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Jökull - 01.01.2015, Qupperneq 23

Jökull - 01.01.2015, Qupperneq 23
Delimiting Bárðarbunga and Askja volcanic systems with Sr- and Nd-isotope ratios Figure 3. Histogram of Nd isotope ra- tios in same basalts as in Figure 2. – Súlurit fyrir Nd samsætuhlutfall í sömu basöltum og á 2. mynd and tephra are given in Table 2, excluding the three old lavas from the Veiðivötn area. Restricted spread in isotope composition is evident with 87Sr/86Sr and 143Nd/144Nd in the narrow ranges of 0.70307– 0.70315 and 0.51306–0.51307, respectively. Askja range When compiling published results for the Askja vol- canic system, it becomes clear that three samples have most likely been erroneously associated with Askja. These are samples H25, collected along the track from Hrossaborg to Herðubreiðarlindir, just south of Ferjuás, and H24, collected east of Herðubreiðartögl (Kokfelt et al., 2006). Both these are olivine tholei- ites that display the highest 87Sr/86Sr and lowest 143Nd/144Nd of all the analyses examined here and major element composition significantly less evolved than the Askja basalts (Kokfelt et al., 2006). Their origin remains unknown but we note that they have near identical values to those reported from Kverk- fjöll by Hémond et al. (1993). A single Mg-rich and relatively old (>6000 a) basalt (sample# IA1709) col- lected just south of Herðubreiðartögl by Kuritani et al. (2011) is of unknown origin, displaying both low and high 87Sr/86Sr and 143Nd/144Nd, respectively. Be- fore associating these curious basalts to Askja, a more thorough study of the Dyngjufjöll basalts is needed. Alternatively, the Askja basalts exhibit a much larger range in radiogenic isotope ratios than any other vol- canic system studied so far in Iceland. Excluding these three anomalous basalts, Table 2 gives the range, median, mean and standard deviation for both Sr and Nd isotope ratios for Askja. The respective ranges are 0.70316–0.70326 and 0.51301–0.51304, only slightly but nevertheless significantly different from those ob- served for Bárðarbunga volcanic system. JÖKULL No. 65, 2015 23
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