FLUID INCLUSIONS 257 of the inclusion walls This usually occurs without any change in volume Crystallization of material on the walls of inclusions in hydrothermal and metamorphic minerals after trapping is usually considered to be insignificant because of the low solubilities of host minerals, such as quartz, in aqueous or CO2-rich fluids In highly water-soluble minerals, such as halite, or in melt inclusions formed in slow cooling plutonic environments, significant crystallization may occur on the walls of the cavity Although there will be some apparent change in volume after trapping, the situation can be reversed by heating the sample at an appropriate rate in the laboratory The internal recrystallization processes responsible for changing the shape of large irregular inclusions can also cause the inclusion to split up or ‘neck down’ into a number of smaller inclusions This is not usually a problem if necking down has occurred before the development of vapour bubbles or daughter phases in the inclusions, because each necked-down inclusion will still retain the original P–V–T–X properties at the time of trapping Problems arise when necking down has taken place after heterogenization, in which case inclusions with variable phase proportions can develop (Figures 2B and 6) Can a particular fluid inclusion assemblage be linked to a specific stage of crystal development or geological process? If primary (or pseudosecondary) inclusions are present, given the caveats above, they Figure Schematic representation of necking down pro cesses in aqueous fluid inclusions after trapping, showing the development of contraction bubbles at different stages in the necking down process Modified from Roedder E (1984) Fluid inclusions Mineralogical Society of America, Reviews in Mineralogy 12, and Shepherd TJ, Rankin AH, and Alderton DHM (1985) A Practical Guide to Fluid Inclusion Studies London: Blackie and Sons Figure Schematic representation of four different gener ations of fluid inclusions in a quartz cemented sandstone The earliest inclusions (1) are contained within the quartz grains and represent fluid processes prior to sedimentation and diagenesis The latest inclusions (4) cross cut both the sand grains and the quartz cement and are also unrelated to the cementation pro cess Inclusions related to early cementation (2) may occur at the quartz grain boundary and along healed microfractures in the quartz grains Inclusions related to later cementation processes (3) occur within the centre of the cement should represent the primary crystallization conditions and processes responsible for the formation of the host mineral If only secondary inclusions are present, all that can be inferred are the P–V–T–X conditions during some recrystallization processes Several generations of secondary inclusions, each representing a different stage in the evolutionary history of a rock or mineral sample, may be present, and careful petrographical examination is required to distinguish between them Cathodoluminescence studies of quartz or carbonate hosts can sometimes be used to establish relative fluid inclusion chronologies With care and patience, it is sometimes possible to link them to particular stages of crystallization or fluid activity (Figure 7) Fluid inclusion studies on hydrothermal ore deposits are usually carried out on transparent gangue minerals because the associated ore minerals are usually opaque Even when primary inclusions are present, it is critically important to establish a clear mineral paragenesis to ensure that a particular generation of transparent gangue is coeval with a particular stage of ore mineralization Principles of Fluid Inclusion Geothermometry and Geobarometry One of the most important applications of fluid inclusion studies is their use as palaeogeothermometers and palaeogeobarometers This involves the careful measurement of the temperatures at which the heterogeneous contents of the inclusion became