Jökull - 01.12.1986, Síða 39

Table 7.
Results of calculations using the olivine from Maelifell to predict the MgO and FeOcontent of the parental
picritic liquid and its estimated temperature of crystallization. Taken from Hardardóttir (1983).
anal. no 11084 11166 11147 11066 11174 11014 11043
olivine group I KD 1(2) I II II II
Fo mole% 89.1 87.2 87.1 80.4 91.3 90.3 88.0
MgO (liq) (cat.% (3)) 17.7 15.0 15.0 11.5 23.5 20.3 16.0
(ox.% (4)) 16.2 13.6 13.6 10.2 21.4 18.5 14.4
FeO (liq) (cat.% (3)) 6.4 6.5 6.5 8.2 6.4 6.4 6.4
T°C 1264 1223 1219 1173 1334 1298 1236
Ni ppm 375 664
Mg/(Mg+Fe2+) 0.73 0.70 0.70 0.58 0.79 0.76 0.71
(1) Cr-Al-endiopside in intergrowth with (or moulded) on the olivine.
(2) Solid inclusion of plagioclase included in the olivine.
(3) Based on mole % of cation units.
(4) Based on mole % of oxides.
Equation (2) and (3) are from Hanson & Langmuir
(1978) and no. (4) from Hart & Davis (1978). The
results of these calculations from Hardardóttir (1983)
are shown in Table 7. The melt that equilibrated with
the reversely zoned olivine (group I) was relatively
evolved, containing 18 mole% MgO (approximately
13 wt.%), 6.4 mole% FeO, and 375 ppm Ni at a tem-
perature of approximately 1264°C. The magma that
equilibrated with normally zoned olivine contained
23.5 mole% MgO (approximately 18 wt.%, see also
Table 1 and Fig. 2), 6.4 mole% FeO and 664 ppm Ni
at a temperature of approximately 1334°C. A liquidus
temperature of 1334°C for the primitive melt (ap-
proximatlely 18 wt.%) at Maelifell is close to the ex-
perimentally determined value 1360°C obtained by
Maaloe á Jakobsson (1980) for a Reykjanes Penin-
sula oceanite of similar composition (18 wt.% MgO;
sample RE78). The agreement between the present
calculations and the experimental determination in-
spires confidence in their validity. The other major
elements in the high-MgO liquid (18 wt.%) can be cal-
culated from the whole-rock composition, giving that
it has accumulated 26% olivine (Fo88, Table 1). This
calculated liquid composition compares well with the
average composition of picrite lavas on the western
Reykjanes Peninsula (Jakobsson et al. 1978). This
relatively primitive Maelifell magma crystallized
Cr-spinel and Cr-Al-endiopside as well as normally
zoned olivine (Fo88_91), whereas the relatively evolved
magma at Maelifell crystallized olivine (Fo86_89) and
plagioclase (An85_89), found as inclusions in the oli-
vine). After mixing, the olivine from the evolved melt
continued to crystallize, as did the olivine in the
primitive melt; by continued crystallization, the Fo
and Ni contents in the olivine from the evolved melt
increased toward the rim, whereas the opposite oc-
curred for group-II olivine. One of the analyzed
reversely zoned olivines in Table 7 (11166) is partly
surrounded by Cr-Al-endiopside. This olivine gives an
approximate inferred temperature of 1223°C, indicat-
ing that the mixing must have occurred at a tempera-
ture higher than this. The presence of such a mixing
event suggests as well that the rounded shape of the
Cr-Al-endiopside may by due to post-mixing resorp-
tion owing to disequilibrium.
Do the whole rock picrites represent primitive or
primary liquid compositions, more primitive than
inferred from the olivine composition or do they repre-
sent magmas which have accumulated phenocrysts?
This question is not easily resolved because there
appear to be no reliable textural criteria for distin-
guishing between these two alternatives. As shown
37