Synthesis of majorite from omphacitic glass at deep mantle conditions and the rheology of the subducting oceanic crust

The transport of chemical elements through subduction into the Earth’s transition zone and lower mantle is strongly affected by the stability of mineral phases at high pressure that, in turn, controls the rheology of the subducting slab. During subduction of the oceanic crust, clinopyroxene (Cpx) reacts with coexisting garnet to form majorite. However, the slow solid-state diffusion associated with this reaction could cause metastable Cpx to be stable up to P and T conditions where it may alternatively decompose to form lower mantle mineral assemblages with important implications for the density of the subducted oceanic crust. Previous experimental studies on the stability of omphacitic Cpx as a function of pressure showed that decomposition occurs to an assemblage of majorite + Ca-perovskite + stishovite, or formation of an unknown post-clinopyroxene phase. However, experiments were performed at the high temperatures of the convecting mantle rather than slab thermal regimes. Here, we discuss recent results of multi anvil experiments on the crystallization products of synthetic omphacitic glass at ~18 and ~25 GPa and 1000 °C to simulate P-T regimes of cold subduction. Synthesis runs were carried out using the Walker-type multi anvil press available at the National Institute of Geophysics and Volcanology (INGV) in Rome. The recovered samples were analyzed by field-emission scanning electron microscopy, powder X-ray diffraction and Raman spectroscopy for chemical and textural characterization. Single crystal X-ray diffraction on selected grains allowed to study in more detail the structure of the recovered minerals. Our results show evidence of direct crystallization of Na-, Si-rich majorite in contrast with the generally proposed formation of high-pressure phases like Ca-perovskite, Mg-perovskite (bridgmanite) and stishovite. To understand the effect of the Cpx → Na,Si-rich transformation on the rheology of the subducted slab, we used our results to model the density, longitudinal (VP) and shear (VS) wave velocities of mid-ocean ridge basalt (MORB) at the pressures of the mantle transition region using elastic parameters of major minerals available in the literature. This study provides the evidence that an increase in pressure during cold subduction of a clinopyroxenerich portions of the slab results in majorite crystallization through isochemical transformation, which causes an increase in local density by 6% favoring, therefore, the continued floating of the slab.