Ocean zircons! Constraints on spreading rates, cooling and faulting histories, and modes of crustal accretion

Author(s): John, Barbara; Cheadle, Michael; Baines, Graham; Grimes, Craig; Schwartz, Joshua; Wooden, Joseph L.
Author Affiliation(s): Primary:
University of Wyoming, Department of Geology and Geophysics, Laramie, WY, United States
Adelaide University, Australia
Stanford University, United States
Volume Title: Geological Society of America, 2007 annual meeting
Source: Abstracts with Programs - Geological Society of America, 39(6), p.526-527; Geological Society of America, 2007 annual meeting, Denver, CO, Oct. 28-31, 2007. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0016-7592 CODEN: GAAPBC
Note: In English
Summary: 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 ∼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°20' fracture zone, the Kane Megamullion at 23°N and IODP Hole 1309D at 30°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 206Pb/238U age is ∼0.2 my older than the estimated magnetic age of Hole 735B, implying that magnetic remanence was acquired ∼3 km off-axis, and crustal cooling rates of >1000°C/myr from 850-550°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.
Year of Publication: 2007
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: (U-Th)/He; 03 Geochronology; 18 Geophysics, Solid-Earth; Absolute age; Accretion; Ar/Ar; Atlantic Ocean; Atlantis II fracture zone; Biotite; Cenozoic; Cooling; Crust; Dates; Denudation; Diorites; Expeditions 304/305; Faults; Fission tracks; Gabbros; IODP Site U1309; Igneous rocks; Indian Ocean; Integrated Ocean Drilling Program; Ion probe data; Leg 209; Magnetization; Mass spectra; Mica group; Mid-Atlantic Ridge; Nesosilicates; North Atlantic; ODP Site 1270; ODP Site 1275; ODP Site 735; Ocean Drilling Program; Orthosilicates; Paleomagnetism; Plutonic rocks; Quartz diorites; Quaternary; Remanent magnetization; SHRIMP data; Sea-floor spreading; Sheet silicates; Silicates; Southwest Indian Ridge; Spectra; Thermal history; Tonalite; U/Pb; Zircon; Zircon group
Coordinates: N301000 N301100 W0420600 W0420700
N144300 N144300 W0445300 W0445300
N154400 N154400 W0465400 W0465400
S324327 S324318 E0571618 E0571557
Record ID: 2011008566
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