Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 235

mineralchemistry and relationships
231
tion of the individual rocks. That most of the apatite containing samples
belong to this category might indicate that these apatites are not found in
samples where they have crystallized in equilibrium with the liquid (repre-
sented by the rock composition). If this is the case, the lack of correlation
between volatile composition of the apatites and the D.I. (or some such
parameter) of the whole rock is to be expected. As regards how far from
equilibrium with the present rock composition these apatites are, the
significant aspects are which one of the mixing components the apatite
comes from and what the volume relations and compositions of the mixing
components are.
l'he high OH-low F group apatites may thus be expected to have
crystallized from relatively basic liquids or eventually to be relatively late
tninerals to crystallize from more evolved liquids. The OH poor-F rich
group apatites may then similarily be expected to have crystallized from
tnore evolved liquids or to be relatively late crystallization products of the
basic liquids. Such a F poor apatite in a basic rock (Jan 329 & 70) would be
of early crystallization origin from the liquid presented by the rock composi-
don, while a F rich apatite in a basic rock (Jan 87 & 69) would bc either one
oí late crystallization from the rock or introduced into the rock through
mixing of an evolved liquid. The later possibility is more likely in this case,
as the apatite is a phenocryst phase and the plagioclases of the samples
indicate mixing. A F rich apatite in an evolved rock (Jan 175 & 324) should
be crystallized from that liquid (late or early). 'Fhe apatites of these two
samples are rich in Mn relative to the other apatites. The iron-titanium
oxides of these rocks (closely associated with the apatites) are Mn rich as
well. Mn rich iron-titanium oxides crystallize from evolved magmas only.
lhis similarity of the apatites and oxides supports the picture presented
here. A F poor apatite in an evolved rock (Jan 37 & 168) would either
fepresent a late crystallization phase or more likely be introduced into the
rock through mixing of a basic liquid.
The crystallization conditions of these apatites regarding temperature
and pressure are not readily determined. Their relations to the iron-titan-
ium oxides and the feldspars are probably the only indications or restric-
ttons in this respect.
In the hydrous mineral xenolith (Jan 329) the euhedral relatively big
apatite grains are closely associated with the iron-titanium oxides. I hese
oxide grains are in most cases sandwich type lamellae of ilmenite and
magnetite as previously described. These lamellae give a temperature of
formation of 1210°C, but the lamellae origin is rather uncertain. A tempera-
ture around 1200°C as the crystallization temperature ofthe earliest apatites
of the rock series may thus, at least, be suspected. Most of the basic rocks,
though, only contain titanomagnetite phenocrysts and no ilmenites. I he
crystallization temperature of these has been estimated to be mostly
between 1100°C and 1150°C but to extend down to 1040°C in cases. The late