Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 82

7. ROCK CHEMISTRY
A. INTRODUCTION
The analyses of major and trace elements of the Jan Mayen rocks carried
out in this study are presented in the tables of this chapter together with
their CIPW norms. In the case of the rock suite each table includes one rock
group. The analyses are arranged in order of decreasing MgO content.
Table 13 contains all the xenolith analyses. Table 11 lists the concentrations
of F, S, and Hg (analysed by: Óskarsson (unpubl.), Gunnlaugsson (1977)
and Coderre & Steinthorsson (1977)) in a few of the Jan Mayen samples.
Analytical procedures and the “correction” of the iron ratio of the analyses
carried out before the norm calculations are described by Imsland (1980).
In the tables of analyses the analysed Fe203 and FeO concentration are
reported. Older analyses of Jan Mayen rocks are not listed here and have
only been plotted in a few diagrams. From these diagrams it is clear, that the
older analyses, without exception, are of the same rock suite as the new
analyses.
B. THE COMPOSITION OF THE ROCK SUITE
Only a short description of the chemistry of the rock suite is given here.
The evolution of the suite is discussed in chapter 9.
I. Major element oxide variations
The rock suite ranges from ankaramites to trachytes with a bimodal
distribution so that low-silica intermediate rocks are scarce. In spite of this
the rock suite is chemically continuous, as shown in the variation diagrams.
MgO is one of the most variable components of the rock suite, ranging
from >18 wt. per cent in the most mafic ankaramites down to < 0.3 per
cent in the most silicic trachytes. The rock suite is divided into rock groups
which are characterized by restricted mineralogical and chemical ranges.
The MgO overlap of the rock groups is relatively small except in the case of
the pl-ol-cpx porphyritic basalts, which overlap the entire MgO range of the
other basalt groups. Thus the basalts as a whole have a restricted range
compared to the other rock groups and MgO serves quite well for compara-
tive purpose in the variation diagram. The MgO range of the rock groups is