Effect of low-temperature phase change on the re-equilibration of brine inclusions hosted in quartz minerals

Abstract

Microthermometric measurement of fluid inclusions should follow the sequence of measuring homogenization temperature (Th) first, followed by the measurement of final freezing point temperature (Tm). However, in practice, this measurement sequence has not always been strictly followed. Many researchers may repeatedly measure the homogenization temperature of fluid inclusions even after the measurement of the final freezing point temperature. This may result in erroneous microthermometric data being obtained. In order to highlight this issue and to emphasize the importance of following the correct microthermometric measurement sequence, this study carried out repeated microthermometric measurements of brine inclusions and quantitatively compared the differences in the microthermomitric data obtained from different microthermomitric measurement sequences. The measurement exercise was aided by petrographic observation of brine inclusions and microscopic laser Raman analysis and fluid inclusion PVTx modelling. The results indicate that prior to the measurement of the final freezing point temperature, the homogenization temperature of brine inclusions can be repeatedly measured with the Th data showing no obvious deviation. During the measurement of the final freezing point temperature, the brine inclusion may undergo stretching or even leakage, and any subsequent Th measurements may yield elevated values, but Tm value remains largely unchanged. Therefore, low-temperature (freezing) phase-change measurement may cause resetting of the original PVTx information recorded. Compared to calcite minerals, brine inclusions hosted in quartz have stronger resistance to the influence of re-equilibration, and higher vapor filling degree and high salinity can offset the influence of brine inclusion re-equilibration to a certain extent. In the process of low-temperature phase change temperature measurement, due to the inflation caused by water freezing, it is common for brine inclusions to undergo stretching or even leakage, leading to a general increase in the homogenization temperature of brine inclusions.

Keywords

• brine inclusion
• microthermometric analysis
• re-equilibration
• pvtx modeling
• vapor filling degree
• salinity