HR: 11:50h
AN: V11I-07    [PDF]
TI: Comparison of U-series ages with (U-Th)/He apatite ages at Damavand Volcano, Iran
AU: * Davidson, J P
EM: j.p.davidson@durham.ac.uk
AF: University of Durham, Department of Earth Sciences, University of Durham,, Durham, DH13LE United Kingdom
AU: Thomas, L E
EM: L.E.Thomas@open.ac.uk
AF: The Open University, Department of Earth Sciences, The Open University,, Milton Keynes, MK7 6AA United Kingdom
AU: Stockli, D F
EM: stockli@ku.edu
AF: University of Kansas, 3Department of Geology, University of Kansas, 1475 Jayhawk Boulevard,, Lawrence, KS 66045-7613 United States
AU: Hassanzadeh, J
EM: jamshidh@khayam.ut.ac.ir
AF: University of Tehran, Department of Geology, University of Tehran, PO Box 14155-6466, Tehran, 14155-6466 Iran (Islamic Republic of)
AB: Damavand Volcano, Alborz Mountains, N. Iran is an isolated voluminous ($>$400km$^{3}$) composite volcano, built from small-volume eruptions of trachyandesite. The stratigraphy has been calibrated using Ar-Ar and (U-Th)/He apatite dating to establish a broad volcanic history. Eruptions as young as 7,000 yrs and as old as 445,000 years bracket activity at Damavand; although rocks as old as 1.8 Ma are believed to represent a precursor cone occupying a similar footprint. The lavas and pyroclastic deposits are uniformly porphyritic (fspr + oxide + apatite + px ñ amph ñ bi), representing a very restricted compositional range 57-63% SiO$_{2}$. The ubiquitous presence of large apatite crystals has allowed us to compare potential crystallization ages with those of eruption. Apatite strongly concentrates Th, leading to low U/Th ratios (0.13-0.17) compared with the magmas from which they crystallize (0.24-0.28). By analyzing the ($^{230}$Th/$^{232}$Th) and ($^{238}$U/$^{232}$Th) 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. $70\deg$C - i.e. the age of eruption. The results suggest that\\1. Damavand magmas are characterized by initial ($^{230}$Th/$^{232}$Th) = 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 low ($^{230}$Th/$^{232}$Th) cumulate material lying on the equiline into the whole rock - a suggestion which is consistent with petrographic observations and geochemical data.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting