Jökull - 01.12.2006, Page 70

David W. McGarvie et al.
Figure 5. Five new Ar-Ar ages, plot-
ted against past global mean temper-
atures. Ages are from either rhyo-
lite whole-rock samples (r) or from
feldspars (f). The error bars repre-
sent 1σ uncertainties. The temper-
ature curve is based on a smoothed
dataset from Vostok ice core (Petit et
al., 1999). – Samanburður á 5 aldurs-
greiningum af Torfajökulssvæðinu við
fornhitastig samkvæmt Vostok ískjarn-
anum af Suðurskautslandinu.
eruption wholly during OI stage 11 and by implica-
tion the Holsteinian interglacial, this implication is
the subject of ongoing debate (e.g. Nitychoruk et al.,
2006).
Taking the above into account plus information
from preceding sections, it can be concluded that
Ar-Ar dating of sustained tuya-forming eruptions,
even when they have been eroded, are likely to pro-
vide the most unequivocal palaeoclimatic informa-
tion. Smaller rhyolite eruptions are less useful, be-
cause glaciovolcanic relationships are more equivo-
cal. Finally, a powerful means of understanding the
rapidity with which land-based ice accumulates and
melts would be to correlate the Ar-Ar ages of vol-
canic rocks with the subdivisions of the oxygen iso-
tope record, such as the specific substages within OI
stage 5. However this will only be achieved if uncer-
tainties can be reduced to ±2 ka or less, which will
require a breakthrough in the Ar-Ar dating method.
Geochemical evolution
Geochemical data for the five Ar-Ar dated samples
(Table 3) plus one sample from each of the two
youngest rhyolite eruptions at Torfajökull (1477 AD
and c. 874 AD) are plotted on Figure 6, which uses Zn
(which is strongly incompatible) as an index of mag-
matic evolution against which are plotted data for two
incompatible elements (Nb and Y) and one compat-
ible element (Sr). Near-linear trends are apparent for
all three trace elements (especially Nb and Y), with Sr
showing a less regular trend due to variable feldspar
contents in the porphyritic rhyolites (especially those
with <220 ppm Zn). These near-linear trends have
been noted before (e.g. McGarvie et al., 1990), and
have been interpreted as indicating a co-genetic re-
lationship between the rhyolites erupted at Torfajök-
ull, and also, by using what relative stratigraphy ex-
ists, as indicating that (in general) older rhyolites at
Torfajökull are geochemically more evolved than the
younger rhyolites.
68 JÖKULL No. 56