Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 315

SUMMARY AND CONCI-USIONS
311
Towards the end of the rock suite evolution the salites become depleted in Ti
and A1 again, while Fe increases to form a ferroaugite. The early chromian
diopside seems to have started to crystallize somewhat before olivine in most
samples, but mostly these two minerals coprecipitate in the mafic liquids.
The clinopyroxene, though, seems to crystallize in greater amount. As the
rock suite evolves the titan-salite takes over from the chromian diopside. In
a short interval in the most evolved basalts the clinopyroxene disappears as
a phenocryst phase and does not appear again until in the tristanites,
evolving from salite to ferroaugite. In most cases the clinopyroxene has
enough A1 to more than fill the tetrahedral position along with Si. In the
most evolved rocks though, there is generally not enough Si+Al to fill this
position, which then is apparently filled by Fe3+.
3) The spinels are mostly Cr rich phases of early crystallization in the
mafic magmas and are thus primarily found in the wehrlite and ankara-
mites. At later stages, Cr poorer, Fe3+ enriched varieties are present in the
rock suite, as well as Cr free A1 rich spinels. Ti is a typical minor element in
these spinels, showing strong enrichment in the Fe3+ enriched late spinels.
Except for the spinel end member cations and Ti, these spinels generally
contain Si, V, and Mn in trace element concentrations. Of these both V and
Mn tend to be concentrated in late spinels relative to early ones, while no
such systematics have been found regarding Si. Calculations based on
rather coarse assumptions indicate that around 1 per cent of early spinel was
crystallized out of the primitive liquids. The later crystallized spinels are
nearly negligible in amount.
4) The iron-titanium oxides are primarily titanomagnetites and ilmen-
ites. These have undergone oxidation in cases, leading to a variety of
alteration types. Thus there are present in the rock: unaltered separate
titanomagnetites and ilmenites, as well as granular composite intergrowths
of these, sandwich lamellae composite oxides, pseudomorphically oxidized
varieties, titanomaghemites, and pseudobrookite-rutile-titanohematite
intergrowths of post trellis oxidation. The primary oxides have crystallized
from the basaltic and more evolved magmas. In relatively few cases is the
two-oxide thermometer of any use, because of a lack of common coprecipita-
tion of ilmenite and magnetite in separate phases. Titanomagnetite cryst-
allized in the rock suite from ~1150°C to at least 1040°C; at the lower
temperatures occasionally accompanied by ilmenite. The two-oxide cryst-
allization mostly took place at shallow depths, shortly before eruption, while
the higher temperature crystallization of the titanomagnetite alone vent on
at somewhat deeper levels. The iron-titanium oxides are high in total
amount of trace- and minor elements, the Mg- and A1 content being
especially prominent. A covariation is found between crystallization
temperature and the amount of most common minor- and trace elements.
The amount of trace elements in iron-titanium oxides of alkaline rocks is
found to be generally high, relative to tholeiitic rocks. This is especially the