Metamorphic Reactions and Mineral Assemblages
Metamorphic reactions consume and produce minerals as pressure and temperature change, generating the assemblages that record a rock's metamorphic history.
Definition
The chemical reactions among minerals during metamorphism and the equilibrium mineral assemblages they produce, used to deduce the pressure-temperature conditions and paths of metamorphic rocks.
Scope
This topic covers the types of metamorphic reactions, dehydration, decarbonation, solid-solid, and continuous reactions, the construction of petrogenetic grids, the AKFM and ACF chemographic projections used to portray assemblages, and geothermobarometry, the use of mineral compositions to quantify the pressure and temperature of equilibration.
Core questions
- What kinds of reactions drive mineral changes during metamorphism?
- How do petrogenetic grids predict stable assemblages?
- How are chemographic diagrams used to read assemblages?
- How does geothermobarometry quantify peak conditions?
Key theories
- Continuous and discontinuous reactions
- Metamorphic minerals change through discontinuous reactions that occur at fixed conditions for a given assemblage and continuous reactions in solid-solution minerals that proceed over a range, together defining the petrogenetic grid for a bulk composition.
- Geothermobarometry
- Because the partitioning of elements between coexisting minerals and the positions of reaction equilibria depend on pressure and temperature, measured mineral compositions can be inverted to estimate the conditions of equilibration.
Clinical relevance
Reaction analysis and geothermobarometry are the quantitative core of metamorphic petrology, enabling reconstruction of pressure-temperature-time paths that reveal burial and exhumation rates and constrain the dynamics of mountain building.
History
The thermodynamic treatment of metamorphic reactions, developed through the mid-twentieth century, was synthesized in petrogenetic grids and internally consistent thermodynamic datasets; Spear's 1993 treatise codified phase equilibria and pressure-temperature-time path analysis for metamorphic rocks.
Key figures
- Frank S. Spear
- Kurt Bucher
- Bernard Yardley
Related topics
Seminal works
- spear1993
- bucher2011
- winter2013
Frequently asked questions
- What is a dehydration reaction?
- A metamorphic reaction that breaks down hydrous minerals such as micas and amphiboles and releases water as temperature rises, producing anhydrous minerals and a fluid phase.
- How can a rock record its pressure-temperature path?
- Mineral zoning, inclusions, and partially reacted assemblages preserve evidence of earlier conditions, which combined with geothermobarometry let petrologists reconstruct the path of burial and exhumation.