Syn‐Shearing Deformation Mechanisms of Minerals in Partially Molten Metapelites

• Published in: Geophysical research letters 2021-11, Vol.48 (22), p.n/a
• Main Authors: Dutta, Dripta, Misra, Santanu, Mainprice, David
• Format: Article
• Language: English
• Subjects: Aggregates; Backscatter; Biotite; Cordierite; CPO; Crystal growth; Crystallography; Crystals; Deformation; Deformation mechanisms; Diffraction; Earth Sciences; EBSD; Electron backscatter diffraction; Feldspars; Grain boundary sliding; Grain size; High pressure; Ilmenite; metapelite; Mica; Minerals; misorientation analysis; Mullite; Muscovite; partial melt; Plastic deformation; Quartz; Rock; Rocks; Scanning electron microscopy; Sciences of the Universe; Shear strain; Shearing; Strain; Stress transfer; torsion experiment
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL094667

We investigated an experimentally sheared (γ = 15, γ˙ = 3 × 10−4 s−1, 300 MPa, 750°C) quartz‐muscovite aggregate to understand the deformation of parent and new crystals in partially molten rocks. The scanning electron microscope and electron backscatter diffraction analyses along the longitudinal axial section of the cylindrical sample suggest that quartz and muscovite melted partially and later produced K‐feldspar, ilmenite, biotite, mullite, and cordierite. Quartz grains became finer, and muscovite was almost entirely consumed in the process. With increasing γ, melt and crystal fractions decreased and increased, respectively. Among the new minerals, K‐feldspar grains (highest area fraction and coarsest) nucleated first, whereas cordierite and mullite grains, finest and least in number, respectively, nucleated last. Fine grain size, weak crystallographic preferred orientations, low intragranular deformation, and equant shapes suggest both initial and new minerals deformed dominantly by melt‐assisted grain boundary sliding, which is further substantiated by higher misorientations between adjacent grains of quartz, K‐feldspar, and ilmenite. Plain Language Summary The processes governing the deformation of minerals in partially molten rocks are poorly understood as we generally only see the end product. To focus light on this, we sheared quartz and muscovite aggregate to a large shear strain at high pressure and temperature, where these two minerals underwent partial melting and produced new minerals. Electron backscatter diffraction based microstructural investigations of an experimentally sheared partial melt reveal that even at elevated pressure and temperatures, and significant magnitude of deformation, the presence of melt, together with strain partitioning and low intergranular stress transfer, inhibited intragranular plastic deformation in the remaining starting materials and the newly grown crystals. Key Points In an HPT torsion experiment (γ = 15), quartz‐muscovite melted partially and produced K‐feldspar, ilmenite, biotite, mullite, and cordierite Quartz grain size reduced, muscovite was consumed entirely, K‐feldspar grains nucleated first while mullite/cordierite nucleated last Melt‐assisted grain boundary sliding was the dominant deformation mechanism for the reactants and “in‐situ” melt‐crystallized phases