Identifying Good Geobarometer Reactions

A geobarometer in geology is a mineral or a mineral assemblage that changes predictably with variations in pressure. These changes allow scientists to estimate the pressure conditions under which rocks formed or were altered. A reaction is considered a good geobarometer if its equilibrium is highly sensitive to pressure changes, involves phases with significantly different molar volumes, and is experimentally well-calibrated.

Analyzing the Reactions

Let's examine each reaction to determine its suitability as a geobarometer:

• Option 1: Andalusite = Sillimanite

  This reaction involves the transformation between two polymorphs of aluminum silicate (Al$_2$SiO$_5$). These polymorphs have different crystal structures and densities, meaning the reaction involves a volume change and is sensitive to pressure. Sillimanite is generally stable at higher pressures than andalusite. However, the stability fields are also strongly temperature-dependent, and solid solution effects can complicate precise barometry.

  Chemical Equation: Al$_2$SiO$_5$ (Andalusite) $\rightleftharpoons$ Al$_2$SiO$_5$ (Sillimanite)

• Option 2: Hematite = Magnetite + O$_2$

  This reaction involves iron oxides and the release or consumption of oxygen. The equilibrium of this reaction is primarily controlled by oxygen fugacity ($f_{O_2}$) rather than pressure. While pressure can have a minor effect, it's not the dominant factor, making it unsuitable as a primary geobarometer.

  Chemical Equation: 2 Fe$_2$O$_3$ (Hematite) $\rightleftharpoons$ 2 Fe$_3$O$_4$ (Magnetite) + 1/2 O$_2$

• Option 3: Annite + Pyrope = Phlogopite + Almandine

  This represents a reaction within the garnet-biotite system. Garnet and biotite compositions are sensitive to both temperature and pressure (geotherms and geobarometry). The partitioning of elements like Mg and Fe between garnet and biotite depends on these conditions. While used in thermobarometry, it often requires complex models and data for multiple components.

  Chemical Equation (simplified end-members): KFe$_3$AlSi$_3$O$_{10}$F$_2$ + Mg$_3$Al$_2$Si$_3$O$_{12}$ $\rightleftharpoons$ KMg$_3$AlSi$_3$O$_{10}$F$_2$ + Fe$_3$Al$_2$Si$_3$O$_{12}$

• Option 4: Anorthite = Grossularite + Kyanite + Quartz

  This reaction involves the breakdown of the feldspar anorthite (CaAl$_2$Si$_2$O$_8$) into garnet (grossularite, Ca$_3$Al$_2$Si$_3$O$_{12}$), the high-pressure polymorph kyanite (Al$_2$SiO$_5$), and quartz (SiO$_2$). The key indicator here is kyanite, which is the densest Al$_2$SiO$_5$ polymorph and forms under significantly higher pressures compared to andalusite or sillimanite. The equilibrium of this specific reaction is strongly dependent on pressure because the products (especially kyanite) generally occupy less volume than the reactant (anorthite) under high-pressure conditions. This makes the reaction shift significantly with pressure changes, providing a reliable basis for pressure estimation in metamorphic rocks.

  Chemical Equation: CaAl$_2$Si$_2$O$_8$ (Anorthite) $\rightleftharpoons$ Ca$_3$Al$_2$Si$_3$O$_{12}$ (Grossularite) + Al$_2$SiO$_5$ (Kyanite) + SiO$_2$ (Quartz)

  This reaction is well-studied and calibrated, making it a classic example of a useful geobarometer in metamorphic petrology.

Conclusion on Geobarometry Suitability

The reaction involving Anorthite breaking down to form Grossularite, Kyanite, and Quartz is widely recognized as a reliable geobarometer. The formation of the high-pressure indicator mineral Kyanite makes this equilibrium highly sensitive to pressure variations, allowing for quantitative estimates of the pressure conditions during metamorphism.