When a mantle plume interacts with a mid-ocean ridge, both are noticeably a
ffected. The mid-ocean ridge can display anomalously shallow bathymetry, ex
cess volcanism, thickened crust, asymmetric sea-floor spreading and a plume
component in the composition of the ridge basalts(1-4). The hotspot-relate
d volcanism can be drawn closer to the ridge, and its geochemical compositi
on can also be affected(3,5-7). Here we present Sr-Nd-Pb isotopic analyses
of samples from the next-to-oldest seamount in the Hawaiian hotspot track,
the Detroit seamount at 51 degrees N, which show that, 81 Myr ago, the Hawa
iian hotspot produced volcanism with an isotopic signature indistinguishabl
e from mid-ocean ridge basalt. This composition is unprecedented in the kno
wn volcanism from the Hawaiian hotspot, but is consistent with the interpre
tation from plate reconstructions(8) that the hotspot was located close to
a mid-ocean ridge about 80 Myr ago. As the rising mantle plume encountered
the hot, low-viscosity asthenosphere and hot, thin lithosphere near the spr
eading centre, it appears to have entrained enough of the isotopically depl
eted upper mantle to overwhelm the chemical characteristics of the plume it
self. The Hawaiian hotspot thus joins the growing list of hotspots that hav
e interacted with a rift early in their history.