The mysteries of jadeite jade, a monitor of subduction-zone fluids

Jadeite jade is the rare variety of jade at present known only from 20 localities of high-pressure, subduction-zone serpentinite mélange. Jadeite jade is tough, hard and brilliant when polished. It has been of special mystic meaning in Mayan and Chinese cultures for up to 8000 years. Much discussion surrounds jadeite-jade genesis: origin by direct precipitation from a high-pressure aqueous fluid? Or metasomatism of a precursor rock like trondhjemite/plagiogranite? Present evidence suggests both processes are actually closely related; densification and decrease in volume during reaction of plagiogranite to jadeite jade is a key factor. Until recently, the plagiogranite protoliths were considered to be of mid-ocean-rift origin and brought into subduction zones by cold descending oceanic crust. Jadeite jade was assumed to be mainly formed at T < 500°C. Recent work (e.g., Angiboust et al., J. Metam. Geol., 39, 473-500, 2021) indicates a surprising second source: Adakitic melting of basaltic crust at depth can produce trondhjemite intrusions that initiate the formation of jadeite jade at high temperatures as well. Since the 1950’s, jadeite is known to be a high-pressure mineral, but only in silica-saturated environments. Thus it is enigmatic why quartz is rarely found in jadeite jade (Harlow et al., Short Course Handbook, Min. Ass. Canada, 44, 305-374, 2014). Recent studies (e.g., Hertwig et al., Russ. Geol. Geoph., 62, 496–524, 2021) indicate the answer. Remnants of quartz are found as tiny, inconspicuous relict inclusions in jadeite crystals, but quartz has been removed from the rock matrix by later desilication.