Optimizing Raman Spectral Collection for Quartz and Zircon Crystals for Elastic Geothermobarometry
Authors: Cizina Mayara F. 
Published: January 2021

Raman microspectroscopy is widely used to identify and characterize organic and inorganic
compounds. In the geosciences, Raman microspectroscopy has been used to identify mineral
and fluid inclusions in host crystals, as well as to calculate pressure-temperature (P-T)
conditions using mineral inclusions in host crystals, such as quartz-in-garnet barometry
(QuiG). For thermobarometric applications, the reproducibility of Raman peak position
measurements is crucial to obtain accurate P-T estimates. In this study, we explored how
to optimize Raman spectral collection of quartz and zircon inclusions and reference crystals
by monitoring machine stability and by varying spectral parameters. We also monitored a
reference Hg atomic-emission line derived from fluorescent lights. Factors that we varied
independently included laser source [442 nm (blue), 532 nm (green), 633 nm (red)],
power density (1 to 100%) and acquisition time (3 to 270s). Drifting up to 1 cm-1
occurred within the first hour of powering the laser source, after which spectra were usually
stable for several hours. However, abrupt shifts in peak positions can occur subsequently
that can be either positively or negatively correlated to changes in room temperature
greater than 0.1 °C. The Hg-line showed highly correlated but attenuated directional shifts
compared to quartz and zircon peaks. Varying spectral parameters did not shift Raman peaks
of either quartz or zircon grains. However, some zircon inclusions were damaged at higher
power levels of the blue laser source, likely because of laser-induced heating. We also
used Raman spectra of a quartz inclusion in garnet collected with blue, green, and red
lasers to calculate inclusion pressures ("Pinc"), which were then used to calculate
inclusion entrapment pressures ("Ptrap"). The published maximum pressure for this
rock is c. 0.7 GPa based on thermodynamic calculations. Using a combination of 1, 2, or 3
peaks to calculate Pinc and consequently Ptrap, showed that use of the
blue laser source resulted in the most reproducible Ptrap values for all methods
(0.59 to 0.68 GPa), with precisions for a single method as small as ±0.03 GPa, 2σ). Using the
green and red lasers, some methods of calculating Ptrap gave nearly identical
estimates as the blue laser with similarly good precision (±0.02 GPa for green laser, ±0.03 GPa
for red laser). However, using 1- and 2-peak methods to calculate Ptrap can yield
values that range from 0.52 GPa and 0.53 GPa up to 0.93 GPa and 1.00 GPa for green and red
lasers, respectively. For optimal measurements, we recommend: 1) delaying data collection
approximately one hour after laser startup, or leave the laser on; 2) collecting the Hg-line
simultaneously with Raman spectra of mineral inclusions to correct partially for externally-induced
shifts in peak positions, and either 3a) using the blue laser for either quartz or zircon
crystals for P-T calculations, but for zircon, using very low laser power ( < 12 mW) to avoid
overheating and damaging of zircon inclusions or 3b) using either the green or red laser for P-T
calculations, but to restrict calculations to specific methods. Implementation of our
recommendations should contribute to better precision in elastic geothermobarometry, especially QuiG barometry.