Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 193

mineral chemistry and relationships
189
and sandwich- and composite intergrowths with titanomagnetites in coarse-
grained xenoliths. The scarce data show the same tendency of Mg to be
higher in the early crystallized ones than in the late crystallized ones, as is
the case with the titanomagnetites. In spite of the fact that the Jan Mayen
magmas have crystallized out a number of minerals rich in Mg (ol, cpx,
spin) prior to and at the same time as these Mg rich oxides, the ilmenites
show a crude correlation regarding Mg content to the whole rock Mg
content; the more magnesian the rock, the more magnesian both the early
and late ilmenites it contains tend to be. This indicates a strong dependence
of the amount of Mg in ilmenites on the composition of the liquid it
crystallizes from. A tholeiitic rock from Thingmuli (Carmichael, 1967),
containing comparable Mg to an alkaline rock from Jan Mayen, contains an
ilmenite of much lower Mg content than does the Jan Mayen one. This
indicates that the Mg, Ti and Fe contents are not the only determining
parameters of the ilmenite composition. The most contrasting difference of
these two rock suites is the alkali content, especially that of K. The alkali
contents of magmas are thus very likely to affect the composition of ilmenites
and other oxide minerals, as has been discussed by i.a. Carmichael &
Nicholls (1967). For the Jan Mayen rocks though, there is no simple
correlation between alkalies in the rock and the ilmenite composition, so this
is most likely a rather complex interplay.
High Mg content of ilmenites has long been considered pressure related
according to Haggerty (1976b), who believes the mineral composition to
depend mostly on the composition of the primary magma. Evans & Moore
(1968) report ilmenites from 7, 62 and 71 feet depth in the prehistoric
Makaopuhi tholeiitic lava lake, which contain 1.3, 1.9 and 1.9 wt. per cent
MgO respectively. This shows an increase in Mg content with increasing
depth, but the data are scarce and the concentration and depth ranges
small. The magnesian ilmenites of Jan Mayen are mostly groundmass
grains crystallized in the lavas en route to, or on the surface, but also
crystals brougth up from greater depths. This shows that magnesian ilmen-
ites are not necessarily an expression of high pressure. Other factors may
control or influence the compositions as well.
In Fig. 105 the temperature of contemporaneous crystallization of ilmen-
ite and titanomagnetite in the rock suite has been plotted against the
concentrations of Mn, Al, Mg and V in these minerals. A covariation exists
in all cases except for V in ilmenites. One temperature determination which
does not plot on this covariation trend is the late overgrowth on the
sandwich type megacryst of Jan 27.
The Mn contents are similar for both minerals and decrease with
increasing temperature. Buddington & Lindsley (1964) concluded on the
basis of a pattern showing increasing ratio of Mn in ilmenite to Mn in
titanomagnetite and accompanying temperature variations, that the
temperature is a major factor in controlling the distribution of Mn between