How precisely can the temperature of a fluid event be constrained using fluid inclusions?

Abstract

Fluid inclusions in clearly defined fluid inclusion assemblages (FIAs) from various geologic environments were examined to assess the uncertainty associated with determining the temperature of a fluid event based on fluid inclusion homogenization temperatures (Th). A fluid event is defined as a physical or chemical process such as the healing of a microfracture or the formation of a growth zone in a crystal that occurs in the presence of a fluid phase and results in trapping of fluid inclusions to form an FIA. Examination of data from a large number of fluid events collected within a rigorous temporal and spatial (paragenetic) framework forms the basis for developing a complete fluid pressure-temperature-composition (PTX) history. The range in homogenization temperatures of fluid inclusions within well-constrained FIAs was determined, and the minimum (smallest) range in Th, the median range in Th, and the first quartile (Q1 at 25%) and third quartile (Q3 at 75%) of the median Th ranges were calculated for different fluid environments, including the following: 1. Low-permeability sedimentary environments: 49 out of 144 FIAs show a range in Th of ≤1°C; the median range = 2°C (from Q1 = 1°C to Q3 = 3.7°C). 2. Mississippi Valley-type deposits: 11 out of 137 FIAs show a range in Th of ≤1°C; the median range = 4.1°C (from Q1 = 2.3°C to Q3 = 8.3°C). 3. Epithermal deposits: 102 out of 923 FIAs show a range in Th of ≤1°C; the median range = 9°C (from Q1 = 3.8°C to Q3 = 19°C). 4. Porphyry-type deposits: 24 out of 271 FIAs show a range in Th of ≤ 1°C; the median range = 15°C (from Q1 = 8°C to Q3 = 30°C). 5. Orogenic Au deposits: 21 out of 231 FIAs show a range in Th of ≤1°C; the median range = 8.7°C (from Q1 = 4°C to Q3 = 20°C). While all environments show some FIAs in which all the fluid inclusions homogenize at essentially the same temperature (range = ≤1°C), we propose that the median range in Th reported here represents a reasonable and achievable constraint on the uncertainty associated with the temperature of a fluid event in the environments examined. In summary, the temperature of a fluid event, as represented by the range in Th of the fluid inclusions within an individual FIA, can be constrained to better than 15°C in all environments examined, and in Mississippi Valley-type and low-permeability (deep) sedimentary basin environments, the range in Th can be constrained to better than 2°C. The processes that produce variability in Th of fluid inclusions within an FIA are many and include natural variations in temperature, pressure, or fluid composition during trapping of the FIA, trapping of immiscible fluids, various forms of reequilibration in nature such as necking, stretching, and leakage, and modification of the inclusions during sample preparation and data collection. If the range in homogenization temperature for an individual FIA is found to be greater than the median range determined here for that environment, then assessment of the cause of the variability might provide useful information concerning the trapping and post-trapping history of the sample.