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The timing and origin of the earliest human colonization of the Americas has been the subject of great debate over the last 100 years and is still a matter of heated discussion today (Renne et al., 2005; Gonzalez et al., 2006a). It is widely accepted that the Clovis culture was the first to migrate into the New World at 13.1 ka (Waters and Stafford, 2007). However, archaeological evidence, in the form of stone tools, linguistics, craniometrics and genetics all suggest that the first Americans were ethnically diverse, and a few sites dated to 15e16 ka challenge the ‘Clovis First’ model (Goebel et al., 2008). Perhaps the most spectacular challenge to the ‘Clovis First’ model was the reported presence of human footprints within a basaltic ash (Xalnene Ash, Valsequillo Basin, Central Mexico), dated to 38.04 8.57 ka using optically stimulated luminescence (OSL; Gonzalez et al., 2006a). However, Renne et al. (2005) challenged the validity of the footprints by dating lapilli from the Xalnene Ash using 40Ar/39Ar and reported an age of 1.30 0.03 Ma (2s). Renne et al. (2005) also reported a reversed palaeomagnetic polarity for the ash, consistent with deposition during chron C1r.2r (Cande and Kent, 1995). Such antiquity casts considerable doubt on the interpretation of the impressions as human footprints. Gonzalez et al. (2006b) questioned the 40Ar/39Ar age and highlighted the heterogeneous nature of the lapilli as a potential problem for 40Ar/39Ar geochronology. The lapilli contain phenocrysts and include xenocrysts. Olivine phenocrysts can be contaminated with excess Ar (ArE; McDougall et al., 1969) and hence the dating of