New relative paleointensity results from mafic lava flows erupted in quick succession during four Neogene geomagnetic reversals show that volcanic transition zones can provide a favorable setting for the pseudo-Thellier technique. The most compelling data are from a normal-to-reverse (N-R) transition zone in the 4 my old basaltic lava flows of the Olokele Formation on Kauai (Hawaii). Following stepwise alternating field (AF) demagnetization of each sample’s natural remanent magnetization, an anhysteretic remanent magnetization (ARM) was imparted in a 50 uT bias field and then AF demagnetized. Very similar behavior of the 35 mT to 90 mT coercivity fractions suggest that the NRM (interpreted to be a primary thermoremanent magnetization (TRM)) and ARM reside in the same subpopulation of magnetic grains. With the ARM intensity serving as an analog for TRM susceptibility, the 35 mT-90 mT TRM/ARM ratios provide a relative measure of the ancient field intensity. Good correlation of flow-mean TRM/ARM ratios with a small set of double-heating paleointensities from companion samples allows an approximate scaling of the TRM/ARM ratios to absolute intensity. As the geomagnetic polarity switched, the field strength apparently dropped by 91% compared to the maximum recorded by the flows. This decrease is comparable to that observed in other paleomagnetic studies and numerical simulations. During an interval of NE-down directional stasis recorded by a succession of 10 flows in the lower half of the section, the geomagnetic intensity rose to over 20 uT and then decreased to near 5 uT. This distinctive behavior could mark the waxing and waning of a stationary, inward-directed flux patch on the core mantle boundary centered NE of Kauai. Within-flow variability of relative paleointensity was higher in two other Pliocene transition zones on Kauai (both R-N), and in the R-N transition recorded by 15.2 my old lavas in the Sheep Creek Range (Nevada). Nevertheless, all three records document a reduction in the ancient geomagnetic field intensity similar in magnitude to that seen in the Olokele flows. The relative uniformity of magnetic mineralogy may help explain why the pseudo-Thellier technique was able to capture the large transitional intensity changes recorded by these rapidly-erupted flow sequences.

Laboratory reanalysis of paleomagnetic samples from two sets of superposed ~ 4.5 m.y. old lava flows (N=64) collected on Kauai (Hawaii) in the 1960s, when combined with more recently published results from two additional flow sequences (N=59) on the island, yields a mean paleomagnetic pole that differs significantly from the spin axis. The new laboratory work was motivated by the fact the 60% of the published flow mean directions on the old collection derived from measurements of natural remanent magnetization without any demagnetization treatment. In our reanalysis, we subjected all specimens to stepwise alternating field (AF) demagnetization (13 steps; 90 mT peak AF) and modern line-fit analysis. As has been found in many restudies of this sort on young, well-behaved basaltic lavas, modern demagnetization technique produced little change in the flow-mean directions (an average change of just over 2° in this case.) To minimize the bias introduced when rapidly erupted flow sequences sample a single geomagnetic field direction, we thinned the combined data by grouping superposed flows with similar paleomagnetic directions. The procedure requires choosing a minimum angle between directions that will be considered independent samples of the ancient geomagnetic field. Previous studies have commonly used a 6° threshold. We found, however, that much larger values (10° to 16°) were needed to eliminate significant serial correlation (measured in several ways) in the thinned directional sequence. Our conclusion is that the 123 lava flows in this study provide only 33 independent estimate of the ancient field direction. The mean normal polarity paleomagnetic pole resulting from the analysis (Lon=076.8, Lat=82.2, N=13) is almost perfectly antiparallel (179°) to the mean reverse polarity pole (Lon=258.4, Lat=-83.2, N=20). The grand mean of 33 normal and reverse polarity data is Lon=077.3 Lat=82.8 k=31.8 α95=4.5°. The VGP dispersion (without correction for within-site dispersion, of order 0.5° for these very well-behaved paleomagnetic recorders) is 14.3° about the mean pole and 15.9° about the spin axis, comparable to that found in other analyses of Hawaiian paleomagnetic data.