Ch. Lo et al., AN ASSESSMENT OF AR-40 AR-39 DATING FOR THE WHOLE-ROCK VOLCANIC SAMPLES FROM THE LUZON ARC NEAR TAIWAN/, Chemical geology, 114(1-2), 1994, pp. 157-178
From electron microprobe, petrographic and image analyses of samples f
rom the northern tip of the Luzon Arc, the effects of various alterati
on reactions, phenocryst and groundmass assemblages on the whole-rock
Ar-40/Ar-39 step-heating spectra have been delineated. The alteration
of primary hornblende to an assemblage of zoisite + chlorite + sphene,
the palagonitization of glass and the crystallization of phillipsite
tends to cause discordant Ar-40/Ar-39 dates in the low-temperature ste
ps of the age spectra. Discordant Ar-40/Ar-39 dates in the high-temper
ature steps appear to result from degassing of inherited or trapped Ar
-40 from coarse-grained phenocrysts. Samples in which most of the K re
sides in groundmass plagioclase tend to yield meaningful plateau dates
, whereas samples in which most of the K resides primarily in palagoni
tized glass tend to yield meaningless plateau and isochron dates. Subs
tantial loss of Ar-39 (13-40%) occurred from irradiated samples prior
to Ar-40/Ar-39 step-heating analysis, which appears to be correlated w
ith the abundance of groundmass and the degree of alteration. This arg
on loss is attributed to thermally-induced argon diffusion from the fi
ne-grained groundmass plagioclase, glass and alteration phases during
the pre-step-heating bakeout.