Stanford University, United States

The recognition of igneous zircon in evolved gabbroic rocks (including Fe-Ti oxide and/or amphibole-bearing gabbro, quartz diorite, and tonalite) that comprise a significant portion of oceanic crust exposed at slow and ultraslow spreading mid-ocean ridges, provides an unprecedented opportunity for U/Pb zircon SHRIMP II dating. U/Pb ages, with a resolution of approximately 2-3% (as low as 0.02-0.03 my in 1 Ma crust), provide detailed information on spreading rates, help calibrate marine magnetic anomalies, provide detailed constraints on the timing of both magmatism and denudation, and contribute to knowledge of the rates and duration of crustal growth in slow spreading environments. Further, when used in conjunction with other thermochronometers, the full crustal cooling history can be determined. To date, our work has focused on the footwalls to large-slip faults at several sites including Atlantis Bank and ODP Hole 735B (SW Indian Ridge), ODP Holes 1270D and 1275D along the 15 degrees 20' fracture zone, the Kane Megamullion at 23 degrees N and IODP Hole 1309D at 30 degrees N, all on the Mid-Atlantic Ridge. We have determined plate spreading rates at Atlantis Bank and the Kane Megamullion that suggest that oceanic detachment faults form during periods of highly asymmetric spreading (Atlantis Bank detachment fault accommodated roughly 80% of plate spreading). We note that the average (super 206) Pb/ (super 238) U age is approximately 0.2 my older than the estimated magnetic age of Hole 735B, implying that magnetic remanence was acquired approximately 3 km off-axis, and crustal cooling rates of >1000 degrees C/myr from 850-550 degrees C. We have recognized zoned zircon crystals that record up to 2.5 my of inheritance testifying to the complexity of crustal growth at slow spreading ridges. Dating of zircons from deep ODP/IODP vertical boreholes has allowed us to determine a detailed chronology of the emplacement of up to 1500 meters of gabbroic crust, illuminating not only crustal architecture, but the timing and scale of vertical crustal accretion, and the cooling/denudation history of slow spreading crust. These data test models of crustal accretion at mid-ocean ridges, including the gabbro-glacier and many sill models. We have also combined zircon U/Pb ages with zircon U/Th-He and fission track ages, sphene fission track ages, and biotite Ar-Ar ages to define a complete cooling history for Atlantis Bank. Off axis heating seems to be required.