Ar/Ar ages from transitionally magnetized lavas on La Palma, Canary Islands, and the geomagnetic instability timescale

摘要: [1] A detailed study of 43 lava flows comprising two stratigraphic sequences exposed along the north and south walls Barranco de los Tilos on island La Palma, Canary Islands, reveals a complex, temporally segmented record geodynamo behavior that contains no less than three distinct geomagnetic events. The Matuyama-Brunhes (M-B) reversal is recorded in five transitionally magnetized from (TN) section. isochrons obtained lower four M-B lavas are defined by 14 incremental heating experiments that, together with previous age determination, yielded weighted mean 798.4 ± 6.2 ka (all uncertainties ±2s). In addition, 780.3 10.3 isochron was determined for overlying flow, indicating it erupted during distinctly younger portion transition. Near base (TS) section one finds sequence weakly associated virtual pole (VGP) positions southwest Indian Ocean between latitudes 56� S 65� S, suggesting instability field beyond typical secular variation. 40 Ar/ 39 Ar these flows, 11 separate experiments, gave 822.2 8.7 ka. This anomalous 24 kyr prior to may coincide an event featured several marine sediment records. Directly above normal polarity ( 751.9 18.1 675.0 15.7 ka) nine having magnetization directions low midlatitude VGPs spanning 23� –60� N. These then capped single flow possessing polarity. Based 12 lavas, uppermost 580.2 7.8 this period transitional behavior. From same Quidelleur et al. [1999] reported unspiked K-Ar ages 602 proposed new called ‘‘La Palma’’ excursion. However, presented here times more precise indistinguishable at 95% confidence level Snake River Plain, Idaho, originally Big Lost event. Transitional similar observed astronomically dated cores further establishes Palma indeed global extent. Rigorous temporal constraints additional periods last million years will comprise timescale can be factored confidently into models dynamo process. INDEX TERMS: 1560 Geomagnetism Paleomagnetism: Time variations—secular long term; 1520 Magnetostratigraphy; 1035 Geochemistry: Geochronology; 1513 Geomagnetic excursions; KEYWORDS: argonargon, dating, excursion, paleomagnetism,