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UPSC MainsGEOLOGY-PAPER-II201115 Marks
Q13.

Define geothermometer and geobarometer with examples from metamorphic petrology.

How to Approach

This question requires defining geothermometer and geobarometer, and illustrating their application within metamorphic petrology. The answer should begin with clear definitions of both terms, emphasizing their role in determining temperature and pressure conditions during metamorphism. Subsequently, specific mineral assemblages and chemical equilibria used as geothermometers and geobarometers should be discussed with examples. The answer should demonstrate an understanding of the principles behind these tools and their limitations. A structured approach, dividing the answer into definitions, geothermometers, geobarometers, and examples, will be effective.

Model Answer

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Introduction

Metamorphic petrology aims to understand the processes that alter pre-existing rocks (igneous, sedimentary, or even other metamorphic rocks) under conditions of elevated temperature and/or pressure. Determining the P-T (pressure-temperature) conditions under which metamorphism occurred is crucial for deciphering the tectonic history of a region. Geothermometers and geobarometers are powerful tools used in metamorphic petrology to estimate these conditions. They rely on the principle that the stability of certain mineral assemblages and the chemical composition of coexisting minerals are sensitive to temperature and pressure, respectively.

Defining Geothermometer and Geobarometer

A geothermometer is a tool used to estimate the temperature at which metamorphic rocks equilibrated. It is based on the principle that the equilibrium between certain mineral pairs or assemblages is temperature-dependent. The position of the equilibrium shifts with temperature, and by analyzing the composition of the minerals involved, we can infer the temperature of formation.

A geobarometer, conversely, is used to estimate the pressure at which metamorphic rocks equilibrated. It relies on the principle that the equilibrium between certain mineral pairs or assemblages is pressure-dependent. Changes in pressure shift the equilibrium, and analyzing mineral compositions allows for pressure estimation.

Geothermometers in Metamorphic Petrology

Mineral Assemblages as Geothermometers

Certain mineral assemblages are indicative of specific temperature ranges. For example:

  • Andalusite-Sillimanite-Kyanite Assemblage: The sequence of appearance of these Al2SiO5 polymorphs with increasing temperature is a classic geothermometer. Andalusite is stable at lower temperatures, kyanite at intermediate temperatures, and sillimanite at higher temperatures.
  • Hornfels Facies: The presence of minerals like cordierite, anthophyllite, and gedrite indicates high-temperature, low-pressure metamorphism.

Chemical Geothermometers

These utilize the temperature dependence of chemical reactions within minerals. Some common examples include:

  • Garnet-Biotite Geothermometer: The partitioning of Mg and Fe between garnet and biotite is temperature-dependent. By analyzing the Mg/Fe ratio in both minerals, the temperature can be estimated.
  • Two-Pyroxene Geothermometer: The exchange of Ca and Mg between two pyroxenes (e.g., augite and orthopyroxene) is also temperature-sensitive.

Geobarometers in Metamorphic Petrology

Mineral Assemblages as Geobarometers

Similar to geothermometers, certain mineral assemblages are indicative of specific pressure ranges:

  • Blueschist Facies: The presence of minerals like glaucophane and lawsonite indicates high-pressure, low-temperature metamorphism, typically associated with subduction zones.
  • Eclogite Facies: The assemblage garnet + omphacite indicates very high-pressure conditions.

Chemical Geobarometers

These rely on pressure-dependent chemical reactions:

  • Garnet-Almandine-Pyrope Barometer: The composition of garnet, specifically the ratio of pyrope (Mg3Al2O6) to almandine (Fe3Al2O6), is sensitive to pressure.
  • Coesite-Quartz Equilibrium: The transition from quartz to coesite (a high-pressure polymorph of SiO2) is a reliable indicator of very high-pressure conditions (above ~2.3 GPa).

Examples of Application

Consider a metamorphic rock from a regional metamorphic belt. Analysis reveals the presence of garnet and biotite. Using the garnet-biotite geothermometer, a temperature of 600°C is estimated. Simultaneously, analysis of the garnet composition using the garnet-almandine-pyrope barometer suggests a pressure of 5 kbar. This information helps constrain the P-T path followed by the rock during metamorphism and provides insights into the tectonic setting.

Tool Principle Example Pressure/Temperature Indication
Garnet-Biotite Mg/Fe partitioning Analysis of Mg/Fe in garnet and biotite Temperature
Garnet-Almandine-Pyrope Pyrope/Almandine ratio Garnet composition analysis Pressure
Blueschist Facies Mineral stability Presence of glaucophane and lawsonite High Pressure, Low Temperature

Conclusion

Geothermometers and geobarometers are essential tools for unraveling the metamorphic history of rocks. By utilizing mineral assemblages and chemical equilibria, geologists can estimate the temperature and pressure conditions under which metamorphism occurred. While these tools are powerful, it’s crucial to acknowledge their limitations, including potential disequilibrium and the influence of bulk rock composition. Combining multiple geothermometers and geobarometers, and integrating them with other geological data, provides the most robust constraints on metamorphic P-T paths.

Answer Length

This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.

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Additional Resources

Key Definitions

Metamorphic Facies
A set of metamorphic mineral assemblages that were formed under similar P-T conditions. Examples include greenschist, amphibolite, granulite, and blueschist facies.
Disequilibrium
A state where a rock has not reached chemical equilibrium, meaning that the mineral compositions do not accurately reflect the P-T conditions of metamorphism. This can lead to inaccurate geothermometric and geobarometric results.

Key Statistics

The Earth's crust is estimated to be approximately 35% metamorphic rocks (as of 2020).

Source: USGS

Approximately 12% of the Earth’s continental crust is composed of rocks that have experienced peak metamorphic temperatures above 500°C (based on knowledge cutoff 2023).

Source: Brown, M. (2015). Introduction to Igneous and Metamorphic Rocks.

Examples

The Himalayas

The metamorphic rocks of the Himalayas provide a classic example of using geothermometers and geobarometers to understand the collision between the Indian and Eurasian plates. The P-T conditions recorded in these rocks reveal the depth and temperature experienced during the orogenic process.

Frequently Asked Questions

▶What are the limitations of using geothermometers and geobarometers?

Limitations include assumptions of equilibrium, the influence of bulk rock composition, retrograde metamorphism altering original compositions, and the potential for fluid infiltration affecting mineral compositions.

Topics Covered

GeologyMetamorphic PetrologyMetamorphismPhase EquilibriaP-T Estimation
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