Thellier

Thellier refers to a set of paleomagnetic laboratory procedures for determining the absolute intensity of Earth's ancient magnetic field from rocks and archaeological materials. It is named after Claude Thellier, who, with colleagues in the 1960s–1970s, developed a systematic approach to recover paleointensity from thermoremanent magnetization. The method is often used in conjunction with the Thellier–Coe protocol, a widely adopted variant.

In practice, samples with remanent magnetization are subjected to stepwise demagnetization by heating in zero field to progressively higher temperatures, removing components of magnetization. After each heating step, the sample is given a known laboratory field to impart a partial thermoremanent magnetization (pTRM) in the same temperature interval. The portion of the original magnetization lost during demagnetization is plotted against the pTRM gained; a Thellier plot is created. In a well-behaved sample, a linear segment yields a slope that is proportional to the ancient field strength, after applying necessary corrections for factors such as non-ideal pTRM acquisition and alteration. Multiple checks, including pTRM checks and consistency tests, are used to assess reliability.

Thellier-type approaches have played a central role in paleointensity research, enabling constructions of age-dependent field intensity curves and aiding geomagnetic reversal studies. Limitations include potential alteration during heating, magnetic mineralogical changes, anisotropy of magnetization, and varying cooling rates, which can introduce biases if not properly controlled.