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Short Description:
Damavand Volcano, Alborz Mountains, N. Iran
Long Description:
Damavand Volcano, Alborz Mountains, N. Iran is an isolated voluminous (>400km3) composite volcano, built from small-volume eruptions of trachyandesite. The stratigraphy has been calibrated using Ar-Ar and (U-Th)/He apatite dating to establish a broad volcanic history. Eruptions as young as 7,000 yrs and as old as 445,000 years bracket activity at Damavand; although rocks as old as 1.8 Ma are believed to represent a precursor cone occupying a similar footprint. The lavas and pyroclastic deposits are uniformly porphyritic (fspr + oxide + apatite + px ñ amph ñ bi), representing a very restricted compositional range 57-63% SiO2. The ubiquitous presence of large apatite crystals has allowed us to compare potential crystallization ages with those of eruption. Apatite strongly concentrates Th, leading to low U/Th ratios (0.13-0.17) compared with the magmas from which they crystallize (0.24-0.28). By analyzing the (230Th/232Th) and (238U/232Th) activity ratios of coexisting apatite-whole rock pairs we can generate a 2-point isochron, which, in the case of simple closed-system crystallization, should represent the age of apatite crystallization. These ages can then be compared with ages from (U-Th)/He apatite analyses, which represent the time at which alpha particles from U and Th decay begin to accumulate as diffusion is effectively stopped when cooling through temperatures of c. 70° C - i.e. the age of eruption. The results suggest that 1. Damavand magmas are characterized by initial (230Th/232Th) = 0.65 - 0.90. 2. There is no significant "residence time" of apatite in the magmas prior to eruption 3. Some of the samples have apparent crystallization ages younger than that of eruption, clearly not a realistic scenario. The young isochron "ages" can be reconciled with eruption ages, if the isochron slopes represent mixing rather than simply a crystallization event. Given that the whole rock samples are de facto mixtures, the most likely scenario is one of mixing relatively
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