Alteration of undisturbed igneous material used for argon dating work, is the most common cause of incorrect (low) estimates of the time of crystallization. Identification of alteration has relied on qualitative and subjective (optical) methods. For 40 Ar/ 39 Ar dating, I introduce a new parameter—the alteration index (A.I.)—to quantitatively assess alteration. This looks to the quantity of 36 Ar (atmospheric argon) released in such studies. A non-dimensional parameter is used, relating the 36 Ar levels to that of 39 Ar for K-rich phases (K-feldspar, biotite, whole-rock basalt), and to 37 Ar for Ca-rich phases (plagioclase feldspar and hornblende). Water contains large amounts of dissolved argon derived from the atmosphere. During chemical weathering, 36 Ar carried by water is introduced into the silicate phases of rocks. All common alteration minerals contain water; their 36 Ar contents are ∼100–1000 times higher than in anhydrous silicate phases. Incipient alteration, undetected by current tests, is unequivocally recognized by the A.I. method. In 40 Ar/ 39 Ar stepheating studies, the plateau steps (if any) release argon from the least altered sites. The A.I. of plateau steps for fresh, subaerial, material yields the cut-off value for detecting alteration. Partial loss of 40 Ar* from altered samples may result in statistically acceptable plateaus that underestimate the true crystallization age by ∼2–10%. Many ages are invalid as accurate estimates of the age of crystallization (a) based on statistical analysis of the apparent ages on plateau/isochron plots and/or (b) ages derived from altered phases within the sample. At subduction zones, the hydrated slab cycles substantial quantities of (atmospheric) argon into the mantle. Monitoring 36 Ar levels in fresh (mafic and intermediate) rocks should serve as a sensitive tool in elucidating the role of water driven off the subducted slab in triggering magmatism in convergent zone settings.