Protracted timescales of lower crustal growth at the fast-spreading East Pacific Rise

The formation of oceanic crust at mid-ocean ridges is a fundamental component of plate tectonics. A large fraction of the new crust is created when magmas in the lower crust cool to form gabbroic rocks. The duration of magmatism during formation of the new gabbroic crust is expected to vary with plate-spreading rate and has been constrained by dating gabbroic rocks at the slow-spreading Southwest Indian Ridge and Mid-Atlantic Ridge(1-4). Here we present high-precision U-Pb dating of zircon minerals from gabbroic rocks formed at the fast-spreading East Pacific Rise and exposed at Hess Deep. We find that the zircons formed between 1.420 and 1.271 (+/-0.006-0.081) million years ago. Within individual samples, the zircon minerals exhibit a range in formation dates of up to 0.124 Myr, consistent with either protracted crystallization from a magma or assimilation into the magma of older zircons from adjacent rocks. The variability of zircon dates is comparable to that measured at the slow-spreading Mid-Atlantic Ridge(3). We conclude that the timescales of magmatic processes in the lower crust may be similar at slow-and fast-spreading ridges, implying that the duration of crust formation at mid-ocean ridges is not only controlled by spreading rate.