Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 127

MINERAL CHEMISTRY AND RELATIONSHIPS 123
xenoliths respectively tend to have a somewhat lower Ca content than the
average for these minerals occurring in the lavas.
Si02 decreases with decreasing Mg/Fe ratio in the chromian diopsides
and titan-salites. In the titan poor salites and ferroaugites it is slightly
higher than in most of the titan-salites. The concentrations are mostly
between 51 and 54 wt. per cent in the chromian diopsides, mostly between
44 and 51 per cent in the titan-salites and between 49 and 52 in the titan
poor salites and ferroaugites. The titan-salite of the gabbro xenoliths and the
titan poor salites of the hydrous mineral xenolith contain slightly more Si
than the bulk of the titan-salites and titan poor salites occurring in the lavas,
while the Si content of both the chromian diopsides and the ferroaugites
occurring in lavas and plutonic xenoliths is similar.
Al203. Compared to Si, AI2O3 shows the reverse behaviour. It increases
from around 2 wt. per cent in the chromian diopsides and up to 10 wt. per
cent in the most aluminous titan-salites. The titan poor salites show a
decrease again towards around 1 wt. per cent in the ferroaugites. The titan-
salites and ferroaugites occurring in plutonic xenoliths are poorer in A1 than
the bulk of these minerals occurring in the lavas.
Ti02 shows the same pattern as does Al. It increases from low values
(0.3—0.5 wt. per cent) in the chromian diopsides and up to 3 wt. per cent
maximum in the titan-salites. The titan poor salites contain around or <1
wt. per cent and the ferroaugites mostly cluster around 0.25 wt. per cent
ri02. The titan-salites and the ferroaugites of the plutonic xenoliths are
slightly lower in Ti than those of the lavas.
The bulk of the titan-salites contain between 1.5 and 3 wt. per cent Ti02.
These clinopyroxenes are thus not as Ti rich as titan-salites or titan-augites
reported from some other alkaline rocks and the titan-salite name may be
questioned in the Jan Mayen case. Yagi & Onuma (1967) f.i. set the lower
limits of Ti02 in titan-salites and titan-augites at 2 wt. per cent, while
Huckenholz (1965a) set the limits at 3 wt. per cent. Nevertheless, Ti and A1
are the elements which characterize these pyroxenes in the Jan Mayen
rocks.
The closer relationships of the elements Si, Ca, A1 and Ti and their
bearing on the crystallization environment and history will be discussed in
the next section of this chapter.
Cr203 content is high in the chromian diopsides but shows a considerable
scatter. In the wehrlite xenolith the Cr2Os concentration of the diopside is
rather constant; between 1 and 1.3 wt. per cent, while in the ankaramites it
is mostly between 0.5 and 1.05 wt. per cent. In the titan-salites it is mostly
below 0.5 wt. per cent and in the titan poor salites and ferroaugites it occurs
at very low concentrations, mostly <0.05 wt. per cent. The titan-salites of
the gabbro xenoliths are poorer in Cr than the titan-salites of the lavas,
while the titan poor salites and ferroaugites of the plutonic xenoliths are not
any different from those of the lavas as regards Cr content.