Jade (Nephrite and Jadeitite) and Serpentinite: Metasomatic Connections

Abstract

The lapidary term "jade" refers to two very tough, virtually monomineralic rocks used for ornamental carvings or gems. Both have metasomatic origins that are intimately connected with their host serpentinite bodies and convergent-margin petrotectonics. Amphibole jade is nephrite, a tremolite-actionalite rock with a felted, microcystalline habit; pyroxene jade is jadeite rock (jadeitite), which varies from micro- to macrocrystalline textures. Most nephrite occurs along fault contacts between serpentinite and mafic to felsic igneous rocks or metagraywacke in obduction settings. It forms by Ca- and Si-rich, aqueous fluid-mediated metasomatic replacement of serpentinite, typically antigorite, at greenschist-facies or lower P-T conditions. Other nephrite bodies reflect contact metasomatic replacement of dolomite by Si-rich aqueous fluids during felsic pluton emplacement. Like most nephrite, jadeitite is hosted by antigorite-dominated serpentinite bodies. However, these serpentinites are associated with HP/LT metamorphic terranes, in which jadeitite occurs as isolated tabular bodies or tectonized blocks. Based on textural evidence, particularly clear from cathodoluminescence studies, nearly all jadeitite bodies appear to have formed originally as vein crystallization of an aqueous fluid, most readily interpreted as Na-Al-Si-rich fluid at HP/LT conditions in subduction/collisional settings. The host serpentinite influences jadeitite compositions by lowering fluid aSiO2 during serpentinization, and contributing Ca + Mg ± Cr to late-stage jadeitite-forming fluids. Thus, although both types of jade form in convergent-margin tectonic settings, jade has two distinct primary modes of origin: (1) by siliceous replacement of already serpentinized ultramafic rock at low-P, low- to moderate-T conditions following obduction (nephrite); or (2) by the interaction of serpentinizing peridotite and Na-Al-Si fluids at HP/LT conditions during active subduction/collision (jadeitite).