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Many geological and geodynamical studies of metamorphism in subduction zones have relied upon worldwide compilations of modelled slab-top pressure-temperature (P-T) conditions, although recent evaluation of such data sets suggests that these predictions are ~100-300°C colder at any given pressure than the conditions recorded by exhumed metamorphic rocks. As such, geochemical, petrological and geo-

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In contrast to low‐T eclogites (garnet growth zoning preserved) or high‐T eclogites (garnet diffusionally homogenized at peak conditions), medium‐temperature eclogites pose additional challenges to P–T determinations due to the partial preservation of garnet zoning. The Dulan area, in the southeastern part of North Qaidam ultrahigh‐pressure terrane, exposes minor eclogites hosted by ortho‐ and paragneisses. Four fresh, medium‐temperature eclogites contain the paragenesis Grt+Omp+Rt+Qz/Coe+Ph±Ky±Zo. In all samples, garnet XMg shows little zoning, suggesting diffusional modification, and precludes the use of pyrope+almandine+grossular isopleth intersections to determine a P–T path. However, in one sample, sharp zoning in grossular content suggests that grossular growth compositions are preserved. Since garnet pyrope+almandine compositions appear to be modified, we instead use the intersections of grossular and garnet volume isopleths to define a prograde P–T path. This approach yields a path from ~17 kbar and ~410°C to ~35 kbar and ~625°C with a gradient of ~5–9°C/km through the lawsonite stability field. Peak P–T conditions determined from the intersection between Si pfu in phengite and Zr‐in‐rutile isopleths are ~26–33 kbar and ~625–700 °C for the four eclogites. These conditions are close to the limit of the lawsonite stability field, suggesting that fluid released from lawsonite breakdown may have promoted re‐equilibration at these conditions. These peak conditions are also in good agreement (within 3 kbar and 50°C) with garnet–omphacite–phengite (±kyanite) thermobarometry in three of the four samples. We regard the phengite–rutile constraints as more reliable, because they are less sensitive to uncertainties associated with ferric iron compared to conventional thermobarometry. Phase equilibrium modelling predicts that the retrograde assemblage of amphibole+zoisite formed at ~60 km. Infiltration of external fluids was likely required for the growth of these hydrous minerals. Based on the comparison of P–T estimation methods applied in this study, we propose that the garnet grossular+volume isopleths can recover the prograde P–T path of medium‐temperature eclogites, and that the combination of phengite+rutile isopleths represents a more robust approach to constrain peak P–T conditions.

Arc volcanism and trace-element recycling are controlled by the devolatilization of oceanic crust during subduction. The type of fluid—either aqueous fluids or hydrous melts—released during subduction is controlled by the thermal structure of the subduction zone. Recent thermomechanical models and results from experimental petrology argue that slab melting occurs in almost all subduction zones, although this is not completely supported by the rock record. Here we show via phase equilibrium modeling that melting of either fresh or hydrothermally altered basalt rarely occurs during subduction, even at water-saturated conditions. Melting occurs only along the hottest slab-top geotherms, with aqueous fluids being released in the forearc region and anatexis restricted to subarc depths, leading to high-SiO2 adakitic magmatism. We posit that aqueous fluids and hydrous melts preferentially enhance chemical recycling in “hot” subduction zones. Our models show that subducted hydrothermally altered basalt is more fertile than pristine basaltic crust, enhancing fluid and melt production during subduction and leading to a greater degree of chemical recycling. In this contribution, we put forward a petrological model to explain (the lack of) melting during the subduction of oceanic crust and suggest that many large-scale models of mass transfer between Earth’s surface and interior may require revision.