Textural Development and Melt Topology in Spinel Lherzolite Experimentally Deformed at Hypersolidus Conditions

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Abstract
Knowledge of the geometry and distribution of mantle-derived fluids at depth in the Earth is crucial to our understanding of processes of metasomatism, melting, magma mobilization, and transport, and the geophysical characterization of the Earth's lithospheric and asthenospheric mantles. This study is based on the observed textural development and the distribution and geometry of the melt phase in natural and hot-pressed spinel lherzolites experimentally deformed in compression at hypersolidus conditions in the P-T-fO2 stability field of the assemblage. Under hydrostatic ‘wet’ conditions and upon attainment of hypersolidus temperatures, a melt phase forms within minutes, principally distributed heterogeneously as melt pools associated with Cr-diopside and spinel. The steady-state deformation of spinel lherzolite, in the presence of small melt fractions (1–7 vol.%) and stresses ranging from 100 to 400 MPa, involves a process of fluid-assisted dynamic recrystallization by subgrain rotation and rearrangement of the melt phase as an interconnected network between newly recrystallized neoblasts. Steady flow in the partially melted rocks exhibits power law rheology with stress exponent n = 3. It has been established that the stresses are borne by the solid phases, which deform by dislocation creep and dynamic recrystallization. A unique anisotropic melt topology is observed in the specimens deformed for up to 750 h and longitudinal strains exceeding 30%. The melt forms a network of channels anisotropically wetting recrystallized grain boundaries, at ∼30° to the maximum principal compressive stress σ1 These textures were not observed in previous experimental studies conducted under hydrostatic conditions and/or using isotropic synthetic materials. The results are interpreted as resulting from deformation and the development during steady-state flow of an olivine preferred crystallographic orientation and grain shape anisotropy. Combined with the commonly observed textures and strong olivine crystallographic fabric in naturally deformed mantle samples, this suggests that existing theories on mantle melt morphology may not be valid for mantle conditions and that texturally and/or deformation controlled melt topologies may contribute to anisotropic physical properties of a partially molten zone in the Earth's mantle.

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