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 δ18O and Ti in zircons, and bulk δ18O data for the jadeitite whole-rock from four terranes: Osayama serpentinite mélange, Japan; Syros mélange, 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 δ18O: featureless or weakly zoned zircons with δ18O = 3.8 ± 0.6‰ (2 SD, VSMOW), and zircons with oscillatory or patchy zoning with higher δ18O = 5.0 ± 0.4‰. Zircons in phengite jadeitite from Guatemala and a jadeitite block from Syros have similar δ18O values to the latter from Osayama: Guatemala zircons are 4.8 ± 0.7‰, and the Syros zircons are 5.2 ± 0.5‰ in jadeitite and 5.2 ± 0.4‰ in associated omphacitite, glaucophanite and chlorite-actinolite rinds. The δ18O 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 ± 0.8‰) and measured in bulk by laser fluorination of zircons in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6‰). Titanium concentrations in these zircons vary between 1 and 19 ppm, within the range for igneous zircons worldwide. Values of δ18O (whole-rock) ≅ δ18O (jadeite) and vary from 6.3 to 10.1‰ in jadeitites in all four areas. These values of δ18O and Ti are higher than predicted for hydrothermal zircons, and the δ18O 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.
- Oxygen isotopes