971
The Canadian Mineralogist
Vol. 46, pp. 971-980 (2008)
DOI : 10.3749/canmin.46.4.971
EXPERIMENTAL STUDIES OF THE SYSTEM Na2Ca(CO3)2 – NaCl – KCl
AT 0.1 GPa: IMPLICATIONS FOR THE DIFFERENTIATION AND LOW-TEMPERATURE
CRYSTALLIZATION OF NATROCARBONATITE
Roger H. MITCHELL§
Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1
Bruce A. KJARSGAARD
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
Abstract
The system [Na2Ca(CO3)2 ]100–x – (50 wt.% NaCl + 50 wt.% KCl)x (10 < x < 80 wt.%), a join in the quaternary system Na2CO3
– CaCO3 – NaCl – KCl, has been investigated at temperatures ranging from 450° to 900°C at 0.1 GPa. Liquidus temperatures
are on the order of 825°C, with a depression on the liquidus to 750°C at about 23 wt.% halide corresponding to a piercing point.
Liquids that are poor in halides relative to the composition of the piercing point have a nyerereite-like (Na,Ca,K) carbonate as
the primary liquidus phase; it coexists with a one-phase halide-rich liquid. Liquids with ~ 30–70 wt.% halide initially precipitate calcite as the primary liquidus phase. Below 700°C, calcite reacts with liquid to form nyerereite-like (Na,Ca,K) carbonate.
From 600° to 550°C, this carbonate coexists with two immiscible halide liquids. Below the solidus at ~550°C, discrete exsolved
inclusions of (Na,K)Clss occur in (K,Na)Clss and of (K,Na)Clss in (Na,K)Clss. These data are relevant to the differentiation and
crystallization of the natrocarbonatitic melt at Oldoinyo Lengai under hypabyssal and plutonic conditions; they predict the formation of cumulate rocks composed of nyerereite, gregoryite, halite and sylvite. Disaggregation of such assemblages could lead to
formation of sylvite–fluorite macrocrystal natrocarbonatite.
Keywords: nyerereite, sylvite, halite, calcite, immiscibility, natrocarbonatite, Oldoinyo Lengai, Tanzania.
Sommaire
Nous avons étudié le sous-système [Na2Ca(CO3)2 ]100–x – (50% NaCl + 50% KCl)x (10 < x < 80%, poids) du système quaternaire Na2CO3 – CaCO3 – NaCl – KCl entre 450° et 900°C à 0.1 GPa. Les températures du liquidus sont de l’ordre de 825°C, avec
une diminution à 750°C à environ 23% de halogénure, ce qui correspond à un point de perçage. Les liquides appauvris en halogénures par rapport à la composition de ce point de perçage possèdent une phase carbonatée (Na,Ca,K) semblable à la nyéréréite
comme phase primaire sur le liquidus, et elle coexiste avec un liquide riche en halogénures. Les liquides contenant entre ~30 et
70% de halogénures précipitent d’abord la calcite comme phase primaire sur le liquidus. Au dessous de 700°C, la calcite réagit
avec le liquide pour former un carbonate (Na,Ca,K) semblable à la nyéréréite. Entre 600° et 550°C, ce carbonate coexiste avec
deux liquides immiscibles riches en halogénures. En dessous du solidus à environ 550°C, on trouve des inclusions exsolvées
distinctes de (Na,K)Clss dans le (K,Na)Clss et de (K,Na)Clss dans le (Na,K)Clss. Ces résultats sont pertinents pour comprendre la
différenciation et la cristallisation du magma natrocarbonatitique à Oldoinyo Lengai à des conditions subvolcaniques et plutoniques. Ils prédisent la formation de roches cumulatives composées de nyéréréite, grégoryite, halite et sylvite. La désagrégation
de tels assemblages pourrait mener à la formation de natrocarbonatite contenant des macrocristaux de sylvite–fluorite.
(Traduit par la Rédaction)
Mots-clés: nyéréréite, sylvite, halite, calcite, immiscibilité, natrocarbonatite, Oldoinyo Lengai, Tanzanie.
§
E-mail address: [email protected]
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the canadian mineralogist
Introduction
The volcano Oldoinyo Lengai, in Tanzania, is
unique with respect to the eruption of natrocarbonatite
lavas (Dawson 1962, 1989). The natrocarbonatites are
composed dominantly of gregoryite [(Na,Ca,K)2CO3]
and nyerereite [Na 2Ca(CO3)2] phenocrysts set in a
matrix of a gregoryite-like phase, barian nyerereite,
halite–sylvite, fluorite, potassian neighborite, and
a Ba–Sr–Mg–Na–Cl–F-rich carbonate known as
phase X (Dawson et al. 1995). Various aspects of the
geology, petrology and mineralogy of these lavas have
been described by Peterson (1990), Keller & Krafft
(1990), Dawson et al. (1995), Church & Jones (1995),
Mitchell (1997, 2006), and Mitchell & Belton (2004).
Of particular interest with respect to this study is the
observation by Mitchell (1997) that the late-stage
residual liquid separates into two immiscible liquids: a
Na–K–Ca–CO2–Cl-rich liquid crystallizing gregoryite,
sodian sylvite, potassian neighborite and phase X, and
a Na-rich Cl-poor carbonate liquid approximating to
a nyerereite–gregoryite cotectic. Recently, Mitchell
(2006) has described the occurrence of unusual natrocarbonatite lavas containing cryptogenic macrocrysts of
sylvite and fluorite. These macrocrysts are considered
to be possibly derived by the fragmentation of rocks
formed by the crystallization of natrocarbonatite under
hypabyssal conditions.
To understand the low-temperature crystallization
and differentiation of natrocarbonatitic liquids, we
have commenced a long-term program of experimental
investigation of parts of the system Na2CO3 – CaCO3
– NaCl – KCl – MgF2 – CaF2. This system, apart from
phase X, encompasses most of the phases encountered
during crystallization of natrocarbonatitic magma. In
this work, we present experimental data at 0.1 GPa for
parts of the pseudobinary join Na2Ca(CO3)2 – (50 wt.%
NaCl + 50 wt.% KCl).
some bulk compositions analogous to natrocarbonatite can initially precipitate gregoryite-like (Na2,Ca)
CO3, followed by coprecipitation of this compound
together with nyerereite along a cotectic from 785°
to 640°C (Fig. 1). This cotectic was considered by
Niggli (1919) to terminate at a reaction point, where
nyerereite plus liquid forms (Na2,Ca)CO3 plus NaCl
and liquid. Note that the temperature of this reaction
point is very close to that of the eutectic (638°C) on
the Na2CO3 – NaCl join (Fig. 1). As Niggli (1919)
noted that the experimental results for this ternary
system are “unsatisfactory”, and given the potential
uncertainties in temperature determination (not given)
in this older work, it is possible that this reaction point
is actually a ternary eutectic; further investigations are
desirable. Note that in modern studies, temperature
uncertainties are on the order of ±2–5°C using either
cold-seal (Cooper et al. 1975, Gittins & Tuttle 1964)
or rapid-quench apparatus (Signorelli & Carroll 2000).
Importantly, the work of Niggli (1919) suggests that on
the join Na2Ca(CO3)2 – NaCl, the liquidus is defined by
the appearance of calcite, except for extremely NaClrich compositions (>85 wt.% NaCl). This is a reflection
of the position of the NaCl–CaCO3 eutectic at ~95 wt.%
NaCl (Fig. 1).
The boundary binary systems for parts of the
quaternary system Na2CO3 – CaCO3 – NaCl – KCl are
illustrated in Figures 2 and 3. In the system Na2CO3
– CaCO3 (Cooper et al.1975, Jago & Gittins 1991),
nyerereite occurs as a congruently melting (817 ± 2°C)
compound. There are two eutectics, located at 78.5 wt.%
Na2CO3 and 47 wt.% Na2CO3, at ~725°C and 813 ±
2°C (Fig. 2). In the KCl–NaCl system at 1 bar (Scheil
& Stadelmeir 1952), there is a binary solid-solution
with a minimum at ~660°C at ~50 mol.% NaCl. In
the subsolidus region, there is a solvus with an upper
consolute point at a temperature of ~500°C at ~66
mol.% NaCl (Fig. 3).
Previous Work
Experimental Procedures
The system Na2Ca(CO3)2 – (50 wt.% NaCl + 50
wt.% KCl) is a join in the quaternary system Na2CO3
– CaCO3 – NaCl – KCl. This system has not been
previously investigated. In a related system, Na2CO3 –
K2CO3 – NaCl – KCl, Nyankovskaya (1952) indicated
that there is a thermal maximum on the Na2CO3 – KCl
join at 588°C that divides this three-component reciprocal salt system into two subsystems: Na2CO3 – KCl
– NaCl and Na2CO3 – KCl – K2CO3, with liquidus
minima at 568°C and 558°C, respectively. Neither the
liquidus phase-assemblages nor liquid immiscibility
were described by Nyankovskaya (1952). Part of a
related ternary system, Na2CO3 – CaCO3 – NaCl, was
investigated at 1 bar CO2 pressure by Niggli (1919); it
includes a eutectic on the binary join NaCl–Na2CO3
at 638°C and ~41 wt.% NaCl (Fig. 1). In this system,
The starting compositions (Table 1) were prepared
along the pseudobinary join nyerereite – (50 wt.% NaCl
+ 50 wt.% KCl). We did not attempt to investigate
compositions with more than 80 wt.% halide, because
of experimental costs, and as these have little relevance
to natural natrocarbonatite.
All starting compositions were prepared by mixing
dried (140°C) analytical grade NaCl, KCl, Na2CO3, and
CaCO3 under alcohol in an agate mortar. All experiments were conducted in 3 mm diameter Au tubes that
had been previously degreased with alcohol, cleaned in
boiling HCl and annealed. To eliminate adsorbed water,
all starting materials were heated for 24 hours at 140°C
prior to loading the samples for the experiments.
Approximately 0.05–0.10 g of the starting mixture
was loaded into the Au tubes, which were sealed by
experimental studies of the system na2ca(co3)2
– nacl – kcl
973
Fig. 1. Phase relationships in the system Na2CO3 – CaCO3 – Na2Cl2 at one atmosphere
pressure of CO2 (after Niggli 1919).
arc-welding. Experiments were performed at a pressure
of 0.1 GPa over the temperature interval 900 to 450°C.
All experiments were taken from room temperature
to the final run temperature, i.e., without reversals
from supraliquidus temperatures. The experimental
charges were run in either Tuttle-type, externally
heated Nimonic 105 pressure vessels or rapid-quench,
externally heated Stellite pressure vessels. The rapidquench apparatus employed is similar to that shown in
Figure 1 of Matthews et al. (2003), but with an Iconel
sample holder and support rod, and a fixed magnet
configuration. All experiments were conducted in the
Experimental Petrology Laboratory of the Geological
Survey of Canada (Ottawa). Argon was used as pressure
medium for all experiments. Run times ranged from 6 to
146 hours. Temperature was measured with a stainlesssteel-sheathed type-K thermocouple located in a well at
the base of the pressure vessel. A temperature correction
for each pressure vessel utilized was employed on the
basis of a calibration with an internal thermocouple.
Reported temperatures are considered to be accurate
to ±5°C. Pressure was measured using a Bourdon tube
gauge (Astrogauge) calibrated against a Heise laboratory standard gauge, and is considered to be accurate
to ±0.005 GPa. Tuttle-type vessels were quenched at
the end of the experiment by a jet of compressed air.
Rates of quenching for these vessels were on average
300°C/minute for the first minute. For the rapid-quench
vessels, the samples are quenched to 15°C in less than
a minute. By comparing runs of the same composition
in Tuttle-type versus rapid-quench vessels, we observed
that the rate of quenching did not influence the observed
phase-assemblages.
All experimental products were investigated as
polished sections prepared using kerosene. These were
characterized at Lakehead University by back-scattered
electron (BSE) imagery and X-ray energy-dispersion
spectrometry using a JEOL 5900 LV scanning electron
microscope. The compositions of carbonates were
determined with a LINK ISIS analytical system incorporating a Super ATW Light Element Detector. Raw
EDS spectra were acquired for 150 s (live time) with
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Fig. 2. Phase relationships in the system Na2CO3 – CaCO3 (Cooper et al. 1975) with
addition data for Na2CO3 – CaCO3 – F from Jago & Gittins (1991). Note the significant depression of the liquidus with increasing F contents. It is not known if Cl acts
in a similar manner. NCss = (Na,Ca)2CO3, nyr: nyerereite, cal: calcite, sh: shortite, L:
liquid, V: vapor.
an accelerating voltage of 20 kV and beam current of
0.475 nA on a Ni standard. The spectra were processed
with the LINK ISIS SEMQUANT quantitative software package with full ZAF corrections applied. All
analyses were undertaken using a rastered beam over
an area typically of 10,000 mm2. Jadeite, orthoclase
and wollastonite were used as standards for Na, K and
Ca, respectively.
Experimental Results
Experimental data for the join (nyerereite)100–x – (50
wt.% NaCl + 50 wt.% KCl)x (10 < x < 80 wt.%) are
given in Table 1. Figure 4 illustrates the phase relationships established for this join. Liquids in this system
do not quench to a glass, but form skeletal crystals
of gregoryite-like sodium carbonates set in a very
fine-grained mixture of diverse sodium carbonates and
halides (Fig. 5). The experimental data demonstrate that
all bulk compositions investigated have a long interval
of crystallization and that the solidus for compositions
with 30–60 wt.% halide must lie approximately between
600 and 550°C, and for compositions with 70–80 wt.%
halide, between 550 and 500°C. Phase assemblages
obtained at 450°C are considered to be illustrative
of solidus assemblages, as there is no difference in
experimental studies of the system na2ca(co3)2
– nacl – kcl
975
the modes of these assemblages over the temperature
range from 500 to 450°C. Although we did not attempt
to locate the solidus for compositions with <30 wt.%
halide, experiments at 10 wt.% halide reveal consistent
assemblages from 600 to 450°C, and experiments at 20
and 30 wt.% halide reveal consistent assemblages from
550 to 450°C, in accordance with the solidus moving to
higher temperatures with decreasing halide content (Fig.
4). In the binary system NaCl – Na2CO3, which is the
closest chemical analogue to the Na2Ca(CO3)2 – (NaCl
+ KCl) pseudobinary join studied, the solidus is at
650°C (Niggli 1919); hence 650°C has been utilized as
the solidus temperature at 0 wt.% halide in Figure 4.
Figure 4 shows that liquidus temperatures typically
are on the order of 825°C. There is a depression in
the liquidus temperature to ~750°C at about 23 wt.%
halide, which corresponds to a piercing point. Liquids
reaching this point can evolve to more K-, Na, and
Cl-rich compositions in the pseudoquaternary system
Na 2CO 3 – CaCO 3 – KCl – NaCl. Liquids poor in
halides (<30 wt.% halide) relative to this piercing point
composition have a nyerereite-like (Na,Ca,K) carbonate
as the primary liquidus phase (Fig. 6). Liquids rich in
halide relative to the piercing point composition initially
Fig. 3. Phase relationships in the system KCl–NaCl (after
Scheil & Stadelmaier 1952). SS: continuous solid-solution
of KCl and NaCl.
Fig. 4. Experimentally determined phase-relationships in the system Na2Ca(CO3)2 – (KCl
+ NaCl) at 0.1 GPa. Nyr: nyerereite, Cal: calcite. The solidus at 650°C at 0 wt.% halide
is inferred from the system NaCl – Na2CO3 (Niggli 1919). Phase relations at 100 wt.%
halide are from the 1 bar study of Schiel & Stadelmeir (1952). The phase relationships
between 81 and 99 wt.% halide are inferred.
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the canadian mineralogist
precipitate calcite as the primary liquidus phase (Figs. 7,
8). Note that the liquidus is essentially isothermal from
40 to 80 wt.% halide (Fig. 4). In the halide-rich experiments, calcite commonly forms crystals with a rounded
habit (Fig. 8), a feature noted in other experimental
studies by Cooper et al. (1975) and Gittins & Tuttle
(1964). In experiments poorer in halide, calcite crystals
form prismatic laths with rounded terminations (Fig. 7).
At lower temperatures (<700°C) for compositions with
~30 to 80 wt.% halide, calcite reacts with liquid to form
a nyerereite-like (Na,Ca, K) carbonate phase; this coexists with one liquid phase (Fig. 8).
Quenched liquids in the (Na,Ca, K) carbonate plus
liquid field consist of irregular patches of halide and
Fig. 6. Primary liquidus nyerereite-like Na-Ca-K carbonate
(Nyr) set in a single-phase halide-rich groundmass (Na,K)
Cl. Nyr + 20 wt.% (KCl + NaCl) at 650°C. Back-scattered
electron image.
F ig . 5. Quench sodium carbonates in a halide-bearing
groundmass. Nyr + 30 wt.% (KCl + NaCl) at 900°C. Backscattered electron image.
Fig. 7. Primary liquidus calcite (CC) set in a carbonate-rich
quenched groundmass. Nyr + 40 wt.% (KCl + NaCl) at
750°C. Back-scattered electron image.
experimental studies of the system na2ca(co3)2
minor (Na,Ca)2CO3. The halides are typically heterogeneous in BSE images and consist of small (<5 mm)
irregular to amoeboid grains of (K,Na)Cl set in a (Na,
K)Cl matrix. These intergrowths are considered to have
formed from a single-phase halide precursor. None of
the experiments in the (Na,Ca, K) carbonate plus liquid
field contain discrete crystals of halite and sylvite (see
below). The textural relationships between the quenched
carbonate and halide fractions are difficult to interpret.
However, we find no conclusive evidence for liquid
immiscibility sensu stricto.
Below ~550°C for most compositions with 30
to 80 wt.% halide, the assemblage nyerereite-like
(Na,Ca, K) carbonate plus liquid is replaced by the
assemblage nyerereite-like (Na,Ca, K) carbonate plus
two crystalline halides and minor (Na,Ca,K)2CO3. The
texture of these run products is granuloblastic (Fig. 9),
and the halides consist of discrete grains of (K,Na)
Cl and (Na,K)Cl solid solutions. Each of these phases
has apparently undergone unmixing to form discrete
– nacl – kcl
977
exsolved inclusions of (Na,K)Clss in (K,Na)Clss and
(K,Na)Clss in (Na,K)Clss (Fig. 9).
Experiments at 600°C with 50–80 wt.% halide and
at 550°C with 70–80 wt.% halide (Table 1, Fig. 4),
contain the nyerereite-like (Na,Ca, K) carbonate phase
set in a quench matrix consisting of Na carbonate and
two halides. In contrast to the two-halide assemblage
described above, these are homogeneous in BSE images
(Fig. 10). We consider that these halides might represent
bona fide immiscible halide liquids, as their textural
relationships are very different from those observed
in other experiments of the same bulk composition at
lower or higher temperature (Table 1, Fig. 4).
Representative compositions of liquidus nyerereitelike (Na,Ca,K) carbonate are given in Table 2 and illustrated in Figure 11. These data show that the carbonates
contain increasing amounts of K with increasing
amounts of halide in the bulk composition. For a given
bulk composition, K2O and CaO contents increase and
decrease, respectively, with decreasing temperature.
Compositions are all slightly Na-poor relative to nyerereite occurring as phenocrysts and groundmass crystals
in Oldoinyo Lengai natrocarbonatite (Fig. 11).
Discussion
Bearing in mind the differences in bulk compositions, the experiments are in general agreement with
some aspects of the low-temperature crystallization
of natrocarbonatite and can be used to make some
predictions regarding the low-temperature evolution
and differentiation of natrocarbonatitic magma. Note
that the eruption temperatures of natrocarbonatite lavas
(510–600°C; Keller & Krafft 1990, Dawson et al.
1995) are commensurate with many of the subliquidus
Fig. 8. Primary liquidus calcite (CC) set in a single-phase
halide-rich groundmass with quench sodium carbonates.
Nyr + 70 wt.% (KCl + NaCl) at 750°C. Back-scattered
electron image.
Fig. 9. Primary liquidus nyerereite-like Na–Ca–K carbonate
(dark grey) set in a two-phase halide-rich groundmass.
Note that both KCl and NaCl have undergone exsolution
Bulk composition Nyr + 80 wt.% (KCl + NaCl) at 500°C.
Back-scattered electron image.
978
the canadian mineralogist
experiments (Fig. 4), although the actual temperatures
of extrusion of natrocarbonatitic lava are lower than
those of the experiments, likely owing to the presence
of fluorine in the natural lavas. Thus, in common with
Fig. 10. Primary liquidus nyerereite-like Na–Ca–K carbonate (nyr) set in quenched immiscible halide liquids. Nyr +
50 wt.% (KCl + NaCl) at 600°C. Back-scattered electron
image.
natrocarbonatite, the initial crystallization of nyerereite
is followed by the formation of quenched halide-rich
liquids and gregoryite-like carbonate. Quench textures
in the nyerereite-rich parts of the system resemble those
of the natrocarbonatite groundmass, and carbonate and
halide liquids are clearly not miscible.
In the portion of the Na2Ca(CO3)2 – (NaCl + KCl)
pseudobinary join investigated, apart from the piercing
point, there is little change in the liquidus temperature
of the majority of the halide-rich (40–80 wt.%) bulk
compositions. Exceptionally flat liquidus surfaces have
been observed in many other experimental systems, e.g.,
BaO – SiO2, albite – fayalite, orthoclase – diopside, and
leucite – fayalite – SiO2, and these have been attributed
to the presence of a stable or metastable two-liquid
field (Roedder 1979). In the nyerereite–salt system,
the observed presence of a metastable two-liquid field
(i.e., below the liquidus) from 40 to 80 wt.% halide is
consistent with the observed flat liquidus surface from
40 to 80 wt.% halide.
The piercing point represents the intersection of the
binary nyerereite–calcite cotectic in the pseudoquaternary system Na2CO3 – CaCO3 – NaCl – KCl with the
pseudobinary join nyerereite – salt. The large primary
field of calcite as a liquidus phase for bulk compositions
enriched in halides is consistent with the earlier work of
Fig. 11. Compositions (mol.%) of experimentally formed nyerereite plotted in the system Na2CO3 – K2CO3 – CaCO3. Data points for nyerereite crystallized from the bulk
compositions with 10 wt.% (open squares) and 20 wt.% (solid circles) halide (50:50
NaCl:KCl wt.%) are shown as a function of temperature of crystallization. Compositions of nyerereite formed from bulk compositions with 70 and 80 wt.% halide are
independent of temperature and are plotted as the dark shaded field. Compositions of
phenocrystal and groundmass nyerereite in Oldoinyo Lengai natrocarbonatite are from
Peterson (1990) and Mitchell (2006). Thermal divide and cotectic are from Cooper et
al. (1975).
experimental studies of the system na2ca(co3)2
Niggli (1919). Liquids leaving the piercing point follow
the nyerereite–calcite cotectic in the pseudoquaternary
Na2CO3 – CaCO3 – NaCl – KCl. These liquids, with a
lower temperature, must follow boundary curves that
terminate at a quaternary eutectic involving (Na,K)2CO3
– (Na,K)2Ca(CO3)2 – NaCl – KCl.
The occurrence of calcite as a primary liquidus
phase for many bulk compositions is interesting in
that it demonstrates that halide-rich liquids have the
potential to form calcite “cumulates”, as in many of
the experiments calcite commonly is concentrated in
one part of the Au capsule. Figure 4 shows that calcite
could be precipitated over a wide range of temperatures from natural natrocarbonatitic magmas that have
become sufficiently enriched in halides (see below).
Segregation and accumulation of such calcite would
form rocks that could be described as calcite carbonatite. This can only occur if there is perfect fractional
crystallization at temperatures greater than 625°C,
because at lower temperatures, calcite is in a reaction
relationship with liquid to form nyerereite (Fig. 4). As
yet, neither primary calcite nor calcite carbonatites have
been described from Oldoinyo Lengai, and moreover,
we have no desire to claim that common calcite carbonatites are derived in this manner from natrocarbonatitic
parents. However, we can predict that there is the
potential for the formation of calcite-bearing natrocarbonatite in the magma chambers at Oldoinyo Lengai,
where differentiation and assimilation of previous
halide-rich natrocarbonatite undoubtedly occur. Typically, Oldoinyo Lengai natrocarbonatite contains only
about 5 wt.% (F + Cl), although aphanitic varieties can
contain up to 10 wt.% (F + Cl) (Keller & Krafft 1990).
However, assimilation of halide-rich differentiates in
relatively unevolved batches of natrocarbonatite might
lead to enrichment in halides such that calcite could
become the liquidus phase.
Our data have a bearing on the crystallization of
natrocarbonatitic magma under hypabyssal and plutonic
conditions, as the low-temperature assemblages
observed in the experiments predict the formation of
rocks composed of nyerereite- and gregoryite-like
carbonates together with halite and sylvite. The addition of fluorine to this system will undoubtedly result
in the formation of fluorite with this assemblage. Thus,
it is probable that low-temperature (<450°C) crystallization of natrocarbonatite in magma chambers will
result in “plutonic” rocks consisting of these mineral
assemblages. Such material can be easily fragmented
and assimilated by subsequent batches of hotter natrocarbonatitic magma, and lead to the sylvite–fluorite
macrocryst assemblage described by Mitchell (2006).
Acknowledgements
This paper is dedicated to John Gittins in recognition of his pioneering experimental studies of
– nacl – kcl
979
carbonatite-like systems. This research is supported
by the Natural Sciences and Engineering Council of
Canada, the Geological Survey of Canada, and Lakehead University. Ann Hammond and Allan MacKenzie
are thanked for assistance with sample preparation and
analytical electron microscopy, respectively. Constructive reviews of an earlier version of this manuscript by
Anton Chakhmouradian, Karen Moore, and Ilya Veksler
are appreciated. Bob Martin is thanked for editorial
comments.
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