Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 20

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PÁLL IMSLAND
is unlikely. A short rest during ascent should be sufficient to produce these
scarce trachyandesites. The trachytes on Sör-Jan on the other hand could
point to the presence of a short lived magma chamber there.
Flower (1969) gives an analysis of phlogopite from Jan Mayen and
concludes that this mineral represents a minor phase in the upper mantle
below Jan Mayen, which has locally escaped assimilation by the basalt
magmas ascending to the surface.
In a description of the eruption on the north eastern flanks of Beerenberg
in 1970, Siggerud (1972) gives 2 analyses of the lava, but does not study the
rocks further.
Weigand (1972) gives 4 analyses of the lava erupted in 1970 and 27
analyses of its clinopyroxenes as well as partial analyses of olivines and
opaques. He concludes that the chromian diopsides represent portions of
dissociated and mostly resorbed ultramafic nodules and may therefore
represent fragments of the upper mantle, where the host basalts were
generated from a pyrolite mantle as envisaged by Green & Ringwood
(1967). These were alkali olivine basaltic in composition formed at 35 to 70
km depth. Separation at lower pressures of olivine and clinopyroxene
altered this primitive parental composition towards the MgO depleted and
alkali enriched composition of the lava. Weigand et al. (1972) give analyses
of 20 trace elements of the lava erupted in 1970.
Lussiaa-Berdon-Polve & Vidal (1973) analysed 7 rock samples from
Nord-Jan (1 ankaramite, 4 basalts and 2 trachyandesites) for major ele-
ments, Rb, Sr, and Sr isotopes. According to the authors the isotopes show a
typical, homogenous mantle ratio, slightly higher than most oceanic
tholeiites. Their conclusion is that the basalts are formed from a uniform
source region, have the same origin and have undergone the same evolution.
O’Nions & Pankhurst (1974) in a paper on isotope and trace element
variations in oceanic volcanic rocks, report Rb, Sr, Na20, KzO and Sr
isotopes of 3 samples of the 1970 lava and 1 older rock sample from Jan
Mayen. They conclude that the isotope ratios are essentially constant
throughout the eruptive history of the island. The constancy of the isotope
ratios within each volcanic island leads them to the conclusion that petro-
genetic relationships between various rock types on oceanic volcanic islands
are simple and crustal contamination insignificant.
Goles (1975) compared Jan Mayen basic rocks and basalts from Kenya.
He gives 10 analyses including concentrations of 7 trace elements, the rocks
being ankaramites and basalts. He concludes that the Jan Mayen magmas
were formed at depths of perhaps 70 km in portions of the upper mantle
which had not been previously stripped of basaltic liquids to an appreciable
degree and then transported to shallower levels where extensive crystal
fractionation took place. The ankaramites are, he says, probably enriched in
ferromagnesian trace elements and depleted in incompatible trace elements
by extensive accumulation of clinopyroxene and olivine. The composition of
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