LASER RAMAN SPECTROSCOPY OF DIFFERENTIAL PARTITIONING IN MIXED-GAS CLATHRATES IN H2O-CO2-N2-CH4 FLUID INCLUSIONS: IMPLICATIONS FOR MICROTHERMOMETRY

Abstract

Combined microthermometric and laser Raman microspectroscopic techniques were used to investigate the compositional effects of gas hydrates (clathrates) on the residual fluid in aqueo-carbonic fluid inclusions. By attaching a Linkam heating-freezing stage to a Raman spectrometer, it was possible to analyse the volatile content of a single inclusion, (composition 49% CO2, 42% N2, 9% CH4, aqueous fill = 0.8) at a variety of temperatures, under the normal conditions of microthermometric analysis. The results reveal that, not only does the volatile composition of the fluid change dramatically on clathration, but it also becomes progressively more depleted in CO2 relative to N2 and CH4 with decreasing temperature, resulting in a composition of 5% CO2, 86% N2, and 9% CH4 at -60°C. This has a major effect on the microthermometric properties of the fluid. The preferential incorporation of CH4 into the clathrate relative to CO2 described by previous workers was not observed here, and the recorded change in composition of the residual fluid is far greater than has previously been described.The reaction kinetics of clathrates, generally assumed to be ''sluggish'', are in fact rapid enough to cause major compositional changes during microthermometric analysis. Besides temperature and pressure, the degree of aqueous fill, equilibration time, and temperature cycling can all affect the composition of the clathrate, and therefore, also the residual fluid.