An Evaluation of the Complex Age Progression along the Cook-Austral Islands Using High-resolution 40Ar/39Ar Incremental Heating Ages

Abstract:

Until recently, the hotspot hypothesis has been generally accepted to explain the presence of linear volcanic chains. The hypothesis predicts a linear age progression along each chain, as well as consistent angular rotation velocities for all chains on a single plate. While such age progressions have been observed at places such as Hawaii and Louisville, several young (0-30 Ma) volcanic chains that formed on the Pacific plate show age progressions and associated angular velocities that are in disagreement with one another. The Cook-Austral island chain has age distributions that are particularly difficult to resolve based on the hotspot hypothesis, due to its location on the “hotspot highway” (Jackson et al. 2010) and a wide geographic range of recent volcanism. Several of these islands were previously studied by Turner and Jarrard (1982) who interpreted this age progression to suggest the presence of three active hotspots positioned along a “hot line” above a sheet-like upwelling area in the mantle as opposed to a singular “hot spot”. However, this study was performed using the K/Ar dating method, and it has been shown that K and/or Ar loss (and addition of K) due to weathering and alteration can have significant effects on the age and uncertainty of samples dated with this technique. Here we present high-resolution ⁴⁰Ar/³⁹Ar incremental heating ages for several of the same samples previously analyzed in this study, as well as some unpublished samples. Analyses were conducted using the ARGUS-VI multicollector mass spectrometer, employing incremental heating procedures that provide more precise and more accurate ages compared to K/Ar and total fusion ⁴⁰Ar/³⁹Ar measurements. These new data will be used in conjunction with existing plate motion models and geochemical data to assess whether they support a point source or line source hypothesis. This in turn will allow us to improve our overall knowledge of mantle anomaly geometry and absolute plate motion.