JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 12
PAGES 2177^2186
2008
doi:10.1093/petrology/egn064
Liquid Immiscibility and Evolution of Basaltic
Magma: Reply to S. A. Morse, A. R. McBirney
and A. R. Philpotts
ILYA V. VEKSLER1,2*, ALEXANDER M. DORFMAN3,
ALEXANDER A. BORISOV4, RICHARD WIRTH1 AND
DONALD B. DINGWELL3
1
HELMHOLTZ CENTRE POTSDAM^GFZ GERMAN RESEARCH CENTRE FOR GEOSCIENCES, SEKTION 4.1,
TELEGRAFENBERG, 14473 POTSDAM, GERMANY
2
TECHNICAL UNIVERSITY BERLIN, DEPARTMENT OF MINERALOGY AND PETROLOGY, ACKERSTRASSE, 71^76, D-13355
BERLIN, GERMANY
3
EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY OF MUNICH, THERESIENSTRASSE 41, 80333 MUNICH,
GERMANY
4
INSTITUTE OF GEOLOGY OF ORE DEPOSITS, PETROGRAPHY, MINERALOGY AND GEOCHEMISTRY RUSSIAN
ACADEMY OF SCIENCES, STAROMONETNY 35, 109017 MOSCOW, RUSSIA
RECEIVED NOVEMBER 6, 2008; ACCEPTED NOVEMBER 10, 2008
ADVANCE ACCESS PUBLICATION DECEMBER 9, 2008
We are pleased that our experimental study (Veksler et al.,
2007) received critical comments from three of the best
experts on silicate liquid immiscibility and gabbroic intrusions. Our disagreements are less important than the
shared belief that studies of layered intrusions need a new
impulse, and existing ideas a thorough revision. This discussion is a good opportunity to emphasize and clarify
the key points, better formulate current problems and disagreements, and touch upon some topics that were mentioned only briefly or not at all in our original
publication. We continue experimental work on silicate
liquid immiscibility (Veksler et al., 2009), and will mention
here some new results that are relevant for the discussion.
Like Roedder 30 years ago (Roedder & Weiblen, 1970;
Roedder, 1978), we stumbled upon liquid immiscibility by
serendipity. Our interest in magma unmixing was sparked
by unsought and unexpected melt inclusions in apatite
from the Skaergaard intrusion (Jakobsen et al., 2005).
These inclusions were impossible to miss. We realized that,
surprisingly, some important features of Skaergaard rocks
had been overlooked or dismissed by numerous researchers
before us. As we continued our work and established close
collaboration with experts on the Skaergaard intrusion in
Denmark and the UK (see the Acknowledgements) our
experimental study became a part of a broader petrographic and geochemical project. This discussion primarily
deals with experimental evidence and therefore we will
mention petrographic and geochemical observations only
briefly. However, those other types of evidence are certainly important and hopefully they will be covered soon
by our colleagues in their upcoming publications. We do
realize that the idea of early immiscibility in the
Skaergaard intrusion is at odds with broadly accepted
views, and admit that the concerns expressed by our opponents are fair and well based. We started to consider the
immiscibility hypothesis seriously only as the last resort,
because everything else (including the compositional convection model advocated by Professor Morse) failed to
explain some conflicting facts about the Skaergaard intrusion that were briefly outlined in our original paper and
are further discussed below. In this paper, we would also
like to point out some possible tests and promising directions for future research.
*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
I N T RO D U C T I O N
JOURNAL OF PETROLOGY
VOLUME 49
IMMISCIBILITY IN
EXPERIMENTS
Starting compositions
In his criticism of our starting compositions Professor Morse
appears to have overlooked the two pages of detailed explanations and rationale that we presented in our original
paper. Six out of eight of our starting mixtures were based
on compositions of natural immiscible glasses from melt
inclusions and mesostasis of volcanic rocks that we compiled
from previous publications. Perhaps some of those compositions look unconventional, but we decided to exclude nothing on the basis of preconceptions, and tested them all.
Whatever one thinks about appropriate normative compositions of basaltic magma, natural volcanic glasses and their
mixtures are arguably much better samples of magmatic
melts than the plutonic cumulate rocks that Professor
Morse uses as examples in his table 1. The proportions of
immiscible liquids in our charges were arbitrary and of
course may have differed significantly from those in natural
magma. However, if Professor Morse regards experimental
reproduction of immiscibility by McBirney & Nakamura
(1974) as ‘the standard of the industry’, he should have no
objections to our approach. Our approach was exactly the
same as used by McBirney & Nakamura (1974), who, after
failing to experimentally reproduce unmixing of their preferred model liquid of the Skaergaard Upper Zone, mixed
it with variable and arbitrary amounts of a granophyric
liquid, and suddenly had more luck.
NUMBER 12
DECEMBER 2008
The remaining two mixtures MZ-1 and MZ-2 are the
only ones that are directly applicable to the Skaergaard
intrusion. They were based not on natural magmatic
glasses but on experimental products, and mass-balance
calculations. As explained in our original paper, the composition MZ-1 is a replica of a cotectic experimental glass
produced by Toplis & Carroll (1995) at 11058C and log
fO2 ¼ ^956 [i.e. very close to the fayalite^magnetite^
quartz (FMQ) buffer] in a study of a parental Skaergaard
composition. Crystal phases coexisting with the melt comprised olivine Fo62, high-Ca clinopyroxene and plagioclase
An64. The temperature of the experiment was only 98C
above the onset of ilmenite and magnetite crystallization.
Therefore, the melt can be viewed as a close experimental
analogue of the Skaergaard liquid at the transition
between the LZb and LZc [here and below the names
and abbreviations for the Skaergaard stratigraphic units
are according to Wager & Brown (1968)]. It is true that
the liquidus plagioclase of the MZ-1 composition is somewhat more anorthitic than the natural plagioclase An57^63
from the LZb cumulates (McBirney, 1989). However, the
difference can be explained by re-equilibration of natural
plagioclase with inter-cumulus liquid at lower temperature. Our starting composition MZ-2 is very similar to
MZ-1 but somewhat higher in FeO(t). It is close to the
LZc liquid at about 60% crystallization of the
Skaergaard magma that was calculated by Nielsen
(2004a, 2004b) on the basis of masses and volumes of stratigraphic zones of the intrusion. When we started our
Table 1: Liquid compositions of UZb of the Skaergaard intrusion (wt % oxides) based on experiments, mass-balance
calculations and studies of natural melt inclusions
Bulk UZb liquid (emulsion)
experimental
Conjugate immiscible liquids
mass balance
Lfe
Lsi
Lfe
Lsi
6556
melt
SiO2
463
5845
514
655
4067
TiO2
34
14
24
12
186
022
Al2O3
82
1135
67
96
787
1296
FeO
2597
1604
266
119
3085
863
MnO
05
029
n.d.
n.d.
051
013
MgO
19
056
04
03
235
047
CaO
100
589
67
31
897
20
Na2O
24
378
22
20
156
433
K2O
05
163
10
26
103
368
P2O5
13
042
17
03
025
003
Total
10047
1000
965
9594
Ref.
McBirney &
Nielsen
Naslund (1990)
991
McBirney &
(2004b)
Nakamura (1974)
Lfe, Fe-rich melt; Lsi, silicic melt.
2178
9803
Jakobsen
et al. (2005)
VEKSLER et al.
experiments 3 years ago these were the most reliable
reconstructions of the Skaergaard liquid at the peak of Fe
enrichment. A later experimental study of Skaergaardtype dykes by Thy et al. (2006) reproduced very similar
liquids at the point of melt saturation in Fe^Ti oxides.
Despite the concern expressed by Professor Morse, the
presence of minor normative olivine or quartz in our starting and derivative liquids does not look to us very important. Typical tholeiitic liquids (including the Skaergaard
parental melt) are olivine-normative at the start of crystallization, but become quartz-normative at the stage of
olivine^pigeonite peritectic reaction (the lower part of the
Skaergaard MZ formed exactly at this stage). The Grove
projection (Grove, 1993) is a good illustration of the trend
(Fig. 1). The topology of the projection, which is based on
interpolation of experimental mid-ocean ridge basalt
(MORB) and calc-alkaline liquid compositions, implies
no barrier between olivine- and quartz-normative liquids
evolving along the olivine^augite^plagioclase cotectic.
The thermal divide (TD, Fig. 1) is positioned between the
olivine and pigeonite fields, deep inside the region of
quartz-normative compositions. Notably, all experimental
liquids of low-alkali, tholeiitic compositions equilibrated
with olivine, augite, low-Ca pyroxene and plagioclase in
the study by Longhi & Pan (1988) are quartz-normative,
and so is our MZ-1. We also plot in Fig. 1 two alternative
experimental Skaergaard trends (McBirney & Naslund,
1990; Toplis & Carroll, 1995). Notably, many of the liquids
Cpx
[PLAG]
OXYGEN
UNITS
MZ
a
UZc
g
i
t
e
C
P
u
T&
LZa
TD
pi
g
o l i v i n e
op
x
Ol
opx
qz
Qz
Fig. 1. Compositions of Skaergaard experimental liquids in the
Grove projection (Grove, 1993). Open circles connected by continuous
lines indicate trapped liquids in Skaergaard cumulates (McBirney &
Naslund, 1990). Abbreviations for the cumulate zones are according to
Wager & Brown (1968). Dotted curve T & C indicates liquid evolution
at the fO2 conditions of the QFM buffer (Toplis & Carroll, 1995). P,
olivine^pigeonite peritectic point; TD, thermal divide. Phases: opx,
orthopyroxene; pig, pigeonite; qz, quartz.
REPLY
are quartz-normative. The trends do not pass through the
peritectic reaction point, and none of them closely follows
the cotectic and peritectic curves [see Veksler (2009) for
further discussion]. This probably reflects the difference
between Fe-rich tholeiitic liquids and calc-alkaline basaltic
melts. The diagram also clearly shows that the alternative
Skaergaard experimental trends part at the transition from
LZ to MZ.
Stable immiscibility vs unmixing
during quenching
Professor Philpotts raises a serious issue of distinguishing
between super-liquidus immiscibility and unmixing
during quenching. The explanation that he offers for the
chemical gradients developed in some of our centrifuged
glasses is that the gradients were due to settling of FeO
powder at the initial stages of melting at slow rotation. In
the early stages of our study, we worried about this possible
artefact, and therefore tried different loading methods and
sintering protocols to achieve perfect homogenization of
melts before centrifugation. Finally, while carrying out
our most recent series of centrifuge experiments (Veksler
et al., 2009) we learned how to tightly plug inner iron containers and that allowed us to preheat and homogenize
charges at ambient gravity for days. We repeated some of
the key experiments on compositions used by Veksler et al.
(2007) and confirmed that sintering techniques in our previous study had been adequate. Even without that later
test, our original study gave enough circumstantial evidence to prove that centrifugation experiments were not
fundamentally flawed. If they were, macroscopic chemical
gradients would have been present in all the centrifuged
glasses. That certainly was not the case.
When we examined the centrifuged glasses by scanning
and transmission electron microscopy, we did not see any
Fe-oxide particles. As discussed in our paper, we saw
numerous sub-micrometre, non-crystalline, glassy globules
with sharp interfaces, and compositions of typical Fe-rich
and silica-rich immiscible liquids. Professor Philpotts does
not accept our interpretation of those tiny droplets as products of stable immiscibility, even when they clearly move
and coalesce during centrifugation. He is sceptical because
of the very small size of those exsolutions, as most of them
are indeed two or three orders of magnitude smaller than
the micrometre-sized droplets that have been observed in
various unmixed basaltic compositions.
Professor Philpotts states that micrometre-sized droplets
have never been observed at temperatures above 10408C
in experiments on natural rock compositions. This statement is not true. Experiments by Krasov & Clocciatti
(1979) that we mentioned in our original paper provided
an example of high-temperature unmixing, and in fact
that study is not unique. Roedder & Weiblen (1970)
reported micrometre-sized immiscible droplets in experiments on lunar ferrobasaltic and rhyolitic melts in
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JOURNAL OF PETROLOGY
VOLUME 49
a temperature interval of 1045^11358C. The consolute temperature was not determined, but Roedder & Weiblen
claimed that it certainly was above 1135 and below
13508C. In the published photographs, the droplets meet
all the morphological criteria of stable exsolutions proposed by Professor Philpotts. Notably, tridymite was
reported together with two liquids in products of shorter
runs at 11358C, but was not mentioned in products of slow
cooling quenched from 10458C. This looks like another
case of metastable tridymite nucleation, the same as in
our experiments.
The examples that Professor Philpotts presents in his
comments give an impression that stable immiscible droplets nucleate and grow easily. Unfortunately, this is not
always the case. As mentioned above, McBirney &
Nakamura (1974) could not, according to their own
account, reproduce immiscibility in pre-homogenized Ferich Skaergaard melts, and had to mix them with silicic
granophyre to achieve unmixing. Longhi (1990) had to
use different loading methods and noted that sharp
liquid^liquid interfaces were absent in some of his experimental products. It seems that liquid immiscibility is better
reproduced during slow cooling (e.g. Dixon & Rutherford,
1979) than in isothermal experiments. In any case, one
would expect the nucleation density and size distribution
of the droplets to vary broadly from one experiment to
another in response to variable over-saturation, diffusion
rates, viscosity, interfacial energy, and other physical
properties.
Our disagreements with Professor Philpotts are indeed
very similar to those between Visser & Koster van Groos
(1976, 1977), Roedder (1977) and Freestone & Hamilton
(1977), who argued about the correct interpretation of opalescent glasses and sub-micron exsolutions in the classical
system K2O^FeO^Al2O3^SiO2. We worked only briefly
on that system (Veksler et al., 2009) and will not comment
on that case. However, we were aware of conflicting views
on the origin of opalescent glasses, and that was exactly the
reason why we used centrifugation. Settling of colloidal
emulsions by in situ centrifugation is a straightforward
and reliable way of distinguishing them from quench exsolution. However, in some cases one can arrive at a correct
interpretation simply by careful optical examination of
opalescent glasses recovered from conventional static runs.
Let us look, for example, at a fragment of a polished bead
of glass (Fig. 2) produced in our recent static experiment
(Veksler et al., 2009) on a composition that was taken from
a region of stable, low-temperature immiscibility in the
system K2O^CaO^FeO^Al2O3^SiO2 (Hoover & Irvine,
1978). After 24 h at 10908C the charge quenched to turbid,
opalescent glass composed of sub-micron dispersed Fe-rich
glassy droplets in a silica-rich matrix glass. We interpret the
emulsion as formed by stable unmixing rather than exsolution during quenching. Our conclusion is based on the
NUMBER 12
DECEMBER 2008
0.5 mm
Fig. 2. A fragment of polished opalescent glass from a quench experiment in reflected light (see text for further details). Arrows show the
direction in which air bubbles moved during the experiment.
pattern of flow textures around slowly rising air bubbles
trapped inside the charge. One can see that the bubbles
pushed plumes and clouds of dispersed Fe-rich phase
upwards and aside leaving behind a tail of almost pure
and clean silica-rich liquid. Such flow textures could not
form instantly in a few seconds of quenching time and
therefore imply protracted existence of the emulsion at
high temperature. Furthermore, centrifugation experiments on this composition at the same temperature confirmed stable unmixing beyond reasonable doubt.
Centrifugation greatly enhanced coalescence of the droplets, and after 4 h of forced separation nearly half of the
emulsion settled down to a condensed bottom layer with a
razor-sharp interface [see Veksler et al. (2009) for further
details].
Professor Philpotts brings up examples of metastable
sub-micron exsolution in technological glasses, but does
not mention that these are not always due to quenching.
In fact, some examples of colloidal exsolution have been
2180
VEKSLER et al.
shown to form slowly in under-cooled liquids above the
glass transition. For instance, Toplis & Reynard (2000)
observed time-dependent structural changes in a P-bearing
Na aluminosilicate melt using in situ Raman spectroscopy,
and attributed them to sub-liquidus immiscibility above
the glass transition. Unmixing developed within 3 h at
700^7758C, slower at higher temperatures, and faster at
greater under-cooling. Notably, glasses that were quenched
after dwelling times of a few hours were optically transparent and showed no heterogeneity under scanning electron
microscope at a resolution of 1 mm. Another instructive
example comes from the classical glass-forming system
Na2O^CaO^SiO2 that is used for manufacturing
common window glass. Burnett & Douglas (1970) performed a series of annealing experiments on under-cooled
melts in the region of sub-liquidus immiscibility of the
system. At 6408C they observed nanoscale emulsions and
documented three distinct stages of sub-micron coarsening
that lasted for at least 27 h in total.
Of course, the above are examples of metastable (subliquidus) immiscibility, whereas we were investigating
immiscibility above the liquidus. Nevertheless, with
regard to kinetics, we see no fundamental difference
between sub- and super-liquidus unmixing. As long as
crystals do not nucleate, silicate liquids do not know
whether they are above or below the liquidus. What really
matters for the kinetics and resulting emulsion morphology are the relationships between super-saturation, viscosity, interfacial energy, and diffusion rates that define the
rates of nucleation and growth, and may provide favourable conditions for long-term stability of sub-micron droplets. Investigations of liquid unmixing in materials
science are more advanced, and they have already
revealed high-temperature immiscible sub-micron silicate
emulsions that remained stable for hours and even days.
In situ spectroscopic methods developed in glass technology, such as small-angle X-ray scattering (SAXS;
Mazurin & Porai-Koshits, 1984), may be used for observation of initial, latent stages of unmixing of natural magmatic melts. Future progress, in our view, would require
better experimental constraints on the kinetics of immiscibility, the stability factors of immiscible emulsions at high
temperatures, direct measurements of interfacial energy,
and advanced models of liquid^liquid and crystal^liquid
phase relationships for geologically relevant melt
compositions.
Comparisons with other experimental
studies of Skaergaard liquids
There would have been no need for us to perform centrifugation experiments on Skaergaard liquids if the problem of
magmatic evolution at Skaergaard had been successfully
resolved by other, conventional experimental methods.
Unfortunately, it was not. We do not mind when our opponents call our experiments questionable and we share their
REPLY
concerns with regard to chemical equilibrium.
Experiments on ferrobasaltic and rhyolitic liquids at temperatures below 11008C are anything but easy and the
results should certainly be treated with caution. Reaching
chemical equilibrium in silicate systems at low temperatures has been recognized as a great challenge for decades.
For example, Kennedy (1948) showed that 240 h in air was
not long enough to reach ferrous^ferric equilibrium in
partly crystallized basalt at 11008C. Crystal^liquid equilibria are also notoriously difficult at lower temperatures.
For instance, neither Toplis & Carroll (1995) nor Thy et al.
(2006) were able to reproduce the reappearance of fayalitic
olivine in the final stages of crystallization of Skaergaard
model liquids. The important lesson from our study is that
low-temperature silicate liquid immiscibility is not
immune to nucleation and kinetic problems either.
Relationships between extreme Fe-enrichment and
liquid immiscibility appear to be complex. One of us
(Veksler) has recently searched through experimental
databases for cases of extreme Fe-enrichment in basaltic
liquids, specifically, melts with total FeO contents above
22 wt %. A detailed review of those cases will be published
elsewhere (Veksler, 2009). In a nutshell, the conclusions are
simple. Extreme Fe-enrichment up to 30 wt% FeO(t) and
even higher has been documented in melts equilibrated
with fayalitic olivine, high-Ca pyroxene, plagioclase, and
Fe^Ti oxides at reducing conditions, atmospheric pressure,
and temperatures slightly above 10008C (e.g. Longhi &
Pan, 1988). The enrichment may result from crystallization
combined with liquid immiscibility, or fractional crystallization alone. However, the latter is possible only in bulk
compositions with very low total alkalis. Alkali contents
in the parental Skaergaard magma [e.g. broadly accepted
starting compositions compiled by Nielsen (2004a)] seem
to allow Fe-enrichment only to about 22 wt % FeO(t)
regardless of redox conditions. This is exactly the maximum FeO(t) that Toplis & Carroll (1995) and Thy et al.
(2006) reported in their experiments on parental
Skaergaard liquids. Any further Fe-enrichment would
require redistribution (removal) of alkalis by some process
other than fractional crystallization, and silicate liquid
immiscibility appears to be the only sensible option.
Phase equilibria limitations are compounded by massbalance constraints. The sequence of Skaergaard liquids
produced by partial melting of cumulates (McBirney &
Naslund, 1990) shows no pronounced enrichment in total
alkalis (see the AFM diagram in Fig. 3), and this is a clear
violation of mass balance. Hunter & Sparks (1990) completely dismissed those experiments and the trend as fundamentally flawed, but we are not so sure [see Veksler (2009)
for further discussion]. We believe that the experiments on
cumulates may carry an important message about the real
Skaergaard inter-cumulus liquids. However, some features
of the McBirney & Naslund (1990) trend are puzzling.
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JOURNAL OF PETROLOGY
VOLUME 49
F
(FeOt)
UZc
TN
Th
C
T& i
l
mu
ing
MZ
LZa
des
sca
Ca
A
(Na2O+K2O)
weight percent
M
(MgO)
Fig. 3. An AFM projection showing the compositional variations of
the Skaergaard liquids, and volcanic series of the Thingmuli volcano,
Iceland and the Cascades, USA (Carmichael, 1964). Dotted curve T &
C and open circles connected by continuous lines show the same
experimental trends as in Fig. 1. Bold dashed curve TN is the liquid
line of descent calculated by Nielsen (2004a, 2004b).
One has to explain, for example, why the sixfold increase
in P2O5 from 02 wt% in LZa to 12 wt % UZa is not
accompanied by a similar rise of the K2O concentrations
(the latter increased only by a factor of 17). By all
accounts, K is approximately as incompatible as P, the element that McBirney & Naslund (1990) used to control the
degree of melting in their experiments. Although fractional crystallization in the absence of apatite and/or
alkali feldspars cannot fractionate K from P, these elements are easily separated by silicate liquid immiscibility.
Therefore, apparent decoupling of K and P in the partial
melts of Skaergaard cumulates (McBirney & Nalsund,
1990), may provide circumstantial evidence for early
unmixing and liquid^liquid fractionation starting from
LZb^LZc, in agreement with our centrifuge experiments
on the composition MZ-1.
I M M I SC I BI L I T Y I N T HOLEI I T IC
M AG M A
Our original paper dealt with experimental evidence
and was not meant to discuss implications for the
Skaergaard or any other gabbroic intrusion in detail. Our
critics have a superb knowledge of mafic intrusions and
other products of basaltic magma. However, at times they
criticize things that we never actually said, and we feel that
we have to respond and clarify our position.
Liquid immiscibility vs compositional
convection
Professor Morse believes that compositional convection
in large basaltic intrusions always ‘trumps’ liquid
NUMBER 12
DECEMBER 2008
immiscibility. With all due respect, we do not see how
such trick can be pulled off. We view immiscibility and
compositional convection as processes that are, in certain
ways, opposite to each other. In the model favoured by
Professor Morse (his Discussion), compositional convection is driven by chemical gradients at crystallization
fronts and it works towards equilibration and complete
homogenization of the liquid. In contrast, immiscibility
splits a homogeneous melt into two liquid phases, and
therefore
works
towards
greater
heterogeneity.
Immiscibility is a self-sufficient, equilibrium process of differentiation, totally independent of crystallization.
Liquidus crystals and immiscible droplets can be kept in
equilibrium indefinitely, and with time phase separation
by gravity should only improve. Furthermore, because
both conjugate liquids are required to be in thermodynamic equilibrium with the same liquidus crystal assemblage, percolation and separation of immiscible liquids in
a crystal mush appears to be a perfect way of making
modal layering. We do not see how any other process can
rival liquid immiscibility in this respect.
We never proposed that immiscible liquids in
Skaergaard or any other intrusion separated like oil and
water into two major, condensed, continuous layers. We
experimented on crystal-free compositions but never
claimed that unmixing of natural magma was superliquidus. Crystal phases were certainly present in the
Skaergaard magma already during its emplacement. In
natural magmatic systems, we view immiscibility as a continuous process that goes in parallel to crystallization and
cooling. At any given moment, we expect to have emulsion
clouds mingling and interacting with crystals. For simplicity, immiscible droplets can be viewed as another crystal
phase, more buoyant than plagioclase, and with a chemical
composition between that of alkali feldspar and pure silica.
We agree with Professor Morse that there are clear signs
of significant vertical redistribution of FeO, alkalis and
silica in the Skaergaard magma chamber, and do not
deny that compositional convection may have played a
role before immiscibility started. Rejected silicic components may go up either in plumes (Professor Morse proposes that), or as clouds of immiscible silicic droplets.
However, as soon as the liquid hits the liquid^liquid binodal (solvus), one cannot have it both ways. When Professor
Morse asks: ‘Will the flux of light solute physically
[and chemically] interfere with the evolving liquid pairs?’,
the question strikes us as very strange. We do not see how
light plumes of rejected solute could form in the presence of
two immiscible liquids. As soon as the Fe-rich liquid
crystallizes a few mafic crystals, it would also nucleate a
few silica-rich immiscible droplets. As crystallization and
immiscibility proceed, the evolving Fe-rich liquid is likely
to become even more Fe-rich and dense because of
the broadening of the miscibility gap with falling
2182
VEKSLER et al.
T, C
LZb
ich
Fe
UZb
n
-r
s io
40
e
ul
1050
lk
m
liq
uid
bu
h liquid
Si - ric
1100
50
60
70
SiO2, wt. %
Fig. 4. A schematic T^SiO2 wt % diagram showing mass balance
between immiscible Skaergaard liquids at different stages of crystallization. Continuous tie-lines connect the compositions of Fe-rich and
Si-rich conjugate melts; dashed curve is a hypothetical binodal; continuous grey arrow traces the changing bulk composition of the emulsion. (See text for discussion.)
temperature (Fig. 4). We believe that emulsion clouds are
more effective carriers than convection plumes because
the former are stable and do not dissipate with time.
Plumes in a convecting liquid blend in and eventually
vanish, whereas stable emulsion droplets would grow even
bigger as they move.
The thermodynamic model that Professor Morse
presents in his fig. 7 (see his Discussion) is dimensionless
and too schematic for a serious discussion. We do not find
the analogy with atmospheric circulation very useful
because air, rain clouds and silicate melts differ dramatically in viscosity, density, heat conductivity, and a
number of other important physical properties. We doubt
that the P^T cycle of coupled over- and under-saturated
adiabats can be sustained at the scaling parameters of
the Skaergaard magma chamber. For instance, the establishment of adiabats implies, by definition, that melt and
crystals in descending plumes sink faster than they
exchange heat with the ambient magma. Such a scenario
does not seem to us physically plausible. In effect,
Professor Morse proposes in his Fig. 7 that melt and crystals in small, fast-cooling intrusions are in thermal and
chemical equilibrium throughout the magma volume,
whereas in large, slowly cooling intrusions equilibrium is
reached only at the top and bottom crystallization fronts.
This notion seems to us paradoxical and counter-intuitive,
if not plain wrong.
Plagioclase issues
Professor Morse discusses plagioclase buoyancy, adcumulus crystal growth and compositional zoning. These are no
doubt interesting and important subjects, but they have
little to do with the main topics of our experimental study.
REPLY
We do not see why the variations in plagioclase zoning that
Toplis et al. (2008) documented in the Skaergaard Layered
Series contradict the idea of liquid immiscibility and
liquid^liquid fractionation starting at the top of the LZ.
Furthermore, the extensive reverse zoning that Stripp
et al. (2007) documented in plagioclase crystals from reactive symplectites is believed to arise from liquid immiscibility and liquid^liquid fractionation in the crystal mush.
However, a serious discussion of plagioclase zoning in
Skaergaard gabbros should be better left for another
paper. Here we would only express our disagreement with
the statement that ‘the two-liquid hypothesis cannot
account for the presence of cumulus plagioclase in the
rocks of a dense, Fe-rich liquid layer’ (Morse, Discussion).
Our model Fe-rich Skaergaard liquid MZF yields 38 wt %
of normative An40, and it crystallized early liquidus plagioclase An55. Therefore, plagioclase is produced by the
Fe-rich immiscible liquid early and in abundance. Density
calculations imply that plagioclase crystals would float in
the Fe-rich Skaergaard liquid, immiscible or not, but
plagioclase intergrowths with mafic minerals and Fe^Ti
oxides would sink (see also Philpotts & Dickson, 2000).
In general, we do not see how immiscibility should be
inconsistent with cumulus plagioclase.
Immiscibility in the Skaergaard intrusion
As mentioned above, McBirney & Nakamura (1974)
demonstrated liquid immiscibility in experiments on mixtures of silicic granophyre and Fe-rich partial melts of the
Skaergaard UZ. They proposed that certain types of melanogranophyre segregations in the UZc formed by immiscibility. However, the trend of extreme Fe-enrichment
throughout the MZ after the start of Fe^Ti oxide crystallization remained, as discussed above, contentious. The
case for immiscibility at Skaergaard was further weakened
when it was found that the distribution of excluded trace
elements between melanogranophyre and host ferrogabbro
(or ferrodiorite) was inconsistent with experimental
liquid^liquid partition coefficients (McBirney, 2002). The
discovery of contrasting silicic and Fe-rich melt inclusions
in apatite (Jakobsen et al., 2005) was so important because
it finally confirmed liquid immiscibility in the Skaergaard
UZ rocks beyond reasonable doubt. The inclusions proved
immiscibility but, again, they explained neither the continuous Fe enrichment during the formation of MZ and UZa,
nor how (and when) the Skaergaard magma had managed
to reach the miscibility gap. That is why we decided to revisit experimental evidence on immiscibility and Feenrichment.
It is certainly true that immiscibility in the Skaergaard
magma, as everything else, started from nothing and
evolved into something (Morse, Discussion). However, it
would be helpful to be more specific. The Skaergaard
magma approached the miscibility gap from the mafic,
Fe-rich side, and unmixing must have started by nucleation
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of a few droplets of silicic liquid in a large volume of ferrobasaltic melt. Mass balance between the bulk composition
MZ-1 and the conjugate liquid compositions Lfe and Lsi
from our centrifuge experiments (Veksler et al., 2007)
implies 15^20 wt % of the silicic liquid. This gives a
rough idea about the proportions of immiscible liquids at
the start of immiscibility when crystallization of the
Layered Series proceeded from LZb to LZc and 40^45
wt % of the Skaergaard intrusion remained molten
(Nielsen, 2004a). However, by the time of UZb formation
the proportions must have been different. As temperature
further decreased by 508C (McBirney & Naslund, 1990),
the Skaergaard magma crystallized large amounts of
mafic cumulates, and the degree of crystallization rose to
90 wt % (Nielsen, 2004a). Everybody seems to agree that
at this late stage the remaining liquid portion of magma
was a mixture of two immiscible melts. Professor Morse
suggests that the Fe-rich melt remained predominant in
the UZ but our mass-balance calculations do not support
such view. For the mass-balance calculation in the UZ, one
needs to know the compositions of coexisting liquids and
the bulk composition of the emulsion. For the former, one
can take the compositions of the experimental immiscible
glasses reported by McBirney & Nakamura (1974), or the
most contrasting compositions of apatite-hosted melt inclusions found in UZb and UZc (Jakobsen et al., 2005). For the
latter, one can use the partial melt of the UZb cumulates
(McBirney & Naslund, 1990), or the bulk UZb liquid calculated by mass balance (Nielsen, 2004a, 2004b). All these
data are presented in Table 1. As it turns out, the experimental immiscible liquids (McBirney & Nakamura, 1974)
and the UZb partial melt (McBirney & Naslund, 1990) do
not balance at all. The UZb partial melt clearly cannot
represent the bulk of the emulsion because it is lower in
silica than the experimental Fe-rich immiscible liquid Lfe,
and therefore mass balance for silica would result in negative fractions of Lfe. On the other hand, the compositions
of apatite-hosted melt inclusions, and the bulk UZb liquid
calculated by Nielsen (2004a, 2004b) fit the mass-balance
equations reasonably well. Silica, alumina, FeO(t) and
Na2O give consistent Lfe fractions of 25^33 wt%.
Therefore, it follows that the Fe-rich immiscible liquid was
not predominant during UZb formation. These mass-balance relationships are illustrated in Fig. 4 in a form of
pseudo-binary T^x diagram. The diagram shows that,
starting from LZc, massive crystallization of Fe^Ti oxides
and other mafic minerals drove the bulk composition of
the emulsion towards silica enrichment, while the Fe-rich
liquid continued to evolve towards further Fe-enrichment
and silica depletion as a result of the widening of the miscibility gap with falling temperature. This is how, in our
view, liquid immiscibility resolves the issue that many
people historically called the Fenner^Bowen Skaergaard
controversy.
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DECEMBER 2008
After a few hours of centrifugation, only a small fraction
of the silicic liquid floated to the top of the charge in our
experiments. In a similar way, we believe that only a
small fraction of the silicic conjugate melt reached the
Upper Border Series (UBS) in the Skaergaard intrusion.
However, the evidence from the UBS is very important
and probably deserves a revision. Some encouraging hints
towards the effects of immiscibility and large-scale liquid^
liquid fractionation can be found in the paper by Naslund
(1984), who noted that the UBS rocks ‘are systematically
richer in K2O and SiO2 . . . than their Layered Series
counterparts’ and the differences in K2O and SiO2 ‘do not
appear to be explicable by any mixture of cumulus minerals or cumulus minerals and trapped Layered Series
liquid’. The rest of the silica-rich immiscible liquid should
have contributed to the cumulate zones of the Layered
Series, and formed the melanogranophyre segregations
that Professor McBirney describes in his comments. Like
the UBS rocks, Skaergaard melanogranophyres may
deserve a more detailed study. Judging by the bulk-rock
analyses that have been published so far, we do not see
any significant, systematic difference between the granophyric zones in pegmatites from the LZ and melanogranophyres from UZc with regard to their P2O5 and alkali
contents. Pegmatitic granophyres with P2O5 concentrations varying from 002 to 09 wt % (Larsen & Brooks,
1994) do not look more P-rich than segregation and podlike melanogranophyres that contain 007^081 wt %
P2O5 (McBirney, 1989). Furthermore, the statement that
granophyric zones in pegmatites are ‘normal products of
crystal fractionation’ (McBirney, Comments) certainly
contradicts the opinion of Larsen & Brooks (1994), who
wrote that contact relations between the gabbroic and
granophyric zones of pegmatites were ‘consistent with the
production of the granophyric component by liquid immiscibility followed by gravity separation from the conjugate
mafic melt of the pegmatitic system’. Larsen & Brooks
(1994) viewed pegmatitic pockets in the lower zones of the
Layered Series as ‘micro-magma chambers’ that followed
the same path of crystallization and immiscibility as the
whole Skaergaard intrusion.
Immiscibility in the Holyoke basalt
Thanks to the extensive research efforts of Professor
Philpotts, large tholeiitic lava flows in Connecticut (USA)
have become the best studied examples of differentiated
volcanic bodies that underwent crystallization, crystal settling, and immiscibility. Basaltic lava in Connecticut is
compositionally close to the Skaergaard magma but the
former cooled and crystallized much faster. As a result,
unquestionable traces of liquid immiscibility have been
preserved in the basaltic groundmass as micron-sized Ferich globules dispersed in rhyolitic matrix glass (Philpotts,
1979). We value the keen observations and careful experiments by Philpotts (1979) and Philpotts & Doyle (1983)
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VEKSLER et al.
but we are not entirely sure that those studies correctly
bracketed the onset of immiscibility. According to
Philpotts (1979), microphenocrysts in the groundmass
show strong compositional zoning. This implies that the
groundmass crystals and melts may have been out of equilibrium and became compromised during the original fast
cooling of the lava flow. Reheating of such disequilibrium
groundmass crystal^glass assemblages would not necessarily reproduce the equilibrium course of crystallization
and immiscibility. To better clarify the sequence of events
in the cooling lava flow, one should probably investigate
in detail the extent of crystal zoning in the groundmass,
and carry out reheating experiments also on plagioclasehosted melt inclusions that apparently show immiscibility
at higher temperature. In our view, with regard to the
timing of immiscibility in the Holyoke basalt the jury is
still out.
CONC LU DI NG R E M A R K S
Although our involvement with the Skaergaard intrusion
started only recently and our knowledge of subtle geological details may be incomplete, objections from our more
experienced opponents has not convinced us so far that
early immiscibility in the Skaergaard magma is a bad
idea. We do not deny that some of the results of our original
study are unexpected or controversial, and we have tried to
address those issues in new experiments since the original
paper was published. However, we suspect that experimental methods in application to ferrobasalts at low temperatures may be already pushed to the limits and the ultimate,
decisive evidence in favour of or against early unmixing in
the Skaergaard and similar intrusions is more likely to
come from natural rocks rather than experimental studies.
AC K N O W L E D G E M E N T S
We have been fortunate in recent years to closely collaborate with Dr Marian Holness, Miss Gemma Stripp,
Dr Charles Kent Brooks, Dr Jakob K. Jakobsen, Dr Troels
F. D. Nielsen, and Dr Christian Tegner. This collaboration
started at the Kent Brooks symposium in 2004, and we
would like to emphasize the crucial role that Kent played
in launching this joined Skaergaard enterprise. Without his
encouragement, and also indispensable contributions from
other colleagues in the group, we would never dare to challenge the paradigms of the classical Skaergaard intrusion.
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