Jökull - 01.12.2006, Page 72

David W. McGarvie et al.
Figure 6. Zn versus Nb, Y, and Sr
variation diagram. The seven large
filled symbols represent the five Ar-Ar
dated samples plus the two most recent
Holocene rhyolite eruptions at Torfa-
jökull (data from McGarvie, 1985).
Small open symbols for Nb and Sr are
from an additional dataset of 23 analy-
ses of Pleistocene rhyolites from Torfa-
jökull (D.W. McGarvie, unpubl. data).
Zn is used to trace changes in the liq-
uid composition as Zn is consistently
incompatible in Torfajökull rhyolites
(McGarvie et al., 1990; Macdonald et
al., 1990). – Hlutfall Zn, Nb, Y og Sr
í 30 súrum bergsýnum af Torfajökuls-
svæðinu.
Figure 7. Age versus Zn, Nb, Y, and Sr.
The five new analyses are plotted along
with tie-lines between the data points
to emphasise relationships. The sharp
step from 83 ka to the 72/67 ka ring-
fracture rhyolites is particularly pro-
nounced for Zn, Nb, and Y, but less so
for Sr due to feldspar variability (see
Figure 5). Also shown are data from
the two most recent Holocene rhyo-
lite eruptions at Torfajökull (data from
McGarvie, 1985), that emphasise the
less-evolved character of recent rhy-
olitic compositions erupted at Torfa-
jökull. – Breytingar á hlutfalli Zn, Nb,
Y og Sr með aldri í 7 súrum bergsýnum
af Torfajökulssvæðinu.
ements between the three oldest compositions span-
ning the 384–83 ka period show near-linear trends;
and third, between 83 ka and 71.4 ka , a pronounced
change occurs that is marked by a sudden decrease in
the concentrations of the incompatible elements (and
a less convincing increase in Sr). It is important to
note that the latter two trends are only revealed when
examined in a stratigraphic context. The potential sig-
nificance of these trends is now discussed.
The first trend, from older and more-evolved to
younger and less-evolved rhyolites at Torfajökull,
confirms earlier work by McGarvie et al., (1990), and
also by Ívarsson (1992) who showed that the compo-
sitions of older rhyolites were always more-evolved
than those of the Holocene rhyolites. The second
trend, which shows a near-linear trend between the
three dated samples from 384 ka to 83 ka (Figure 7),
may only show strong linearity because of the small
70 JÖKULL No. 56