Alumina ceramic as a mounting medium for electron microprobe analysis and ⁴⁰Ar/³⁹Ar laser microprobe dating of mineral grains

Using a commercial ceramic adhesive, we have developed a technique by which loose mineral grains may be mounted for electron microprobe (EMP) analyses and imaging prior to ⁴⁰Ar/³⁹Ar laser microprobe (LMP) dating. Successively applied and cured layers of the adhesive encase mineral grains aligned along the surface of a glass slide; this slide is subsequently ground off to expose the mineral grains for polishing and EMP analysis. Following removal of the conductive coating, irradiation, and heating to 500°C in vacuo (to drive off atmosphere, hydrocarbons, and moisture) the entire grain mount is introduced into the LMP line. Tests of the ceramic's properties were performed by subjecting 125-500μm-diameter grains of a 35.04±0.84-Ma amphibole to the full preparation and analytical regimen. Resulting isotopic data reveal that the ceramic does not interfere with the Ar systematics of the enclosed grains, but does contribute a large component of atmospheric Ar. Curing the grain mounts in a nitrogen atmosphere, and treating them at 700°C in vacuo prior to dating reduces the atmospheric contribution to negligible levels; using laser energies necessary to fuse amphibole grains, heating of the nitrogen-cured, 700°C-treated ceramic fails to evolve significant quantities of any Ar isotopes. This technique is designed for use in the dating of detrital and authigenic grains, each of which may have a different composition and range of inclusion and alteration phases. It is also applicable to the study of other minerals which must be concentrated or processed as grain separates prior to ⁴⁰Ar/³⁹Ar LMP analysis.