In situ quantitative analysis of individual H2O–CO2 fluid inclusions by laser Raman spectroscopy

Raman spectral parameters for the Raman ν1 (1285 cm−1) and 2ν2 (1388 cm−1) bands for CO2 and for the O–H stretching vibration band of H2O (3600 cm−1) were determined in H2O–CO2 fluid inclusions. Synthetic fluid inclusions containing 2.5 to 50 mol% CO2 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=ICO2 / ( ICO2+IH2O)) is related to the CO2 concentration in the inclusion according to the relation: Mole% CO2 ¼ e−3:959 IR2 þ8:0734 IR where ICO2 is the intensity of the 1388 cm−1 peak and IH2O 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% CO2. The CO2–H2O 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=ACO2/(ACO2+AH2O), where ACO2 is the integrated area under the 1388 cm−1 peak and AH2O is the integrated area under the 3600 cm−1 peak. The relationship between peak area ratio (AR) and the CO2 concentration in the inclusions is given as: Mole% CO2 ¼ 312:5 AR The equation relating peak area ratio and composition is valid up to 25 mol% CO2 and from 300 to 450 °C. The relationship between linear peak intensity ratio and composition should be used for inclusions containing ≤50 mol% CO2 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% CO2. 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 (ν1 at 1285 cm−1 and 2ν2 at 1388 cm−1) were slightly poorer than those using only the 2ν2 (1388 cm−1) member owing to interference from quartz peak at approximately 1160 cm−1

Cryptic metasomatism in clino- and orthopyroxene in the upper mantle beneath the Pannonian region

Clino- and orthopyroxenes in anhydrous spinel peridotite xenoliths from Pliocene alkali basalts of the western Pannonian Basin have been analysed for trace elements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Clinopyroxenes show highly variable mantle normalized REE (rare earth elements) patterns but basically can be classified into three major groups: LREE-depleted, LREE-enriched and U-shaped patterns. As the REE patterns of clinopyroxenes usually reflect the REE patterns of the host peridotite, the three major REE patterns define three geochemically different groups of xenoliths. LREE-depleted xenoliths generally have undeformed protogranular textures, while the more deformed xenoliths with porphyroclastic and equigranular textures have LREE-enriched trace element patterns. The U-shaped pattern is very distinctive and is generally associated with poikilitic textures. The HREE content of the clinopyroxenes suggest that most of the xenoliths experienced less than15% partial melting, with the lowest degree occurring in the LREE-depleted xenoliths, and the highest degree in LREE-enriched xenoliths. Cryptic metasomatism frequently accompanies deformation. Metasomatic enrichment of incompatible trace elements can be observed not only in clinopyroxenes but also in coexisting orthopyroxenes. The metasomatic agents were probably alkaline mafic melts of asthenospheric origin and some may relate to upper Cretaceous alkali lamprophyre magmatism. Geochemical signatures of subduction-related melts or fluids have not been found in the anhydrous LREE-enriched xenoliths, although poikilitic xenoliths with U-shaped normalized REE patterns may indicate the influence of subduction-related melts

Raman spectra of water in fluid inclusions: II. Effect of negative pressure on salinity measurement

International audienceSalinity of fluid inclusions is usually determined by microthermometry, but it becomes unsuitable in case of metastability of the aqueous fluid because of thermodynamics indetermination. Raman microspectrometry of water in individual aqueous fluid inclusions can provide chemical information about fluid composition, in particular the concentration of chloride ions. The regular method consists in correlating the deformation of the OH stretching vibration band of liquid water in the region assigned to hydrogen-bonded OH groups, with chloride concentration. In order to evaluate the ability of Raman spectroscopy to determine salinity of metastable fluid inclusions, the Raman spectra of water trapped in two natural fluid inclusions were recorded at various temperatures in two physical states of the liquid phase, at equilibrium with vapor or metastable at negative pressure. The difference in salinity measured in the two states increased when temperature decreased, i.e. when the intensity of metastability increased. Metastability was then expressed in negative pressure scale (MPa) by thermodynamic modeling of the fluid trapped in the inclusions and correlated to salinity relative difference. The quantification of this effect led us to conclude that salinity expressed in mass% NaCleq. was overestimated of about 1% per 10-15MPa of negative pressure. If the negative pressure can be quantified, it is thus possible to determine the salinity of metastable fluid inclusions by Raman spectroscopy