Grenville Turner is best known for his use of isotopes in the investigation of Solar System history. He invented (with Craig Merrihue) the argon-40/argon-39 method of radioactive dating and pioneered its application to terrestrial and extraterrestrial samples. By this method, Grenville obtained the first precise potassium/argon ages of the Apollo rocks, on which the remarkably complete chrononology of the early events in lunar history has been based, especially the early bombardment and basin formation, and the evolution of mare basalts.

Grenville also invented the argon-38/argon-37 method of cosmic ray dating, which provided ages of young impact craters, making the photogeological age scale of lunar surface evolution quantitative. He developed similar methods for combining noble gas and halogen measurements in terrestrial ground waters. Grenville (with John Reynolds) discovered nucleosynthetic isotope anomalies in xenon present in meteoritic nanodiamonds, which predate the formation of the Solar System. He developed a new mass spectrometer based on resonance ionisation and used it to measure iodine-129/iodine-127 ages of primitive meteorites and to discover xenon from extinct plutonium-244 in ancient terrestrial zircons.