.3 5.8 251.9 4 of 4 21 16B 123.0 894.0F 27.0 104.6 � 30.1 5.7 182.2 304.5 � 18.9 6.1 159.0 5 of 5

65 32 231.7 344.1 40.6 3.8 599.9 137.4 74.5 3.8 599.9 4 of 4 20 16A 121.6 215.3 � 24.2 5.8 60.5 121.7 � 55.3 4.5 99.7 13 of 13 13

64 31 211.1 338.8 48.8 4.5 203.2 134.0 69.8 4.5 203.2 7 of 10 1 19 16agg 114.0 188.8 � 43.3 5.5 151.7 83.5 � 80.7 6.2 119.2 6 of 6

63 30 207.0 353.9 36.9 8.7 36.3 116.0 84.1 9.0 33.4 9 of 11 18 16 106.5 915.0F 10.0 187.9 � 34.3 4.1 491.4 127.1 � 82.3 4.0 517.6 4 of 4

62 29 198.8 9.3 32.2 4.4 229.0 312.9 80.6 3.7 328.9 6 of 8 17 15A 105.0 180.3 � 37.8 4.6 393.5 52.6 � 89.5 3.4 750.6 4 of 4

61 28 188.0 763.0F 18.0 354.8 32.5 2.6 399.4 82.2 84.2 2.2 548.7 9 of 12 16 15 100.0 942.0F 29.0 190.5 � 31.2 6.3 216.2 133.6 � 79.3 6.2 223.1 4 of 4

60 27B 185.8 756.0F 6.0 356.5 38.5 4.7 202.5 130.7 86.5 4.7 200.5 6 of 6 15 14AB 98.2 195.5 � 38.7 3.4 315.7 106.6 � 75.5 3.2 365.3 7 of 7

59 27A 184.0 774.2F 3.6 190.2 41.6 7.2 91.1 190.8 � 44.2 6.9 97.5 6 of 7 1 14 14 94.5 194.0 � 40.5 6.5 198.1 100.8 � 76.8 7.1 169.3 4 of 4

58 27 182.0 778.3F 5.2 188.9 36.7 3.1 340.6 191.3 � 47.9 2.8 411.8 8 of 9 4 13 13 82.6 183.8 � 37.2 10.2 57.2 109.6 � 86.6 9.9 61.1 5 of 6

57 26D 181.3 196.9 28.8 4.6 246.9 177.8 � 50.2 3.8 352.8 6 of 7 3 12 12 74.3 191.9 � 40.9 13.4 49.4 98.2 � 78.7 12.5 55.0 4 of 4

56 26C 181.0 195.1 26.6 6.8 79.8 179.7 � 52.0 5.4 125.1 7 of 7 11 11 65.2 194.3 � 36.2 2.8 1103.0 114.3 � 76.6 2.6 1258.7 4 of 5

55 26B 180.6 186.4 40.1 7.9 95.1 195.3 � 45.7 7.1 117.5 5 of 7 10 10 51.4 188.8 � 28.0 4.6 208.9 147.5 � 79.8 4.1 133.5 6 of 7

54 26A 179.9 189.9 44.1 7.2 166.0 191.7 � 42.4 8.8 109.3 4 of 5 9 9 41.3 958.0F 24.0 189.9 � 33.2 7.5 120.6 128.1 � 80.3 6.4 158.3 5 of 10 2

53 26 177.6 193.8 42.2 4.3 101.8 186.6 � 42.6 4.3 104.0 12 of 13 8 8 25.7 191.1 � 22.8 3.8 210.6 152.0 � 76.1 3.9 199.4 8 of 8

52 25A 176.2 782.4F 26.4 212.9 35.8 3.0 85.0 163.3 � 38.2 2.6 111.3 28 of 29 6 7 7 22.9 187.8 � 25.7 2.8 288.7 156.4 � 79.6 2.4 416.9 10 of 10

51 24A 174.6 229.2 54.0 8.7 49.2 163.2 � 16.7 10.5 33.8 7 of 7 6 6 14.7 172.0 � 40.8 6.3 211.8 314.2 � 82.2 6.0 235.5 4 of 6

50 25 173.2 778.7F 6.9 345.6 45.9 4.1 96.8 142.8 75.7 4.5 79.5 14 of 22 5 5 11.0 195.4 � 36.0 5.5 277.1 114.2 � 75.6 5.0 332.2 4 of 4

49 24CCV 171.7 351.9 45.2 2.7 508.6 154.1 80.5 2.8 458.0 7 of 7 4 4 7.3 188.1 � 35.0 4.6 405.8 123.3 � 82.3 4.9 353.4 4 of 4

48 24CC 170.9 327.3 53.2 39.6 10.7 152.0 56.6 36.1 12.7 3 of 3 3 3 4.6 159.0 � 41.8 3.2 823.8 307.5 � 70.3 3.4 744.2 4 of 4

47 24BB 170.2 328.2 55.6 11.4 118.2 150.0 58.1 14.3 75.7 3 of 4 2 2 3.7 156.6 � 38.0 2.5 1331.3 299.5 � 68.1 2.0 2154.9 4 of 4

46 24AA 169.5 351.1 66.2 7.4 153.6 192.1 60.9 10.1 83.3 4 of 4 1 1 0.0 961.4F 23.4 160.8 � 30.8 4.2 254.7 284.1 � 71.4 4.0 277.3 6 of 6

45 24 167.3 785.1F 8.0 4.8 51.8 4.3 244.5 222.8 77.5 4.7 202.8 6 of 6

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.

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222(2004)667–684

670

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 best-

fit 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 demagnetiza-

tion, 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 full-

polarity 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 direc-

tion 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

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 tech-

nique, 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 sam-

ples (Table 1, flow 20). Although the mean is well

defined (a95 = 5.8j), the real accuracy could be con-

siderably less because no stable-endpoint ChRMs

contributed to the mean direction.

4. Paleomagnetic results

In Table 1, we give the flow-mean data, strati-

graphic positions and 40Ar/39Ar ages for every flow

that was sampled. Of the 588 samples demagnetized,

about half by AF and half by thermal demagnetiza-

tion, 516 yielded useable results. The majority of the

samples excluded, mainly because of severe remagne-

tization 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 errati-

cally 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

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. Addi-

tional 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

a95 = 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 geomag-

netic 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

a95 < 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 nor-

mal and reversed VGP means for the data sets with

a95 < 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 6.8jfor 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

Table 2a

Mean directions, VGPs and statistics of full-polarity flows

Polarity Directions VGPs Antipodal?a

N Dec Inc a95 k s R Longitude Latitude A95 K S R Diff b Probc

Normal 30 3.4 38.3 4.5 35.1 13.7 29.173963 266.6 86.8 3.8 48.7 11.6 29.404064

Reversed 19 184.3 � 36.0 4.8 49.9 11.5 18.639199 122.3 � 86.0 5.1 43.9 12.2 18.589909 2.3 35.1

Both N and R 49 3.8 37.4 3.3 40.1 12.8 47.802731 282.2 86.7 3.0 47.3 11.8 47.984668

N (a95 < 10) 24 3.1 40.4 4.5 43.3 12.3 23.468875 246.1 86.3 4.2 50.6 11.4 23.545775

R (a95 < 10) 17 183.9 � 35.6 5.3 45.6 12.0 16.649150 127.4 � 86.4 5.7 40.0 12.8 16.599978 3.8 61.2

Both (a95 < 10) 41 3.4 38.4 3.4 43.6 12.3 40.083159 270.4 86.8 3.3 45.7 12.0 40.124768

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 aver-

age 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

Fig. 5. Plots of magnetic inclination, declination and latitude of the virtual

units at Haleakala volcano. Flows that were dated are shown in open symb

weighted mean ages are given for lavas thought to record the Kamikatsur

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 com-

prising these swings have intermediate directions,

defined here as having VGP latitudes less than 60j.

geomagnetic pole (VGP) for the stratigraphic succession of 89 flow

ols. 40Ar/39Ar isochron ages are in ka with F 2r uncertainties. The

a event and Matuyama–Brunhes reversal.

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-than-

normal 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 remagne-

tization-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 40Ar/39Ar

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 en-

tirely consistent with the stable-endpoint directions of

the other samples, both for thermal and AF demag-

netization (Fig. 7). Thus, we do not think that unre-

moved 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 geody-

namo—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

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 a95 Ang diff Proba

58/1 189.0 39.8 4 617.4 4.3

58/2 189.2 33.7 4 370.8 5.6 6.1 97.7

57/1 196.9 27.3 3 133.9 12.5

57/2 196.3 30.8 3 772.6 7.3 3.5 45.3

56/1 199.1 23.5 3 129.4 10.9

56/2 191.9 28.8 4 72.2 10.9 8.4 74.8

53/1 197.4 43.3 4 94.3 9.5

53/2 191.2 45.7 5 191.2 5.5 5.0 64.9

50/1 339.1 41.6 6 139.6 5.7

50/2 351.2 48.1 8 178.8 4.2 10.7 99.9

43/1 41.9 59.1 12 30.2 8.0

43/2 40.8 54.3 8 31.3 10.1 4.8 39.4

52/1 214.1 41.5 6 98.1 7.1

52/2 221.3 33.1 4 690.5 4.8

52/3 209.8 35.6 7 49.3 8.7

52/4 211.7 35.5 11 267.5 2.8 7.1* 78.9

Average 6.5 71.6

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 paleomag-

netic 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.

5. 40Ar/39Ar 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


cores drilled out of the 2.5-cm-diameter cores used

for paleomagnetic analysis, whereas for the few

olivine or clinopyroxene-phyric flows the holocrys-

talline groundmass was separated at the 200-Am size

fraction and wrapped in copper foil.

The 40Ar/39Ar ages are calculated relative to

standard minerals including sanidine from the 28.34

Ma Taylor Creek rhyolite (TCs) or 1.194 Ma Alder

Creek rhyolite (ACs) that have been calibrated

aga ins t a common pr imary s tandard , the

98.79F 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 27.92

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 sub-

samples 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 Cadmi-

um-Lined In-Core Irradiation Tube (CLICIT). Cor-

rections for undesirable nucleogenic reactions on 40K

and 40Ca are [40Ar/39Ar]K = 0.00086, [36Ar/37Ar]

Ca = 0.000264, [39Ar/37Ar]Ca = 0.000673 [27]. In-

verse-variance weighted mean plateau ages and

uncertainties are calculated according to [28]. Pre-

cision 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

Jager [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 39Ar 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 assump-

tion about the trapped argon component. To improve

precision, multiple sub-samples from several lavas

were measured. The resulting isochrons—each calcu-

lated 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.

6. 40Ar/39Ar results

Given the aim of this study to resolve paleomag-

netic 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 40Ar/39Ar dating standard with uncertainties

that arise solely from the analytical procedures and

intercalibration of our standards to the primary stan-

dard [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. Uncer-

tainty in the age of the primary standard and 40K

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 40Ar/39Ar standard or to chronometers that

are independent of the 40Ar/39Ar system, including

for example, U–Pb, U–Th/He, or astronomical

methods.

Forty-three of the 52 incremental heating experi-

ments 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,

Fig. 8. 40Ar/39Ar 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 40Ar/36Ari 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).

R.S.Coeet

al./Earth

andPlaneta

ryScien

ceLetters

222(2004)667–684

678


suggesting that the effects of argon loss, or extrane-

ous 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 uncer-

tainties, the isochron ages calculated from selected

portions of these experiments on the three flows are

not inconsistent with their stratigraphic positions and

the 40Ar/39Ar 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 40Ar/36Ar 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 flow-

mean 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

961.4F 23.4 to 915F 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 transition-

ally magnetized units (20–35), six of which gave ages

between 894.0F 27.0 and 911.0F 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 900.3F 4.7 ka (MSWD= 0.95)

gives the best age estimate for this period of transi-

tional 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 40Ar/39Ar ages that are based on 20

separate incremental heating experiments (Table 3).

The ages of these six flows are between 773.0F 3.0

and 785.1F 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 775.6F 1.9 ka

(MSWD=2.6) for the six isochrons is not signifi-

cantly different from the 782.4F 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 775.6F 1.9 ka for this

period of transitional behavior is a little younger and

considerably more precise than the 795F 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 125F 5 kyr later than that recorded by

the almost directly underlying sequence of transition-

ally magnetized flows that we have dated at

900.3F 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 transi-

tionally magnetized flows, two normally magnetized

flows 60 and 61 gave isochron ages of 756.0F 6.0 and

763.0F 18.0 ka, respectively (Table 3; Fig. 8A and B).

Thus, these two flows with a weighted mean age of

756.7F 5.7 ka erupted near the base of the Brunhes

Chron, ca. 20 kyr later than the underlying transition-

ally magnetized lavas that presumably record the M–B

reversal (Fig. 5). Further up section, normally magne-

tized flows 67, 85, 87, 88 and 89 yielded isochron ages

of 577 .0 F 9.1 , 530.6 F 20 .1 , 509 .5 F10.8 ,

146.4F 3.8 and 136.5F 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 40Ar/39Ar

dating leads us instead to consider that two entirely

unrelated transitional episodes of the geomagnetic

Table 3

Summary of 40Ar/39Ar dataa 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)

39Ar

(%)

AgeF 2r(ka)

MSWD N MSWD 40Ar/36ArF2r intercept

AgeF 2r(ka)

89 UW18H75 0.55 130.9F14.6 775–1250 100.0 145.8F17.8 0.33 6 of 6 0.45 298.1F6.2 129.2F52.4

UW18H74 0.54 144.9F19.8 875–1265 100.0 139.0F18.1 0.50 5 of 5 0.43 295.0F5.8 151.8F75.8

weighted mean isochron age from two experiments: 11 of 11 0.41 296.8F3.9 130.0F13.0

88 UW18H71 0.561 141.7F4.0 875–1270 100.0 143.7F3.4 1.06 4 of 4 0.25 293.4F2.5 146.1F4.4

UW18H72 0.602 142.9F5.4 825–1300 100.0 144.4F4.1 0.77 5 of 5 0.60 292.9F4.5 >147.2F6.4

weighted mean isochron age from two experiments: 9 of 9 0.34 293.3F2.2 146.4F3.8

87 UW18H69 0.35 501.0F9.5 750–1275 100.0 504.0 F7.9 0.50 6 of 6 0.09 293.5F2.6 509.5F10.8

85 UW18G63 0.46 525.1F10.3 850–1225 100.0 525.2 F9.7 0.28 6 of 6 0.26 294.7F2.7 530.6F20.1

67 95gec47 1.07 563.9F6.8 650–980 98.5 570.1 F4.4 1.50 8 of 9 1.38 292.5F3.9 580.0F12.0

MB5f0085 0.68 576.2F22.5 590–1250 100.0 573.3 F9.6 0.58 5 of 5 0.72 295.8F1.0 572.8F14.0

weighted mean isochron age from two experiments: 13 of 14 0.61 577.0F9.1

61 MB5f0056 0.91 830.0F20.8 710–1215 86.9 781.0 F13.0 1.60 5 of 7 1.60 297.7F2.9 763.0F26.0

MB5f0089 1.06 780.9F18.6 495–1055 93.3 767.7 F16.2 0.17 6 of 7 0.16 296.5F4.4 763.9F25.0


60 95gec06 1.04 773.4F6.0 710–950 74.3 760.8 F4.8 0.92 10 of 17 0.79 300.2F7.6 757.9F6.4

MB5f0048 1.06 743.3F10.2 495–710 68.1 749.0 F10.0 1.03 3 of 6 0.35 302.1F9.4 740.0F18.0


59 MB5f0050 0.78 794.1F9.8 590–1330 92.7 785.4 F7.9 0.24 6 of 7 0.37 296.5F5.9 782.0F23.0

MB5f0046 0.86 776.1F8.0 565–1120 87.0 774.7 F4.3 1.09 8 of 12 1.40 297.3F1.8 770.7F6.1

95GEC14y 0.93 785.6F6.0 650–1160 90.7 784.9 F6.0 1.50 17 of 20 1.50 296.2F2.2 782.5F9.9

UW08M127 0.83 772.0F7.0 750–1340 100.0 771.4 F6.9 0.51 6 of 6 0.47 293.0F6.2 773.6F8.6

UW08M107 0.79 773.3F6.4 800–1260 97.5 775.8 F6.2 0.89 5 of 6 1.20 295.6F8.7 775.7F8.9

UW10G147 0.46 786.4F89.8 875–1450 100.0 771.3 F7.1 0.96 8 of 8 1.02 293.1F5.8 773.1F8.4

weighted mean isochron age from six experiments: 50 of 59 0.96 774.2F3.6

58 MB5f0053 0.86 776.9F10.6 880–1160 58.7 761.0 F13.0 0.27 3 of 6 0.27 299.0F12.0 739.0F73.0

95GEC20y 1.13 798.7F10.0 725–1115 79.4 785.7 F10.0 1.15 8 of 10 1.42 296.6F3.4 780.9F18.6

95GEC21y 0.91 798.8F8.2 776–1170 59.3 791.6 F6.0 0.50 11 of 17 0.52 296.9F2.2 782.4F9.8

UW08M106 0.47 784.9F11.0 700–1320 100.0 780.7 F6.8 0.60 6 of 6 0.76 295.5F1.8 780.8F8.5

UW10G146 0.46 765.6F9.7 800–1450 100.0 767.8 F8.4 1.16 6 of 6 0.96 294.4F2.1 770.7F9.9

weighted mean isochron age from five experiments: 34 of 45 1.16 778.3F5.2

52 MB6f0065 0.92 774.8F8.0 640–1150 78.0 779.1 F12.0 5.94# 8 of 13 11.73# 293.4F7.2 782.4F26.4

50 MB6f0083 0.85 784.3F9.8 635–1150 82.1 780.7 F6.4 0.52 9 of 14 0.34 296.8F1.5 778.7F6.9

45 MB6f0045 1.19 777.7F7.8 590–1120 85.1 783.2 F7.5 1.62 9 of 14 1.38 293.9F0.8 785.1F8.0

37 MB6f0049 1.12 774.2F7.1 440–1140 99.3 775.3 F2.7 1.30 15 of 16 2.00 294.8F1.9 776.0F4.7

UW08M145 0.40 773.8F5.8 750–1200 90.0 770.6 F5.3 0.97 6 of 7 1.21 295.4F3.8 770.7F7.0

UW10F113 0.37 767.9F6.6 990–1375 76.9 770.0 F7.0 1.81 4 of 7 1.97 295.1F1.0 771.0F7.4

UW10F112 0.38 769.3F7.3 900–1400 99.0 766.6 F7.4 1.55 5 of 6 1.85 296.2F2.3 765.6F8.4

UW08M125 0.45 788.9F7.8 900–1270 76.3 786.4 F7.7 0.69 9 of 13 0.37 301.1F6.6 778.6F12.1

UW08M105 0.91 771.5F11.0 880–1250 83.9 782.7 F10.7 0.33 8 of 12 0.24 300.1F9.9 777.2F16.1


35 UW08M103 0.22 893.6F10.7 700–1270 100.0 897.7 F8.5 0.33 6 of 6 0.07 293.8F2.9 902.7F12.0

MB6f0048 0.33 1058.6F36.0 520–1200 100.0 963.7 F32.0 7.27# 11 of 12 3.08# 300.6F1.6 899.7F26.0


34 MB6f0050 0.44 891.6F10.0 440–1140 98.5 893.3 F8.0 1.37 13 of 14 1.19 293.7F1.6 896.0F8.0

28 MB6f0064 0.47 882.2F10.0 630–930 60.9 900.1 F8.0 0.44 6 of 12 0.47 297.6F10.1 898.7F10.0

24 MB6f0062 0.66 916.3F14.0 630–990 68.3 899.8 F8.0 0.30 7 of 14 0.52 295.2F6.2 900.7F16.0

22 MB5f0088 0.41 985.8F50.0 490–1060 97.0 958.0 F42.0 1.23 6 of 7 0.99 287.5F19.0 950.6F40.0

MB5f0086 0.43 907.4F20.0 710–1060 68.2 891.5 F196.0 0.08 3 of 6 0.11 296.0F3.6 888.0F36.0

MB5f0055 0.44 892.8F24.0 490–1160 100.0 900.9 F22.0 1.12 5 of 5 1.24 292.6F3.6 928.6F44.0

95gec27 0.44 924.0F22.0 700–1060 59.2 901.8 F14.0 0.28 10 of 11 0.40 295.5F2.4 902.2F24.0

weighted mean isochron age from four experiments: 24 of 29 1.70 911.0F12.0


Table 3 (continued)

Sample Experiment K/Ca Total fusion Age spectrum Isochron analysis

site no. (total) age

(ka)Increments

used (jC)

39Ar

(%)

AgeF 2r(ka)

MSWD N MSWD 40Ar/36ArF2r intercept

AgeF 2r(ka)

21 MBf50052 0.44 866.8F16.0 490–1160 100.0 889.4 F22.0 6.65 6 of 6 7.24# 289.0F20.6 896.0F32.0

95gec34 0.31 930.4F22.0 780–1200 96.3 918.0 F18.0 2.12 11 of 15 2.49# 297.9F3.6 887.4F50.0


18 95gec41 0.33 941.9F18.2 700–1200 100.0 927.7 F15.0 1.59 15 of 15 1.78 298.3F2.2 900.3F28.0

MB5f0049 0.29 913.5F16.0 490–1330 100.0 912.5 F12.0 0.69 6 of 6 0.84 295.4F3.8 913.5F14.0

UW09Q139 0.10 921.6F33.0 650–1325 99.3 919.3 F22.0 0.15 7 of 8 0.17 295.8F2.2 916.3F31.7

UW08M99b 0.29 912.7F25.3 650–1250 100.0 922.3 F18.6 0.72 12 of 12 0.67 294.5F1.8 931.1F24.2


16 MB5f0018 0.38 975.8F20.0 490–8500 82.3 912.5 F15.4 0.50 4 of 6 0.19 450.6F458.9 854.3F159.0

MB5f0087 0.39 922.4F32.6 590–1160 100.0 938.3 F34.1 1.42 5 of 5 0.63 286.0F9.4 945.3F29.8


9 MB5f0090 0.25 989.1F33.0 590–1160 92.6 965.3 F22.2 0.89 5 of 6 1.19 295.9F3.0 964.6F25.6

MB5f0083 0.37 848.4F42.6 590–880 84.5 913.1 F23.4 1.39 3 of 5 2.62 296.6F28.6 911.5F66.8


1 95gec52 0.43 934.0F6.6 730–930 63.9 954.1 F6.3 1.01 6 of 12 1.28 288.0F23.8 961.4F23.4

a All ages calculated relative to sanidine from 28.34 Ma Taylor Creek rhyolite, or 1.194 Ma Alder Creek rhyolite [26].y Data from Singer and Pringle [9] recalculated to revised age of 28.34 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 interpreta-

tion that demands careful evaluation.

The earlier group of 16 transitional flows, numbers

20–35 in Table 1, dated at 900.3F 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 40Ar/39Ar

age determined from a transitional flow on Tahiti [10].

Together, they give a revised age of 900.4F 4.6 ka

(MSWD=0.81) for the Kamikatsura event, which now

stands as a well-established, high-resolution paleomag-

netic 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

775.6F 1.9 ka of this group of 24 flows compares well

with the astronomical age of the M–B reversal deter-

mined 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 full-

polarity 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). Al-

though admittedly crude, the average ChRM intensity

of sequences of basalt flows has proven useful in the

absence of successful absolute paleointensity determi-

nations as a qualitative indicator of relative paleointen-

sity (e.g., [35]). (ii) Precisely dated sequences of M–B

transitional lava flows in several parts of the world are

significantly older than the 775.6F 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 40Ar/39Ar incremental heating methods and

standards [9,11,15,39]. Nine isochrons from three

flows on Tahiti yield an inverse-variance weighted

mean age of 791.7F 5.6 ka (MSWD= 0.58); 19

isochrons from eight flows in Chile yield an age of

792.0F 3.0 ka (MSWD= 0.43); and 14 isochrons

from three flows from La Palma, Canary Islands, yield


an age of 798.5F 6.5 ka (MSWD=0.70). Moreover,

the flow directly overlying this latter sequence on La

Palma yields an age of 770.1F16.3 ka, more compa-

rable with the 775.6F 1.9 ka age of the 24 Maui flows.

Thus, there is the strong presumption that these flows

from Maui record a later part of the M–B reversal.

Interpreting these ages at face value, it appears that the

M–B reversal may have taken f 16 kyr or more to

complete, two to three times longer than most esti-

mates for reversal duration (e.g., [40]).

Although undoubtedly fragmentary, our younger,

Maui M–B record is nonetheless the most detailed yet

discovered in lava flows (Fig. 6). It shows for the first

time a reversed-to-normal segment with VGPs on or

near the Americas, followed by an oscillation to poles

in the vicinity of New Zealand and back again to high

northern latitude. The most precisely dated flows

(flows 37 and 59, Table 1) occur near the beginning

and end of the M–B sequence and are statistically

indistinguishable (773.0F 3.0 and 774.2F 3.6 ka,

respectively). The nearest other M–B record in lavas

is that from Tahiti [38], with mean age 791.7F 5.6 ka

discussed above, and consists of a reversed-to-normal

VGP path with poles that cluster west of Australia 75jwest of the transitional Maui poles. The Chilean M–B

VGPs form a single cluster in central Australia [39]

about 30jwest of the closest Maui poles, and are about

16 kyears older [9]. The M–B VGP path from La

Palma [15] does lie mainly near the Maui path, but part

of it progresses in the opposite direction and all but the

uppermost flow are about 22 kyears older. Thus, it

seems that, besides taking longer, the M–B polarity

transition was also more complex than it and other

reversals are generally depicted, in agreement with the

unusually complex M–B record from rapidly deposit-

ed sediment drift deposits in the North Atlantic found

by Channell and Lehman [41].

Of course, it could be argued that this later unstable

behavior recorded in theMaui sequence and also by the

single flow on La Palma represents a separate, post-

reversal excursion. Or, for that matter, one could

consider instead that the older transitional sequences

of flows from Tahiti, Chile and La Palma record aM–B

precursory excursion. These appear to be more seman-

tic than real physical possibilities. In light of the 30–

50-kyear post-reversal inhibition period inferred from

analysis of the polarity time scale [31], it makes more

sense to consider all these transitional records as part of

the M–B reversal process. An interesting question

remains, however: Did the reversal take 16 kyears or

more to occur everywhere, or was the reversal time-

transgressive, starting and ending at different times at

different places over the globe? Time-transgressive

behavior is exhibited in the geodynamo simulations

of Glatzmaier et al. [5], especially in the comparably

long and very complex, second reversal of the tomo-

graphic heat-flux model [6]. It seems likely that the

signatures of polarity transitions are quite varied.

We are encouraged by the fact that 40Ar/39Ar results

from these young lavas obtained in three different

laboratories over a sustained period of analysis are in

excellent agreement, both at the scale of sub-samples

measured from an individual lava flow, and also among

the 23 dated lava flows, which do not violate strati-

graphic order within the 95% confidence error limits.

Further, we contend that the temporal resolution gen-

erated during this effort, i.e., ages precise to better than

1% at the 95% confidence level, is necessary to

delineate the paleomagnetic record preserved in Pleis-

tocene lava flow sequences like this one and to resolve

questions such as the duration and degree of simulta-

neity or time-transgressiveness of individual reversals.

8. Conclusions

The northwest wall of Haleakala volcano preserves

a concatenated record of two temporally distinct

periods of transitional behavior of the Earth’s mag-

netic field within the Kamikatsura event and the M–B

polarity transition, brought to light by a detailed

paleomagnetic investigation of the entire section and

a sustained effort to date many basaltic lava flows at a

precision of better than 1% using the 40Ar/39Ar

incremental heating technique.

The full-polarity reversed and normal flows com-

prising the bottom and top parts of the section,

respectively, collectively span 680 kyears and carry

stable directions of remanence that pass a reversal test.

The average of their VGPs does not differ significant-

ly from the rotation axis and their angular standard

deviation is consistent with the mean value for other

0–5 Ma lava flows in the same latitude band.

The flows that lie between the upper and lower

full-polarity sequences carry intermediate, reversed

and normal directions with distinctly lower intensities


of NRM after cleaning away secondary components at

200 mT or 250 C. The precision parameters of the

flow-mean directions for these transitional flows are

not significantly less than those for the full-polarity

flows. Moreover, the differences in direction between

sites in the same transitional flow up to 100 m apart is

small, indicating that the effects of local magnetic

anomalies due to irregularity of shape and magneti-

zation of the flow and the flows underlying it are not

important.

The VGP path contains five large jumps of 50–

100j, each occurring where the geology indicates at

least a small interlude between flows. The within-flow

directions reveal neither systematic streaking nor any

other indications of extraordinarily rapid field

changes.

The Kamikatsura event is recorded by 16 flows, 15

of which have VGPs clustered in central South

America. The age determinations of six flows cover-

ing the time span of the cluster are not significantly

different from each other and yield an average age of

900.3F 4.7 ka. Together with another flow from

Tahiti dated with identical techniques and standards,

the best age for the Kamikatsura event is 900.4F 4.6

ka, which now stands as a well-established, high-

resolution paleomagnetic stratigraphic marker in the

late Matuyama Chron.

Twenty-four flows record a portion of the M–B

polarity transition, showing for the first time a re-

versed-to-normal swing of the VGP over the Amer-

icas and an oscillation to a position east of New

Zealand and back to normal. This is the most detailed

record of this reversal yet discovered in lava flows.

Six flows spanning these swings yield 40Ar/39Ar ages

that are indistinguishable within experimental uncer-

tainty and give an average age of 775.6F 1.9 ka, in

agreement with several astronomical estimates for the

age of this reversal.

When compared to other M–B lava flow sequen-

ces from La Palma, Chile and Tahiti dated using the

same 40Ar/39Ar techniques and standards, these lavas

at Haleakala are f 16 kyears younger and thus

preserve a snapshot of what is probably the waning

stages of the reversal. Taken together, these lava-flow

records suggest that the M–B reversal was more

complex and may have taken considerably longer to

complete than is generally considered to be the case.

They raise the possibility that the transition was time-

transgressive, starting and ending at different times at

different places around the globe.

Acknowledgements

We are indebted to Scott Rowland, Dan Miller,

Scott Miller, Carol English and Greg Baker for help in

the field and in the paleomagnetic laboratory and to

Brian Jicha for assistance with the argon analyses and

figures. Reviews by Scott Bogue and Terry Spell

helped to clarify several issues and are much

appreciated. This work was supported by NSF grants

EAR-8417639, EAR-9526553, EAR-9909309, EAR-

0114055 and EAR-0310316. [KF]

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