A quantitative tool for detecting alteration in undisturbed rocks and minerals; I, Water, chemical weathering, and atmospheric argon
A quantitative tool for detecting alteration in undisturbed rocks and minerals; I, Water, chemical weathering, and atmospheric argon (in Plates, plumes, and planetary processes, Gillian R. Foulger (editor) and Donna M. Jurdy (editor))
Special Paper - Geological Society of America (2007) 430: 285-303
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 (super 40) Ar/ (super 39) Ar dating, I introduce a new parameter-the alteration index (A.I.)-to quantitatively assess alteration. This looks to the quantity of (super 36) Ar (atmospheric argon) released in such studies. A non-dimensional parameter is used, relating the (super 36) Ar levels to that of (super 39) Ar for K-rich phases (K-feldspar, biotite, whole-rock basalt), and to (super 37) Ar for Ca-rich phases (plagioclase feldspar and hornblende). Water contains large amounts of dissolved argon derived from the atmosphere. During chemical weathering, (super 36) Ar carried by water is introduced into the silicate phases of rocks. All common alteration minerals contain water; their (super 36) Ar contents are approximately 100-1000 times higher than in anhydrous silicate phases. Incipient alteration, undetected by current tests, is unequivocally recognized by the A.I. method. In (super 40) Ar/ (super 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 (super 40) Ar (super *) from altered samples may result in statistically acceptable plateaus that underestimate the true crystallization age by approximately 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 (super 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.