Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 209

mineralchemistryand relationships
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less Sr than later formed crystals (Table 32). Groundmass grains and scarce
microphenocrysts of plagioclase are the only feldspars of the ankaramites.
f he analyses of these usually reveal no SrO (i.e. <0.005 wt. per cent) in
spite of 200—600 ppm Sr in the whole rock. Whether the early crystallized
alkali feldspars are richer in Sr than the later formed ones is not clear
because of low concentrations. Where alkali feldspars and plagioclases occur
together in the intermediate rocks, the plagioclases are always richer in Sr
than the alkali feldspars.
Sun et al. (1974) and Drake & Weill (1975) have carried out experimental
mvestigations on the distribution of Sr between plagioclase and liquid.
According to these authors the partition coefficient or distribution coeffi-
cient of Sr between plagioclase and liquid, (Dgr = of *s Sreater than
1 at all geologically reasonable temperatures and is strongly dependent
npon the temperature. At high temperatures plagioclase takes less Sr than at
lower temperatures. (In the experiments ofDrake & Weill (op.cit.) the DSr
varies from 1.18 at 1400°C to 3.06 at 1150°C). According to Sun et al.
(op.cit.) the distribution coefficient is independent of ÍD2 but dependent on
the bulk composition; but it is independent of Sr concentrations (“from
mfinite dilution to well above concentrations observed in igneous rocks”)
according to Drake & Weill (op.cit.). Drake & Weill predicted that under
most circumstances plagioclase fractionation will impoverish the liquid in Sr
°n an absolute basis since DSr>l at temperatures less than 1450°C. They
claim this prediction to be in accordance with observations on actual
^gneous differentiation trends. Sun et al. (op.cit.) concluded that there will
be no universal best distribution coefficient which can be used for diíferent
kinds of volcanic rocks in partial melting or fractional crystallization
calculations and that the experimentally determined distribution coeffi-
cients should be used with caution. According to Drake & Weill (op.cit.)
stoichiometric considerations indicate that Sr in plagioclases substitutes for
Ca only in the concentration range of observed natural rocks.
Plagioclase is thus enriched in Sr relative to the liquid it crystallizes from
and increasingly so with falling temperature. The first plagioclases of each
sample to crystallize should thus be poorer in Sr than later plagioclases if the
temperature difference is great. If the first plagioclase crystallization takes
place at relatively low temperature, the liquid may become so depleted in Sr
that later plagioclases will be poorer in Sr than the previous ones even
though formed at lower temperatures.
As shown by Table 32, the cores of the phenocrysts of the Jan Mayen
basalts are usually lower in Sr than the margins, which again are lower than
the groundmass grains. Qualitatively therc is thus an agreement between
the Sr pattern observed in the plagioclases of the Jan Mayen basalts and the
prediction of the distribution coefficient changes, provided that there was a
relatively great diíference between the temperature of the magma at the time
of phenocryst crystallization and the late- or groundmass crystallization.