Rit (Vísindafélag Íslendinga) - 01.06.1984, Qupperneq 263

PETROGENETIC RELATIONSHIPS 259
one evolves towards the composition of the less primitive one, which evolves
further along the trend.
The most widely used indicator of primitiveness of magmas is the 100
Mg/Mg+Fe+2 ratio, (Mg value). The higher this value is, the more
primitive is the magma. Silica and alkalies have frequently been used as
indicators in this matter. These elements, in general, increase as the Mg
value decreases. In certain cases chemical composition of rocks may vary
considerably without changes of the Mg value. In such cases the amount of
the incompatible elements is frequently taken as an indicator of primi-
tiveness — low incompatible element contents indicating more primitive
magmas. Some of these incompatible elements may in cases reach the status
of becoming compatible major elements.
Primary magmas need not be primitive, if primary magma only refers to a
magma, unmodified since the time of generation, by melting of a solid rock.
But if to this defmition is added the constraint of generation within the
mantle, then the primary magmas are in most cases restricted to relatively
primitive ones. Without this constraint, on the other hand, even highly
evolved primary magmas may be formed. The major controlling factors are
the composition and mineralogy of the parental solid rock and the degree of
its melting. The most primitive of the primary magmas are those generated
at high temperatures well within the mantle. These are formed by the
melting of ultramafic parental rocks which contain only mafic minerals
stable at high temperatures and high pressures. The magmas generated
under these conditions are thus especially rich in Mg, Cr and Ni and poor in
the incompatible elements. How primitive these primary magmas can be is
however disputed. In the literature they are given a MgO range of from 8 to
over 20 wt. per cent.
As has been shown by the mineral relationships, summarized in Fig. 126
above, the earliest minerals of the Jan Mayen rock suite to precipitate from
magma are the wehrlite minerals; i.e. Cr rich spinel, chromian diopside and
Fo rich olivine. This crystallization takes place at mantle depths and high
temperatures (>18 kb & >1300°C). In chapter 8 the most Fo rich olivine
and the chromian diopside have been shown to have crystallized out of
liquids of similar Mg and Fe content as that of the less magnesian ankara-
mites (e.g. Jan 10 & 30). Ultramafic liquids of this composition are thus
available at depths in the Jan Mayen magma system. There are indications
among the mineral relationships that a slightly more primitive liquid is in
existence and capable of crystallizing both chromian diopside and the most
Cr rich spinel before these minerals are accompanied by the olivine. This
means that the most primitive Jan Mayen magma is close to the more
magnesian ankaramites in composition (Jan 12 & 166). The Mg values of
these two liquids are around 72 and 82 respectively. These magmas never
reached the surface in a totally liquid state. They are only represented on
the surface as ankaramites. The most primitive magma to reach the surface