Geology Reference
In-Depth Information
13.2.4.2 Investigation on an Orendite, Wyomingite and a Madupite
from Leucite Hills
Water-saturated melting relations to 0.5 GPa on three Leucite Hill lavas were
carried out by Barton and Hamilton (1978). Composition of these three lavas are
given in Table
13.2
. The wyomingite is characterized by the presence of micro-
phenocryst of phlogopite and diopside in a
fine-grained groundmass comprising
leucite, diopside, K-richterite, apatite and glass. The orendite was constituted of
microphenocrysts of phlogopite set in a groundmass containing leucite, diopside,
K-richterite, apatite, sanidine and sometimes rare amount of glass, which in some
samples are absent. The madupite had microphenocryst of diopside (often enclosed
by poikilitic crystals of phlogopite) set in a groundmass containing leucite and
diopside.
The water-saturated phase relationship of orendite, wyomingite and madupite is
shown in Fig.
13.6
a
figures show that leucite is a primary liquidus phase
in case of both orendite and madupite at low pressures (<0.05 GPa). In the wy-
omingite, leucite olivine and clinopyroxene co-precipitate within a very small
temperature interval. Olivine and clinopyroxene are the primary liquidus phases
from 0.05 to 0.5 GPa and with increase of pressure the proportion of near liquidus
olivine decreases relative to clinopyroxene. Phologopite is the primary liquidus
phase at a pressure close to 0.1 GPa in both the orendite and wyomingite. In case of
wyomingite, clinopyroxene is the liquidus phase at all pressures, whereas in the
orendite, clinopyroxene is the liquidus phase at pressure close to 0.5 GPa. Barton
and Hamilton noted that like the madupite, the relative proportion of near liquidus
olivine decreases with increasing pressure in both the wyomingite and orendite,
compared to one atmospheric liquidus temperature, there is a drop of 100
-
c. These
140
°
Cat
-
0.1 GPa (Fig.
13.6
).
Barton and Hamilton noted that experimentally determined order of phase
appearance is in agreement with the inferred crystallization sequences for natural
lavas. They found that phlogopite is the major primary liquidus phase of orendite at
pressures above
0.1 GPa and joined by clinopyroxene only at pressures close to
0.5 GPa. The orendite contains microphenocrysts of phlogopite and clinopyroxene
as groundmass phases. Initial crystallization of these lavas might have started at
pressures between 0.1 and 0.5 GPa. Phologopite and clinopyroxene co-precipitate
at the liquidus of wyomingite at 0.1 GPa . They further concluded that absence of
olivine in some wyomingites and orendites may be related to reaction relationship
between olivine and liquid, resulting in the resorption of olivine with crystallization
of phlogopite.
Experimental results on the madupite indicate that the clinopyroxene crystallizes
with phlogopite and olivine above 0.3 GPa. Olivine + liquid appear as a liquidus
phase at or above 0.05 GPa, whereas phlogopite + olivine coprecipitate as primary
phases above 0.1 but below 0.3 GPa. The presence of leucite suggests that the
groundmass of the lavas and their hypabyssal equivalents crystallize at low pres-
sures. The abundance of amphibole at low temperature in the near solidus tem-
perature is in agreement with the observation that this phase is a late crystallization
*
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