LARRYN W. DIAMOND
University of Bern, Baltzerstrasse 1, CH-3012 Bern, Switzerland
The following sequences of phase transitions are deduced to be feasible upon heating multicomponent fluid inclusions:
(1) liq -> liq1 + liq2 -> liq;
(2) liq + vap (or liq2) -> liq -> liq + vap (or liq2);
(3) liq + vap -> liq1 + liq2 + vap -> liq + vap;
(4) liq + vap -> liq1 + liq2 + vap -> liq1 + liq2;
(5) sol + liq + vap -> sol + liq -> sol + liq + vap;
(6) liq1 + liq2 -> liq -> liq1 + liq2 -> liq.
Only some of these sequences have been reported so far from natural inclusions. Presumably the remaining sequences have not been found because, following conventional wisdom, systematic observations have never been made at temperatures above the first total homogenisation transition. Investigators are therefore urged to conduct such measurements in the light of this new result and to make use of new technology to inhibit decrepitation.
Fluid inclusions which display the above sequences cannot be interpreted in the same way as inclusions with only one intersection of an immiscibility boundary. If the assemblage of inclusions shows petrographic evidence for homogeneous trapping, there is no way to deduce from the inclusion measurements alone, on which segment of the isochore the inclusion was trapped. Conversely, if petrography indicates heterogeneous trapping, microthermometry does not yield a unique formation temperature. Rather, there may be up to 3 possible P-T points of entrapment.
Based on the concept of global phase diagrams and on analogues with better known fluids, speculations are made on the topology of the CO2-H2O-NaCl and similar systems. It seems likely that the immiscibility field closes at high temperatures (e.g. Figure 1), thereby allowing the region of high-pressure, low-temperature liquid to join the vapour field at low pressures and high temperatures. These systems may therefore exhibit type 6 behaviour listed above.