In situ quantitative analysis of individual H 2O–CO 2 fluid inclusions by laser Raman spectroscopy

Abstract

Raman spectral parameters for the Raman ν 1 (1285 cm − 1 ) and 2 ν 2 (1388 cm − 1 ) bands for CO 2 and for the O–H stretching vibration band of H 2O (3600 cm − 1 ) were determined in H 2O–CO 2 fluid inclusions. Synthetic fluid inclusions containing 2.5 to 50 mol% CO 2 were analyzed at temperatures equal to or greater than the homogenization temperature. The results were used to develop an empirical relationship between composition and Raman spectral parameters. The linear peak intensity ratio (IR = ICO 2/(ICO 2 + IH 2O)) is related to the CO 2 concentration in the inclusion according to the relation: Mole % C O 2 = e − 3.959 IR 2 + 8.0734 IR where ICO 2 is the intensity of the 1388 cm − 1 peak and IH 2O is the intensity of the 3600 cm − 1 peak. The relationship between linear peak intensity and composition was established at 350 °C for compositions ranging from 2.5 to 50 mol% CO 2. The CO 2–H 2O linear peak intensity ratio (IR) varies with temperature and the relationship between composition and IR is strictly valid only if the inclusions are analyzed at 350 °C. The peak area ratio is defined as AR = ACO 2/(ACO 2 + AH 2O), where ACO 2 is the integrated area under the 1388 cm − 1 peak and AH 2O is the integrated area under the 3600 cm − 1 peak. The relationship between peak area ratio (AR) and the CO 2 concentration in the inclusions is given as: Mole % C O 2 = 312.5 AR The equation relating peak area ratio and composition is valid up to 25 mol% CO 2 and from 300 to 450 °C. The relationship between linear peak intensity ratio and composition should be used for inclusions containing ≤ 50 mol% CO 2 and which can be analyzed at 350 °C. The relationship between composition and peak area ratios should be used when analyzing inclusions at temperatures less than or greater than 350 °C (300–450) but can only be used for compositions ≤ 25 mol% CO 2. Note that this latter relationship has a somewhat larger standard deviation compared to the intensity ratio relationship. Calibration relationships employing peak areas for both members of the Fermi diad ( ν 1 at 1285 cm − 1 and 2 ν 2 at 1388 cm − 1 ) were slightly poorer than those using only the 2 ν 2 (1388 cm − 1 ) member owing to interference from quartz peak at approximately 1160 cm − 1 . The technique has been applied to natural low-salinity H 2O–CO 2 inclusions from the Butte, Montana, porphyry copper–molybdenum deposit. Carbon dioxide concentrations obtained range from below detection to 4.2 mol% CO 2, and are in good agreement with concentrations determined previously based on microthermometric and petrographic observations.