CHARACTERIZATION OF SYNDEFORMATIONAL WATER PATHWAYS AND HYDROLYTIC WEAKENING WITHIN NATURALLY DEFORMED QUARTZ GRAINS ALONG A STRAIN GRADIENT

Infiltration of trace amounts of water into quartz grains may lead to hydrolytic weakening and transition from brittle to ductile deformation, which has important consequences for crustal rheology. Here, we investigate pathways of water infiltration in naturally deformed quartzite clasts sampled from diamictite of the Mineral Fork Formation in northern Utah. The diamictite displays increasing strain intensity and quartz veining along a transect related to progressive subsimple shear in the footwall of the Willard thrust fault. Evidence of water infiltration was investigated utilizing three complimentary techniques: standard petrographic microscopy to identify microstructures and interpret deformation mechanisms; scanning electron microscopy cathodoluminescence (SEM-CL) to create maps of healed microfractures and subgrain boundaries related to pathways of fluid infiltration; and synchrotron-source Fourier-transform infrared spectroscopy (FTIR) to create micron-scale maps of water concentration in quartz grains. Integrated data from these techniques allowed for a comprehensive interpretation of water pathways within quartz grains. At low strain, slightly flattened quartz grains exhibit linear fluid inclusion traces, undulose extinction, subgrains, microfractures, and deformation bands, evidencing healed microfractures and limited crystal plasticity. At moderate strain, increasingly flattened quartz grains have more extensive healed microfracture networks, along with increased undulose extinction and subgrains, recording increased water infiltration and crystal plastic deformation. At high strain, flattened quartz grains exhibit extensive subgrains and recrystallization, documenting intensified plastic deformation and resetting of microstructure with fewer microfractures present. Water infiltrated grains through microfractures during progressive deformation. Water then entered the crystal lattice during annealing, leading to increased crystal plasticity and strain softening in the highly strained samples.