Multiple origins of zircons in jadeitite

Jadeitites form from hydrothermal fluids during high pressure metamorphism in subduction environments; however, the origin of zircons in jadeitite is uncertain. We report ion microprobe analyses of delta O-18 and Ti in zircons, and bulk delta O-18 data for the jadeitite whole-rock from four terranes: Osayama serpentinite m,lange, Japan; Syros m,lange, Greece; the Motagua Fault zone, Guatemala; and the Franciscan Complex, California. In the Osayama jadeitite, two texturally contrasting groups of zircons are identified by cathodoluminescence and are distinct in delta O-18: featureless or weakly zoned zircons with delta O-18 = 3.8 +/- A 0.6aEuro degrees (2 SD, VSMOW), and zircons with oscillatory or patchy zoning with higher delta O-18 = 5.0 +/- A 0.4aEuro degrees. Zircons in phengite jadeitite from Guatemala and a jadeitite block from Syros have similar delta O-18 values to the latter from Osayama: Guatemala zircons are 4.8 +/- A 0.7aEuro degrees, and the Syros zircons are 5.2 +/- A 0.5aEuro degrees in jadeitite and 5.2 +/- A 0.4aEuro degrees in associated omphacitite, glaucophanite and chlorite-actinolite rinds. The delta O-18 values for most zircons above fall within the range measured by ion microprobe in igneous zircons from oxide gabbros and plagiogranites in modern ocean crust (5.3 +/- A 0.8aEuro degrees) and measured in bulk by laser fluorination of zircons in equilibrium with primitive magma compositions or the mantle (5.3 +/- A 0.6aEuro degrees). Titanium concentrations in these zircons vary between 1 and 19 ppm, within the range for igneous zircons worldwide. Values of delta O-18 (whole-rock) a parts per thousand... delta O-18 (jadeite) and vary from 6.3 to 10.1aEuro degrees in jadeitites in all four areas. These values of delta O-18 and Ti are higher than predicted for hydrothermal zircons, and the delta O-18 values of most zircons are not equilibrated with the coexisting jadeite at reasonable metamorphic temperatures. We conclude that while some zircons may be hydrothermal in origin, a majority of the zircons studied are best explained as relic igneous crystals inherited from precursor rocks; they were not precipitated directly from hot aqueous fluids as previously assumed. Therefore, U-Pb ages from these zircons may date magmatic crystallization and do not establish the timing of high pressure metamorphism or hydrothermal activity.