Thermochronology

Introduction

Thermochronology is a branch of geochronology that involves the study of the thermal history of rocks, minerals, and sediments. It uses temperature-sensitive radiometric age dating methods to measure the timing of thermal events in the geological past. This field of study is crucial in understanding the thermal evolution of the Earth's crust and the processes that shape it, such as tectonics, erosion, and climate change More on Geochronology.

Principles of Thermochronology

Thermochronology is based on the principle that certain isotopic systems in minerals are sensitive to temperature changes. When a rock cools below a certain temperature, known as the closure temperature, the isotopic system in a mineral within the rock becomes closed, and the parent and daughter isotopes are no longer lost or gained. The time at which this occurs is known as the cooling age, which can be determined by measuring the ratio of parent to daughter isotopes in the mineral. The closure temperature varies for different isotopic systems and minerals, allowing for the determination of a range of thermal histories.

Techniques in Thermochronology

There are several techniques used in thermochronology, each based on different isotopic systems and minerals, and each sensitive to a different range of temperatures. These include (U-Th)/He dating, fission track dating, and Ar/Ar dating More on Fission track dating.

(U-Th)/He Dating

(U-Th)/He dating involves the measurement of helium produced by the decay of uranium and thorium in minerals such as apatite, zircon, and titanite. This technique is sensitive to temperatures between 40 and 200 degrees Celsius, making it useful for studying the uppermost few kilometers of the Earth's crust.

Fission Track Dating

Fission track dating is based on the tracks produced by the spontaneous fission of uranium-238 in minerals such as apatite, zircon, and sphene. This technique is sensitive to temperatures between 60 and 120 degrees Celsius, making it useful for studying the middle to upper crust.

Ar/Ar Dating

Ar/Ar dating involves the measurement of argon produced by the decay of potassium in minerals such as mica, feldspar, and hornblende. This technique is sensitive to temperatures between 300 and 500 degrees Celsius, making it useful for studying the lower crust and upper mantle.

Applications of Thermochronology

Thermochronology has a wide range of applications in the geosciences. These include the study of mountain building, basin formation, fault activity, erosion rates, and climate change.

Mountain Building

Thermochronology can be used to study the timing and rate of mountain building. By dating minerals from different depths within a mountain range, it is possible to determine when and how quickly the mountains were uplifted.

Basin Formation

Thermochronology can also be used to study the formation of sedimentary basins. By dating minerals in sedimentary rocks, it is possible to determine when the basin formed and how quickly it subsided.

Fault Activity

Thermochronology can be used to study the activity of faults. By dating minerals in rocks that have been displaced by a fault, it is possible to determine when the fault was active.

Erosion Rates

Thermochronology can be used to study erosion rates. By dating minerals in sediment, it is possible to determine how quickly the landscape is eroding.

Climate Change

Thermochronology can be used to study past climate change. By dating minerals in sedimentary rocks, it is possible to determine past temperatures and precipitation levels.