Jökull

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Jökull - 01.01.2015, Qupperneq 24

Jökull - 01.01.2015, Qupperneq 24
Sigmarsson and Halldórsson Table 2. Statistics. – Tölfræðileg gögn. Volcanic system Bárðarbunga Askja Range in 87Sr/86Sr 0.70307–0.70315 0.70316–0.70326 n 37 23 Median 0.70311 0.70322 Mean 0.70311 0.70322 Standard Deviation 2.22 x 10−5 2.23 x 10−5 Range in 143Nd/144Nd 0.51306–0.51307 0.51301–0.51304 n 15 25 Median 0.51307 0.51303 Mean 0.51307 0.51303 Standard Deviation 1.59 x 10−6 7.74 x 10−6 n denotes number of analyses from this study and published results of Kokfelt et al. (2006), Kuritani et al. (2011) and Manning and Thirlwall (2014). DISCUSSION The irregular geographical variations of 87Sr/86Sr and 143Nd/144Nd in basalts at the northern edge of Vatna- jökull glacier illustrate the complex overlapping of eruptive fissures belonging to Bárðarbunga and Askja volcanic systems. However, a critical evaluation of published results for Sr- and Nd-isotope composition in lavas from Bárðarbunga-Veiðivötn and Askja vol- canic systems reveals marked difference between iso- tope ratios in products from these volcanic systems. This allows determining the provenance of both re- cent and older lava fields in this complex region as well as discussing the delimitation of these volcanic systems. Source provenance of the Holuhraun lava fields The four 2014–2015 Holuhraun samples and a sam- ple of the older Holuhraun unit fall within the ranges of Sr- and Nd-isotope ratios observed in Bárðar- bunga lavas, strongly suggesting an affiliation with the magma system of Bárðarbunga. The results confirm the limited spread of both Sr- and Nd-isotope ratios in Bárðarbunga-Veiðivötn volcanics. This is further supported by similar high-field-strength element con- centrations and ratios of the older Holuhraun unit and Veiðivötn basalts (Hartley and Thordarson, 2013). Source provenance of the Krepputunguhraun and Fjallsendahraun lava fields The provenance of Krepputunguhraun has been a mat- ter of debate. Based on the presence of large pla- gioclase phenocrysts Sigbjarnarson (1988, 1995) pro- posed an origin from the Bárðarbunga volcanic sys- tem, possibly from the Gígöldur crater row, whereas Guðmundsson et al. (2013) preferred an origin at the Kverkfjöll volcanic system. Kokfelt et al. (2006) measured its composition in a sample collected at Hvannalindir. Its Sr- and Nd-ratios are very differ- ent from those of Gígöldur and Kverkfjöll (Figures 2 and 3; Hémond et al., 1993), but they are well within the range measured for Bárðarbunga-Veiðivötn products thus confirming its provenance. In addi- tion, we note that the large Krepputunguhraun (∼500 km2, Sigbjarnarson, 1988; >7 km3, Thordarson and Self, 1998), which is approximately 8000 years old, is partly overlain by the younger Trölladyngja shield volcano and both these large formations have near- identical Sr- and Nd-isotope ratios (Kokfelt et al., 2006). A single sample from Fjallsendahraun (Fram- bruni), analysed for Nd isotope ratio by Koornneef et al. (2012), is similar to those of Askja basalts. How- ever, our Hf-Pb isotope measurements of this lava (Halldórsson, unpublished) are indistinguishable from 24 JÖKULL No. 65, 2015
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