Jökull - 01.01.2017, Blaðsíða 44

Holocene lavas in Bárðardalur, NE-Iceland
postglacial lavas than lavas erupted later during the
Holocene (Hansen and Grönvold, 2000; Breddam,
2002; Stracke et al., 2003; Sims et al., 2013). The
same appears to hold for the tephra record for the
Bárðarbunga volcanic system, where less evolved
(high-MgO) tephras are generally more frequent ear-
lier in the Holocene (Óladóttir, 2009). We noted a
clear chemical gap (from MgO 6.56 to 7.07) within
the Dyngjuháls data (Figure 4a,c). The fact that two
of the more evolved Dyngjuháls samples were col-
lected from fresh looking and presumably young vol-
canic units possibly reflects temporal chemical vari-
ation within the Bárðarbunga magma plumbing sys-
tem. The residence time of melts within the crustal
magma system beneath Bárðarbunga is critical as
short residence time, perhaps due to greater maturity
of the plumbing system and/or increased magma sup-
ply, may decrease opportunities for melts to mix, as-
similate and undergo crystal fractionation processes,
resulting in generally less evolved melts being erupted
(Gee et al., 1998). This may well have been the case
for the early Holocene large-volume lava flows from
Bárðarbunga, which should then be characterized by
higher MgO contents, wider range of incompatible el-
ement contents, and possibly also a wider range of
radiogenic isotopic ratios, as melts were less mixed
compared to younger lavas associated with the Bárð-
arbunga system. This idea lends support to the pro-
posal that the youngest lava unit of this study, the his-
torical Frambruni lava, is also the most evolved lava
flow in the Bárðardalur region in terms of chemistry
(e.g., Figure 4b,d). To test this idea, detailed mapping
of the lava units at Dyngjuháls is necessary, particu-
larly by means of tephrochronology.
Rather clear temporal variations have for exam-
ple been noted in the chemical and isotopic com-
position of the erupted basalts of Þeistareykir were
a clear difference is for example evident between
samples formed during deglacial and early-Holocene
(Maclennan et al., 2002; Sims et al., 2013). To show
this and to test if this observation extends to other
parts of the NRZ, we plot the composition of sam-
ples associated with the volcanic systems Þeistareykir,
Krafla, Askja, and Bárðarbunga, including groups of
table mountains and shield volcanoes in 208Pb/204Pb
vs. 206Pb/204Pb space in Figure 11a-d (see details in
Svavarsdóttir, 2017). It becomes evident that spa-
tial and temporal variations exists among the studied
eruptive products throughout the entire NRZ. In turn,
this implies that the assumption that a given volcanic
system, which are generally viwed as a central vol-
cano and an associated fissure swarm, delivers mag-
mas towards the surface with a near uniform chem-
ical and isotopic composition does not hold, at least
when considering volcanic systems of the NRZ. We
therefore suggest that only when considering high-
resolution geochemical tracers, such as high-quality
Pb-isotopic datasets, and include only geologically
well characterized, but temporally and spatially vari-
able eruptive units (i.e., O’Hara and Herzberg, 2002),
are we likely to enhance our understanding of the rel-
ative importance of different processes, deep versus
shallow, in the petrogenesis of Icelandic basalts.
CONCLUSIONS
A detailed chemical and isotopic study of Holocene
lavas in the Bárðardalur region has revealed the prim-
itive compositional nature of these lava flows. The
bimodal chemical characteristics of the Bárðardals-
hraun lava support its division into at least two chem-
ically distinct lava units, Bár-1 and Bár-2. How-
ever, these units likely originate from the same source
region.
A comparison of the chemical and isotopic char-
acteristics of the lavas in Bárðardalur with eruptive
units and tephra layers associated with the Bárðar-
bunga volcanic system suggests a Bárðarbunga source
for all lavas found in the Bárðardalur region. Data
for Bárðarbunga basement rocks and several volcanic
units situated in the region north of Vatnajökull show
that the Bárðarbunga volcanic system shares the prim-
itive character of the Bárðardalur lavas. However,
such primitive composition appear less pronounced
among basalts erupted on the southern segment of
the Bárðarbunga fissure swarm and are notably ab-
sent in the bulk composition of Bárðarbunga’s most
recent eruptive products. A compiled dataset of pub-
lished Sr-Nd-Hf-Pb isotopic ratios from the NRZ re-
veals sharp distinctions between the volcanic systems
of interest but also highlights where more data are
JÖKULL No. 67, 2017 39