Rit (Vísindafélag Íslendinga) - 01.06.1984, Side 265
PETROGENETIC REEATIONSHIPS
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covariation is a final restriction to the possibility of the process operating or
not. No matter how accurate a calculation program for the fractionation
model is used, or how perfect a fit is obtained, the minerals to be fraction-
ated must be real and in equilibrium relations to the liquids, if the process is
to represent a real case.
There are numerous indications, mentioned in the discussions on the
mineral relationships and summarized in Fig. 126 above, of a wide range of
crystallization temperature and pressure of the minerals comprising thejan
Mayen rocks. Furthermore the petrology frcquently indicates complications
such as mixing and resorption or other phenomena. The model to be tested
for fractional crystallization must be in accordance with these indications. A
model for fractionation in a single closed magma chamber does not apply to
this magma system, and what is called for could be described as “a network
of magma channels” open for both crystal transport and magma injection
and thus allowing mixing of magmas and even their contamination in some
cases. This is in fact far to complex a model to be tested with any reliability
without a lot of restrictions not at hand. On the other hand, an evolutionary
picture is indicated by the frequently encountered heterogeneous nature of
both the phenocryst assemblage and groundmass structure.
In the following chapters crystal fractionation will be tested by a least-
squares calculation method, for the major elements (Bryan et al., 1969). The
minerals used in the calculations are chosen such that they give a best total
fit between the “analysed and the calculated daughter” compositions, when
subtracted from the “analysed mother”. A further restriction on the minerals
used is that they are found in thc rocks considered. They are thus not
necessarily in equilibrium with the “liquids” of the calculations, but as close
to it as one can get by this crude kind of approximation. The calculations
start by subtracting the wehrlite minerals from the most mafic rock composi-
tions to produce less mafic ones, which are then used as “mother liquids” to
still less mafic ones and so on, until the most evolved rocks are derived as
“daughter liquids".
II. The early fractionation
In the case of the Jan Mayen rocks, the early crystallization of chrome-
spinel, chromian diopside and Fo rich olivine (a wehrlite) from primitive
liquids has already been discussed. The primitive liquid has been found in
these early stages to be of “ankaramitic” composition, i.e. of high Mg
content and high Mg values. These are the characteristics of the ankaramite
lavas themselves.
Further compositional restrictions on this primitive early liquid are small.
The high Mg content and high Mg values are shown by the wehrlite mineral
compositions (see Figs. 84—86 and 94). Taking the most magnesian ankara-
mite composition (Jan 166) as a substitute for the primitive liquid, and