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U-Pb dating of large zircons in low-temperature jadeitite from the Osayama serpentinite melange, southwest Japan: Insights into the timing of serpentinization

January 1, 2005

Crystals of zircon up to 3 mm in length occur in jadeitite veins in the Osayama serpentinite mélange, Southwest Japan. The zircon porphyroblasts show pronounced zoning, and are characterized by both low Th/U ratios (0.2-0.8) and low Th and U abundances (Th = 1-81 ppm; U = 6-149 ppm). They contain inclusions of high-pressure minerals, including jadeite and rutile; such an occurrence indicates that the zircon crystallized during subduction-zone metamorphism. Phase equilibria and the existing fluid-inclusion data constrain P-T conditions to P > 1.2 GPa at T > 350°C for formation of the jadeitite. Most U/Pb ages obtained by SHRIMP-RG are concordant, with a weighted mean 206Pb/238U age of 472 ± 8.5 Ma (MSWD = 2.7, n = 25). Because zircon porphyroblasts contain inclusions of high-pressure minerals, the SHRIMP U-Pb age represents the timing of jadeitite formation, i.e., the timing of interaction between alkaline fluid and ultramafic rocks in a subduction zone. Although this dating does not provide a direct time constraint for serpentinization, U-Pb ages of zircon in jadeitite associated with serpentinite result in new insights into the timing of fluid-rock interaction of ultramafic rocks at a subduction zone and the minimum age for serpentinization.

Citation Information

Publication Year 2005
Title U-Pb dating of large zircons in low-temperature jadeitite from the Osayama serpentinite melange, southwest Japan: Insights into the timing of serpentinization
DOI 10.2747/0020-6814.47.10.1048
Authors T. Tsujimori, J. G. Liou, J. Wooden, T. Miyamoto
Publication Type Article
Publication Subtype Journal Article
Series Title International Geology Review
Series Number
Index ID 70027883
Record Source USGS Publications Warehouse
USGS Organization