Simultaneous determination of Mg# and residual pressure in olivine using micro-Raman spectroscopy

Abstract

We measured the Raman spectra of Fe-Mg olivine grains with Mg# [= 100 × Mg/(Mg + Fe)] values ranging from 80-100 under a pressure of up to 4 GPa using a diamond anvil cell (DAC). We focused on a relatively weak peak at around 547 cm-1, denoted as peak 0, and two intense peaks at around 825 cm-1 (peak 1) and 857 cm-1 (peak 2). The peak position (κ) of each peak shifted upward linearly as Mg# and pressure increased; the Mg# derivatives (∂κ/∂Mg#) for peak 0, peak 1, and peak 2 under ambient pressure were 0.496, 0.184, and 0.227 cm-1/Mg#, respectively. The pressure derivatives (∂κ/∂P) for peak 0, peak 1, and peak 2 were 1.558-2.08, 3.38- 3.46, and 3.20-3.35 cm-1/GPa, respectively. The value of ∂κ/∂P for peak 0 decreased as Mg# increased, whereas those for peak 1 and peak 2 were almost constant against Mg#. The peak positions of peak 0, peak 1, and peak 2 were formulated as function of mg# and pressure based on the result. we found that when the ranges of Mg# and pressure are limited to 85-100 and 0-1 GPa, the combination of peak 1 reproduced the orginial observed data with an error of ± 0.9 for Mg# and an error of ± 0.09 GPa for residual pressure. We applied these functions to a natural olivine inclusion in a diamond obtained from Internationalnaya pipe, Yakutia, Siberia, Russia, and obtained reasonable values of Mg# and pressure-91.6 ± 0.6 Mg# and 0.32 ± 0.05 GPa, respectively.