Some aspects of fluid inclusion geothermometry with particular reference to British fluorites.

The development of fluid inclusion geothermometry is described from its nineteenth century beginnings down to the present time. A method for the preparation of samples for fluid inclusion work and the experimental technique using the Leitz 1350 heating stage are described. The problems involved in obtaining representative geothermometric data are outlined. When a sample consists of well-formed crystals it is relatively easy to determine the thermal history of the sample but not so with massive material. A preliminary investigation was undertaken to assess the variation of data obtained from different sections cut from the one sample and on arbitrary grounds a minimum number of sections and of temperature measurements is suggested so that approximately representative data may be obtained. An investigation of the relationship between filling temperatures of fluid inclusions and the thickness of the enclosing section is described. It was found that on reducing the thickness of a section, the filling temperatures of inclusions decreased. Statistical tests showed the decrease in filling temperatures to be significant and it is suggested that the rate of heating adopted by the writer is too fast to allow equilibrium between the inclusion and the heating stage to be achieved and that the observed filling temperatures are somewhat higher than the actual filling temperatures. A preliminary investigation into the problems of leakage in fluid inclusions is described. Experiments showed that moderate overheating above the filling temperature of an inclusion did not cause leakage but that heating to 100°C. above the filling temperature usually induced leakage. A theory that over-heating may cause fluid to migrate into inter-atomic spaces and that, on cooling, this fluid may move back into the inclusion cavity was not proved. All the recent published works on British geothermometry are reviewed and the published data are compiled as an appendix.