JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 12
PAGES 2171^2175
2008
doi:10.1093/petrology/egn061
Comments on: Liquid Immiscibility and the
Evolution of Basaltic Magma
ANTHONY R. PHILPOTTS*
DEPARTMENT OF GEOLOGY AND GEOPHYSICS, YALE UNIVERSITY, NEW HAVEN, CT 06520-8109, USA
RECEIVED FEBRUARY 26, 2008; ACCEPTED MARCH 20, 2008
ADVANCE ACCESS PUBLICATION DECEMBER 9, 2008
Veksler et al. (2007) present the results of interesting experiments that purport to demonstrate that fractionating
basaltic magma can split into immiscible liquid fractions
at temperatures above 11008C, which is considerably
higher than has been found in all previous experimental
studies for melts of similar composition. Previous studies
showed that immiscibility occurred just above 10008C and
could therefore play a role during only the late stages of
fractionation of basaltic magma. Veksler et al., however,
claim that the higher temperature immiscibility demonstrated in their experiments shows that immiscibility
could play a significant role at much earlier stages of differentiation. If correct, this is an important finding. Having
worked on silicate liquid immiscibility since the 1960s, I
would be only too pleased if their conclusion were correct.
Unfortunately, I believe their results do not indicate stable
high-temperature immiscibility but rather metastable
phase separation developed during quenching of their
experimental charges that were run in a centrifuge.
All binary systems involving SiO2 with FeO, MnO,
MgO, CaO, and TiO2 exhibit liquid immiscibility at temperatures near 17008C (e.g. the binary SiO2^Fayalite in
Fig. 1). With addition of small amounts of alumina and
alkalis to these binary systems, the two-liquid field disappears in ternary or more complex systems (Fig. 1a).
However, Roedder (1951) showed that the two-liquid field
reappears at lower temperatures in the system K2O^FeO^
Al2O3^SiO2 (Fig. 1a). It is important to recognize that,
although the low-temperature two-liquid field is separate
from the high-temperature field, they are both intersections of the same two-liquid solvus with the liquidus surface in this system (Visser & Koster van Groos, 1979).
To illustrate this phase relation, Fig. 1c shows a schematic
perspective view of the system, where the two-liquid
solvus can be visualized as analogous to an anticline plunging from high temperatures on the SiO2^Fayalite binary to
low temperatures in the ternary system. This solvus
plunges below the liquidus on leaving the SiO2^Fayalite
binary and hence becomes metastable. However, at lower
temperatures along the Fayalite^Tridymite cotectic, the
solvus reappears above the liquidus and again becomes
stable. The two-liquid solvus then plunges beneath the
liquidus again and becomes metastable as it approaches
the Fayalite^Leucite binary. It is important to recognize
that even where the two-liquid field is not stable, this field
is lurking just beneath the stable liquidus. In their experiments, Visser & Koster van Groos (1979) were able to trace
the metastable two-liquid field to compositions beyond the
Fayalite^Orthoclase join (Fig. 1c). If a single-phase liquid is
quenched rapidly enough to prevent crystallization, it can
easily intersect the metastable two-liquid field, or its spinode, and develop fine-scale phase separation. Indeed,
this metastable phase separation is taken advantage of
commercially by Corning to create some of the desirable
physical properties of their PyroCeramÕ, which is
used, for example, in the nose-cones of rockets and
CorningwareÕ for cooking. Given the similarity of the supposed immiscibility textures described by Veksler et al.
(2007) to the metastable immiscibility textures found by
Visser & Koster van Groos (1979) in the system K2O^
FeO^Al2O3^SiO2, it is surprising that they make no reference to this paper.
We can draw three important conclusions from the
nature of immiscibility in the system K2O^FeO^Al2O3^
SiO2 that are relevant to the discussion of the Veksler et al.
paper. First, the top of the two-liquid solvus and the
*Corresponding author. E-mail: [email protected]
ß The Author 2008. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oxfordjournals.org
VOLUME 49
NUMBER 12
Fa
17
00
Fa
Fayalite
°C
DECEMBER 2008
(c)
Two
-liqu
A
~17
Cr
15
Tr
°C
BA
30°C
Lc
A 11
Twoliquids
(b)
50 µm
Cr
Or
Lc
B A
P E
Or
Leucite
SiO2
10 µm
Tr
Lc
P E
Two liquids
s
C
°C
30
11
Fa
olvu
00°
12
Fe
Si
id s
Temperature
JOURNAL OF PETROLOGY
SiO2
Orthoclase
(a)
(d)
Fig. 1. (a) System Fayalite^Leucite^SiO2 showing the high-temperature and low-temperature immiscibility fields (after Roedder, 1951).
(b) Polished section of an experimental charge run in the middle of the low-temperature immiscibility field in this system. The iron-rich immiscible droplets (Fe) are more reflective and sit in a silica-rich glass (Si), which is dark opalescent blue colour in thin section because of the presence of minute immiscible droplets formed during quenching. (c) Schematic perspective diagram of the system Fayalite^Leucite^SiO2, showing
the two-liquid solvus extending stably above the liquidus near the Fayalite^SiO2 join and near the Fayalite^Tridymite cotectic; elsewhere the
solvus is beneath the liquidus and hence metastable (after Visser & Koster van Groos, 1979). (d) Immiscible iron-rich (dark brown in thin section) and silica-rich (clear in thin section) liquids coexisting with plagioclase (An50), augite, and pigeonite formed at 9958C (Ni^NiO buffer)
from an initially homogeneous liquid formed by fusing equal amounts of jotunite and quartz mangerite (from Philpotts, 1981a).
liquidus are close over a wide range of composition, and
consequently, addition of components that affect the
solvus or the liquidus, even slightly, can cause significant
changes in the extent of stable immiscibility. For example,
addition of small amounts of Ti, P, or Fe3þ enlarge the
two-liquid field, whereas addition of H2O decreases it
(Freestone, 1978; Naslund, 1983). Second, because the metastable two-liquid field is never far below the liquidus,
metastable two-phase glasses are a common experimental
run product, even when quenching is rapid (a few seconds).
Third, stable immiscible liquids produce globules
up to 2 mm in diameter in runs of only a few hours
(Fig. 1b), which contrasts with the submicroscopic metastable phase separation that produces opalescent glasses,
which are commonly blue (Visser & Koster van Groos,
1979).
Immiscible glasses have been found in a wide range of
tholeiitic basalts, in some andesites, and even in alkaline
basalts (e.g. De, 1974; Philpotts, 1979, 1982), and in lunar
basalts (Roedder & Weiblen, 1971). The glassy globules in
the mesostasis of these rocks typically have diameters
up to 10 mm. Initially, these two-phase glasses were considered to be of metastable origin (Biggar, 1979); that is,
they formed only because there was insufficient time
for pyroxene and plagioclase to crystallize. Subsequent
experimental studies, however, confirmed that these
immiscible phases had formed stably. Stable immiscible
liquids have been produced experimentally by partially
melting and cooling natural rocks (Dixon & Rutherford,
1979; Philpotts, 1979, 1981a; Philpotts & Doyle, 1983) and
from melts formed by fusing mixtures of genetically related
contrasting rocks (McBirney & Nakamura, 1974;
McBirney, 1975; Philpotts, 1967, 1981a).
Three important conclusions can be drawn from these
experimental studies, vis-a'-vis the Veksler et al. paper. First,
stable immiscibility in natural rock compositions occurs at
temperatures approximately 1008C lower than those in the
low-temperature immiscibility field in the system K2O^
FeO^Al2O3^SiO2. Second, these immiscible liquids have
been homogenized on heating to form a homogeneous
liquid at temperatures that never exceed 10408C. For
example, the consolute temperature in a mid-ocean ridge
basalt was found to be 10108C (Dixon & Rutherford, 1979);
the consolute temperature in an iron-rich tholeiite is
10158C (Philpotts & Doyle, 1983); in a fused mixture of
ferrodiorite and melanogranophyre from the Skaergaard
the consolute temperature is 10108C, which is 5^108C
above the liquidus (McBirney, 1975); and in a fused
2172
PHILPOTTS
PHILPOTTS COMMENTS
mixture of jotunite (hypersthene ferrodiorite) and quartz
mangerite (hypersthene quartz monzonite) the consolute
temperature is 10408C (Philpotts, 1981a). Third, in all of
these studies, no difficulties were encountered in growing
immiscible droplets that were large enough to analyze by
electron microprobe; that is, droplets with diameters 410
mm are common. Even at the low temperature of 9958C,
droplets with a diameter of 15 mm (Fig. 1d) were generated
from an initially homogeneous glass formed by fusing
equal proportions of jotunite and quartz mangerite
(Philpotts, 1981a). It should also be emphasized that,
during quenching of experimental runs, these stable
immiscible globules develop minute immiscible droplets,
even when quenching of experimental runs occurs within
a few seconds (see stippled nature of large droplet in
Fig. 1d).
Veksler et al. use 10 different starting compositions for
their experiments. The results for one of these compositions (SF-1), which is intermediate between the average
Fe-rich and Si-rch immiscible glasses in tholeiitic basalts
(Philpotts, 1982), were reported previously (Veksler et al.,
2006). Consequently, the details of these experiments are
omitted from Veksler et al. (2007), but the comments below
apply equally to the experiments described by Veksler et al.
(2006) and (2007).
Veksler et al. (2007) present the results of their static
experiments first. The charges were held at temperature
under controlled oxygen fugacities in vertical quench furnaces. They report finding no ‘unambiguous macroscopic
signs of liquid immiscibility’. They conclude, therefore,
that immiscible droplets have difficulty nucleating in
these experiments. However, the lack of evidence of immiscibility has a far simpler explanationçthe experiments
were run at temperatures above the two-liquid field. This
is easy to demonstrate in the case of composition SF-1,
which is almost identical to the composition of the glass
formed from homogenizing the immiscible liquids in the
tholeiitic Holyoke basalt (Philpotts & Doyle, 1983). The
mesostasis of this basalt contains pyroxene-rich spheres in
a silica-rich glassy host. On heating the basalt, the pyroxene spheres first melt to globules of an iron-rich liquid in
an immiscible silica-rich liquid. On further heating,
these immiscible globules homogenize at 10188C under the
magnetite^wu«stite oxygen buffer. The consolute temperature under the quartz^fayalite^magnetite oxygen buffer is
slightly higher at 10228C. The lowest temperature that
Veksler et al. (2007) report for experiments on SF-1 is
11238C, which is 1008C above the stable consolute temperature found by Philpotts & Doyle (1983). Veksler et al.
use equally high temperatures for their static experiments
on the other starting compositions. It is not surprising,
therefore, that these experiments did not generate immiscible liquidsçthey were performed above the consolute
temperature.
Having failed to produce immiscible droplets in their
static experiments, Veksler et al. undertake the centrifuge
experiments in hope of circumventing the supposed kinetic
problem. These experiments were carried out at the same
high temperatures as the static experiments; that is,
1008C above the known consolute temperatures for
these compositions. It is not surprising, therefore, that in
none of their centrifuge experiments were stable macroscopic immiscible globules formed. Quenching in the
centrifuge was much slower than in vertical quench furnaces, taking up to 2 min to drop below 8008C. During
quenching, the centrifuge continued to spin the charge.
Because of the slow quench, many of the experimental
charges consist of opalescent submicroscopic two-phase
glasses, which appear identical to the metastable immiscible glasses reported by Visser & Koster van Groos (1979).
Veksler et al., however, found compositional gradients
from top to bottom of their charges, which they claim indicate that the liquid must have consisted of two phases
at high temperature that were separated by the high
gravitational acceleration of the centrifuge (1000g). These
gradients are interesting, but they can be explained without having to resort to high-temperature stable
immiscibility.
The first possible source of a gradient in the charges
results from the way in which the charges are prepared.
First, all components other than iron were prepared as a
homogeneous glass by repeated fusing and grinding. This
is a standard technique and should produce a homogeneous charge. The iron, however, was then added to this
mixture as FeO powder. This mechanical mixture was
then fused while the centrifuge was spinning. The charges
were not ground and re-fused. A wide range of liquid compositions with widely different densities would have been
present before the charge fused to a homogeneous mass.
Although major redistribution of components during this
fusion process is unlikely, small gradients at the top and
bottom of the charges such as those shown in fig. 5 of
Veksler et al. (2007) could result. Veksler et al. claim that
‘complete melting and homogenization was checked by a
few test runs in which containers were opened and the
glasses were examined after heating at slow rotation’.
However, this does not demonstrate that they did not
have a chemical gradient near the top or bottom of the
charge. The results of experiments on a model Skaergaard
mixture (MZ-1) at 1110^11208C appear to confirm that
such gradients were present in the charges even when they
are described as being ‘optically homogeneous’. Even metastable phase separation produces glasses that are clearly
stippled or opalescent and are not ‘optically homogeneous’.
It is surprising therefore that even when Veksler et al.
describe a glass as being ‘optically homogeneous’, they
interpret a compositional gradient in it as resulting from
stable immiscibility.
2173
JOURNAL OF PETROLOGY
VOLUME 49
In runs where submicroscopic metastable immiscible
globules form during quenching, it is possible that the
high gravitational acceleration of the centrifuge may
cause some small-scale redistribution of phases during the
2 min quenches. Although Stokes’ Law indicates that single
submicron globules would not be able to travel a significant
distance during the 2 min quench, clumps of these globules, which are revealed by transmission electron microscope, might be able to move a sufficient distance to
produce the observed gradients.
In experiment C-112 (Veksler et al., 2007, fig. 5), Veksler
et al. state that ‘a narrow but clear and distinct layer of
silicic glass formed at the top’ of the charge, which was
run at 11708C. This is interpreted to be an accumulation of
a buoyant silica-rich immiscible liquid. However, the lower
boundary of this layer is diffuse and is not marked by a
sharp meniscus, which has been found in all other experimental studies of stable silicate immiscibility (e.g. Fig. 1).
In an attempt to show that the high-temperature immiscible liquids obtained in the centrifuge experiments are
stable, Veksler et al. undertake so-called ‘reversal experiments’ on model Skaergaard iron- and silica-rich liquids.
In these experiments, half the charge was filled with
the iron-rich glass and the other half with a silica-rich
glass. The glasses were then fused at 13508C for a few
minutes and then held at 1090 or 11148C for 1^3 days.
Although the two liquids completely homogenized in
only one experiment, electron microprobe analyses
showed narrow diffusion boundaries between the two
liquids. Unfortunately, because of the initial preheating at
13508C, it is not possible to say whether this diffusion
occurred at this high temperature or during the longer
low-temperature run. What would have constituted a true
test of reversibility would have been to take an initially
homogeneous liquid and lower its temperature to
10008C where sizable immiscible globules could have
formed. This then could have been used as the starting
material for experiments in which temperatures were
raised until the immiscible globules dissolved in each
other. It is in this way that the consolute temperature has
been determined in previous experimental work (e.g.
Philpotts & Doyle, 1983).
Veksler et al. correctly draw attention to the fact that
their experiments do not produce the ‘spectacular immiscibility textures of the natural’ volcanic rocks. They ignore
the fact that other experimental studies have produced
such textures with ease in a matter of hours (e.g. Fig. 1d).
Instead, they claim that the formation of immiscible droplets is hindered by a kinetic barrier. This is most definitely
not the case. For compositions near the two-liquid field, it
is almost impossible to quench runs fast enough to prevent
nucleation and growth of quench immiscible droplets. Both
of the experimental run products shown in Fig. 1b and d
were quenched within seconds by dropping from a vertical
NUMBER 12
DECEMBER 2008
tube furnace into water. In Fig. 1b, all of the small dark
droplets in the larger Fe-rich droplets (light color) and the
extremely small droplets in the darker Si-rich host, which
give it an opalescent blue color, formed during quench. In
Fig. 1d, small Si-rich droplets can be seen to have formed in
the larger dark brown Fe-rich droplet during quench. Nor
is there any problem in coarsening droplets if held at temperature for a sufficient length of time. The 15 mm diameter droplet shown in Fig. 1d was formed after holding
the initially homogeneous glass at 9958C for 290 h
(Philpotts, 1981a). At higher temperatures, where diffusion
is more rapid, coarsening takes place more rapidly. For
example, Visser & Koster van Groos (1979) were able to
grow 2 mm diameter immiscible droplets in the system
K2O^FeO^Al2O3^SiO2 at 11308C in only a few hours.
The argument that the kinetic barrier to phase separation
at high temperature is related to a decrease in the interfacial energy between the two liquids cannot be valid,
because the interfacial energy decreases to zero at the consolute temperature, regardless of whether this is at high or
low temperature.
Natural examples of relatively magnesium-rich immiscible glasses trapped as melt inclusions in phenocrysts are
cited by Veksler et al. as evidence for high-temperature
immiscibility. Most of these inclusions occur in plagioclase
phenocrysts in tholeiitic basalt and andesite, and, as shown
by Philpotts (1981b), they form metastably as a result of the
failure of pyroxene to crystallize in the inclusions. In the
groundmass surrounding these phenocrysts where pyroxene did crystallize, immiscible droplets are present in the
final mesostasis, but they are iron-rich. Veksler et al. refer
to a study by Krasov & Clocchiatti (1979) of similar Mgrich inclusions in plagioclase phenocrysts (An70) in an
andesite from Kamchatka. The iron-rich glass had crystallized to pyroxene, but on heating, it melted to an immiscible liquid in a silica-rich liquid, with the consolute
temperature being 12808C. It is hard to see how a homogeneous liquid could have been trapped at this unnaturally
high temperature. The alumina content of the immiscible
glasses in these inclusions is low and it is possible that
they do not represent a typical magma composition.
The main argument against immiscibility occurring at
elevated temperatures in basaltic and andesitic magmas is
that the field evidence for such phase separation is completely lacking. This is not to say that evidence for immiscibility is lacking. Indeed, it is very common in tholeiitic
pahoehoe flows (not aa flows), but it is invariably restricted
to the mesostasis, which typically forms after 75% crystallization. If magmas split into immiscible liquids at earlier
stages of crystallization, there is no reason for these phases
not to be preserved as glassy globules in some lavas. So far,
such immiscible glasses have not been found.
In conclusion, I believe that the experiments by Veksler
et al. (2006, 2007) demonstrate that a metastable two-liquid
2174
PHILPOTTS
PHILPOTTS COMMENTS
field covers a wide compositional range corresponding to
magmas intermediate between siliceous and basaltic compositions, but this two-liquid field does not extend stably
above 10408C in fractionated basaltic magmas and definitely does not extend above 11008C as they claim. Liquid
immiscibility can therefore play a role only in generating
late-stage granophyres from fractionated tholeiitic basaltic
magma.
AC K N O W L E D G E M E N T S
I would like to thank Ilya Veksler for taking the time to
show me the results of his many interesting experiments,
and for his lively and enthusiastic defense of liquid immiscibiliity. I hope he continues his investigations of the liquid
state.
R EF ER ENC ES
Biggar, G. M. (1979). Immiscibility in tholeiites. Mineralogical Magazine
43, 543^544.
De, A. (1974). Silicate liquid immiscibility in the Deccan traps and its
petrogenetic significance. Geological Society of America Bulletin 85,
471^474.
Dixon, S. & Rutherford, M. J. (1979). Plagiogranites as late-stage
immiscible liquids in ophiolite and mid-ocean ridge suites: an
experimental study. Earth and Planetary Science Letters 45, 45^60.
Freestone, I. C. (1978). Liquid immiscibility in alkali-rich magmas.
Chemical Geology 23, 115^123.
Krasov, N. F. & Clocchiatti, R. (1979). Immiscibility in silicate melts
and its possible petrogenetic importance, as shown by study of melt
inclusions. Transactions (Doklady) of the USSR Academy of Sciences 248,
92^95.
McBirney, A. R. (1975). Differentiation of the Skaergaard intrusion.
Nature 253, 691^694.
McBirney, A. R. & Nakamura, Y. (1974). Immiscibility in late-stage
magmas of the Skaergaard intrusion. Carnegie Institution of
Washington Yearbook 73, 348^352.
Naslund, H. R. (1983). The effect of oxygen fugacity on liquid immiscibility in iron-bearing silicate melts. AmericanJournal of Science 283,
1034^1059.
Philpotts, A. R. (1967). Origin of certain iron^titanium oxide and apatite rocks. Economic Geology 62, 303^315.
Philpotts, A. R. (1979). Silicate liquid immiscibility in tholeiitic
basalts. Journal of Petrology 20, 99^118.
Philpotts, A. R. (1981a). A model for the generation of massif-type
anorthosites. Canadian Mineralogist 19, 233^253.
Philpotts, A. R. (1981b). Liquid immiscibility in silicate melt
inclusions in plagioclase phenocrysts. Bulletin de Mine¤ ralogie 104,
317^324.
Philpotts, A. R. (1982). Compositions of immiscible liquids in volcanic
rocks. Contributions to Mineralogy and Petrology 80, 201^218.
Philpotts, A. R. & Doyle, C. D. (1983). Effect of magma oxidation state
on the extent of silicate liquid immiscibility in a tholeiitic basalt.
AmericanJournal of Science 283, 967^986.
Roedder, E. (1951). Low temperature liquid immiscibility in the system
K2O^FeO^Al2O3^SiO2. American Mineralogist 36, 282^286.
Roedder, E. & Weiblen, P. W. (1971). Petrology of silicate melt inclusions, Apollo 11 and Apollo 12, and terrestrial equivalents. In:
Proceedings of the Second Lunar Science Conference. Geochimica et
Cosmochimica Acta, Supplement 2, 1, 507^528.
Veksler, I. V., Dorfman, A. M., Danyushevsky, L. M., Jakobsen, J. K.
& Dingwell, D. B. (2006). Immiscible silicate liquid partition coefficients: implications for crystal^melt element partitioning and
basalt petrogenesis. Contributions to Mineralogy and Petrology 152,
685^702.
Veksler, I. V., Dorfman, A. M., Borisov, A. A., Wirth, R. &
Dingwell, D. B. (2007). Liquid immiscibility and the evolution of
basaltic magma. Journal of Petrology 48, 2187^2210.
Visser, W. & Koster van Groos, A. F. (1979). Phase relations in the
system K2O^FeO^Al2O3^SiO2 at 1 atmosphere with special
emphasis on low temperature liquid immiscibility. AmericanJournal
of Science 279, 70^91.
2175