Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 191

MINERAL CHEMISTRY AND RELATIONSHIPS
187
analyses of iron-titanium oxides of volcanic rocks except for the ilmenites of
kimberlites. The ranges of total minor element concentrations of the magne-
tites and ilmenites of the tholeiitic Skaergaard intrusion published by
Vincent & Phillips (1954) e.g. are 3.19—5.77 and 2.33—4.70 wt. per cent
respectively. Corresponding values for the oxides of the tholeiitic rock suite
of Thingmuli (Carmichael, 1967) are 2.44-6.04 and 1.25—4.92. Anderson
(1968) reports only three minor elements, Mg, Mn and Al, in the iron-titan-
tum oxides of the alkaline lavas of Tristan da Cunha and these give only
slightly lower total minor element values than thejan Mayen oxides. On the
other hand, the ilmenites of kimberlites may contain still higher total minor
elements than do the Jan Mayen ilmenites. For instance analyses of
kimberlite ilmenites reported by Mitchell (1973) contain minor elements,
which range from 8.93—22.35 wt. per cent. The statement ofCarmichael et
al. (1974, p. 278) that: “magnesian ilmenites have not been found in
volcanic rocks, but they are characteristically associated with kimberlites”,
was overdue. The Tristan da Cunha lavas contain ilmenites with up to 11.11
wt. per cent MgO (Anderson, op.cit.) and the Jan Mayen ilmenites contain
up to 9.51 wt. per cent MgO. Ilmenites of tholeiitic rocks, on the other hand,
seem to be generally low in Mg.
The elements which are primarily responsible for the minor element
increase in the iron-titanium oxides are Mg and Al, and in some cases Mn.
In the case of titanomagnetites this increase is thus probably most ade-
quately described as increased solid solution ofspinel in titanomagnetite. In
the case of ilmenite, on the other hand, A1 is apparently always of minor
importance and the increase of Mg may be adequately described as
increased solid solution ofgeikielite (MgTi03) in ilmenite. Similarly the less
common Mn increase may be described as increased solid solution of
pyrophanite (MnTi03). The amount of the geikielite and pyrophanite solid
solution in the Jan Mayen ilmenites is shown in Fig. 103. From the
description above and numerous articles in the literature on iron-titanium
oxides of volcanic rocks, there seems to be a tendency for the total amounts
oí minor and trace elements in titanomagnetites to increase from subalkaline
to alkaline rocks. For ilmenites such an increase goes even further towards
kimberlites. This increasing solid solution of geikielite in ilmenites, from
subalkaline lavas through alkaline to kimberlites is graphically shown in
Fig. 104. The figure also shows that the abyssal or ocean bottom tholeiites
contain ilmenites, which contain still lower geikielite amounts than the
ilmenites of oceanic island tholeiites. The sole analysis of ilmenite from a
rock belonging to a transitional rock suite (between subalkaline and alkal-
ine) plots where the subalkaline and alkaline rock ilmenites join.
In the previous section the decrease in some minor elements of the
titanomagnetites from early to late crystallizing grains was described in
I’able 29. In the case of ilmenites the grains are mostly late crystallized. A
few early ilmenites occur as inclusions in silicate phenocrysts, as megacrysts