Jökull - 01.01.2019, Page 92

Eyjafjallajökull ankaramites, South Iceland
En
HdDi
E
n
5
0 E
n
4
0
Wo40
F
s1
0
a
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
70 75 80 85 90 95
C
a
T
s
(C
a
A
lA
lS
iO
)
6
Mg#cpx
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.040
70 75 80 85 90 95
J
d
(
N
a
A
lS
i
O
)
2
6
0.0
0.5
1.0
1.5
2.0
2.5
70 75 80 85 90 95
T
iO
(w
t%
)
2
b
e f
Brattaskjól - core
Brattaskjól - rim
Hvammsmúli - core
Hvammsmúli - rim
Brattaskjól - intra-crystal variation
19.0
19.5
20.0
20.5
21.0
21.5
22.0
22.5
70 75 80 85 90 95
C
a
O
(
w
t%
)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
70 75 80 85 90 95
A
l
O
(w
t%
)
2
3
c d
ol
cpx
ol
cpx
plg plg
Mg#cpx Mg#cpx
Mg#cpx
Mg#cpx
Figure 4. Clinopyroxene macrocryst compositions from the Brattaskjól and Hvammsmúli ankaramites. The core com-
positions are average compositions determined by three analyses from each clinopyroxene core, whereas the crystal rim
compositions are averages of two to three analyses from near the crystal rims. The clinopyroxene macrocrysts include
compositionally variable domains and this intra-clinopyroxene variability in all the studied crystals is illustrated with grey
squares (unpublished data). Black arrows are approximated compositional vectors of olivine (ol), Ca-augite (cpx) and pla-
gioclase (plg) fractionation, calculated assuming KdMg−Fe
Ol−Liq = 0.30, KdMg−Fe
Cpx−Liq = 0.27, DCa
Ol−Liq = 0, DCa
Cpx−Liq = 1.7,
DAl
Ol−Liq = 0 and DAl
Cpx−Liq = 0.15–0.25 (.005 increase with every mol% removed from melt to simulate the T-dependent
partitioning of Al in clinopyroxene). Plg fractionation lowers CaO and Al2O3 but does not affect Mg number (Mg#cpx
= cation ratio Fe/(Fe+Mg)). Stippled arrow is the compositional vector of ol + cpx fractionation in equal proportions. a)
Clinopyroxene quadrilateral; Di = Diopside, Hd = Hedenbergite, En = Enstatite, Wo = Wollastonite, Fs = Ferrosilite; b)
Ca-Tschermak’s (CaTs) component vs. Mg#cpx; c) CaO vs. Mg#cpx; d) Al2O3 vs. Mg#cpx; e) Jadeite (Jd) component vs
Mg#cpx; d) TiO2 vs. Mg#cpx.
JÖKULL No. 69, 2019 91