MANTLE SUCCESSES AND RHYOLITE FAILURES OF PETROLOGY’S NEXT BIG THING: MINERAL-INCLUSION RAMAN THERMOBAROMETRY

Raman thermobarometry can resolve PT conditions of igneous systems. Entrapped and pressurized mineral-hosted solid inclusions have different Raman peak positions than at ambient conditions. This peak shift is used to derive entrapment pressures. We applied this technique to calculate, for the first time, entrapment pressures of 5.2 - 6.3 GPa for forsterite inclusions in diamonds from Guyana, South America; correlated with δ¹³C values of -5.43±1.42 ‰, consistent with upper mantle paragenesis. We present a first application of Raman thermobarometry to clinopyroxene, fayalite, and zircon hosted apatite inclusions from the rhyolitic Lava Creek Tuff, Yellowstone caldera. We analysed 84 apatites using a ThermoScientific Raman spectrometer at Baylor University. Unpressurized matrix and exposed apatite were used to establish standard conditions. Apatite in clinopyroxene and fayalite yielded mean entrapment pressures of >3000 MPa and >1800 MPa respectively, corresponding to deep crustal conditions. Zircon-hosted apatite yielded variable entrapment pressures with a mean of 4990 MPa, and -400 - 13000 MPa range. Raman thermobarometry fails to reproduce the pre-eruptive storage conditions of Yellowstone’s rhyolitic magmas which is likely ~800⁰C and 100 - 300 MPa. Our study provides the first opportunity to document why the technique may fail. First, apatite composition is difficult to measure and is spectrally species specific. Matrix and liberated apatite Raman peaks are inconsistent and may represent spatial or temporal compositional variation throughout Yellowstone’s history, making choosing a suitable reference peak difficult. Second, apatite and host habit differences create anisotropic stresses, unaccounted for in the elastic model. Third, a semi-coherent grain boundary between the host and apatite, can cause apatite to stretch beyond its capacity, detaching it from the host, resulting in anomalous residual pressures. Finally, both apatite and zircon are radioactive minerals, which lattices suffer alpha decay radioactive damage. Apatite 964 cm^-1 peaks were observed to shift to wavenumbers as low as 954 cm^-1 due to radioactive damage. Despite this failure, Raman thermobarometry is still a powerful tool to derive PT conditions without a need for thermodynamic equilibrium or touching mineral pairs.