A young source for the Hawaiian plume

Sobolev et al. show that highly radiogenic strontium in melt inclusions from lavas of the Mauna Loa shield volcano in Hawaii have an isotopic composition matching that of sea water aged 200 million to 650 million years old. They infer that such sea water must have contaminated the Mauna Loa source rock prior to subduction, imparting a unique 'time stamp' and indicating that mantle circulation has an average speed of about 2 centimetres per year, which is more rapid than previously thought. Recycling of oceanic crust through subduction, mantle upwelling, and remelting in mantle plumes is a widely accepted mechanism to explain ocean island volcanism1. The timescale of this recycling is important to our understanding of mantle circulation rates. Correlations of uranogenic lead isotopes in lavas from ocean islands such as Hawaii or Iceland, when interpreted as model isochrons, have yielded source differentiation ages between 1 and 2.5 billion years (Gyr)2,3,4,5. However, if such correlations are produced by mixing of unrelated mantle components6 they will have no direct age significance. Re–Os decay model ages take into account the mixing of sources with different histories7,8, but they depend on the assumed initial Re/Os ratio of the subducted crust, which is poorly constrained because of the high mobility of rhenium during subduction9. Here we report the first data on 87Sr/86Sr ratios for 138 melt inclusions in olivine phenocrysts from lavas of Mauna Loa shield volcano, Hawaii, indicating enormous mantle source heterogeneity. We show that highly radiogenic strontium in severely rubidium-depleted melt inclusions matches the isotopic composition of 200–650-Myr-old sea water. We infer that such sea water must have contaminated the Mauna Loa source rock, before subduction, imparting a unique ‘time stamp’ on this source. Small amounts of seawater-derived strontium in plume sources may be common but can be identified clearly only in ultra-depleted melts originating from generally highly (incompatible-element) depleted source components. The presence of 200–650-Myr-old oceanic crust in the source of Hawaiian lavas implies a timescale of general mantle circulation with an average rate of about 2 (±1) cm yr−1, much faster than previously thought.