HR: 0800h
AN: V41F-1535    [Abstracts]
TI: Widespread Occurrence of Zircon in Slow- and Ultraslow Spreading Ocean Crust: A Tool for Studying Ocean Lithospheric Processes
AU: * Grimes, C B
EM: cgrimes@uwyo.edu
AF: University of Wyoming, Dept 3006 1000 University Ave., Laramie, WY 82071 United States
AU: John, B E
EM: bjohn@uwyo.edu
AF: University of Wyoming, Dept 3006 1000 University Ave., Laramie, WY 82071 United States
AU: Cheadle, M J
EM: cheadle@uwyo.edu
AF: University of Wyoming, Dept 3006 1000 University Ave., Laramie, WY 82071 United States
AU: Schwartz, J J
EM: schwartz@uwyo.edu
AF: University of Wyoming, Dept 3006 1000 University Ave., Laramie, WY 82071 United States
AB: The presence of igneous zircon in oceanic gabbro and peridotite provides a new opportunity to constrain absolute ages, and the processes and rates of crustal accretion in oceanic environments. Our recent investigations show zircon to be common in slow and ultraslow spreading oceanic crust including several locations along the Mid-Atlantic Ridge (MAR) and Southwest Indian Ridge (SWIR), and in rock types ranging from trondjhemite dikes to peridotite. Zircon is typically found in felsic intrusions and oxide gabbro, and in many cases may be due to late stage saturation in small pockets of residual melt. We report the morphologic and chemical characteristics of zircon grains collected from >100 rock samples recovered both from the seafloor by manned submersible and ROV, and with depth by ODP/IODP drilling. Grains range from euhedral and faceted to anhedral and fractured, with internal zonation that may be homogeneous, concentric, or patchy, and rarely contain relict cores. Sizes range from <5 μm to >1 mm. Measurements of major, minor, and trace element concentrations and high-resolution Pb/U ages were collected with the SHRIMP-RG. Chondrite-normalized rare earth element (REE) patterns for more than 50 zircon grains are uniform in shape and closely resemble patterns for known terrestrial igneous zircon. This is in contrast to mantle affinity zircon (e.g. kimberlite), which typically show depleted and relatively unfractionated patterns. Observed total REE concentrations range from 330-3765 ppm. Patterns are convex upward and rise sharply towards the HREE, with normalized Sm/La ratios = 16-320 and Lu/Gd ratios = 20-51. Positive Ce and negative Eu anomalies are ubiquitous. Hf abundances range from 5988 to 14,266 ppm. Other elements occurring at minor abundance levels include Y (463-6949 ppm), P (253-2288 ppm), U (7-2827 ppm), and Th (3-7403 ppm). Preliminary Ti concentrations range from 13 to 270 ppm, indicating crystallization temperatures of 765 to 1147°C based on Ti in zircon thermometry. Enrichment of U and Th (>100 ppm) is frequently observed, and allows for the application of isotopic dating techniques on rocks collected near ridge axes. SHRIMP-RG Pb/U age determinations reveal resolution on the order of 2% corresponding to 40,000 years in crust as young as 2 Ma, and errors of <2% for samples up to 13 Ma. Attempts to employ the (U-Th)/He dating method on oceanic zircon have also been successful. With a closure temperature (Tc) of 180±20°C, this technique can be used in conjunction with U/Pb dating (Tc = 900±50°C) to bracket magnetic ages (Tc = 450-580°C). By applying multi-temperature chronometers to zircon collected from boreholes we can measure cooling rates related to emplacement, denudation, and/or tectonic rotation. Finally, the relatively common presence of zircon in oceanic crust precludes the simple assumption that detrital zircons in Archean meta-sedimentary rocks demonstrate the existence of continental crust.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1115 Radioisotope geochronology
DE: 3035 Midocean ridge processes
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005