Examination of 46 thin-sections of Precambrian metamorphic and igneous rocks from three deep drill holes in northern New Mexico reveals a narrow depth-temperature range for annealing of pleochroic haloes in biotite. Annealing of the haloes is first observed in samples from a depth of 1850 m (134¿C present temperature) and is complete in samples from a depth of 2120 m (151¿C). The observed temperature of annealing of the haloes is in good agreement with earlier observations of fission track annealing in apatite from the same core samples.