Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 30

26
PÁLL IMSLAND
The coarse-grained xenoliths also show a systematic distribution. The
ultramafic xenoliths occur only in the ankaramites and the syenitic ones
only in the intermediate rocks. The gabbros and hydrous mineral xenoliths
occur only in the basalts.
The volume of Jan Mayen island above sea-level is approximately 200
km3. Of this Sör- and Midt-Jan are only ca. 16 per cent or 32 km3. The
trachytes and tristanites have been calculated approximately as being 1
km3, or about 3 per cent of the volume of Sör- and Midt-Jan. This is a
minimum value, as some rocks of this composition may be totally hidden
below other rocks. Trachytes are much more abundant than tristanites. The
basaltic tristanites, as well as the aphyric basalts, seem to be very scarce, but
it is impossible to calculate the volume of these rocks. Likewise it is
impossible to calculate the volume of the ankaramites. They do not occur on
Sör-Jan but are abundant on Nord-Jan. One of the conclusions of Roberts &
Hawkins (1965) dealing with the geology of Nord-Jan is: “on first impres-
sion the proportion of ankaramites is unusually high”. Of the 42 existing
older analyses of Nord-Jan rocks 10 are of ankaramites. A sampling bias
might be suspected to cause this relatively high number, but this is probably
not the case. It should be borne in mind that of these older analyses 8 are of
basalts from the same lava, i. e. from the 1970 eruption.
It was pointed out by Chayes (1963) that among published analyses of
rocks from oceanic islands, there is a gap in the composition of SÍO2
concentration around 55 wt. per cent. The generality of this “Daly gap” has
been questioned by, i. a., Baker (1968) and Cann (1968).
As previously mentioned, basaltic tristanites and tristanites are scarce on
Jan Mayen. Among the analysed rocks there is a bimodal distribution, as
shown in Fig. 4, where only six analyses fall within the “Daly gap”
composition. Thin-section study of the unanalysed samples and field
observations indicate that the gap is real regarding the number of eruptions
of “Daly gap rocks”, but it is less certain that this holds for their volume. It
is impossible to calculate or estimate the volume of these rocks, but
generally each erupted unit, for at least the basaltic tristanites, seems to be
of greater volume than the trachytic ones. The volume of the “Daly gap
rocks” on Jan Mayen might therefore match or exceed that of the trachytes.
The extensive sampling ofjan Mayen rocks excludes a bias in the sampling
(discussed by Baker, 1968) with regard to number of eruptions. The
bimodal frequency of eruptions might be caused by one or more of the
following:
1) The “Daly gap magmas” have difficulties in reaching the surface,
compared with other magmas.
2) The “Daly gap magmas” are more scarce at depths than other magmas
(discussed by Sigurdsson, 1970).
3) Trachytic rocks are remobilized by hotter basic magmas and repeatedly
erupted (discussed by Cann, 1968).