The Effect of Crystal Orientation on <i>In Situ</i> Rb‐Sr Mica Geochronology

Mica crystals have a highly asymmetric structure, arranged in sheets of two tetrahedral layers bonded to a single octahedral layer. The individual sheets are only weakly bonded, leading to perfect basal cleavage. The recent proliferation of in situ laser ablation‐based Rb‐Sr geochronology of mica makes it critical to understand if orientation of the crystal lattice relative to the laser beam may impart a differential ‘matrix’ effect on Rb‐Sr ratios during ablation. Analyses of mica crystals from eight different samples, including biotite, muscovite and phlogopite, mounted with their c ‐axes approximately parallel (flat‐mounted) and perpendicular (vertically mounted) to the incident laser beam, result in dates that are statistically indistinguishable. Moreover, and consistent with previous work, analysis of vertically mounted crystals generally resulted in less within‐spot variation in 87 Rb/ 86 Sr, and is, therefore, the recommended orientation for Rb‐Sr geochronology measurements, where practical.