Simplifying Age Progressions within the Cook-Austral Islands using ARGUS-VI High-Resolution 40Ar/39Ar Incremental Heating Ages

The Cook-Austral islands do not exhibit a simple linear age progression from a single point of active volcanism, in contrast to other Pacific island chains. Explaining this anomaly, however, has been difficult due to the low reliability of the existing K/Ar ages for these islands. Here we present 56 new incremental heating 40Ar/39Ar age determinations for eight of the Cook-Austral islands. We show that these 40Ar/39Ar ages are on average 10-40% different and generally older than the K/Ar ages for the same samples, are more reproducible within single lava flows, exhibit less scatter among data from a single island, and are more reliable than published K/Ar age determinations. Our new data result in two clearly defined and matching age-progressive trends, with origins at the Macdonald and Arago seamounts. This supports the hypothesis that the Macdonald and Rurutu tracks formed by two contemporaneous hotspots aligned in the direction of plate motion. Compared to other volcanic chains on the Pacific plate, the Cook-Austral hotspot tracks record angular rotational plate velocities (0.96 +/- 0.05 to 1.09 +/- 0.04 degrees/Ma) similar to Hawaii (1.15 degrees/Ma) but faster than Samoa (0.63 degrees/Ma). This may imply that hot spot location relative to tectonic boundaries may have an effect on the age progressions recorded by volcanic chains. Specifically, the Cook-Austral and Hawaii hot spots have over the last 30 Myr been located truly intraplate and thus far away from any tectonic boundary, while Samoa has been proximal to the active Tonga-Kermedec subduction zone.