Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 106

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PÁLL IMSLAND
high in AI2O3 (12.29 per cent), extremely high in FeO1 (28.05 per cent), but
low in MgO (1.66 percent),CaO (3.71 per cent) and SiOz (36.26 per cent).
Another point (5), also close to a clinopyroxene crystal, is much higher in
Si02 (43.88 per cent), MgO (6.22 per cent) and CaO (15.36 per cent) but
lower in F’eOt (11.12 per cent). A point between two olivine crystals (11) is
relatively high in FeO1 (20.20 per cent), A1203 (13.24 per cent) and CaO
(9.86 per cent) but low in MgO (2.48 per cent). The glass is thus not
systematically depleted in the elements the adjacent minerals take up from
the liquid during crystallization and is therefore clearly not a residual liquid.
This view is supported by the non-cumulus allotriomorphic texture of
unzoned minerals in the gabbro. On the other hand the analyses show the
glass to be rich in the elements of the adjacent minerals in many cases, as
e.g. the high A1 and Ca contents in analyses 2 (close to a plagioclase) given
above. The glass of the xenolith is thus most probably the quenched liquid
produced by the first remelting of the gabbro. The gabbro contains ilmenite-
magnetite grains of composite intergrowths. Temperature estimates for the
crystallization of these (obtained by extrapolation of Buddington &
Lindsleys’ (1964) hematite curves) gives 1050°C. The temperature of sepa-
rate ilmenite-titanomagnetite microphenocrysts in the basalt, where the
gabbro xenolith occurs, is 1060—1080°C. The temperature of the basic
magma containing the xenolith is thus apparently higher than the crys-
tallization temperature of the oxides of the xenolith.
The generally high A1 and Ca contents of the glass analyses and the high
Fe and Ti in most of the analyses indicate that the remelting was primarily
at the expense of the plagioclase and iron-titanium oxides. Mg in the glass is
mostly low, but the iron-titanium oxides of the gabbro contain 4 to 9 per
cent MgO. Thus it seems that the clinopyroxene and olivine have contri-
buted relatively little to the liquid composition, even though glass occurs at
the boundaries of these minerals. The P content of the glass as well as K, is
high in some of the analyses, indicating that an intergranular film contain-
ing these elements was present in the gabbro before the remelting started.
The great range of P205 (0.06 to 1.15 per cent) in the analyses might
indicate that this film was discontinuous or even that P was retained in
minute apatite crystals. This is supported by the presence of the vesicles,
which result from the volatile expansion.
If, in fact, the clinopyroxene and olivine crystals have contributed little to
the liquid, the glass film found at grain boundaries of these minerals must
have, at least partly, moved into these intergrain channels from the site of
generation. In this case the distance is on a mm scale only. The heter-
ogeneity of the glass is thus the result of a simultaneous melting of minerals
of different compositions, a disequilibrium or incongruent melting, yielding
liquids which not have had the time or opportunity to mix or homogenize.
Small amounts of light brown glass occur in the hydrous mineral xenolith.
Kaersutitic amphibole, biotite and plagioclase crystals show corrosion