SYNTHETIC H2O-CO2 FLUID INCLUSIONS IN SPONTANEOUSLY NUCLEATED FORSTERITE, ENSTATITE, AND DIOPSIDE HOSTS - THE METHOD AND APPLICATIONS

Synthetic H2O-CO2 fluid inclusions have been produced in forsterite, diopside, and enstatite hosts during the spontaneous nucleation and growth of these phases at T = 800-980-degrees-C and P = 6100-7800 bars via the reactions 2 TREM + 18H2O = 4 DIOPSIDE + 3 FORSTERITE + 5Si(OH)4.2H2O (1) TREM + 3H2O = 2 DIOPSIDE + 3 ENSTATITE + Si(OH)4.2H2O. (2) The inclusions are large (10-40-mu-m), abundant, and commonly of regular shape making them ideal as standards for microthermometric and spectroscopic analyses. The technique provides the opportunity to study fluid inclusion trapping mechanisms and their potential effect on inclusion chemistry. The fluid inclusions exhibit a small degree of compositional scatter about the ambient composition. Deviations between the ambient and inclusion composition increase for experiments performed in the presence of low (3-8-degrees-C) thermal gradients (along the capsule length). This observation suggests that compositional heterogeneities may develop in the absence of fluid convection in the capsule. The effect on the inclusion composition, however, does not exceed 3 mol% in XCO2 for all inclusions studied. The advantages of this technique are (1) it provides a means of producing fluid inclusions of known compositions in hosts other than quartz, and (2) inclusion formation is relatively slow, allowing ample time for the P-T-X(fluid) conditions of the run to equilibrate. The disadvantages are that the small grains produced can be difficult to work with, and the host-producing reactions are viable over a relatively narrow range of P-T-X(fluid) conditions.