JADE: OCCURRENCE AND METASOMATIC ORIGIN
" (Coleman, 1961). Except for the Polar Urals occurrence, which is probably Devonian in age, and the Itoigawa source, which is probably Permian, jadeitite deposits are associated with post-Cretaceous geology.
Jadeitite is a very uncommon rock type restricted to primary occurrences in bodies of subduction-related serpentinite along major fault zones. Pure jadeite jade is white, ¡°Imperial¡± emerald green is due to Cr 3+ , leak-green from Fe 2+ , blue-green from mixed Fe 2+ and Fe 3+ and mauve from Mn 2+ . Blue jadeite/omphacite from the Itoigawa area is due to Ti (with TiO2 ¡Ü6 wt%, Matsubara, personal communication, and color probably resulting from intervalence charge transfer with Fe, as in sapphire) and a blue jadeitite from Guatemala has not as yet been studied. Pyroxene compositions in jadeite jades range from Jd100 to omphacite usually with Ac5-10. Jadeitite occurs as veins (Myanmar, California) or tectonic blocks (Guatemala, Polar Urals) generally with surrounding albitite, actinolite schist and a blackwall rind bounding serpentinite. Minerals coexisting with primary jadeite include sodic amphibole (eckermannite to pargasite-glaucophane in Myanmar and Japan) or mica (phengite and/or paragonite in Guatemala) ¡À albite, titanite, rutile, zircon, apatite, chromite, pyrite, and graphite¡ªquartz is absent except in a single non-gem sample reported by Smith and Gendron (1997). Crystals of jadeite in jadeitites are cryptically to rhythmically zoned, indicating crystallization from an aqueous fluid. Thus, most jadeitites probably began as vein crystallizations, although a partial metasomatic replacement origin is difficult to rule out. The host serpentinite is either highly faulted or a melange and is always associated with a major fault which is usually a lateral fault on the wedge side of a fossilized convergent margin. Consequently fluid flow in faulted serpentinite and emplacement of serpentinite along faults active during subduction or collision (continent-continent or fragment-continent) are critical features to jadeitite occurrences.
Jadeite is a high pressure mineral, but with PH 2 O=Ptotal and no quartz, P is only > 5-6 kbar (bounding reaction is Jd+W=Anl) rather than > 8-11 kbar (bounding reaction Jd+Qtz=Ab) for the low T environments (200 to 400 ¡ãC based on assemblages). Nevertheless, this represents substantial depth (>16 - 20 km) for a fluid that deposits jadeite in serpentinite (serpentinizing peridotite) by rising through an accretionary wedge or back along subduction-related faults. Fluid inclusions and O/H isotopic systematics infer the predominance of a seawater-like fluid entrained during subduction rather than the product of dehydration of deep metamorphic minerals, at least for Guatemala (Johnson and Harlot, 1999). Trace element studies of jadeite from jadeitites manifest considerable heterogeneity, suggesting diverse fluid trajectories for different jadeitites in the same deposit, although overall trends suggest some deposits may be derived primarily from sediments (perhaps in Guatemala) while others may record a significant felsic-igneous component (perhaps Myanmar) (Sorensen and Harlot, 1998, 1999, and in prep.). The high-pressure origin of jadeitites associates them in the belts of eclogites and blueschists around the world ¨C Fig. 2.
Jadeitite formation requires devolatilization, primarily dewatering of sediments, within a subducting slab at depths down to the blueschist-to-eclogite transition. Such fluids will be enriched in components that are nearly saturated with jadeite (e.g., Manning, 1998). These fluids may become channelized through overlying serpentinized peridotite (of unknown provenance) which can diapirically rise along a fore-arc transform/lateral fault, which may be related to final oblique convergence of continental (Myanmar) or island arc (Guatemala) terrane. Crystallization of jadeite provides a focus for brittle fracture, fluid flow, and further deposition of jadeite during serpentinite diapirism and faulting. Fluid travels down P but up T which is key to the sequence jadeitite followed by albitite found at all jadeitite occurrences. Fractionation by jadeite crystallization and decrease in P progressively enriches the diopside content in the rising fluid, so pyroxene crystallization trends to omphacite at crystal rims. Increased silica activity at shallower depths leads to the co-crystallization of albite + omphacite or diopside. Interaction of the fluid with tectonic blocks entrained in the host serpentinite can lead to jadeitization, or in the case of basaltic blocks, the formation of Fe-rich omphacitites or omphacite-rich amphibolites. At the top of the system tremolite may saturate¡ªwith possible formation of nephrite. The diapiric rise of serpentine, perhaps enhanced by the collision process, exposes fossil jadeitite, however the rapid uplift may also result in a short duration of exposure, explaining the paucity and young age of most jadeitite deposits.
The formation of most nephrite and jadeitite deposits records events at convergent margins that involve fluid interactions in and around serpentinizing peridotite at depths from perhaps > 50 km to the near surface. Preservation of the jade is a relatively rare event that may require special tectonic conditions and a limited range of peridotite hosts for jadeitite and perhaps nephrite. Jades are thus unique probes of convergent margins and fluids derived from subduction zone devolatilization¡ªprofoundly interesting geologically as well as materially and archaeologically. "