Crystallization of Superfast-Spreading Oceanic Crust (ODP Hole 1256D, Pacific Ocean): Constraints From Zircon Geochronology

Studies of oceanic crust, which covers a large proportion of the Earth's surface, have provided significant insight into the dynamics of crustal accretion processes at mid-ocean ridges. It is now recognized that the nature of oceanic crust varies fundamentally as a function of spreading rate. Ocean Drilling Program (ODP) Hole 1256D (eastern Pacific Ocean) was drilled into the crust formed at a superfast spreading rate, and hence represents a crustal end member. Drilling recovered a section through lava and sheeted dykes and into the plutonic sequence, the study of which has yielded abundant insight into magmatic and hydrothermal processes operating at high spreading rates. Here, we present zircon U-Pb dates for Hole 1256D, which constrain the age of the section, as well as the duration of crustal accretion. We find that the main pulse of zircon crystallization within plutonic rocks occurred at 15.19 Ma, consistent with magnetic anomalies, and lasted tens of thousands of years. During this episode, the main plutonic body intruded, and partial melts of the base of the sheeted dykes crystallized. One sample appears to postdate this episode by up to 0.25 Myr, and may be an off-axis intrusion. Overall, the duration of crustal accretion was tens to several hundreds of thousands of years, similar to that found at the fast-spreading East Pacific Rise and the slow-spreading Mid-Atlantic Ridge. This indicates that crustal accretion along slow- to superfast-spreading ridges occurs over similar time scales, with substantially longer periods of accretion occurring at ultraslow-spreading ridges characterized by thick lithosphere.