Pyrite grains can act as K-Ar time capsules, by armoring the minute inclusions hosted within the pyrite, thus preventing loss of their radiogenic argon. This pyrite clock provides the means to apply single-crystal laser 40Ar-39Ar dating to rocks that typically resist straightforward dating, notably sedimentary rocks. We have applied the technique to a banded iron formation from Earth's oldest sequence of sedimentary and volcanic rocks, the Isua greenstone belt of west Greenland. We find that, in spite of having been subjected to several metamorphic and metasomatic events, the mineral inclusions in pyrites from these rocks give remarkably well-defined and reproducible ages at 3.86 ¿ 0.07 Ga, in agreement with previous determinations of the age of their host rocks. Moreover, the isotopic data from these pyrites yield distinct indications of inclusions giving an age of 4.31 ¿ 0.06 Ga, possibly providing the first direct evidence for a much earlier crust at Isua.