Abstract
High-pressure, low-temperature metamorphic rocks record subduction of oceanic crust and sediments into the mantle, but the timing of subduction initiation is commonly difficult to determine. However, the appearance of fluid activity in the mantle wedge above a subduction zone can be correlated with incipient subduction. Here we describe jadeitite veins up to 5 m wide, which are hosted in serpentinites of the Syum-Keu ultramafic complex, Polar Urals, Russia. The jadeitites consist of both jadeite (86–88 mol% jd) and omphacite (54–75 mol% jd). Multi-stage growth of the jadeite is indicated by variations in color and structure, such as growth zoning and the distribution of fluid inclusions. We consider that the jadeitite veins formed from Na–Al–Si-rich fluids under HP/LT conditions during active subduction, and that the source material was subducted ocean crust. SHRIMP U–Pb dating of zircons from the jadeitite yielded a concordant weighted mean 206Pb/ 238U age of 404 ± 7 Ma, which we interpret as the age of peridotite serpentinization and jadeitite formation in the suprasubduction zone mantle. This age indicates that subduction was initiated about 40 Ma earlier than peak metamorphism (360–355 Ma) of eclogite in the Polar Urals.
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