Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 246

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PÁU. imsi.and
Mg/Fe ratio might indicate that this phlogopite crystallized at somewhat
higher temperatures than those of the basalts, but the F concentration
indicates a similar stage of evolution (or primitiveness) of the liquid. 'Fhe
origin of this very minor amount of intergranular liquid in the wehrlite is
uncertain. Whether it is a restliquid, the liquid of first melting, or a liquid
introduced from the ankaramite magma carrying the xenolith, is not clear,
but the heterogeneous composition of this phlogopite seems to exclude a
mantle crystallization.
Eruption temperature of the 1970 lava of NE Beerenberg was around
1010— 1030°C as previously mentioned. It has been concluded that the
groundmass plagioclase of the basic lavas ofjan Mayen crystallized below
1040°G. The phlogopite of these lavas falls under the groundmass category
and consequently crystallized at similar temperatures. The grains embed-
ded in the groundmass itself may be concluded to have crystallized between
1000 and 1040°C. In the cases where the phlogopite has crystallized in the
vesicles of the lavas its crystallization temperature is probably somewhat
below 1000°C. The higher Mg/Fe ratio of these vesicle phlogopites occur-
ring in the wehrlite has above been taken as an indication of a slightly higher
temperature of crystallization, which is approximately estimated to be
around 1050°C.
The biotite phenocrysts occurring in the tristanites and the trachytes are
usually accompanied by feldspar phenocrysts, which in most cases are
present in amounts far exceeding that of the biotite itself. The crystallization
of the alkali feldspar of these rocks starts at temperatures of 995—1025°C
according to the arguments presented in the feldspar chapter above, while
the plagioclases are of still higher temperatures (1130— 1195°C) in the cases
where they preceed the crystallization of the alkali feldspar.
The trachyte sample, Jan 77, contains relatively few, euhedral, unaltered
biotite phenocrysts of considerable size and more abundant euhedral small
phenocrysts of oligoclase to anorthoclase compositions in a flow-bandcd
groundmass of alkali feldspar and clear glass. Such features clearly indicate
an equilibrium crystallization of these two phenocryst phases and further
indicate that the crystallization of the biotite started prior to that of the
alkali feldspar.
The history of the crystallization of the biotite thus seems to be a short
one. This crystallization leads to early formed euhedral phenocrysts of
rather limited amounts which are accompanied or succeeded by the earliest
alkali feldspars. Thus the crystallization temperature of the biotites of these
tristanites and trachytes seems to start well above 1000°C but it most
probably does not exceed 1100°C. The depth of this crystallization is
unknown but it appears to be a shallow one, as it takes place at tempera-
tures higher than in the case of the alkali feldspars but lower than for the
early plagioclases.
A postcrystallization breakdown of the biotite has taken place in many of