Isotope geochemistry of the Darwin Rise seamounts and the nature of long‐term mantle dynamics beneath the south central Pacific

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Abstract
The Darwin Rise region in the west central Pacific Ocean contains an extraordinarily large number of Cretaceous seamounts with isotopic compositions spanning a range nearly equal to that of the global ocean island basalt data set. These edifices formed in what is now the south central Pacific, which is characterized by anomalously shallow lithosphere and concentrated intraplate volcanism. We present Sr, Nd, and Pb isotopic data for Darwin Rise seamount basalts recovered by Ocean Drilling Program Leg 143 and Deep Sea Drilling Project Leg 7. Comparison of chemical and isotopic data for Darwin Rise and French Polynesian volcanoes confirms that the same range of distinct sources has been tapped by intraplate volcanism in the south central Pacific since the Early Cretaceous. Moreover, the increased data coverage reveals that most of the isotopic variation within Darwin Rise volcanic chains can be explained by shallow mixing between plumes and asthenosphere or oceanic lithosphere. On the basis of geophysical and geochemical evidence we propose an alternative model for volcanism in the south central Pacific from the Cretaceous to the present. Instead of the conventional hypothesis involving a few long‐lived mantle plumes, we propose that numerous hotspots formed and died out in the region over the past 130 Myr. We envision that these hotspots are the surface expression of “plumelets” continually separating from a shallow thermochemical boundary layer at the top of a large heterogeneous body of material which rose from the lower mantle. These plumelets rise and melt beneath the south central Pacific lithosphere, producing chains of volcanic edifices which terminate when they become exhausted of melt. The surfacing of multiple plumelets in proximity can explain the complex temporal, spatial, and geochemical patterns of volcanism observed in most volcanic chains in the south central Pacific and Darwin Rise.