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

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  • The Cook-Austral island chain has been the center of debate for many years. Contrary to the classical hotspot hypothesis, this volcanic island chain does not exhibit a linear age progression with a single node of active volcanism, but instead shows evidence of young volcanism at several points along the chain. While several hypotheses have been put forth to explain these age systematics, including multiple mantle plumelets, small-scale convection and lithospheric extension, exploring these different possibilities has been limited by the uncertainty surrounding the reliability of the age database for these islands. The vast majority of the ages that have been published for the Cook-Australs were obtained using the K Ar method, a technique that has been shown to be susceptible to the effects of weathering and alteration, with concurrent loss of radiogenic Ar. Here we present 56 new Ar age determinations for eight of the Cook-Austral islands. This incremental heating technique is both more accurate and more precise than the Ar and total fusion Ar techniques. We found that these new ages are on average 10-40% different from and generally older than the K Ar ages for the same samples. We show that these ages are more reproducible within a single lava flow, as well as exhibit less scatter among ages from a single island, and therefore are expected to be more reliable than published Ar age determinations. With less variability in the ages at each island, at least two clearly defined and matching age-progressive trends with origins at Macdonald and Arago seamounts appear in the data, supporting the hypothesis that the Macdonald and Rurutu hotspot tracks were formed by multiple, contemporaneous mantle plumelets aligned in the direction of plate motion. In relation 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° Ma) that are similar to that of Hawaii (1.15° Ma) and faster than that of Samoa (0.63° Ma). Over the last 30 Myr both the Cook-Austral and Hawaii hotspots have been located truly intra-plate and thus far away from any tectonic boundary, as opposed to Samoa's hotspot position alongside the active Tonga-Kermedec subduction zone. This implies that hotspot location relative to tectonic boundaries may have an effect on the age progressions recorded by volcanic chains. Furthermore, the similarity between the primary Hawaiian hotspot, which is thought to have a deep origin, and the shallower secondary hotspots of the Cook-Austral islands suggests that these different types of hotspots may behave more similarly than previously hypothesized and can therefore both be used to reconstruct past plate motion, provided they are located far away from any plate tectonic boundary.
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