New age for the Skálamælifell excursion and identification of a global geomagnetic event in the late Brunhes chron

Sixteen lava flows sampled from isolated hills, including Skálamælifell hill, on the Reykjanes Peninsula in southwest Iceland record excursional geomagnetic field directions that correspond to virtual geomagnetic poles (VGPs) grouped at 12°S, 250°E in the eastern Pacific Ocean. The VGPs are similar to those obtained in earlier studies of these lavas, for which the name Skálamælifell excursion was previously assigned. Based on unspiked K-Ar dating of nine samples from six hills, it has been accepted for more than two decades that the transitionally magnetized lavas reflect the 41 ka Laschamp excursion. However, nine new 40Ar/39Ar incremental heating experiments on four of the transitionally-magnetized lavas give an age of 91.0 ± 13.0 ka. Moreover, 238U-230Th isochrons determined for two of these lava flows yield a weighted mean age of 96.0 ± 10.0 ka (2-sigma). The weighted mean of the 40Ar/39Ar and 238U-230Th ages is 94.1 ± 7.8 ka (2-sigma), which is more than twice as old as the K-Ar data imply. We conclude that the Skálamælifell excursion is not synchronous with the Laschamp excursion. Rather, the Skálamælifell excursion likely corresponds to the post-Blake excursion at 94 ka, which coincides with a globally observed low in paleointensity. These findings highlight the potential shortcomings of K-Ar dating of young (< 100 ka), low-K basalts, and reinforce the need for multiple independent chronometers when determining ages for geologic events throughout Earth's history.