Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 146
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PÁLLÍMSLAND
further discussed and tested by a calculation model for the chemistry in
Chapter 9.
The magma system of Jan Mayen is open for material transport (crystals
and liquids). Primary liquids accordingly have great difficulty in reaching
the surface without modifications. Obvious modifications are caused by
fractional crystallization. Further modifications by mixing of liquids (con-
taining phenocrysts) in the magma column is a likely process but more
difficult to trace.
The primary liquid is most probably similar in composition to the less
magnesian ankaramites as e.g. Jan 10, 30 and 28. This primary liquid is a
subject to crystallization of chromian diopside, olivine of composition
around Fogo and minor chrome-spinel, at depth, between 65 and 80 km.
These minerals accumulate to form a wehrlite, which may be brought to the
surface either as coherent rock fragments or as phenocrysts in the liquid.
The wehrlite fractionation leads to the more magnesian ankaramites (e.g.
Jan 12 and 166) where the minerals accumulate, and to relatively magnesian
basalts (e.g. Jan 66 and 46) where the minerals are subtracted.
The primary liquid itself has not been found on the surface in unmodified
condition. The most primitive liquid to erupt without containing noticeable
amounts of phenocrysts is Jan 26, an ankaramitic basalt containing only
minor amounts of small olivine and clinopyroxene phenocrysts or micro-
phenocrysts.
Further fractionation at shallower depths takes place involving more
fayalitic olivine and titan-salite clinopyroxene accompanied by opaque
oxides and feldspars.
E. OPAQUE OXIDES
I. Introduction
The opaque oxides of the Jan Mayen rocks occur in moderate amounts
throughout the rock suite and in all xenolith types, except the trachytic
glassy xenoliths. Mostly the oxides are primary minerals, but exsolution and
alteration products are common as well. Different rock types characteris-
tically contain different opaque oxide mineral assemblages.
The minerals that occur are members of the (Mg, Fe2+) (Cr, Al, Fe3+)204
-spinel solid solution series and iron-titanium oxides belonging to the
ulvospinel-magnetite solid solution series (titanomagnetites to magnetites
and titanomaghemites), the ilmenite-hematite solid solution series (ferrian-
ilmenites to titanohematites) and the pseudobrookite-ferropseudobrookite
solid solution series. Rutil has not been identified optically, but the micro-
probe analyses indicate its presence as a minor constituent in at least some of
the pseudomorphically highly oxidized mineral grains.