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The Petrogenetic Model, an Extension of Bowen's Petrogenetic Grid*

Published online by Cambridge University Press:  01 May 2009

Peter J. Wyllie
Peter J. Wyllie
Affiliation:
Department of Mineralogy, College of Mineral Industries, The Pennsylvania State University, University Park, Pennsylvania.
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Abstract

Bowen's petrogenetic grid is a PT projection containing univariant curves for decarbonation, dehydration, and solid-solid reactions, with vapour pressure (Pf) equal to total pressure (Ps). Analysis of experimental data in the system MgO–CO2–H2O leads to an expansion of this grid. Three of the important variables in metamorphism when Pf = Ps are P, T, and variation of the pore fluid composition between H2O and CO2. These can be illustrated in a three-dimensional petrogenetic model; one face is a PT plane for reactions occurring with pure H2O, and the opposite face is a similar plane for reactions with pure CO2; these are separated by an axis for pore fluid composition varying between H2O and CO2. Superposition of the PT faces of the model provides the petrogenetic grid. The reactions within the model are represented by divariant surfaces, which may meet along univariant lines. For dissociation reactions, the surfaces curve towards lower temperatures as the proportion of non-reacting volatile increases, and solid-solid reaction surfaces are parallel to the vapour composition axis and perpendicular to the PT axes. The relative temperatures of reactions and the lines of intersections of the surfaces can be illustrated in isobaric sections. Isobaric sections are used to illustrate reactions proceeding at constant pressure with (1) pore fluid composition remaining constant during the reaction, with temperature increasing (2) pore fluid composition changing during the reaction, with temperature increasing, and (3) pore fluid changing composition at constant temperature. The petrogenetic model provides a convenient framework for a wide range of experimental data.

Type
Articles
Information
Geological Magazine , Volume 99 , Issue 6 , November 1962 , pp. 558 - 569
Copyright
Copyright © Cambridge University Press 1962

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Footnotes

*

Contribution No. 62–5 from The College of Mineral Industries.

References

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