Precise determination of Mg/Fe ratios applicable to terrestrial olivine samples using Raman spectroscopy

The relationship between Mg# [ = 100 Mg/(Mg + Fe) in mol] and the Raman shift was analyzed precisely for olivine [(Mg, Fe)₂SiO₄] samples with Mg# between 100 and 62.8. Two prominent peaks at 826-820 cm ^-1 (peak 1) and 858-849 cm^-1 (peak 2) and three subordinate peaks at 883-881 cm^-1 (peak 3), 920-914 cm^-1 (peak 4), and 967-951 cm^-1 (peak 5) were observed to shift monotonously to lower wavenumbers with decreasing Mg#. The ΔMg#(= Mg# _ref - Mg#) versus Δν(= ν_ref - ν) can be linearly regressed for each peak as ΔMg# = A Δν, where ν is a peak wavenumber of olivine with Mg# ranging from 100 to 62.8, and ν_ref is that of olivine with a reference value of Mg#, namely, Mg#_ref. We set Mg#_ref as 100 (i.e.pure forsterite Mg ₂SiO₄) whereas A is a regression parameter (5.789, 4.294, 12.34, 6.348, and 2.09, respectively,for peaks 1, 2, 3, 4, and 5). This equation enables us to avoid small inter-laboratory differences of wavenumber calibration. The equation for peak 2 yields estimations of Mg# in geologically satisfactory precision, ± 1 Mg# (1σ) in the Mg# range of 100-62.8.