Jökull - 01.01.2019, Side 98

Eyjafjallajökull ankaramites, South Iceland
The diffusion modelling gives diffusion time
scales in the range 9–37 days (Figure 6). This time
frame does not reflect the speed of the ankaramite
magma ascent directly, because of the unknown
olivine residence time in the lava and the fact that the
timing of the magmatic recharge (forming the reverse
zoned high-Fo rims) does not necessarily coincide
with the beginning of the magma ascent. The mag-
matic recharge to the crystal mush did not necessar-
ily act as the eruption trigger and may have occurred
considerably earlier than agitation and upheaval of the
olivine (and clinopyroxene) crystals (see Rae et al.,
2016). Nevertheless, as the unknown time of diffusion
at the surface and the potential time-delay between
magmatic recharge and crystal lift both lengthen the
diffusion time, our 9–37-day estimate for diffusive re-
equilibration can be viewed as a likely maximum time
that elapsed from crystal mush disaggregation to erup-
tion. The 9–37-day range may reflect varying olivine
residence times in a magma ascent conduit or surfi-
cial lava-feeding system after the magmatic recharge
event, or be due to olivine sectioning and anisotropy
effects (Shea et al., 2015).
Moderate-pressure co-crystallization of primitive
clinopyroxene and olivine
The Brattaskjól and Hvammsmúli ankaramites have
abundant olivine and clinopyroxene macrocrysts and
only minor plagioclase macrocrysts, Mg#cpx of
clinopyroxene being as high as 90. This and the
compositional variation in clinopyroxene cores sug-
gest olivine- and clinopyroxene-dominated fractiona-
tion and a late arrival of plagioclase on the liquidus.
Specifically, Ca and Al enrichment with decreas-
ing magnesium number in clinopyroxene grains with
Mg#cpx 84.5–90.0 suggests fractionation of olivine
and clinopyroxene in approximately equal amounts,
and the decrease in Ca and Al, which indicates si-
multaneous plagioclase fractionation from the host
melt, is only seen in clinopyroxene grains with
Mg#cpx<84.5 (Figure 4c and d). Variation in crystal-
lization conditions (T and P) or disequilibrium crys-
tallization processes are unlikely to explain this com-
positional variation because of the unchanged Jd con-
tent in clinopyroxene cores (Figure 4e), which should
be dependent on pressure, temperature and crystal-
lization rate (e.g., Mollo et al., 2010; Hammer et al.,
2016; Welsch et al., 2016; Ubide et al., 2019). More-
over, rapid disequilibrium crystallization is expected
to produce anti-correlation between Ca and Al (Mollo
et al. 2010), contrary to what is seen in our clinopy-
roxene data (Figures 4c and d).
MORB melts typically crystallize in the sequence
of olivine (±spinel) –> olivine + plagioclase –>
olivine + plagioclase + clinopyroxene, leaving be-
hind troctolitic or gabbroic cumulates, not wehrlite
cumulates, such as in the case of the Brattaskjól and
Hvammsmúli ankaramites. In addition, due to the
late arrival of clinopyroxene as a crystallizing phase,
the most primitive clinopyroxene produced in MORB
crystallization experiments typically has Mg#cpx of
∼83 (Grove and Bryan, 1983; Tormey et al., 1987;
Yang et al., 1996), not Mg#cpx 90 like in Bratta-
skjól ankaramite. Geochemical trends indicative of
clinopyroxene fractionation before plagioclase have
been noted from lavas in SEVZ and other parts of
Iceland (Furman et al., 1991; Thy, 1991a; Maclen-
nan et al., 2001; Mattsson and Oskarsson, 2005), and
some Iceland lavas do host high-Mg# clinopyroxene
(up to Mg#cpx 92 in Borgarhraun lava, Winpenny and
Maclennan 2011). In this regard, our findings are in
line with earlier studies. Anyhow, clinopyroxene frac-
tionation from primitive SEVZ lavas has usually been
regarded as indicative of high (>8 kbar) crystallization
pressures near Moho or at upper mantle depths (Fur-
man et al., 1991; Thy, 1991a, b; Mattsson and Oskars-
son, 2005), as the stability of clinopyroxene increases
with pressure (Presnall et al., 1978, 2002; Stolper,
1980). The calculated 3.0±1.4 kbar crystallization
pressures for the primitive Brattaskjól and Hvamms-
múli clinopyroxene macrocrysts (up to Mg#cpx 87)
contradicts this contention and suggest that the mildly
alkaline SEVZ magmas crystallized clinopyroxene
and olivine before plagioclase at depths as shallow as
in the mid-crust. This inference is supported by, and
is in line with, recent experimental work that shows
that "enriched end-member" (Shorttle and Maclen-
nan, 2011) Iceland tholeiite melt crystallized clinopy-
roxene before plagioclase at 3 kbar (Neave et al.,
2019b). Elevated H2O in primitive SEVZ magmas
(Thy, 1991a; Moune et al., 2012) and low amounts of
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