During the neutron irradiation required for 40Ar/39Ar dating, the 39Ar atoms produced recoil with an average distance of 0.08 μm in silicate minerals (Turner and Cadogan, 1974). Thus, fine-grained materials with high surface area-to-volume ratios, like the iddingsite studied here, may lose 39Ar during the irradiation process, resulting in spuriously old ages (Onstott et al., 1995). The dating of fine-grained materials using the 40Ar/39Ar technique thus requires a non-conventional approach, which consists of micro-encapsulation to prevent loss of recoiled isotopes (Dong et al., 1997). To achieve this, we encapsulated twelve ∼1 μg aliquots of the hand-picked iddingsite in evacuated, flame-sealed quartz glass capsules prior to neutron irradiation. Following irradiation, we measured the Ar isotopic composition of the iddingsite in two stages. First, the glass capsule was cracked under vacuum to measure any recoiled 37Ar and 39Ar. Second, the samples recovered from the cracked glass tubes were then fused with a diode laser. The 37Ar and 39Ar measurements from the cracked tubes were added to the total fusion isotope measurements (36Ar, 37Ar, 38Ar, 39Ar, and 40Ar). Data were corrected for backgrounds, mass discrimination, radioactive decay since irradiation, cosmogenic Ar, and trapped Martian atmospheric Ar.