The Joule-Thomson expansion of CO2 and H2O in geothermal and volcanic processes

In kimberlites and other volcanoes whose gas or fluid phase is CO2-rich, it has been generally assumed that gas emission during eruption/emplacement would produce a large cooling effect due to gas free-expansion. However, during protracted eruptions, gas emission may be more analogous to an isenthalpic Joule-Thomson expansion. A review of the chemical engineering data and use of various equations of state discloses that CO2 expansions from the geothermal gradient (or hotter) with P > 600 bars will produce a heating effect. Under similar expansion conditions H2O will produce a cooling effect. The implications for volcanic-and geothermal environments rich in CO2 are discussed.