Jökull - 01.01.2019, Page 95

Nikkola et al.
from Brattaskjól and Hvammsmúli. For these calcu-
lations, we paired the alkali basaltic Brattaskjól melt
inclusions (Table 1) with high-precision Brattaskjól
and Hvammsmúli olivine core compositions (n=160,
Figure 2a) given that they were in Fe-Mg equilib-
rium (KdMg−Fe
Ol−Liq =0.30±0.3; Toplis, 2005) with the
melt inclusions, and assumed 1 wt% H2O in the melt
(Moune et al., 2012). The liquid-only model tempera-
tures for Brattaskjól alkali basalt melt inclusions (with
Mg#melt 56–69, Table 1), in accordance with Eq. 15
in Putirka (2008) and assuming 1 wt% H2O in melt,
are 1155–1222◦C. The melt inclusions in Fo87.9−88.6
Brattaskjól and all Hvammsmúli olivine grains were
not utilized for thermobarometric purposes, as their
low FeOtot contents suggest modification by post-
entrapment solid-state diffusion.
The thermobarometric calculations indicate that
the macrocrysts in the Brattaskjól and Hvamms-
múli ankaramites crystallized at 3±1.4 kbar and over
a ≥100◦C temperature window. Assuming oceanic
crustal density of 2860 kg/m3 (Carlson and Herrick,
1990), the 3±1.4 kbar crystallization pressure of the
studied clinopyroxene crystals corresponds to a depth
of 10.7±5 km. This depth overlaps with the proposed
depth of the brittle-ductile transition below Eyjafjalla-
jökull (Hjaltadóttir et al., 2009) and is a typical pre-
eruption residence depth of basaltic magmas in Ice-
land (Neave and Putirka, 2017). However, considering
the high Mg#cpx (up to Mg#87) values of the clinopy-
roxene crystals, the obtained mid-crustal crystalliza-
tion depth is noteworthy. Due to their primitive char-
acter (magnesian whole-rock and macrocryst compo-
sitions), the Eyjafjallajökull ankaramites have been
envisioned to represent magmas from the deep crust
or shallow mantle (e.g., Loughlin, 1995). This, how-
ever, is not the case for the majority of the clinopy-
roxene grains in the Brattaskjól and Hvammsmúli
ankaramites, the only potential exception being the
primitive clinopyroxene crystals with Mg#cpx 89.7–
89.8 for which crystallization pressures were not de-
termined because of the lack of suitable equilibrium
liquids.
Significance of olivine-hosted spinel inclusions
The spinel inclusions in the Brattaskjól and Hvamms-
múli olivine macrocrysts have higher Cr# and TiO2
than hitherto published for chromian spinels from Ice-
land (Thy, 1983; Sigurdsson et al., 2000; Matthews et
al., 2016; Spice et al., 2016) and their Al2O3 con-
tent is low (Figure 3). According to Kamenetsky
(2001), spinel TiO2 and Al2O3 correlate well with the
host-magma composition; therefore, spinel composi-
tions likely reflect the nature of the parental magma.
We have a poor control on the magma at depth from
which these spinels crystallized, yet Eyjafjallajökull
magmas are generally mildly alkaline with relatively
low Al2O3 and high TiO2. Spinels with high Cr# and
TiO2 and low Al2O3 are typical for oceanic (OIB) and
Large Igneous Province (LIP) basalts, in contrast to
mid-ocean ridge basalts (MORB). In the classification
of Kamenetsky (2001), the Brattaskjól and Hvamms-
múli spinels plot in the OIB field (Figure 3c). The
high TiO2 content in these spinels also discriminates
them from mantle-derived spinels that typically have
<0.2 wt% TiO2 (Kamenetsky, 2001). The Al2O3-poor
and TiO2-enriched nature of the spinels suggests that
the parental melts of the ankaramites were produced
by low-degree melting of deep and enriched mantle
sources, in accordance with the olivine minor and
trace element chemistry (see Nikkola et al., 2019).
Insights into magmatic time scales from olivine
zonation
The wide compositional zonation in the Hvammsmúli
olivine macrocrysts, coarse groundmass, and the fact
that both clinopyroxene and olivine macrocrysts en-
close groundmass minerals around them (Figure 1c)
indicate an extended cooling and crystallization his-
tory for the Hvammsmúli ankaramite. In addition,
the low FeO and MgO contents in the Hvammsmúli
melt inclusions suggest transfer of these elements to
olivine by post-entrapment solid-state diffusion dur-
ing an extended stay of the olivine crystals at mag-
matic conditions. According to Loughlin (1995), the
Hvammsmúli ankaramite outcrop is likely an eroded
lava lake, which is consistent with the diffusive re-
equilibration of melt inclusions and the wide com-
positional zonation in olivine macrocryst. It is pos-
sible that the bulk of the diffusive re-equilibration
in Hvammsmúli macrocrysts took place when they
resided in the postulated lava lake.
94 JÖKULL No. 69, 2019