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

• Main Author: Grimes, Craig B
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2008
• Subjects: Geology
• ISBN: 9781109180077; 1109180071

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.