A POTENTIAL GEOLOGICAL BAROMETER:NEW METHOD TO DETERMINE THE PRESSURE WITH DAUGHTER MINERALS IN FLUID INCLUSION BY RAMAN SPECTROSCOPY

In situ measurement of Raman spectroscopy at high temperature and high pressure indicates that the frequency of the Raman peak of some materials, including minerals, molecules and ions, shifts systematically with increasing pressure and temperature. This property has been used as a pressure gauge for high pressure experiment with diamond anvil cell(DAC). Since the system of fluid inclusion is similar to that of DAC, we propose that it also be used to determine the formation pressure for mineral by measuring the Raman shift of the daughter mineral and ion in fluid inclusion in geological and geochemical studies. The method is firstly to increase the temperature of the fluid inclusion to homogeneous temperature heated by heat stage and then to measure the Raman shift for the mineral or ion with Raman active mode of vibration. For ion there is no problem to get the data of the Raman shift at homogeneous temperature. As for the mineral disappearance at homogeneous temperature, the data of Raman shift can be obtained before the mineral disappears by measuring Raman shift at several lower temperatures and then extrapolating them to the homogeneous temperature. By comparing with the previous geological barometer, such as the barometer based on two co-existing minerals or the barometer on the state function of CO2 volume in fluid inclusion, we believe that the proposed method might be a more rapid, convenient and precise method for determining the geological pressure. Because there exist many possible minerals or materials with Raman active mode in the fluid inclusion in minerals, systematic measurement and study should be done to obtain the relationship between the Raman shift of minerals and the temperature and/or pressure by carrying out in situ experimental study at high temperature and high pressure.