ISOCHORIC PATHS IN IMMISCIBLE FLUIDS AND THE INTERPRETATION OF MULTICOMPONENT FLUID INCLUSIONS

Fluid inclusions in minerals often follow isochoric-isoplethic paths t hrough P-T space, both in nature and during microthermometry. The syst ematics which arise from this behaviour are the basis for interpreting fluid inclusion analyses. Previous studies have claimed that isochori c-isoplethic paths may transect a given phase boundary only once, and natural fluid inclusions have been interpreted accordingly. Herein it is demonstrated on theoretical and experimental grounds that such clai ms are false. Multicomponent fluid inclusions may indeed show the disa ppearance then reappearance of liquid or vapour phases during microthe rmometric heating, without violating the isochoric-isoplethic constrai nts. This result reinstates some formerly discredited observations of natural fluid inclusions; it requires modification of the way in which fluid inclusions are studied by microthermometry; and it opens new po ssibilities to understand the occurrence and geochemical effects of im miscibility at high temperature and pressure in the lithosphere. The f ollowing sequences of phase transitions are deduced to be feasible upo n heating multicomponent fluid inclusions: (1) liq --> liq(1) --> liq( 2) --> liq; (2) liq + vap (or liq(2)) --> liq --> liq + vap (or liq(2) ); (3) liq + vap --> liq(1) + liq(2) + vap --> liq + vap; (4) liq + va p --> liq(1) + liq(2) + vap --> liq(1) + liq(2); (5) sol + liq + vap - -> sol + liq --> sol + liq + vap; (6) liq(1) + liq(2) --> liq --> liq( 1) + liq(2) --> liq. Only some of these sequences have been reported s o far from natural inclusions. Presumably the remaining sequences have not been found because, following conventional wisdom, systematic obs ervations have never been made at temperatures above the first total h omogenisation transition. Investigators are conduct such measurements in the light of this new result and to make use of new technology to i nhibit decrepitation. Fluid inclusions which display the above sequenc es cannot be interpreted in the same way as inclusions with only one i ntersection of an immiscibility boundary. If the assemblage of inclusi ons 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 un ique formation temperature. Rather, there may be up to three possible P-T points of entrapment. Based on the concept of global phase diagram s 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, thereby allo wing the region of high-pressure, low-temperature liquid to join the v apour field at low pressures and high temperatures. Isochores in these systems may therefore exhibit type six behaviour listed above.