Feldspar Raman shift and application as a magmatic thermobarometer

We calibrate the pressure-dependent Raman shift of feldspars by measuring spectra of 9 compositionally diverse plagioclase and alkali feldspars at pressures ranging between 0.1 MPa and 3.6 GPa using a diamond-anvil cell coupled with Raman spectroscopy. We observe up to 12 vibrational modes that are caused by deformation of Si(Al)O-4 tetrahedral chains. The most intense modes are v(22), v(24), and v(25), which are produced by stretching and bending of four-membered Si(Al)-O-Si(Al) rings. Because modes are a product of lattice environments, feldspar composition may relate to mode frequency. We find that the frequencies of the v(25) mode correlates with composition, whereas the other intense bands do not correlate with composition. All feldspar compositions exhibit modes that shift linearly (r(2) > 0.9) to higher frequencies with increasing pressure. Modes v(22), v(24), and v(25) shift to higher frequencies with slopes that range from 1.7 +/- 0.5 to 5.5 +/- 1.6 cm(-1) GPa(-1), and provide the best combination of intensity and pressure-sensitivity. For all compositions the v(22) mode exhibits the most advantageous pressure-dependent (P-T) frequency shift. We use an elastic model, thermodynamic properties, and shear moduli to establish the pressure-temperature dependent sensitivity of feldspar inclusions hosted by garnet, clinopyroxene, and olivine. Raman shifts for all feldspars are <2 cm(-1) for crustal and upper lithosphere conditions. Albitic plagioclase inclusions show the least temperature-sensitive inclusion pressures and provide the best barometers, followed by alkali feldspars and anorthitic plagioclase. Our new calibration allows Raman spectroscopy of feldspars to be used to quantify P-T conditions for crustal magmatic rocks, low-to high-grade metamorphic rocks, and the mantle.