Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 18
2. PREVIOUS STUDIES OF
JAN MAYEN ROCKS
The oldest description oí'Jan Mayen rocks was given a century ago by
Reusch (in Mohn, 1882). A more detailed general description was given by
Berwerth (1886), while the first analyses were made by Scharizer (1884).
These are incomplete analyses of 5 rocks and 4 minerals, which were
separated from beach sand.
In his attempt to gain an overall picture of “the basaltic rocks of the
Arctic region” Holmes (1918) reports 1 analysis of a Jan Mayen basalt.
Tyrrell (1926) describes the rocks of Jan Mayen with 7 analyses which
span the compositional range from ankaramites to trachytes. The series, he
concludes, is “only mildly alkalic”, but the potash concentration is “well
above the average”. He recognizes Jan Mayen as the most potashic part of
the Thulean province. His “initial magma” is of trachybasaltic composition
and has differentiated towards the ankaramites and the trachytes. He
concludes that the ankaramites are “accumulative or proto-enriched rocks,
whose chemical characteristics are mainly due to segregation of the earliest
crystallized constituents”, highly magnesian olivine and clinopyroxene. The
differentiation towards the trachytes “seems to have proceeded according to
Bowen’s reaction scheme” in a relatively dry magma at comparatively low
pressures.
The second part of an unpublished Ph.D. thesis by Kirk (1953) deals with
the petrography and mineralogy of' the lavas and agglomerates of’ the
“Vogelberg vent” onjan Mayen. He describes the petrography of various
trachybasaltic samples and of its glassy inclusions and gives 1 chemical
analysis of each. The mineralogical description is restricted to hornblende
except for very short notes on pyroxene, olivine and biotite. He concludes
that the Fugleberget magma was partially differentiated, the upper parts of
the hyaloclastite containing plagioclase phenocrysts and being richer in
alkalies than the lower parts. Regarding the glassy inclusions, the most
likely explanation he finds is that they are not differentiates of the trachy-
basalt but represent earlier, independently formed trachytes, now partly
fused.
In three papers, Carstens (1961, 1962 and 1963) is concerned with the
rocks of the southern part ofjan Mayen. He describes them and gives 5 rock
analyses, ranging from trachyte to basalt in composition. He does not make
conclusions on their origin, but is of the clear opinion that the ankaramites
are accumulative.
In his article on trachytes and their feldspar phenocrysts, Carmichael