Jökull - 01.12.1986, Síða 38

Table 6. Composition of calculated versus analyzed
olivine in the Maelifell picrite basalt.
sample
mole% Fo mole% Fo
calc. anal.
glass* 85.0 91-86 (group-I & II)
whole rock 94.0 91-86 (group-I & II)
* average composition of the quenched glass.
calculated as SiO,, A1203, Na,0, etc.). I have used
cation units (molecular weight calculated as Si02,
AIO, 5, NaO05, etc.). The results of a calculation of
the predicted olivine composition are shown in Table
6.
According to such a calculation (Table 6) neither
population of olivines was in equilibrium with the
melt composition of Table 1. Furthermore, it appears
that the whole rock is enriched in cumulus olivine,
whereas the quenched glass may be depleted in the
olivine component. The presence of these two popula-
tions of olivine phenocrysts (groups I-II) in the same
rock might indicate that mixing of two melts has oc-
curred. One of these two melts might have been
“evolved”, whereas the other might have been more
“primitive” in composition. The “evolved” one, be-
fore mixing, contained olivine (group I) phenocrysts
and plagioclase, whereas the “primitive” one con-
tained phenocrysts of olivine (group II), Cr-spinel and
Cr-Al-endiopside.
DISCUSSION
The mineralogy and mineral chemistry of the
picrite basalts in the Reykjanes and Grindavik
swarms, western Reykjanes Penisula (Jakobsson et al.
1978) are comparable to those from Maelifell. The
picrites in the western Reykjanes Peninsula contain
phenocrysts of olivine (Fo89_90) that correspond to the
group-II primitive olivine of the Maelifell specimen;
the chromite microphenocrysts correlate in all re-
spects with the Cr-spinel from Maelifell. The plagio-
clase (An90_92 5) they observed is of similar compo-
sition to that found by Grönvold & Óskarsson (pers.
comm. 1983) in other samples of the Maelifell picritic
basalt, but no plagioclase phenocrysts occur in the
picrite rocks included in the present study. The picrite
basalts from the western Reykjanes Peninsula are
believed to represent primary high-MgO melts, having
MgO contents of approximately 18 wt.% (Jakobsson
et al. 1978, Maaloe & Jakobsson 1980).
Cr-Al-endiopside megacrysts have not previously
been reported in Icelandic picrite basalts, but strik-
ingly similar megacrysts are found in the Famous
region (Le Roex et al. 1981, Bender et al. 1978) and at
DSDP sites in the Atlantic and Pacific oceans (Don-
aldson & Brown 1977). Bender et al. 1978 suggested,
on experimental grounds, that in most cases, this
pyroxene represents a phase that crystallized from a
primitive magma (Mg/(Mg+Fe2+)>0.68) at pressures
greater than approximately 10 kbar.
Normally zoned olivine occurs commonly both in
Icelandic basalts (Carmichael 1967, Hermes & Schill-
ing 1976, Jakobsson et al. 1978, Makipaa 1978) and
in mid-ocean-ridge basalts (e.g., Le Roex et al. 1981).
The zonation reflects a progressive increase in the
Fe/Mg ratio of the residual liquid as a result of pro-
gressive crystallization of the magma, and is a con-
sequence of the preferential partition of Mg into the
crystalline fraction. Reversely zoned olivine has ap-
parently not been previously reported in picrite
basalts, a few grains have been found in some Icelandic
tholeiitic hyaloclastites (Makipaa 1978) and in one
basalt sample from the Reykjanes Ridge (Hermes &
Schilling 1976). Olivine found in gabbro nodules,
from the Midfell picrite basalt, observed slight com-
positional zoning with core composition of Fo86 4_87 3
and rim composition of Fo86 4_88, mol%. This zoning
is both normal and reversed (Risku-Norja 1985).
Reversely zoned olivine phenocrysts with a Fo82_83
core and a Fo84_86 rim have been found sporadically
in mid-ocean-ridge basalts, associated with normally
zoned (Fo88_90) olivine, but not in picrite (Donaldson
& Brown 1977, Dungan & Rhodes 1978, Rhodes et al.
1979). These have been taken as strong evidence for
mixing of primitive liquid with a more evolved basic
liquid.
One way to calculate the possible compositions of
the two end-member liquids involved in the mixing
that occurred in the Maelifell picritic basalt is to use
the olivine composition. The temperature and the Ni
content can also easily be calculated by using follow-
ing equations:
K(D) = (FeO/MgO)ol / (MgO/FeO)Hq (1)
K(d) MgO = Col/Cllq (2)
log(Io) K(d) MgO = (3740/T(K) - 1.87) (3)
and
DNi = ((123.13/MgO wt.%) - 0.879) (4)
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