SUBDUCTION ZONE FLUID CRYSTALLIZATION PRODUCTS—THE PERSPECTIVE FROM JADEITITE RESEARCH

Fluids expelled from subduction of seafloor crust play a critical role in the Earth system for flux melting, metasomatism, and other important processes. However, as the fluids are ephemeral, their compositions and evolution are difficult to constrain. In recent years, study of jadeitite – previously of more interest as a gem material or archaeological artifact – has informed about these fluids. Composed predominantly of jadeite (NaAlSi₂O₆), jadeitite is found in serpentinite mélange of exhumed subduction zones with other indicators of high-pressure—low-temperature processing, e.g., eclogite, blueschist, and antigoritite. Conditions of formation range from ~250°C at 5 kbar to ~600° at 26 kbar with most at 10 to 20 kbar. Fluid inclusions in and rhythmic zoning of jadeite together with vein filling crystallization textures document hydrous fluid precipitation in all jadeitite, even those showing a metasomatic replacement origin. Thus, jadeitite records fluid saturation of solutes acquired in the channel.

Jadeite crystal chemistry limits the chemical fingerprint; however, solid solution with Cpx containing Na, Ca, Mg, Fe²⁺, Fe³⁺ permits greater capacity for incorporation of minor elements and REE, as well as provides PT constraints. Minor mica and feldspar-like phases record conspicuous LILE signatures: Cs, Rb, NH₄⁺, Ba, Sr, Pb or the lack thereof. Jadeitite from two contexts in Guatemala demonstrate somewhat different evolutions; however, the whole-rock minor-to-trace-element compositions track remarkably well with global seafloor sediment (GLOSS), including LILE and HFSE, although not generally for K and Pb. The apparent mobility of Zr, Hf, Ta, and Nb is perhaps unexpected. Late-stage crystallization products typically saturate with omphacite or phengite, suggesting both fluid composition and condition changes but are not correlated with advanced exhumation. Interactions with hosting ultramafic mélange are minimal until the latest-stage crystallization (e.g., little Ni or Cr) suggesting H₂O is largely absorbed by serpentinization, which is supported by comparably light δ¹¹B in both jadeitite mica and serpentinite antigorite.

Consequently, jadeitites and their related vein crystallizations offer a window into fluids exiting the subduction channel into brittle mantle wedge.