BROAD TRENDS IN GEOMAGNETIC PALEOINTENSITY ON HAWAII DURING HOLOCENE TIME

Paleointensity determinations have been obtained from 22 basaltic lava flows on the island of Hawaii using the Thelliers' method. Radiocarbo n dating indicates that these flows erupted at intervals ranging from about 200 to 1000 years, and results of the experiments provide an est imate of broad trends in geomagnetic paleointensity during Holocene ti me in the vicinity of Hawaii. Most of the samples were obtained from q uickly cooled flow margins and, as a consequence, typically contain tw o titanomagnetite populations. Only two of the 79 samples analyzed fai led to yield an estimate of the paleofield, demonstrating that paleoin tensities can be obtained from such samples if carefully selected. Vir tual dipole moments calculated for the flows are compared with a publi shed curve of dipole field intensity that was constructed using worldw ide archeomagnetic data. The large nondipole fields that were previous ly postulated for the vicinity of Hawaii are confirmed, and the presen t data indicate that they were present from about 5000 years B.P. to p erhaps as recently as the past 200 years. The data indicate, however, that these nondipole sources must have been virtually absent between a bout 12,000 and 5000 years B.P. as they are at the present time.