Micro-Raman mass spectrometry for measuring carbon isotopic composition of carbon dioxide fluid inclusions in mantle-originated minerals

Carbon isotopic compositions (13C/12C) of CO2 on the earth vary by their origin. Therefore, carbon isotopic compositions serve as a fingerprint that is useful for clarification of the origin and global circulation of CO2. We investigated the applicability of micro-Raman spectroscopy for determining carbon isotopic compositions (13C/12C) of minute CO2 fluid inclusions in minerals. This method is nondestructive and has sufficiently high spatial resolution (1 μm) to measure each fluid inclusion independently. Isotopically substituted 13 CO2 gas was prepared by heating 13 C-contended amorphous carbon with CuO. Carbon dioxide samples with various 13C/12C ratios were prepared by mixing 13 CO2 with CO2 from a tank (15 MPa, 3 m3, δ13CPDB = -31.52 ± 0.14‰). The carbon isotopic compositions of the obtained CO2 fluids were determined using a mass spectrometer (delta S; Finnigan MAT GmbH). Each CO2 fluid was introduced into a high-pressure cell (PC-400MS; Syn Co., Ltd.); high-density CO2 fluids were prepared in the cell, and Raman spectra was measured. Raman spectra of CO2 have 12 CO2-origin peaks at about 1285 cm-1 and 1388 cm-1 (ν[12]- and ν[12]+) and a 13 CO2-origin peak at about 1370 cm-1 (ν[13]+). The relationship between carbon isotopic compositions and peak intensity ratios of ν[12]+ and ν[13]+ was calibrated (Arakawa et al. (2007) Appl. Spectrosc. 61, 701-705). Considering several factors affecting the peak intensity ratio, the error in obtained carbon isotopic composition was 2% (20‰). The reproducibility of intensity ratio at the same experimental environment was 0.5% (5‰). Within these error values, we can distinguish biogenic CO2 from abiogenic CO2. Micro-Raman spectroscopy also provides density information of CO2 fluid in mantle derived minerals, and the depth the rock came from in the mantle can be calculated using the equation of state of CO2. Several applications on simultaneous determinations of density and carbon isotopic composition for CO2 inclusions in mantle xenoliths will be shown.