Collaborative Research: Argon Retentivity and Closure Temperature in Micas and Amphiboles

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EAR-0107054/0106987Foland/DahlPotassium-bearing micas and amphiboles are widely used in 40Ar/39Ar age studies to determine thermochronologic histories and are recognized to have ages that reflect temperature, although the actual mechanism(s) and controls are neither well understood nor well documented. This research will investigate the factors governing Ar retention and 40Ar/39Ar ages of mica and amphibole in slowly-cooled terranes. Relationships in specimens from the Adirondack Lowlands of New York will be studied. The research will test the often-assumed volume diffusion control of argon loss, while also seeking to evaluate alternative mechanisms and factors governing argon retention and ages of individual mica and amphibole crystals. Specific objectives of the research are to clarify: (1) the mechanism(s) of argon loss in nature and the question of volume-diffusion control on argon retention and closure, with specific examination of the roles of stress and fluids (in addition to temperature); (2) the crystal-chemical dependence of retention; and (3) the roles of microstructures and other factors. The research will involve an array of modern microanalytical techniques on individual crystals, including: 40Ar/39Ar and Rb/Sr geochronology, featuring UV laser-ablation Ar microanalysis; chemical microanalysis; transmission electron microscopy for elucidation of microtextures; X-ray diffraction determination of crystal structures; and, analyses of O and H isotopes. This research targets the next level of understanding of these mineral ages and, thus, their use in interpreting, deciphering, and refining crustal histories.