Jökull - 01.12.2006, Síða 71

Pleistocene rhyolitic volcanism at Torfajökull, Iceland
Table 3. Whole-rock XRF analyses of the five dated samples, along with their Ar-Ar ages. Major element data
were obtained from glass discs and trace element data were collected from pressed pellets. A.I. is the Agpaitic
Index (i.e. molecular Na2O+K2O/Al2O3): a value greater than 1.0 indicates peralkalinity. Rock names are based
on FeOt-Al2O3 relationships (Macdonald, 1974). Note that although analysed, trace elements Sc, V, Cr, Co,
Ni, and As are not listed as their concentrations were below detection limits. The XRF analysis were carried out
at The Open University, and errors shown here represent standard deviation among eight repeated analyses of
a known standard. See Stevenson (2005) and Stevenson et al., (2006) for full details pertaining to the methods
of sample preparation, analysis, and analytical errors. – Efnagreiningar á aðalefnum og snefilefnum á 5 súrum
bergsýnum. Sýnatökustaðir eru sýndir á 2. mynd.
Sample TJ97-29 TJ97-18 TJ97-9 TJ97-22 TJ97-14 error
Ar-Ar age 67 ka 72 ka 83 ka 278 ka 384 ka
SiO2 72.43 70.90 72.25 72.76 73.05 0.23
TiO2 0.263 0.337 0.238 0.282 0.295 0.00
Al2O3 12.77 13.31 12.07 11.19 10.34 0.04
Fe2O3(tot) 3.20 3.78 3.95 4.16 5.02 0.01
MnO 0.12 0.15 0.11 0.09 0.12 0.00
MgO 0.02 0.09 0.00 0.00 0.00 0.01
CaO 0.62 0.91 0.53 0.19 0.30 0.01
Na2O 5.43 5.63 5.42 5.56 5.27 0.01
K2O 3.86 3.60 4.22 4.38 4.50 0.02
P2O5 0.02 0.04 0.02 0.02 0.02 0.00
LOI 0.47 0.33 0.37 0.46 0.17 0.00
Total 99.20 99.07 99.17 99.09 99.08
A.I. 1.0 1.0 1.1 1.2 1.3
Name Comendite Comendite Comendite Comendite Pantellerite
Rb 95 86 102 108 115 1
Sr 46 98 33 17 8 1
Y 123 115 144 159 167 0.4
Zr 890 999 1359 1493 1575 2
Nb 162 151 188 211 215 1
Ba 501 555 430 421 110 12
Pb 9 6 11 9 10 1
Th 15 14 20 20 19 1
U 3 5 4 4 6 1
Zn 205 203 278 324 345 1
Ga 32 32 36 37 38 1
Mo 8 8 11 11 12 1
S 25 30 45 33 55 2
The five new Ar-Ar ages provide an absolute (al-
beit skeletal) stratigraphic framework that for the first
time enables the pre-Holocene magmatic evolution of
Torfajökull to be explored in a temporal context. On
Figures 6 and 7 three trends are noteworthy: first, that
for all elements the oldest sample is the most evolved
and the youngest is the least evolved (although Sr
shows more variation, see Figure 6); second, all el-
JÖKULL No. 56 69