A semi-empirical manganese-in-garnet single crystal thermometer

Abstract

Kimberlite-hosted garnet xenocrysts, derived from the disaggregation of garnet peridotite, retain a record of their mantle source rock, including the temperature of its last equilibration. This information is locked into the garnet during its rapid ascent to the Earth's surface in kimberlite magmas. With the proper geochemical tools, diamond explorers can access the information stored within garnet xenocrysts and find economic diamond deposits. Presented here is a semi-empirical thermometer for peridotitic garnet xenocrysts based on the temperature-dependent exchange of manganese (Mn) between garnet and olivine with the formula: T Mn ( ° C ) = − 1635.48 − 184.8 × ( ln a spess − ln a pyr ) calibrated as a single crystal Mn-in-garnet thermometer. The improved accuracy (± 150 °C) of this new calibration allows for the calculation of mantle residence temperatures for large databases of garnet xenocrysts from electron microprobe data when trace-element analyses (LA-ICPMS or PIXE) required for the nickel (Ni)-in-garnet thermometer are not available.