LAUSR

Abstract The 40 Ar/ 39 Ar method was used to refine the dating of a volcanic sequence drilled in the Ko'olau volcano which recorded a period of anomalously shallow inclinations. Previously, this sequence was dated using the unspiked K-Ar method at ≈ 2.1 Ma (Laj et al., 2000). This age was questioned by Ozawa et al. (2005) based on unspiked K-Ar ages of 2.40 and 2.41 Ma for two stratigraphically younger lava flows. Our new 40 Ar/ 39 Ar isochron results date this period of low inclination between 2.52 ± 0.10 and 2.40 ± 0.17 Ma. The combination of our two most reliable ages allows us to propose an age of 2.46 ± 0.13 Ma for the magnetic anomaly that we attribute to the cryptochon C2r.2r-1. This age agrees with previous estimates on the Halawa section of 2.46 ± 0.12 Ma (Singer, 2014) and the Sao Goncalo profile of 2.46 ± 0.08 Ma (Holm et al., 2008; Knudsen et al., 2009). Our 40 Ar/ 39 Ar ages confirm the unspiked K-Ar dating results of Ozawa et al. (2005), which were corrected for the mass-fractionation effect. Along with published data, our new ages permit to bracket the Makapuu late-shield stage between 2.6 and 2.2 Ma.