Duration, rates, and patterns of crustal growth at slow-spreading mid-ocean ridges: Using zircon to investigate the evolution of in situ ocean crust

This dissertation addresses temporal and spatial characteristics of magmatic and tectonic accretion processes occurring at mid-ocean ridges during formation of new ocean lithosphere. The accretion histories of plutonic crust recovered by deep drilling of footwalls to large-displacement normal faults at three locations on the MAR are emphasized. Pb/U zircon dating of evolved gabbros and oceanic plagiogranites show that magmatic accretion typically occurs over 100-200 kyr. Magmatic accretion occurs by emplacement of many short-lived intrusive pulses that are focused at different depths within the crust and mantle. During formation of the Atlantis Massif core complex (30° N, MAR), spreading may have been partitioned up to 100% on the North American plate for at least 200 kyr, implying crustal accretion was highly asymmetric. Cooling rates of 1000-2000° C/m.y. over 900-200° C are documented at Atlantis Massif and two locations from 14-16°N, MAR, consistent with rapid denudation of the footwall sections to the seafloor following emplacement. Spreading rates combined with the time interval over which cooling to <200° C occurred (<600 kyr) indicates that the ~900° and ~200° C isotherms were separated by up to 10 km along these non-conservative fault systems, placing constraints on the length-scale of the fault system, and therefore the possible geometries of the fault beneath the ridge axis.;Temporal and thermal constraints are derived primarily from radiometric dating of the mineral zircon. Zircon is of fundamental importance in many geologic studies because it retains a record of its crystallization age, incorporates a large number of elements that yields petrogenetic information, and is robust enough to persist through several episodes of erosion, weathering, and even magmatism. Therefore, a second emphasis of this dissertation is to characterize zircon recovered from in situ ocean crust. Zircon from ocean crust exhibit very low U/Yb and Th/Yb ratios that effectively distinguish them from zircon crystallized from continental magmas. These ratios are inherited from the source magmas, and provide a method for distinguishing provenance in ancient detrital settings.