Using Raman Spectroscopy to Test Whether Intracrystalline Chemical Variations Induce Pressure Gradients

In theory, chemical gradients in crystals can induce pressure gradients due to localized volume mismatch during the interdiffusion of large vs. small ions. In garnet, calcium (which is a large cation) commonly shows compositional “steps” from high to low concentrations that theoretically should induce pressure gradients. This study uses the pressure-dependence of Raman peak positions to quantify the magnitude of intracrystalline pressure gradients across calcium compositional steps in garnet. Measurements were collected on traverses both along polished surfaces and at 15-50 µm depth. Peak positions at depth are consistently offset from surface peak positions, indicating the presence of pressure gradients, and pressures change systematically along traverses. Stresses are typically up to a few kbar of compression, but range up to ~9 kbar in one sample and are sometimes tensile. All observations are consistent with theory. Such shifts may help explain unexpectedly large scatter in Raman peak positions for quartz inclusions and provide insight on diffusional reequilibration for calcium relative to iron, magnesium, and manganese. Corrections for stress may improve models of cation diffusion in minerals and thermobarometry using mineral inclusions.