Náttúrufræðingurinn - 2009, Side 23

23
Tímarit Hins íslenska náttúrufræðifélags
Þakkir
Náttúrufræðistofnun Íslands er þakkað fyrir basaltsýnið 7637 sem Haukur
Jóhannesson safnaði. Yfirlesurum eru þökkuð vönduð vinnubrögð.
Heim ild ir
Martin, E. & Olgeir Sigmarsson 2007. Crustal thermal state and origin 1.
of silicic magma in Iceland: the case of Torfajökull, Ljósufjöll and
Snæfellsjökull volcanoes. Contributions to Mineralogy and Petrology
153 (5). 593–605.
Moorbath, S., Haraldur Sigurðsson & Goodwin, R. 1968. K-Ar ages of 2.
the oldest exposed rocks in Iceland. Earth and Planetary Science
Letters 4. 197–205.
Haukur Jóhannesson 1982. Yfirlit um jarðfræði Snæfellsness. Árbók 3.
Ferðafélags Íslands 1982. 151–172.
Haukur Jóhannesson 1980. Jarðlagaskipan og þróun rekbelta á 4.
Vesturlandi. Náttúrufræðingurinn 50. 13–31.
Kristján Sæmundsson 1979. Outline of the geology of Iceland. Jökull 5.
29. 7–28.
Guðmundur G. Bárðarson 1932. Jarðmyndanir á Snæfellsnesi. Árbók 6.
Ferðafélags Íslands 1932. 60–66.
Faure, G. 1986. Principles of Isotope Geology. John Wiley & Sons, 7.
New York, 2. útg. 589 bls.
Olgeir Sigmarsson & Sigurður Steinþórsson 2007. Origin of Icelandic 8.
basalts: A review of their petrology and geochemistry. Journal of
Geodynamics 43 (1). 87–100.
Kokfelt, T.F., Hoernle, K., Hauff, F., Fiebig, J., Werner, R. & Garbe-9.
Schönberg, D. 2006. Combined trace element and Pb-Nd-Sr-O isotope
evidence for recycled oceanic crust (upper and lower) in the Icelandic
mantle plume. Journal of Petrology 47 (9). 1705–1749.
Um höfundana
Ingibjörg Snædal Guðmundsdóttir (f. 1981) lauk BS-
prófi í jarðfræði frá Háskóla Íslands árið 2004 og M.Sc.-
prófi frá sama skóla árið 2008.
Olgeir Sigmarsson (f. 1958) lauk doktorsprófi í jarð-
efnafræði frá Université Blaise Pascal í Clermont-
Ferrand, Frakklandi 1990. Hann hefur starfað við
Vísindastofnun Frakklands (CRNS) og Université Blaise
Pascal frá 1992 og við Jarðvísindastofnun Háskólans frá
2002
Póst- og netföng höfunda/Authors’ addresses
Ingibjörg Snædal Guðmundsdóttir
Actavis
Þróunardeild mæliaðferða
Rannsóknarhúsi
Reykjavíkurvegi 80
IS-220 Hafnarfjörður
InGudmundsdottir@actavis.com
Olgeir Sigmarsson
Jarðvísindastofnun Háskóla Íslands,
Öskju, Sturlugötu 7
IS-101 Reykjavík
olgeir@raunvis.hi.is
Summary
The age of the youngest rocks in
Ljósufjöll
Silicic and intermediate lavas from the
Ljósufjöll central volcano on the
Sneafellsnes peninsula were analyzed
for Rb and Sr isotopes. The aim of this
study is to test the possibilities of using
Rb-Sr systematics to date young vol-
canic rocks from Iceland. A few criteria
must be fulfilled for this dating method:
the rocks must be comagmatic and have
highly variable Rb/Sr. Previous research
on the Ljósufjöll samples shows that
their chemical and isotope compositions
are best explained by extensive frac-
tional crystallization. The Ljósufjöll
samples are thus comagmatic. Their
Rb/Sr ranges between 0,116 and 153
and the 87Sr/86Sr is in the range 0,70350–
0,70476. These ratios are the highest
ever measured in Icelandic rocks and
most likely reflect extensive plagioclase
fractionation. On the 87Rb/86Sr–87Sr/86Sr
isochron plot three lines are drawn be-
tween basic and intermediate rocks and
the silicic end-members. These lines may
represent isochrons indicating an age
range of 140 ka to 420 ka for these
youngest lavas in the Ljósufjöll system.
These findings clearly show the potential
of the Rb-Sr dating method for Icelandic
rocks.