U, Th-²¹Ne, a new dating tool: a case study of apatites from Hogenakal carbonatites

Abstract

Nucleogenic ²¹Ne produced through (a, n) reaction on ¹⁸O in U, Th-rich minerals can be used as a dating tool. We have demonstrated the application of this method to apatite in carbonatite from Hogenakal, South India. Sm-Nd whole rock-mineral isochron of the same hand sample has yielded an age of 2400 Ma and Rb-Sr micawhole rock isochron method has yielded a reset age of 1900 Ma. Using nucleogenic ²¹Ne we derive a gas retention age of 845 ± 127 Ma for the apatite from this carbonatite. This age is considerably younger than the reset Rb-Sr mica (blocking temperature: 350°C) age, but is in good agreement with the younger 770 Ma carbonatite- syenite-alkali granite emplacement event in the region. This system therefore has the potential to identify lowtemperature thermal events and to estimate cooling histories of large igneous complexes.