JOURNAL OF PETROLOGY
VOLUME 52
NUMBERS 7 & 8
PAGES 1265^1280
2011
doi:10.1093/petrology/egq069
Experimental Studies of the System
Na2CO3^CaCO3^MgF2 at 0·1GPa: Implications
for the Differentiation and Low-temperature
Crystallization of Natrocarbonatite
ROGER H. MITCHELL1* AND BRUCE A. KJARSGAARD2
1
DEPARTMENT OF GEOLOGY, LAKEHEAD UNIVERSITY, THUNDER BAY, ONTARIO, CANADA P7B 5E1
2
GEOLOGICAL SURVEY OF CANADA, 601 BOOTH STREET, OTTAWA, ONTARIO, CANADA, K1A 0E8
RECEIVED FEBRUARY 10, 2010; ACCEPTED OCTOBER 14, 2010
ADVANCE ACCESS PUBLICATION DECEMBER 2, 2010
Oldoinyo Lengai (Tanzania) is the world’s only active carbonatite
volcano and until recently has been characterized by the eruption
of natrocarbonatite lavas consisting principally of nyerereite
[Na2Ca(CO3)2] and gregoryite [(Na,Ca)2CO3] phenocrysts, in a
mesostasis of nyerereite, gregoryite, halite^sylvite, fluorite, potassium
neighborite [(Na,K)MgF3], and khanneshite. The pseudoternary
system Na2CO3^CaCO3^MgF2 (NC^CC^MF) includes synthetic analogs of nyerereite (NY), gregoryite (NC) and neighborite
(PV). Phase relationships along five pseudobinary joins in the subsystem NC^NY^MF have been determined at 0·1 GPa at diverse
temperatures to establish liquidus phase relationships and elucidate
liquid lines of descent. Additional experiments at subliquidus and
subsolidus temperatures were undertaken to assess the crystallization
sequence of natrocarbonatite magmas and predict the mineralogy of
rocks formed under hypabyssal conditions within the volcano.
Primary liquidus phases encountered in NC^NY^MF include gregoryite, nyerereite, calcite and neighborite. Along the join NC^MF
there is a pseudoeutectic (NC þ PV þ L) at 20 wt % MgF2, and
the subsolidus assemblage is NC þ PVþ eitelite [Na2Mg(CO3)2].
This pseudoeutectic extends into the ternary system as a pseudocotectic leading to a pseudoternary eutectic, involving NC, NY and PV,
with the approximate composition [NC49CC28(MgF2)23] at
5258C. In the more CaCO3 -rich join NY^MF, the phase assemblage is characterized by the crystallization of calcite (CC) for
compositions with 45 wt % MgF2, and PV at 45 wt % MgF2.
In the ternary system NC^NF^MF pseudocotectics involving
CC þ NY and CC þ PV terminate at a reaction point [5758C;
NY46CC30(MgF2)24], where CC reacts with liquid to form NY.
Phase relations on other joins in the ternary system indicate that the
*Corresponding author. E-mail: [email protected]
NC^NY pseudocotectic terminates at the NC^NY^PV pseudoternary eutectic. For compositions with 55 wt % MgF2, this pseudocotectic is an odd reaction curve; for those with45 wt % MgF2, this
pseudocotectic is an even reaction curve. Subsolidus assemblages consist of NC, NY, PV, dolomite and eitelite. For natrocarbonatite
magmas, our phase equilibrium data explain the typical crystallization of gregoryite before nyerereite, the crystallization of nyerereite
before gregoryite, and why their differentiation leads towards
magnesium enrichment and formation of the fluoroperovskite neighborite. Crystallization under hypabyssal conditions could result in
gregoryite^nyerereite cumulates, and dolomite- and eitelite-bearing
assemblages.
KEY WORDS:
natrocarbonatite; nyerereite; gregoryite; neighborite;
Oldoinyo Lengai
I N T RO D U C T I O N
The volcano Oldoinyo Lengai, located in the Gregory Rift
Valley of Tanzania (28450 S; 358540 E) is the world’s only
active nephelinite^carbonatite volcano and is unique
with respect to the eruption of natrocarbonatite lavas
(Dawson, 1962, 2008). The natrocarbonatites are composed dominantly of gregoryite [(Na,K,Ca)2CO3] and
nyerereite [Na2Ca(CO3)2] phenocrysts set in a matrix of
gregoryite, nyerereite, halite^sylvite, fluorite, potassium
neighborite [(Na,K)MgF3] and khanneshite [(Na,Ca)3
(Ba,Sr,Ce,Ca)3(CO3, F,Cl)5; Zaitsev et al., 2008]. Various
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JOURNAL OF PETROLOGY
VOLUME 52
aspects of the geology, mineralogy and petrology of these
lavas have been described by Keller & Krafft (1990),
Peterson (1990), Church & Jones (1995), Dawson et al.
(1995), Mitchell (1997, 2006), Mitchell & Belton (2004) and
Zaitsev et al. (2008, 2009). Unusual aspects of natrocarbonatite lavas are their low eruption temperatures
(490^6008C; Keller & Krafft, 1990; Dawson et al., 1995),
the crystallization of diverse primary halide minerals, formation of late-stage carbonate^halide liquid immiscibility
(Mitchell, 1997) and differentiation towards Mg-rich residua (Gittins & Jago, 1998).
To understand the low-temperature crystallization and
differentiation of natrocarbonatite liquids we have commenced a program of experimental investigations of parts
of the haplo-natrocarbonatite system Na2CO3^CaCO3^
NaCl^KCl^MgF2^CaF2. This system, apart from
khanneshite, encompasses most of the minerals that have
been encountered in erupted natrocarbonatites. Initial
experiments have been directed at understanding the
conditions under which halide^carbonate liquid immiscibility occurs (Mitchell & Kjarsgaard, 2008). This study
attempts to understand the conditions leading to Mg
enrichment, and the formation of the fluoroperovskite
potassium neighborite, and has implications regarding the
crystallization of natrocarbonatite magmas under hypabyssal conditions.
P R E V I O U S E X P E R I M E N TA L
WO R K
There has been no previous experimental work on the
system Na2CO3^CaCO3^MgF2. In our experiments we
did not attempt to define the complete assemblage of liquidus, subliquidus, or subsolidus phase assemblages in the
system Na2CO3^CaCO3^MgF2. In addition, we confined
our investigations to only the Na^Ca-rich parts of the
sub-system Na2CO3^Na2Ca(CO3)2^MgF2, relevant to
natrocarbonatite magmas.
Phase relationships of the boundary binary system
Na2CO3^CaCO3 at 0·1GPa are well established (Cooper
et al., 1975; Jago & Gittins, 1991). This binary system includes nyerereite as a congruently melting compound
(8178C), and two eutectics located at 78·5 wt % Na2CO3
and 47 wt % Na2CO3 at 7258C and 8138C; indicated
as E1 and E2, respectively, in Fig. 1. In contrast, phase relationships for the boundary systems Na2CO3^MgF2 and
CaCO3^MgF2 have not been calculated or experimentally
determined as far as we are aware.
E X P E R I M E N TA L P RO C E D U R E S
Our studies included 28 bulk compositions lying along five
joins within the ternary system Na2CO3^Na2Ca(CO3)2^
MgF2 (Tables 1^5), together with an additional 10 ternary
bulk compositions (Table 6) utilized to locate the primary
NUMBERS 7 & 8
JULY & AUGUST 2011
liquidus phase field boundaries. The bulk compositions
and joins investigated are shown in Fig. 1, and given in
Tables 1^6.
All starting compositions were prepared by mixing
dried (1408C) analytical grade Na2CO3, CaCO3 and
MgF2 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 h at 1408C 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
arc-welding. Experiments were performed at a pressure of
0·1GPa, over the temperature range from 800 to 4508C.
All experiments were taken from room temperature to the
final run temperature (i.e. no reversals from supra-liquidus
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 rapid-quench apparatus employed is similar
to that shown in Fig. 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
36 h. Temperatures were measured with stainless steel
sheathed Type-K thermocouples located in a well at the
base of the pressure vessels. 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 58C.
Pressure was measured using a Bourdon tube gauge
(Astrogauge), calibrated against a Heise laboratory standard gauge and is thought to be accurate to 0·005 GPa.
Tuttle-type vessels were quenched at the end of the experiment by a jet of compressed air. Quenching rates for these
vessels were on average 3008C min1 for the first minute.
For the rapid-quench vessels, the samples are quenched to
158C in less than 1min. It was observed, by comparing
runs of the same composition in Tuttle-type versus
rapid-quench vessels, that the rate of quenching did not
influence the observed phase assemblages.
All experimental products were investigated as polished grain mounts 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 with a LINK ISIS analytical system incorporating a Super ATW Light Element Detector.
Representative BSE images of run products are illustrated in Figures 2^8. These illustrate liquidus NC
(Fig. 2), NY (Fig. 3), PV (Figs 4 and 5), and CC
1266
MITCHELL & KJARSGAARD
NATROCARBONATITE MAGMAS
Fig. 1. Locations of the pseudobinary joins and bulk compositions studied in the ternary system Na2CO3 (NC)^CaCO3 (CC)^MgF2. Numbers
refer to sample numbers in Table 6.
(Fig. 6), and subsolidus assemblages of NC þ PV þ eitelite
(Fig. 7) and PV þ DOL (Fig. 8).
E X P E R I M E N TA L R E S U LT S
The investigated system NC^CC^PV is complex, and not
a true ternary system. The crystallization of NaMgF3 on
the join NC^MF cannot be expressed within the ternary
NC^MF^MC (magnesite, MgCO3) because the compound NaMgF3 does not lie within the composition
plane. Similarly, the phase relations within the NC^CC^
PV ternary cannot be expressed as a true quaternary
system (i.e. NC^MF^MC^CC) because the compound
NaMgF3 does not lie within this four-component composition space. Hence all the studied joins are pseudobinary,
within a pseudoternary system. For the studied joins, subliquidus and subsolidus phase assemblages have been interpreted (in the absence of experimental data points)
utilizing the phase rule (Rhines, 1956; Maaloe, 1985).
The join Na2CO3^MgF2 [NC^MF]
Experimental data for the join Na2CO3^MgF2 (Fig. 1:
NC^MF) are given in Table 1 and the phase relationships
illustrated in Fig. 9. These data show that the liquidus temperatures decrease from the melting point of Na2CO3
(8728C) towards a pseudobinary eutectic occurring at a
composition of NC80MF20 at 6008C. NaMgF3 is the primary liquidus phase (Fig. 5) for compositions richer in
MgF2. NaMgF3 melts congruently at 10308C (Berman &
Dergunov, 1941), and as there is no solid solution between
the compounds NaMgF3 and Na2CO3, a eutectic relationship is expected. Experiments at subsolidus temperatures
(55508C) showed the coexistence of Na2CO3 þ
NaMgF3 þ Na2Mg(CO3)2 (i.e. analogs of gregoryite^
neighborite^eitelite; Figs 7 and 9); the presence of the
latter two phases demonstrates the pseudobinary character
of this join. We interpret our data to indicate that a stability
field for Na2CO3 þ NaMgF3 þ Na2Mg(CO3)2, þ liquid
exists between 550 and 6008C (Fig. 9), between the
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JOURNAL OF PETROLOGY
VOLUME 52
NUMBERS 7 & 8
JULY & AUGUST 2011
Table 1: Experimental data for the join Na2CO3^MgF2
at 0·1 GPa
Table 2: Experimental data for the join (NC80CC20)^
MgF2 at 0·1 GPa
% MgF2
T (8C)
Time
Products
% MgF2
10
700
16
NC þ L
10
750
16
NC þ L
10
800
16
L
15
550
16
15
600
15
650
15
700
6
20
450
36
20
500
16
20
550
20
600
20
T (8C)
Time
Products
5
650
16
NC þ L
5
700
16
NC þ L
5
750
16
NC þ L
NC þ PV þ EIT
10
600
16
NC þ L
16
NC þ PV þ L
10
650
16
NC þ L
16
NC þ L
10
700
6
NC þ L
L
10
800
6
L
NC þ PV þ EIT
15
600
16
NC þ PV þ EIT
15
650
6
16
NC þ PV þ EIT
20
450
36
NC þ NY þ PV þ EIT þ DOL
16
NC þ PV þ L
20
550
24
NC þ NY þ PV þ EIT
650
16
L
20
600
16
L
20
700
16
L
20
700
6
L
20
800
6
L
25
600
16
25
650
25
30
NC þ L
L
L
20
800
6
NC þ PV þ L
25
650
6
PV þ L
6
PV þ L
25
700
6
L
700
6
L
30
600
16
PV þ L
700
6
PV þ L
30
700
6
PV þ L
30
750
6
L
30
800
6
L
30
800
6
L
L, liquid; NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2;
Time, duration of the experiment in hours.
Na2CO3 þ NaMgF3 þ liquid and the subsolidus phase
fields. We did not attempt to determine the precise locations of these phase boundaries as this was not essential
to our study.
The join (NC80CC20)^MgF2 [A^MF]
Experimental data for the join [(NC80CC20)^MgF2]
(Fig. 1; A^MF) are given in Table 2 and the phase relationships illustrated in Fig. 10. The join includes a pseudocotectic between the primary phase fields of Na2CO3 (Fig. 2)
and NaMgF3 at A80MF20 and 5908C. Interpreted subliquidus phase fields include Na2CO3 þ Na2Ca(CO3)2 þ
and
Na2CO3 þ Na2Ca(CO3)2 þ
NaMgF3 þ liquid
NaMgF3 þ Na2Mg(CO3)2 þ liquid (Fig. 10). Subsolidus
assemblages consist of Na2CO3 þ Na2 Ca(CO3)2 þ
NaMgF3 þ Na2Mg(CO3)2, from 5258C to 4758C.
This subsolidus assemblage is joined by dolomite at temperatures below 4508C (Fig. 10).
The join (NC70CC30)^MgF2 [B^MF]
Experimental data for the join [(NC70CC30)^MgF2]
(Fig. 1; B^MF) are given in Table 3 and the phase relationships illustrated in Fig. 11. The join includes a small field
L, liquid; NC, Na2CO3; NY, Na2Ca(CO3)2; PV, NaMgF3;
EIT, Na2Mg(CO3)2; DOL, CaMg(CO3)2; Time, duration of
the experiment in hours.
of nyerereite crystallization for bulk compositions with
55 wt % MF. We did not attempt to determine precisely
phase relationships in this region; however, we consider
that given the addition of MgF2 to the NC^CC join
(i.e. for compositions in the pseudoternary) a peritectic reaction separates the field of NY þ L from that of NC þ L
(Fig. 11). For bulk compositions with greater than 5 wt %
MF, NC is the primary liquidus phase until the pseudocotectic between the primary phase fields of Na2CO3 and
NaMgF3 is reached at B78MF22 and 5508C. Subliquidus phase fields consist of Na2CO3 þ NaMgF3 þ liquid
and Na2CO3 þ NaMgF3 þ Na2Mg(CO3)2, þ liquid. Subsolidus assemblages consist of Na2CO3 þ NaMgF3 þ
Na2Mg(CO3)2, at temperatures below 5108C.
The join (NC60CC40)^MgF2 [C^MF]
Experimental data for the join [(NC60CC40)^MgF2]
(Fig. 1; C^MF) are given in Table 4 and the phase relationships illustrated in Fig. 12. Along this join the primary
liquidus fields of NY and PV are interrupted by a thermal
maxima where calcite (CC) appears as the primary liquidus phase (Fig. 6). Hence, the pseudobinary contains two
pseudocotectics: (1) between the primary phase fields of
1268
MITCHELL & KJARSGAARD
NATROCARBONATITE MAGMAS
Table 3: Experimental data for the join (NC70CC30)^
MgF2 at 0·1 GPa
Table 4: Experimental data for the join (NC60CC40)^
MgF2 at 0·1 GPa
% MgF2
% MgF2
T (8C)
Time
Products
T (8C)
Time
Products
5
650
16
NC þ L
5
650
6
NY þ L
5
700
6
NC þ L
5
750
6
NY þ L
5
750
6
L
5
800
6
NY þ L
10
500
24
NC þ PV þ NY þ EIT
10
650
6
NY þ L
10
550
24
NC þ L
10
750
6
NY þ L
10
600
16
NC þ L
15
600
6
NY þ L
10
625
6
NC þ L
15
650
6
NY þ L
10
650
6
NC þ L
20
600
16
CC þ L
10
800
6
L
20
650
6
20
450
36
NC þ PV þ NY þ EIT
25
600
16
20
500
36
NC þ PV þ NY þ EIT
25
650
6
20
550
24
NC þ L
30
600
16
20
600
6
L
30
650
6
PV þ DOL þ L
25
550
24
PV þ L
30
700
6
PV þ L
25
600
6
PV þ L
25
650
6
PV þ L
25
700
6
L
30
500
16
NC þ PV þ NY þ EIT
30
550
6
PV þ L
30
600
6
PV þ L
30
700
6
PV þ L
L
CC þ L
L
PV þ DOL þ L
L, liquid; NY, Na2Ca(CO3)2; CC, CaCO3; PV, NaMgF3;
DOL, CaMg(CO3)2; Time, duration of the experiment in
hours.
L, liquid; NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2;
Time, duration of the experiment in hours.
Na2Ca(CO3)2 (NY) and CaCO3 (CC) at C17MF83 and
6258C; (2) between the primary phase fields of CaCO3
(CC) and NaMgF3 (PV) at C28MF72 and 5908C. For
compositions containing approximately 425 wt % MgF2
at temperatures below 6508C, CaMg(CO3)2 (dolomite;
DOL) joins PVas a liquidus phase. Interpreted subliquidus
phase fields include CaCO3 þ Na2Ca(CO3)2 þ liquid,
and
NaMgF3 þ
CaCO3 þ CaMg(CO3)2 þ liquid,
CaCO3 þ CaMg(CO3)2 þ liquid (Fig. 12). Experiments
along the joins B^MF and NY^MF (Fig. 13) suggest that
the subsolidus assemblage of C^MF will consist of
NY þ PV þ DOL þ EIT þ CC.
The join Na2Ca(CO3)2^MgF2 [NY^MF]
Experimental data for the join [Na2Ca(CO3)2^MgF2]
(Fig. 1; NY^MF) are given in Table 5 and the phase
relationships illustrated in Fig. 13. The join contains a pseudocotectic between the primary phase fields of CaCO3
(CC) and NaMgF3 (PV) at NY60MF40 and 6008C.
Interpreted subliquidus phase fields include CaCO3 þ
CaCO3 þ MgCa(CO3)2 þ
CaMg(CO3)2 þ liquid,
NaMgF3 þ liquid, and NaMgF3 þ MgCa (CO3)2 þ liquid
(Fig. 13). Subsolidus assemblages below 5508C consist of
CC þ PV þ DOL, and of CC þ PV þ DOL þ EIT below
5258C. The join includes a very small field of NY crystallization for bulk compositions with 55 wt % MgF2.
We did not attempt to determine phase relationships in
this region of the join as they are not essential to this study.
Liquidus phase relationships in parts
of the pseudoternary system
Na2CO3^CaCO3^MgF2
The experimental data for the five joins investigated
(Tables 1^5) are combined with additional experimental
data (Table 6) to construct a liquidus diagram for the
Na^Ca-rich portion of the system Na2CO3^CaCO3^
MgF2. Figure 14 shows that the pseudoeutectic along the
pseudobinary join NC^MF extends into the pseudoternary
system as a pseudocotectic along which NC and PVcrystallize together. This pseudocotectic terminates at a pseudoternary eutectic (E) involving NC, NY and PV, with
the approximate composition [NC49CC28(MgF2)23] at
5258C.
Along the NC^CC join the eutectic E1 (NC þ NY) extends into the ternary system as an odd reaction curve
(NY þ L ) NC þ L) at low MF concentrations (55 wt %
MgF2). At higher MF concentrations NY þ NC
co-crystallize along the pseudocotectic, which terminates
at the pseudoternary eutectic (E). At more calcium-rich
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JULY & AUGUST 2011
Table 5: Experimental data for the join NY^MgF2
at 0·1 GPa
Table 6: Experimental data for the miscellaneous
compositions in the system NC^CC^MgF2 at 0·1 GPa
% MgF2
T (8C)
Products
MF
5
700
6
CC þ L
2
5
750
6
CC þ L
5
800
6
CC þ L
5
850
6
L
10
500
16
10
600
6
CC þ L
10
650
6
CC þ L
10
700
6
L
10
800
6
20
500
16
20
600
20
Time
3
CC þ PV þ DOL þ EIT
4
5
L
6
wt %
T (8C)
Time
Products
NC þ L
5:68:27
700
16
5:68:27
800
6
L
5:65:30
750
6
NY þ L
5:65:30
800
6
15:55:30
650
16
NY þ L
15:55:30
700
16
NY þ L
20:50:30
550
6
20:50:30
600
12
L
L
NC þ NY þ PV þ EIT þ DOL
20:45:30
550
16
NY þ PV þ EIT þ DOL
CC þ PV þ DOL þ EIT
20:45:30
600
12
CC þ L
6
CC þ L
20:45:30
550
16
L
650
6
L
7
25:48:27
600
16
CC þ L
20
700
6
L
8
25:40:35
600
16
CC þ L
20
800
6
L
9
30:40:30
600
16
PV þ DOL þ CC þ L
25
600
6
CC þ L
30:40:30
650
6
L
30
600
6
CC þ L
35:35:30
600
6
PV þ DOL þ L
30
650
6
L
30:35:30
650
6
PV þ DOL þ L
30
700
6
L
30:35:30
700
6
PV þ L
40
600
6
L
45:30:25
650
6
PV þ DOL þ L
40
650
6
L
45:30:25
700
6
PV þ L
PV þ L
45:30:25
750
6
L
10
11
45
700
6
50
500
16
CC þ PV þ DOL þ EIT
50
550
16
CC þ PV þ DOL
50
600
6
L, liquid; NC, Na2CO3; NY, Na2Ca(CO3)2; PV, NaMgF3;
EIT, Na2Mg(CO3)2; DOL, CaMg(CO3)2; Time, duration of
the experiment in hours. MF, sample number as shown
in Figs 1 and 14: wt %, MF compositions in terms of
wt % MgF2:NC:CC.
PV þ L
50
650
6
PV þ L
50
700
6
PV þ L
L, liquid; PV, NaMgF3; EIT, Na2Mg(CO3)2; DOL, CaMg
(CO3)2; CC, CaCO3. Time, duration of the experiment in
hours.
compositions on the NC^CC join the eutectic E2
(NY þCC) extends into the ternary system as a phase
boundary separating the primary liquidus phase fields of
NY þ L and CC þ L (Fig. 14). The field boundary is an
even reaction curve with NY and CC crystallizing from
and in equilibrium with liquid. A pseudocotectic separates
the primary phase fields of PV and CC (Fig. 14). A small
primary phase field of dolomite could exist between
the NY^MF and C^MF joins at 20^30 wt % MgF2;
however, these compositions are too rich in MF to be applicable to the problem of crystallization of natrocarbonatite magma. The NY þCC and PV þCC pseudocotectics
intersect at a reaction point R [5758C; NY46CC30
(MgF2)24], where CC is consumed in the reaction
PV þCC þ NY þ L ) PV þ NY þ L (Fig. 14). Similar
reaction points involving CC and melt have been described
in the systems Na2CO3^CaCO3^CaF2 and Na2CO3^
CaCO3^F by Jago & Gittins (1991). For fractional crystallization any residual liquids must follow the pseudocotectic
from R to E, crystallizing NY þ PV.
A P P L I C AT I O N S T O T H E
C RY S TA L L I Z AT I O N O F
N AT RO C A R B O N AT I T E M A G M A
Natrocarbonatite lavas are F-rich (1·65^5·25 wt % F;
Dawson et al., 1995), hypersodic rocks whose bulk compositions (Keller & Krafft, 1990) are dominated by Na2O,
CaO and CO2 (65 wt % Na2CO3 35 wt% CaCO3), as
a consequence of the high modal abundance of gregoryite
and nyerereite phenocrysts. These bulk compositions are
intermediate to the E1 and E2 eutectics along the NC^CC
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Fig. 2. Rounded primary liquidus crystals of Na2CO3 (NC) set in matrix of Na2Ca(CO3)2 laths and very fine-grained Na2CO3, NaMgF3 and
Na2Mg(CO3)2. Composition (A)95(MF)5 at 6508C. BSE image.
Fig. 3. Laths of primary liquidus crystals of Na2Ca(CO3)2 (NY) set in a matrix of skeletal quench NY (pale grey) and very fine-grained
Na2CO3, NaMgF3 and Na2Mg(CO3)2. Composition (C)90(MF)10 at 6008C. BSE image.
join. Mitchell (2009) has described quenched natrocarbonatite formed by liquid immiscibility between silicate and
carbonatite melts observed as inclusions in a variety of
solid phases. These inclusions, which have not crystallized
primary gregoryite or nyerereite, are also hypersodic, and
have similar compositions to the lavas (20 wt % Na2O
and 22 wt % CaO, to 33 wt % Na2O and 15 wt % CaO).
Thus, their bulk compositions also lie between E1 and E2,
and could represent undifferentiated primary natrocarbonatite magmas (Mitchell, 2009). Accordingly, it is expected
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Fig. 4. Euhedral primary liquidus crystals of NaMgF3 (PV) set in a matrix of quench crystals of Na2CO3 (pale grey) and Na2Mg(CO3)2
(dark grey). Composition (A)70(MF)30 at 7008C. BSE image.
Fig. 5. Euhedral primary liquidus crystals of NaMgF3 (PV) set in a matrix of quench crystals of calcite (CC), Na2Ca(CO3)2 (light grey) and
Na2Mg(CO3)2 (dark grey). Composition MF10 (Table 6) at 7008C. BSE image.
that parental magmas might have compositions close to
the pseudocotectic involving NC and NY in Fig. 14, and
depending on the bulk composition could crystallize
either gregoryite or nyerereite initially, followed by gregoryite plus nyerereite. However, given the predominance of
Na over Ca in natrocarbonatites it is expected that, ideally,
primary magmas will initially crystallize gregoryite.
Fractional crystallization would then drive the bulk composition towards the gregoryite^nyerereite pseudocotectic
and ultimately towards the pseudoternary eutectic E
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Fig. 6. Euhedral laths and rounded primary liquidus crystals of CaCO3 (CC) set in a matrix of very fine-grained NY, Na2Mg(CO3)2 and
NaMgF3. Composition (NY)80(MF)20 at 6008C. BSE image.
Fig. 7. Subsolidus assemblage of Na2CO3 (NC), NaMgF3 (PV) and Na2Mg(CO3)2 (EIT). Composition (NC)75(MF)25 at 5508C. BSE image.
(Fig. 14), at which they will be joined by neighborite. As
neighborite is never found as phenocryst in natrocarbonatites it is evident that the NC þ NY þ PV pseudoeutectic
and the NC þ PV pseudocotectic cannot play a role in the
initial stages of crystallization of natrocarbonatites
Although some natrocarbonatites are characterized by
the initial crystallization of gregoryite followed by
nyerereite, others are dominated by euhedral laths of phenocrystal nyerereite, suggesting that nyerereite crystallized
first. This observation is consistent with our experimental
data; in particular, it should be noted that the field
of NC expands significantly at the expense of NY with
the addition of small amounts of MF (Fig. 14). Hence,
near-identical natrocarbonatite magmas, with only small
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Fig. 8. Near solidus assemblage of NaMgF3 (PV) and CaMg(CO3)2 (DOL). Dark areas are fine grained mixtures of quench Na2Ca(CO3)2
and Na2Mg(CO3)2. Composition MF11 (Table 6) at 6508C. Back-scattered electron image.
Fig. 9. Phase relationships along the pseudobinary join Na2CO3^MgF2 (NC^MF) at 0·1GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT,
Na2Mg(CO3)2; L, liquid.
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Fig. 10. Phase relationships along the pseudobinary join (A^MF) at 0·1GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; NY,
Na2Ca(CO3)2; DOL, CaMg(CO3)2; L, liquid.
Fig. 11. Phase relationships along the pseudobinary join (B^MF) at 0·1GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; NY,
Na2Ca(CO3)2; L, liquid.
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Fig. 12. Phase relationships along the pseudobinary join (C^MF) at 0·1GPa pressure. NY, Na2Ca(CO3)2; PV, NaMgF3; CC, CaCO3; DOL,
CaMg(CO3)2; L, liquid.
variations in MF (or possibly F alone) could crystallize
nyerereite first, or gregoryite first. Further varieties of
natrocarbonatite contain several petrographically distinct
generations of gregoryite and/or resorbed crystals of nyerereite; the latter obviously not being in equilibrium with
their current hosts. Using trace and major element data,
Mitchell & Kamenetsky (2008) and Zaitsev et al. (2009)
have shown that different populations of gregoryite and
nyerereite are present in a given natrocarbonatite lava.
Evidently, natrocarbonatite lavas contain crystals of gregoryite and nyerereite derived from several batches of
magma that have been mixed together prior to eruption.
These observations demonstrate that most of the erupted
natrocarbonatites cannot represent the crystallization
products of a single batch of natrocarbonatite magma,
and must have bulk compositions that are controlled by
rheological and mixing processes.
Temperatures of erupted melts are all low (500^6008C;
Keller & Krafft, 1990; Dawson et al., 1995), and similar
to the liquidus temperatures determined in this study.
This suggests that natrocarbonatite melts are erupted at
temperatures very close to those of the magma solidus,
with rapid quenching preventing development of the subsolidus assemblages encountered in our experiments.
Our data for the pseudoternary system Na2CO3^
CaCO3^MgF2 demonstrates that fractional crystallization
leads to the enrichment of the residual magma in Mg.
This is in accord with the observations of Gittins & Jago
(1998) on trends in the bulk compositions of natrocarbonatites, and the presence of potassium neighborite and
Mg-bearing khanneshite as groundmass phases (Mitchell,
1997; Zaitsev et al., 2008). Enrichment in Mg could eventually lead to the formation of other magnesian minerals in
the groundmass. For example, Keller & Krafft (1990)
have reported the presence of trace amounts of sellaite
(MgF2), although this mineral has not been reported in
other investigations. We did not encounter sellaite in any
of our experiments, and our data indicate, in accord with
recent petrological observations (Church & Jones, 1995;
Dawson et al., 1995; Mitchell, 1997), that potassium neighborite is the common Mg^F-bearing groundmass mineral
rather than sellaite. It should be noted also we did not
encounter any NaF in our experiments, and thus do not
expect this mineral to be present in natrocarbonatites.
Our experiments have shown that nyerereite-rich bulk
compositions can crystallize calcite and/or dolomite.
To date neither of these minerals has been found in natrocarbonatite lavas, suggesting that magma compositions
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Fig. 13. Phase relationships along the pseudobinary join (NY^MF) at 0·1GPa pressure. NY, Na2Ca(CO3)2; PV, NaMgF3; EIT, Na2Mg(CO3)2;
CC, CaCO3; DOL, CaMg(CO3)2; L, liquid.
Fig. 14. Liquidus phase relationships in the pseudoternary system Na2CO3^CaCO3^MgF2 at 0·1GPa pressure. Experimental run compositions
labelled 2^11 (Table 6) were used to define the phase fields. E is a pseudoternary eutectic. R is a reaction point. NC, Na2CO3; NY,
Na2Ca(CO3)2; CC, CaCO3; Liq, liquid.
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Fig. 15. Allotriomorphic granular textured microxenolith consisting of gregoryite (G), nyerereite (NY), and fluorite (F) with minor apatite
(a) and magnetite (m). Natrocarbonatite flow 24 July 2000, hornito T49B, Oldoinyo Lengai, Tanzania.
must always be Ca-poor and Na-rich, and lie within the
primary liquidus field of gregoryite (or nyerereite) near
the NC^NY pseudocotectic.
The experimentally determined gregoryite^nyerereite
cotectic suggests that crystallization of natrocarbonatite
magma under hypabyssal conditions could lead to the formation of coarse-grained intergrowths of nyerereite and
gregoryite. Figure 15 illustrates one such intergrowth consisting of nyerereite, gregoryite and fluorite, which is interpreted here as a disaggregated cumulate. This clast can be
regarded as an example of a hypabyssal natrocarbonatite
and representative of one of the mineralogical assemblages
that can be present within the magma chambers feeding
the eruptive material.
On the basis of the subliquidus and subsolidus assemblages found in our experiments (Figs 7 and 8), it is
possible that potential formation of calcite-, dolomite-,
or eitelite-bearing hypabyssal rocks could occur within
slowly cooling subvolcanic natrocarbonatite magma
chambers. Potential cumulate rocks would include coarsegrained eitelite-bearing assemblages (i.e. gregoryite^
neighborite^eitelite^fluorite carbonatites) although to
date none have been encountered at Oldoinyo Lengai.
In this context, Fig. 16 illustrates the occurrence of a Naand Mg-rich mineral enclosed in khanneshite. Because
of the small size of crystals and excitation of the matrix
we were not able to obtain accurate compositions of this
phase, but consider it to be the first recognition of eitelite
from Oldoinyo Lengai.
CONC LUSIONS
The pseudoternary system Na2CO3^CaCO3^MgF2 is a
useful analog for illustrating the crystallization paths and
evolution of natrocarbonatite magmas. The phase relationships in this system indicate that ideally gregoryite or nyerereite will appear as the first primary liquidus phase,
and that both minerals can crystallize together along a
pseudocotectic leading to a low-temperature (5258C)
pseudoeutectic. Nyerereite, gregoryite and the fluoroperovskite neighborite crystallize together at this pseudoeutectic, reflecting the increase in the Mg content of the
natrocarbonatite liquid during crystallization. The temperatures of the pseudocotectics and pseudoeutectic are in
good agreement with measured temperatures of erupting
natrocarbonatite. It is considered that these lavas are
erupted close to their solidus temperatures and represent
hybrid magmas formed by the crystallization and mingling of several batches of natrocarbonatite in the vent of
the volcano prior to eruption. Na-rich bulk compositions
in the pseudoternary can crystallize eitelite as a subsolidus
phase, whereas nyerereite-rich bulk compositions can crystallize dolomite and calcite as subliquidus and subsolidus
phases. Although these carbonates have not yet been
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Fig. 16. Laths of a Mg-rich carbonate (?eitelite) set in a matrix of khannesite in the groundmass of a natrocarbonatite flow from hornito T37B,
26 July 200, Oldoinyo Lengai, Tanzania. E, eitelite.
found in natrocarbonatites, there is the potential for
the formation of hypabyssal natrocarbonatites containing
eitelite, calcite and/or dolomite in subvolcanic magma
chambers.
Interestingly,Veksler et al. (1998) have described eitelite coexisting with dolomite, nyerereite, and other Na^Ca- and
Na^Mg-carbonates in olivine-hosted crystallized melt inclusions from Kovdor olivine-bearing carbonatites. These
observations suggest that a trend of Mg enrichment can
occur during differentiation of other carbonatite-forming
magmas. Thus, our experimental data might have a wider
petrological applicability than to natrocarbonatites alone.
AC K N O W L E D G E M E N T S
This paper is dedicated to Peter Wyllie in recognition of his
pioneering experimental studies of the phase relationships
of haplocarbonatite systems. Ann Hammond is thanked
for assistance with sample preparation. John Gittins,
Oleg Safonov, Ilya Veksler and an anonymous reviewer
are thanked for constructive criticism of the initial draft
of this manuscript. This is Geological Survey of Canada
contribution 2010^639.
FUNDING
Our research is supported by the Natural Sciences and
Engineering Research Council of Canada, the Geological
Survey of Canada, and Lakehead University.
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