Developing the cosmogenic He chronometer

Helium isotopes have found wide use in geoscience research, notably as chronometers of surface exposure. This thesis comprises two distinct components around the theme of cosmogenic He determinations. In the first section (Chapter 3) I test whether cosmogenic 3He can be used to determine short exposure times (<30 ka) in volcanic rocks that are significantly older than the exposure time (~60 Ma). Helium isotopes were measured in olivine, ilmenite and plagioclase from early Tertiary basalts from Isle of Skye, Scotland in order to determine exposure ages. Correction for radiogenic He was made with three different methods. The upper limit of exposure ages are variable, and slightly lower than previously determined by 36Cl (Stone et al. 1998). In the second part of the thesis I report characterisation of the new ThermoScientific HELIX-SFT mass spectrometer then compare performance against the existing MAP 215-50 instrument. The reproducibility of helium isotope determinations using the HELIX-SFT (~0.78%) is significantly better than the MAP 215-50, though not as good as reported elsewhere. The reasons for this are discussed and the potential for further improvements are highlighted