J. G. Liou J. Wooden T. Miyamoto T. Tsujimori 2005 <p><span>Crystals of zircon up to 3 mm in length occur in jadeitite veins in the Osayama serpentinite m&eacute;lange, 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 &gt; 1.2 GPa at T &gt; 350&deg;C for formation of the jadeitite. Most U/Pb ages obtained by SHRIMP-RG are concordant, with a weighted mean&nbsp;</span>²⁰⁶<span>Pb/</span>²³⁸<span>U age of 472 &plusmn; 8.5 Ma (MSWD = 2.7,&nbsp;</span><i>n</i><span>&nbsp;= 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.</span></p> application/pdf 10.2747/0020-6814.47.10.1048 en Taylor & Francis U-Pb dating of large zircons in low-temperature jadeitite from the Osayama serpentinite melange, southwest Japan: Insights into the timing of serpentinization article