Tl. Spell et al., Systematics of xenocrystic contamination: preservation of discrete feldspar populations at McCullough Pass Caldera revealed by Ar-40/Ar-39 dating, EARTH PLAN, 190(3-4), 2001, pp. 153-165
Single crystal Ar-40/Ar-39 dating of K-feldspars from silicic volcanic rock
s containing xenocrysts often yields a spectrum of ages slightly older than
those of juvenile sanidine phenocrysts. In contrast, feldspars from thin,
low-volume units of the Tertiary (14 Ma) McCullough Pass Tuff define discre
te age populations at similar to 14 Ma, similar to 15 Ma, and similar to 1.
3 Ga, reflecting the time of eruption, xenocrysts from an older ignimbrite
exposed in the caldera wall, and Proterozoic basement K-feldspars, respecti
vely. Conductive cooling and diffusion modelling suggests preservation of s
uch discrete populations is likely only when xenocrystic material is incorp
orated into the magma very near or at the surface, or is engulfed in thin,
rapidly cooled pyroclastic flows during emplacement. Incorporation of xenoc
rysts into the subvolcanic magma chamber, into thick rhyolite domes or lava
flows, or into large, welded ignimbrite sheets will result in partial or t
otal resetting of the K/Ar isotopic system. Similarly, petrographic evidenc
e such as exsolution lamellae may be homogenized under these conditions but
not in thin ignimbrites. Extremely low diffusion rates for disordering of
the Al-Si tetrahedral siting of basement feldspars suggests that they will
retain their ordered structural state given rhyolitic magma temperatures. T
hus, even when petrographic and K/Ar isotopic evidence For xenocrystic cont
amination is obscured, it may be preserved in the form of Al-Si ordering. (
C) 2001 Elsevier Science B.V. All rights reserved.