ABSTRACT

The concept of isotope thermometry, which has been the heart and soul of stable isotope geochemistry since its foundation more than half a century ago, has been constantly tested against the effect of other variables such as pressure, fluid composition and biological processes. Despite the fact that the effect of dissolved salts on isotope partitioning in aqueous systems (the isotope salt effect) was discovered in the early 1950s, soon after carbonate paleothermometry commenced, it took isotope geochemists decades to understand the detail and geochemical significance of the effects, particularly those at elevated temperatures. The effect of pressure, which is potentially large under subduction zone and mantle conditions, has been even more elusive. The 1990s witnessed several significant breakthroughs in experimental and theoretical developments for understanding the effect of these variables on equilibrium isotope partitioning. The controversy of high temperature (>100°C) isotope salt effects was finally resolved. It was demonstrated later that significant pressure effects exist for D/H fractionation between a hydrous mineral and water at 200-600°C and at pressures to 0.8 GPa. Theoretical advances in understanding the isotope pressure effects, on both minerals and water, were also made during this period. Finally, and unexpectedly, the effects of dissolved minerals were proved to be significant on isotopic distributions in aqueous systems at high temperature and pressure (750°C and 1.5 GPa). This article reviews historical development of these "other" effects in isotope thermometry, highlighting significant discoveries of the past decade.