Geology Reference
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quaternary invariant points: kalsilites, leucite, phlogopite, forsterite, orthopyroxene,
quartz, different types of carbonates, liquid and vapour. These univariant reactions
and various singular points represent assemblage similar to different types of K-rich
rocks. These different assemblages are found in various localities, where ultrapot-
assic rocks have been reported. Because of reaction relationship between felds-
pathoids and silica, there are thermal barriers, which inhibit crystallization of silica-
undersaturated and silica-saturated rock types.
Phase equilibria study of the silica-undersaturated portion of the forsterite
-
kalsilite
SiO
2
system under 2.8 GPa (Gupta and Green 1988) illustrates mechanism
by which, even in this simple system, the availability of C
-
fluid may produce
diverse liquids during very low degrees of melting of a model phlogopite-bearing
harzburgitic source rock. In addition, small differences in H
2
O/CO
2
ratio may
control the appearance of phlogopite during fractionation of highly potassic liquids
in the upper mantle. The early appearance of phlogopite will drive liquids towards
silica-oversaturated derivative melts, whereas the same parent magma following a
P
H
O
-
-
field, will fractionate to the silica-undersaturated
minimum at high pressure (kalsilite + sanidine + forsterite or leucite + sani-
dine + forsterite). Luth (1997) observed that a pyroxene and phlogopite-bearing
assemblage yields phlogopite + diopside + garnet + quartz at 6
-
T path avoiding the phlogopite
±
1 GPa. K-richterite
±
joins this assemblage at 8
1 GPa, and at still higher pressure, phlogopite is
completely replaced by K-richterite. The stable assemblage is diopside + gar-
net + forsterite + K-richterite above the line de
ned in the P-T
-
T space by the points
11.5 GPa and 1,000
°
C and 8 GPa and 1,400
°
C.
leucitite, a katungite, a madupite, a
leucite lamproite, a phlogopite mafurite, a olivine ugandite and a phlogopite-
bearing minette, shows that under mantle P-T
Experimental study of a melilite
nepheline
-
-
T conditions all these rock types are
-
represented by a phlogopite
pyroxinite (
±
richterite,
±
apatite
±
spinel
±
rutile
±
pride-
-
rite
±
wadeite
±
paragonite). Only a wolgidite under mantle P-T
T conditions, is
-
represented by a rutile and phlogopite-bearing websterite.
Presence of both compatible (Ni, Co, Cr, and Sc) and incompatible elements
(K, Rb, Ba, Sr and U, F, P, etc.) in these rocks and great variability in
87
Sr/
86
Sr and
18
O/
16
O ratios suggest that these rocks are product of geosphere mixing processes.
Jia et al. (2007) have determined nitrogen concentration for four leucite-bearing
tephrites) and lamproites. They found that the N-content of these rocks varies
between 60 and 270 ppm, a maximum being 270 ppm and possibly this high
concentration is related to nitrogen components added to the mantle by sedimentary
subduction processes. The global
flux of nitrogen in rocks of island arcs is
10
8
mol/year. This is related to large
6.4
×
flux of subducted sedimentary nitrogen,
10
8
mol/year) has a relatively
smaller but measurable subducted components. The volcanic gas and hydrothermal
components have a concentration of mantle-derived nitrogen of up to 9
whereas nitrogen
flux from back-arc basins (5.6
×
30 %,
which is in agreement with those of phenocrysts in subareal volcanic rocks and
-
fluid inclusions. In general nitrogen in island arc samples is dominated by atmo-
spheric and/or sedimentary components.
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