Geology Reference
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sanidine is observed only as ghost-like domains in some of the glass fragments, and
appears to have been formed during quenching. The solid phases crystallized
experimentally are generally compositionally similar to the minerals in the rock.
These similarities and the experimental phase relations support the concept of a
rapid initial magma ascent with only a small temperature drop and crystallization of
olivine, but not of orthopyroxene. At lower pressure, less than
1.2 GPa, it appears
that the magma ascended more slowly with a larger temperature drop suggested by
the similarity of the experimentally determined sequence of assemblage to the
paragenesis of the rock.
*
13.2.13 Phase Relations in a Sanidine Phlogopite Lamproite
under High P-T
T Conditions
-
Mitchell (1995) made melting experiments on a sanidine phlogopite lamproite (for
analysis see Table 13.2 )at4
C
(Fig. 13.22 ). This particular sample was collected from Table Mountain, Leucite
Hills, Wyoming. The lamproite is a silica-rich variety, and has been postulated to be
representative of the magmas which were parental to the Leucite Hills volcanic
7 GPa and temperatures from 1,000 to 1,700
°
-
field. Near-liquidus phases above 5 GPa are pyrope-rich garnet and jadeite-rich
pyroxene. Below 5 GPa, jadeite-poor pyroxene is the only near-liquidus phase.
Near-solidus assemblage consists of clinopyroxene, titanian potassium richterite
and titanian phlogopite with either potassium titanian silicate above 5 GPa or
potassium feldspar below 5 GPa. The potassium titanian silicate is a high-pressure
phase ranging in composition form K 4 Ti 2 Si 7 O 20 to K 4 TiSi 8 O 20 . It coexists at
pressures above 6 GPa at 1,100
Ba-
phosphate is a common near-solidus phase. The phase relationship is interpreted to
suggest that lamproite cannot be derived by partial melting of simple lherzolite
sources. However, it is proposed that sanidine phlogopite lamproites are derived by
high degrees of partial melting of ancient metasomatic veins within a harzburgitic-
lherzolitic lithospheric substrate mantle. The veins are considered to coexist with
phlogopite, K-richterite, K-Ti
1,400
°
C. A previously unrecognized K
-
-
Ti silicates.
The change in slope of phase boundaries at A, B and C would denote change in
mineralogical assemblages, but the phase diagram of Michell (1995, Fig. 13.22 )
does not show any new phase assemblage in the right hand side of the line BC. If
this change in slope is correct careful study of the run products might reveal the
presence of a new assemblage in that region.
Mitchell and Edgar (2002) studied phase relations on a leucite-phlogopite
lamproite from Oscar plug (West Kimberely, Australia, Fig. 13.23 ) at pressures
between 2 to 6.4 GPa and temperature ranging between 800 and 1,700
Ba-phosphate and K
-
-
C.
Their experimental study on a madupitic lampriotes from Middle Table
Mountain, Wyoming, (U.S.A) is summarized in Fig. 13.23 , whereas phase rela-
tionships of a sanidine-richterite lamproite from Cancarix, Murcia-Almeria, Spain
are shown in Fig. 13.24 .
°
 
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