Jökull - 01.01.2019, Síða 88

Eyjafjallajökull ankaramites, South Iceland
directions with respect to the micro-analytical tra-
verses. This is essential for diffusion modelling of
olivine, as the diffusivity of elements (e.g., Fe-Mg
or Ni) in olivine is anisotropic, with diffusion along
the crystallographic c-axis being six times faster than
along the a- or b-axes (e.g. Dohmen and Chakraborty,
2007; Dohmen et al., 2007). Following the proce-
dure of Kahl et al. (2017), we acquired orientation
maps consisting of hundreds of EBSD point determi-
nations for a total of 46 olivine grains from Brattaskjól
and Hvammsmúli. We used the HKL CHANNEL 5
EBSD post-processing software to extract hundreds
of orientation measurements from individual crystals.
The EBSD Euler angles were converted into plunges
and trends of the a-, b- and c-axes of the analysed
olivines using an Excel sheet provided by Dr. D. Mor-
gan (University of Leeds). Finally, the angular rela-
tions between the crystals a-, b- and c-directions and
the micro-analytical traverses were acquired using the
Stereo32 software developed at the Ruhr-Universität
Bochum.
MINERAL AND MELT COMPOSITIONS
Olivine
The Brattaskjól and Hvammsmúli olivine macrocryst
cores show substantial, yet similar, compositional
variability in terms of Fo (Fo81−90) and minor ele-
ments (e.g., Ni, Figure 2a). Both have a population of
Fo88−90 olivine cores showing a large minor element
variability (Ni, Mn, Ca). Minor element variation in
Fo<86 olivine cores, at a given Fo content, is less pro-
nounced (see Nikkola et al., 2019).
All Brattaskjól olivine macrocrysts (n=22) anal-
ysed for their compositional zonation have thin
(<100µm) Fe-Mg zonation around homogenous
cores. Interestingly, olivine grains with Fo>85.7 cores
(n=15) are normally zoned (Fo decreases towards
crystal edges), whereas olivine grains with Fo<84.4
cores have complex reverse zoned rims (n=7, high-Fo
bands near crystal edges, Figure 2d).
The Fo zonation in normally zoned Brattaskjól
olivines exhibits changes in slope (Figure 2b) and
steps (Figure 2c), and as such does not follow the
steady decrease in Fo towards crystal boundaries typ-
ical of diffusion under steady-state conditions (Costa
et al., 2008). The outermost parts of the olivine crys-
tals have the steepest decrease in Fo (B1 and C1 in
Figure 2b and 2c), followed by an inner rim section
with a shallower gradient in the change of Fo (B2 and
C2 in Figure 2b and 2c). These olivine grains also
commonly exhibit high-Fo "shoulders", with an ap-
proximate composition of Fo89.2, near the outer edges
of the otherwise homogenous crystal cores (B3 in Fig-
ure 2b). The complexly reverse zoned Brattaskjól
olivine crystals with homogenous Fo80.9−84.4 cores
have ∼10–30µm thick Fo83.1−85.4 bands near crys-
tal edges (Figure 2d). The outermost edges of the
crystals have the lowest and very variable Fo contents
ranging from Fo73.5 to Fo82.8. Typically, the high-
Fo bands near the crystal edges have relatively broad
diffuse boundaries towards the crystal interior in com-
parison to the steep decrease in Fo towards the outer
edge of the crystals (Figure 2d).
Unlike the Brattaskjól olivine macrocrysts, all
Hvammsmúli olivine macrocrysts show broad (up to
700µm) normal zoning, irrespective of the Fo con-
tent in olivine cores (Figure 2e). Some olivine grains
also exhibit a two-fold division in the Fo zoning pat-
tern (Figure 2e). In these olivine crystals, the Fo zona-
tion pattern in the outermost 200µm of the crystal has
a concave slope (E1), which is indicative of growth-
dominated zoning, followed by up to 500µm convex
slope (E2), more typical to diffusive re-equilibration.
Spinel inclusions in olivine
Spinel inclusions in the Brattaskjól (n = 16)
and Hvammsmúli olivine macrocrysts (n =
22) have Mg#spl (Mg#spl = cation fraction
100Mg/(Mg+Fe2+)) of 38–66, Cr# (Cr# = cation frac-
tion 100Cr/(Cr+Al)) of 52–80, Fe3+/Fetot of 0.15–
0.35, 1–3 wt% TiO2, and 8–22 wt% Al2O3 (Figure
3). The spinel inclusions in Brattaskjól olivine grains
have somewhat higher mean Cr# and Fe3+/Fetot than
in the Hvammsmúli olivine grains, although their
compositions do overlap. In addition, variation in
Al2O3 is less in the Brattaskjól spinels (10–16 wt%
Al2O3). In the Hvammsmúli samples, spinel is also
found in olivine-hosted melt inclusions; these have a
distinct composition with relatively high (>55 wt%)
Al2O3 (Björnsson, 2019).
JÖKULL No. 69, 2019 87