Chocolate in the marble cake: the fate of eclogite and pyroxenite during mantle convection and melting

The presence of lithologies derived from recycled oceanic lithosphere in the convective mantle is an expected consequence of subduction. Geochemical studies have provided compelling evidence of the contribution of recycled eclogite and pyroxenite in the mantle source of oceanic basalts, particularly ocean island basalts (OIB). However, identifying their signatures in mid-ocean ridge basalts (MORB) is challenging due to more intricate melting and mixing processes. Furthermore, the use of elemental and isotopic proxies of different geochemical affinities provides contrasting pictures on their source heterogeneity. Understanding the role of pyroxenite and eclogite during partial melting bears critical information regarding the fate of recycled lithospheric material, the dynamics and timescales of mantle convection and the thermal regime of mid-ocean ridges.

We present a numerical approach based on the thermodynamically constrained Mixed-Source Melting model (MSM3), enabling a coherent assessment of the role of recycled lithologies. Within a comprehensive plate tectonic cycle, the MSM3 model simulates the two-stage recycling of eclogites derived from subducted oceanic crust in a marble-cake mantle.

• Stage 1 corresponds to the formation of secondary pyroxenite from the hybridization of high-degree eclogite-derived melts interacting at high pressure with peridotite in the convective mantle.
• Stage 2 corresponds to the formation of MORB in a triangular melting regime from the adiabatic decompression melting of a 3-lithology source of peridotite, pyroxenite and residual eclogite obtained from stage 1.

To tackle the diversity of geochemical proxies applied to oceanic basalts, MSM3 recovers melt and residual compositions in terms of major elements and sulfur, as well as any lithophile and chalcophile trace elements and isotope systems. This is achieved thanks to the integration of melting models with pMELTS calculations constrained by a thermodynamic parametrization specific to pyroxene-rich lithologies (Melt-PX), calculations of sulfur concentration at sulfide saturation (SCSS), and composition-dependent partition coefficients. To take into account the inherent variability of most parameters (e.g., potential temperature, source proportions, sulfur contents) and avoid arbitrary choices, we use a stochastic approach by running the MSM3 model as an inversion based on adaptative Monte Carlo simulations.

We here demonstrate the flexibility of this approach, even for systems controlled by sulfides. We show that, over potential temperatures ranging between 1280 and 1420 ºC, the generation of 0-10% of pyroxenitic heterogeneities from subducted eclogite, and the contribution of both eclogite and pyroxenite in the melting regime of MORB produce 20-95 % of the melts aggregated at the ridge. Such proportions correspond to up to 30 times the proportion of these lithologies in the mantle. This over-contribution is controlled by the melting regime properties and is enhanced or attenuated by the mass balance specific to the elements and isotope systems considered (concentrations, partitioning behavior, modal evolution of the main host minerals in the different lithologies). In other words, the more-fusible pyroxenite and eclogite act as chocolate in the marble-cake mantle, giving the dominant flavor to its melting products, although different geochemical proxies may "taste" it differently.