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

作者：Jicha Brian R^*; Kristjansson Leo; Brown Maxwell C; Singer Brad S; Beard Brian L; Johnson Clark M

来源：Earth and Planetary Science Letters, 2011, 310(3-4): 509-517.

DOI：10.1016/j.epsl.2011.08.007

• Skalamaelifell
• excursion
• Iceland
• (40)Ar/(35)Ar geochronology
• paleomagnetism

Sixteen lava flows sampled from isolated hills, including Skalamaelifell 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 Skalamaelifell 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 (40)Ar/(39)Ar incremental heating experiments on four of the transitionally-magnetized lavas give an age of 91 +/- 13 ka. Moreover, (238)U-(230)Th isochrons determined for two of these lava flows yield a weighted mean age of 96 +/- ka (2 sigma). The weighted mean of the 40Ar/39Ar and (238)U-(230)Th 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 Skalamaelifell excursion is not synchronous with the Laschamp excursion. Rather, the Skalamaelifell 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.

• 出版日期2011-10-15