Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 317

SUMMARY AND CONCLUSIONS
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present as trace elements in the apatites. Si and Mn are enriched in apatites
ofSi rich rocks relative to Si poor ones. The behaviour of Sr in these apatites
indicates that the distribution coeíficient for Sr between apatite and magma
is highly dependent on the magma composition. The bulk of the apatite
phenocrysts is apparently crystallized ~1100°C. In the case of basic mag-
mas this crystallization may have taken place at depths approaching 40 km
but apparently it happened at less depth than 9 km in the highly evolved
magmas.
7 Micas occur as small flakes in the groundmass and vesicle walls of the
basic lavas and in the wehrlite. These are of phlogopitic composition. Biotite
phenocrysts occur in the intermediate lavas and the same biotite is a major
phase of the hydrous mineral xenolith. This biotite is, along with the alkali
feldspar, the most eífective phase in the evolution of the intermediate
magmas. Both micas are Ti rich. The biotite is F poor, while the phlogopite
is F rich. In each rock sample, F decreases in the phlogopites with
decreasing Mg/Fe ratio and is thus preferentially taken up by the earliest
phlogopites. The F variation of both micas is mostly compensated for by
OH, as C1 is present only in low and generally constant values. The
phlogopite is heterogeneous in composition, even in the wehrlite. This shows
that it is not a mineral crystallized in the mantle but a result of crystalliza-
tion of the rest of the liquid, after extrusion of the magma, at 1000 to 1040°C.
The biotite is apparently crystallized at crustal levels and temperatures
between 1000 and 1100°C, as the first mineral in the trachytic and tristanitic
liquids. These liquids are at this stage relatively poor in F, indicating that
they are not the remnant liquids, after crystal fractionation of “dry mine-
rals”, from the more primitive liquids of the magma system.
8) Amphiboles occur as two different types. Basaltic hornblende is a late
crystallizing phase in the intermediate lavas. Kaersutite occurs as a major
component in the hydrous mineral xenolith, as partly disintegrated xeno-
crysts in lavas and as a phenocryst phase in trachyte. The basaltic
hornblende is relatively Ti- and A1 poor but Mn rich as may be expected of a
late crystallizing Fe containing mineral. It is rich in F relative to the
kaersutite, which is in fact nearly F free. Both minerals contain a small
amount ofCl, the basaltic hornblende slightly more than the kaersutite. OH
is apparently the major component of the hydroxy-halogen position of both
minerals; occupying over 95 per cent of that position in the kaersutite and
~50 to 60 per cent in the basaltic hornblende. The kaersutite appears to
have crystallized at 1060 to 1100°C and at depths of 3 to 7 km, but the
basaltic hornblende at 950 to 1000°C, probably en route to the surface.
C. FRACTIONATION
1) The chrome-spinel, kink-banded olivine, and the chromian diopside,
the wehrlite minerals and ankaramite phenocrysts, are the early crystalliz-