CO2-H2O fluid inclusions in forsterite: An experimental study
Lüttge, Andreas; Metz, Paul; Walther, Johannes; Althaus, Egon; Heinrich, Wllhelm
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 997 - 1014
published: Oct 30, 1996
manuscript accepted: Apr 17, 1996
manuscript received: Aug 1, 1994
ArtNo. ESP147050805010, Price: 29.00 €
Abstract Primary inclusions (10-20 μm) of CO2-H2O fluids were produced in crystals of forsterite synthesized in a conventional hydrothermal apparatus by the reaction: 1 tremolite + 11 dolomite ⇔ 8 forsterite + 13 calcite + 9 CO2 + 1 H2O Stoichiometric mixtures of dolomite and tremolite powders (grain size fraction 80-100 μm) were used to form the forsterite and calcite. Experiments were performed at P = 500 MPa, T = 670°C with a CO2-H2O fluid in the range XCo2 = 0.10-0.16, and at 720QC with an XCo2 of 0.40-0.44. Petrographic microscopy reveals that many of the synthesized forsterite crystals contain fluid inclusions. The results of microthermometric analysis show that these forsterite hosts formed at 670°C trap three different types of inclusions: l) inclusions representing the bulk composition of the fluid phase present during the runs (XCo2 = 0.10-0.16); 2) inclusions having slightly smaller amounts of CO2 (XCo2 = 0.08) than the starting composition, and 3) inclusions having a significantly higher amount of CO2 (XCo2 = 0.26) than the bulk composition at the end of the run. Fluid inclusions in forsterite hosts produced at 720QC always contain a significantly higher CO2 concentration (XCo2 = 0.47-0.60) than the bulk composition of the fluid (XCo2 = 0.40-0.44). The different sites of forsterite nucleation and growth combined with weak convection of the fluid can explain the different fluids trapped in forsterite. The deviation of the trapped fluid from the bulk composition demonstrates that the fluid phase was inhomogeneous over distances of millimeters or tenths of millimeters, indicating that microsystems with fluids of different composition existed within the mineral powders of the experiments at least during the period of formation of the single fluid inclusions. Microsystems with fluids of different composition probably account for the different neighboring microstructures seen in natural rocks.