ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Isothermal or isobaric phase diagram sections as a function of fluid composition (X F) are widely used for interpreting the genetic history of metacarbonate rocks. This approach has the disadvantages that: (1) the influence of a key metamorphic variable, either pressure (P) or temperature (T), is obscured; (2) the diagrams are inappropriate for systems that are not fluid-saturated. These problems are avoided by constructing phase-diagram projections in which the volatile composition of the system is projected onto a P-T coordinate frame, i.e., a petrogenetic grid. The univariant curves of such P-T projections trace the conditions of the invariant points of isothermal or isobaric phase-diagram sections, thereby defining the absolute stability of high-variance mineral assemblages, with and without a coexistent fluid phase. Petrogenetic grids for metacarbonate rocks are most useful for the study of regional metamorphism and for systems in which fluid composition has not been externally controlled. A calculated example of a P-T projection for the system CaO−MgO−SiO2−H2O−CO2 suggests that many assemblages (e.g., calcite +tale, enstatite+fluid, magnesite+tremolite, antigorite+diopside+dolomite, and calcite+forsterite+tremolite) in mixed-volatile systems have stability fields that make them useful as P-T indicators. Consideration of the principles governing projection topology demonstrates that the univariant curves around a fluid present invariant point cannot be oriented independently with respect to the direction of compositional variation in the fluid phase. This has the interesting predictive implication that if the direction of compositional variation along one univariant curve around an invariant point is known, then the direction of compositional variation along the remaining curves can be determined solely from topologic constraints. The same constraints can be applied to systems containing simple mineral solutions or melts in order to predict compositional variations.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00307329
Permalink