Abstract
AbstractThe physical and mechanical processes rooted in the hydrated, serpentinized mantle above subduction zones remain insufficiently explored despite fundamental implications for our understanding of rheology and fluid recycling along subduction interfaces. Through a field‐based investigation, serpentinized peridotites and jadeitite samples from a fossil forearc mantle in the Polar Urals (Russia) are studied here to document fluid–rock interaction processes in the high‐P field, as well as the long‐term evolution of the base of the mantle wedge. Petrographic, geochemical and microstructural observations reveal a complex, protracted evolution of the jadeitite‐forming fluid pathway throughout the gradual cooling of the forearc mantle and increasing serpentinization of the host. It is shown that the jadeitite lenses in the studied locality (a) derive for a large part from a trondhjemitic dyke earlier emplaced in a warm subduction environment, and (b) record the cooling of the subduction hangingwall under high‐P conditions associated with increasing host serpentinization. In the studied locality, the majority of the jadeitites formed at relatively high temperatures (>600°C) by the influx of Na–Al‐rich, slab‐derived metamorphic fluids that were drained along the base of the mantle wedge, parallel to the subduction interface. Changes in bulk‐rock geochemical signatures and in paragenetic sequences also constrain the compositional evolution of the fluid channelized along this drainage, with an increasing sedimentary component. The phlogopite‐bearing walls of the dyke exhibit Rb–Sr and Ar–Ar ages ranging between c. 405 and c. 390 Ma, a range partly overlapping within uncertainty with the previously dated zircons from the jadeitite core (410–400 Ma; U–Pb). This study opens a unique window on the pristine structures formed above the plate interface by melting and fluid–rock interaction in the early subduction stages, as well as their evolution during secular cooling of the base of the mantle wedge.
Highlights
Understanding the pathways of subduction fluids produced by devolatilization reactions within the downgoing plate is of paramount importance to volatile budget and recycling, as well as the rheology of the plate interface (Bebout & Penniston-Dorland, 2016; Hyndman & Peacock, 2003; Prigent et al, 2018; Scambelluri et al, 2019; Schmidt & Poli, 2014)
Long-lasting subduction-related serpentinization events largely erased most of the markers from the early subduction stages, markers only occasionally preserved as dismembered blocks in ‘serpentinite mélanges’ (e.g. Gerya et al, 2002; Lazaro et al, 2009; Sorensen et al, 2010)
We show that the jadeitite body initially formed by the infiltration of alkali-rich fluids/melts before c. 410 Ma, leading to the crystallization of edenite-rich dark granofels and paragonite-bearing trondhjemites
Summary
Understanding the pathways of subduction fluids produced by devolatilization reactions within the downgoing plate is of paramount importance to volatile budget and recycling, as well as the rheology of the plate interface (Bebout & Penniston-Dorland, 2016; Hyndman & Peacock, 2003; Prigent et al, 2018; Scambelluri et al, 2019; Schmidt & Poli, 2014). One of the few places on Earth where such pristine contact can be observed lies along the Main Uralian Thrust (MUT) in the Polar Urals, Russia (Dobretsov & Ponomareva, 1968; Meng et al, 2011; Figure 1) In this contribution, we focus on the Syum-Keu massif, and reconstruct through a petrochronological and geochemical study the evolution of this important jadeitite deposit. Jadeitite bodies form a several-hundred metres long, dyke-like structure, which has been moderately boudinaged during ductile shearing in the host serpentinites (Figure 1b) These authors mention the presence of ‘plagioclasite’ remnants with variable jadeitization imprint in the centre and in the north of the deposit. Zircon crystals separated from the white jadeitite yielded SHRIMP U–Th–Pb ages of 404 ± 7 Ma (Meng et al, 2011) and 409 ± 3.3 Ma (Konovalov & Sergeev, 2015), interpreted by Meng et al (2011) as dating intra-oceanic subduction initiation within the Uralian ocean realm
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