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The Cobb hot spot: HIMU-DMM mixing and melting controlled by a progressively thinning lithospheric lid

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  • The Cobb Seamount Chain in the northeast Pacific basin records the composition of the Cobb hot spot for the past 33 Myr, as the migrating Juan de Fuca Ridge approached and ultimately overran it ca. 0.5 Myr ago. In this first comprehensive geochemical study of the Cobb chain, major and trace element compositions and Sr, Nd, Pb, and Hf isotopic ratios were measured for whole-rock samples from throughout the chain, and He isotopes were acquired for olivine phenocrysts from one seamount. Trace element modeling indicates increased melting along the chain over time, with progressively more depleted lavas as the ridge approached the hot spot. The isotopic data reveal the first evidence of the high μ (μ = ²³⁸U/²⁰⁴Pb) (HIMU) mantle component in the north Pacific basin and are consistent with a progressively decreasing mixing proportion of HIMU melts relative to those from depleted mid-ocean ridge basalt mantle (DMM) in the chain over time. Decreasing lithospheric thickness over the Cobb hot spot due to the approach of the migrating Juan de Fuca ridge allowed adiabatic melting to continue to shallower depths, leading to increased melt fractions of the refractory DMM component in the hot spot and more depleted and MORB-like lavas in the younger Cobb seamounts.
  • Keywords: Oceanic lithosphere, MORB, HIMU, Cobb hot spot, Juan de Fuca Ridge
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  • Chadwick, J., R. Keller, G. Kamenov, G. Yogodzinski, and J. Lupton (2014). The Cobb hot spot: HIMU-DMM mixing and melting controlled by a progressively thinning lithospheric lid. Geochemistry Geophysics Geosystems, 15(8), 3107–3122. doi:10.1002/2014GC005334
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  • 15
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  • 8
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  • The 1999 and 2002 Atlantis/Alvincruises to the Gulf of Alaska werefunded by NOAA WCNURP and NOAAOE, respectively. This study was alsofunded in part with faculty startupfunding from the Department ofGeology and EnvironmentalGeosciences at the College ofCharleston.
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