Jökull - 01.01.2019, Blaðsíða 97

Nikkola et al.
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Ol81_1
t = 22.3 days
Fo-core = 82.9
Ol82_1
t = 9.2 days
Fo-core = 82.45
Ol45_1
t = 36.7 days
Fo-core = 80.8
Ol76_1
t = 17.9 days
Fo-core = 84.4
Ol83_2
t = 33.4 days
Fo-core = 81.65
Ol89_1
t = 23.1 days
Fo-core = 82.9
Ol91_1
t = 10.3 days
Fo-core = 84.0
Fo Fo
Fo Fo
Fo
Fo
Distance ( m)μ Distance ( m)μ
Distance ( m)μ Distance ( m)μ
Distance ( m)μ Distance ( m)μ
Distance ( m)μ
a b
c d
f
g
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Fo
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e
’mixing plateau’
’core plateau’
Figure 6. a-g)Fe-Mg zonation in
the seven complex reverse zoned
olivine macrocrysts used for dif-
fusion modelling. The blue dots
show the measured forsterite (Fo)
content near the olivine rims,
the orange stippled line is the
assumed initial olivine compo-
sition, and the orange curve is
the calculated end result of dif-
fusive re-equilibration after time
(t). – a-g) Fe-Mg-beltun í
sjö ólivíndílum með flókna öfuga
beltun og notaðir voru við líkan-
reikninga efnasveimis. Bláu
punktarnir sýna mælt forsterít-
gildi (Fo) nærri rimum ólivíns.
Gula brotalínan á hverri mynd er
áætluð samsetning ólivíns í upp-
hafi líkanreiknings og heila gula
línan er reiknuð samsetning af
völdum efnasveimis eftir tíma t.
Kahl et al., 2015); however, the timescales are sen-
sitive to the selected model temperature. Consider-
ing the primitive nature of the Brattaskjól ankaramite,
we surmise that 1170◦C is fair, although conservative
(potentially low) estimate of the diffusion T (see Fig-
ure 5). We prefer a lower model temperature to avoid
underestimating the diffusion time scales, because, ir-
respective of the model conditions, the resulting diffu-
sion timescales are the maximum time scales of sub-
surface storage and ascent (after magmatic recharge)
as the crystals are hosted by a lava flow. Lava flow
fields insulate themselves effectively (e.g., Hon et al.,
1994; Self et al., 1998; Harris and Rowland, 2009)
and hence lava-hosted olivine crystals may remain at
high but gradually decreasing temperature for weeks
to months after eruption. We cannot separate diffu-
sion in a lava flow/lava lake and diffusion occurring
at depth during subsurface ascent; this is the caveat
of working with crystalline lava samples instead of
tephra.
96 JÖKULL No. 69, 2019