Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 118

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PÁLL IMSI.AND
been analysed. These occur in the basalts and ankaramites and are titan-
salites.
a. Homogeneity of the clinopyroxenes
The pyroxenes oí the xenoliths are unzoned and relatively homogeneous,
while the pyroxenes of the rock suite show considerable heterogeneity. This
heterogeneity is reflected by both inter- and intra crystal compositional
variations.
The chromian diopside crystals occurring in the ankaramites are anhed-
ral, unzoned and relatively homogeneous in composition. They mostly occur
in this manner enclosed by the groundmass, but may also have a zoned
overgrowth of titan-salite. In the basalts the diopside mostly occurs as
anhedral cores mantled by this overgrowth. Individual crystals or cores are
fairly homogeneous, but each rock sample either contains crystals of similar
compositions or crystals showing considerable compositional differences.
The titan poor salites and the ferroaugite phenocrysts of the intermediate
rocks mostly occur as subhedral to euhedral rather small phenocrysts and
are unzoned or only slightly zoned and relatively homogeneous in composi-
tion. Occasionally they may be overgrown by a rim of amphibole.
The titan-salite phenocrysts occurring as individual crystals and over-
growth on chromian diopside are usually highly zoned with various complex
features in the zoning. Hourglass structure and “patchy” extinction are
commonly observed under the microscope (see Fig. 14 & 15). In certain
cases they are sector zoned. This can be shown by analyses, but no attempt
has been made to study the sector zoning features systematically. The two
analyses given in Table 19 show a typical compositional difference revealed
by analyses within the same zone on different sectors of the same titan-salite
crystal in a basalt. A general somewhat simplified picture of the titan-salite
zoning shows an increase in Ti and A1 and decrease in Cr and in the Mg/Fe
ratio from core to rim and a very thin marginal zone where these relations
are reversed again. Two crystals from different ankaramite samples, both
with a chromian diopside core and titan-salite overgrowth plotted in Fig. 76,
reveal this clearly. The titan-salite crystals of the groundmass of both the
basalts and the ankaramites are slightly richer in Mg relative to Fe, richer in
Cr and slightly poorer in Ti and A1 than the bulk of the titan-salite and thus
show the same trend as the final marginal zone of the phenocrysts (this is
shown by the x in Fig. 76). Thus the zoning of the titan-salite shows a
general iron enrichment of the clinopyroxenes as crystallization proceeds,
which is reversed in the short final stage of crystallization, the quick cooling
after extrusion. This general iron enrichment rule is broken in the case of
clinopyroxenes from one sample. Jan 66, an ankaramitic basalt, contains,
except for titan-salite, faintly greenish iron rich slightly sodic salites, which
are poor in Ti and Al. These crystals have a relatively thin marginal zone of
iron poor salite, rich in Ti and A1 and similar in composition to the bulk of
the titan-salite.