Jökull - 01.01.2019, Síða 87

Nikkola et al.
The Brattaskjól olivine and clinopyroxene macro-
crysts are often embayed and have thin composition-
ally zoned rims in contact with fine-grained ground-
mass mineralogy (Figure 1b). In Hvammsmúli sam-
ples, conversely, the groundmass is coarse and olivine
and clinopyroxene macrocrysts have broad composi-
tionally zoned rims that commonly enclose ground-
mass plagioclase near the macrocryst edges (Figure
1c). In addition to the zonation near crystal edges,
clinopyroxene macrocrysts in both localities exhibit
complex oscillatory and convolute zoning, while sec-
tor zoning was not identified.
The Brattaskjól olivine macrocrysts are com-
monly devoid of spinel and melt inclusions. When
present, the Brattaskjól melt inclusions are found in
the cores of the crystals as primary or pseudosec-
ondary inclusions (following the nomenclature of
Roedder, 1984). Their size varies between 20 and
250µm and they are partially crystallized, containing
dominantly clinopyroxene daughter minerals, silicate
glass and a large bubble phase. These inclusions may
also contain opaque phases, which are generally acci-
dentally trapped spinel crystals. Spinel and melt in-
clusions are more common in the Hvammsmúli sam-
ples, and some Hvammsmúli olivines have small (usu-
ally <1µm) oxide exsolutions. The textural appear-
ance of melt inclusions is similar to those observed in
olivine grains from Brattaskjól. These inclusions con-
tain abundant clinopyroxene, minor orthopyroxene, il-
menite and spinel daughter minerals, a vapour phase
and interstitial silicate glass (see Björnsson, 2019). In
backscattered electron images, we commonly observe
a bright Fe-rich diffusion halo in olivine around the
melt inclusions in Hvamsmmúli samples. This halo is
missing around Brattaskjól melt inclusions.
ANALYTICAL AND EXPERIMENTAL
METHODS
Melt inclusion homogenization experiments
As melt inclusions from both localities were partially
crystalline, we tried to homogenize them. Handpicked
olivine crystals were placed in a graphite crucible and
mixed with graphite powder in order to avoid oxi-
dation during the heating experiments. We carried
out three sets of experiments at temperatures of 1200,
1220 and 1240◦C. The precision of the temperature
reading, calibrated to the melting points of Au, Ag and
NaCl, was ±5◦C. Crucibles were placed in the high-
temperature oven at the target temperature and kept
there for 5 minutes. After 5 minutes, the samples were
quenched in room temperature water. Olivine was re-
covered after quenching and mounted in epoxy. Melt
inclusions were exposed by manual polishing and in-
spected under the polarizing microscope. More than
90% of the melt inclusions from Brattaskjól were ho-
mogenized at 1200±5◦C, and all of them at 1220◦C.
By contrast, a few crystals were observed in Hvamms-
múli melt inclusions even after heating to 1240◦C.
Electron microprobe analysis
We separated olivine and clinopyroxene macrocrysts
from crushed and sieved (Ø = 0.1–4.0 mm) rock sam-
ples and determined their major and minor element
compositions using a JEOL JXA-8230 electron mi-
croprobe at the University of Iceland. Clinopyrox-
ene was analysed in a thin section made from the
Hvammsmúli sample. In total, we analysed 192
olivine and 51 clinopyroxene crystals for their core
compositions, 38 spinel and 21 melt inclusions in
olivine, and 47 concentration profiles (with 4–10µm
spacing between analysis spots) across olivine zona-
tion. Acceleration voltage and a beam current of 15
keV and 20 nA, respectively, were used for analysis of
clinopyroxene, spinel and olivine zonation, whereas
10 nA beam current was used for analyses of melt in-
clusions. For the high-precision trace element analy-
ses of olivine cores, we used a modified version of the
analysis protocol by Batanova et al. (2015) with ac-
celeration voltage of 20 keV and a high beam current
of 500 nA (see Nikkola et al., 2019; for details). Crys-
tals of known composition were analysed to check for
instrumental drift. The mineral and melt compositions
are available from the authors on request.
EBSD analysis
Crystallographic orientations of olivine macrocrysts
were resolved with electron backscatter diffraction
(EBSD, Prior et al., 1999) using FEI Quanta SEM
at the University of Leeds (UK). The EBSD analy-
ses were performed to constrain the crystallographic
86 JÖKULL No. 69, 2019