Abstract

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 + PV + L) at 20 wt % MgF2, and the subsolidus assemblage is NC + PV+  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 ∼525°C. In the more CaCO3-rich join NY–MF, the phase assemblage is characterized by the crystallization of calcite (CC) for compositions with >5 wt % MgF2, and PV at ∼45 wt % MgF2. In the ternary system NC–NF–MF pseudocotectics involving CC + NY and CC + PV terminate at a reaction point [∼575°C; ∼NY46CC30(MgF2)24], where CC reacts with liquid to form NY. Phase relations on other joins in the ternary system indicate that the NC–NY pseudocotectic terminates at the NC–NY–PV pseudoternary eutectic. For compositions with <5 wt % MgF2, this pseudocotectic is an odd reaction curve; for those with >5 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.

INTRODUCTION

The volcano Oldoinyo Lengai, located in the Gregory Rift Valley of Tanzania (2°45′S; 35°54′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 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 (∼490–600°C; 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.

PREVIOUS EXPERIMENTAL WORK

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 0·1 GPa are well established (Cooper et al., 1975; Jago & Gittins, 1991). This binary system includes nyerereite as a congruently melting compound (817°C), and two eutectics located at 78·5 wt % Na2CO3 and 47 wt % Na2CO3 at ∼725°C and ∼813°C; 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.

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. 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.

EXPERIMENTAL PROCEDURES

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 liquidus phase field boundaries. The bulk compositions and joins investigated are shown in Fig. 1, and given in Tables 1–6.

Table 1:

Experimental data for the join Na2CO3–MgF2 at 0·1 GPa

% MgF2 T (°C) Time Products 
10 700 16 NC + L 
10 750 16 NC + L 
10 800 16 
15 550 16 NC + PV + EIT 
15 600 16 NC + PV + L 
15 650 16 NC + L 
15 700 
20 450 36 NC + PV + EIT 
20 500 16 NC + PV + EIT 
20 550 16 NC + PV + EIT 
20 600 16 NC + PV + L 
20 650 16 
20 700 16 
20 800 
25 600 16 NC + PV + L 
25 650 PV + L 
25 700 
30 700 PV + L 
30 750 
30 800 
% MgF2 T (°C) Time Products 
10 700 16 NC + L 
10 750 16 NC + L 
10 800 16 
15 550 16 NC + PV + EIT 
15 600 16 NC + PV + L 
15 650 16 NC + L 
15 700 
20 450 36 NC + PV + EIT 
20 500 16 NC + PV + EIT 
20 550 16 NC + PV + EIT 
20 600 16 NC + PV + L 
20 650 16 
20 700 16 
20 800 
25 600 16 NC + PV + L 
25 650 PV + L 
25 700 
30 700 PV + L 
30 750 
30 800 

L, liquid; NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; Time, duration of the experiment in hours.

Table 2:

Experimental data for the join (NC80CC20 )–MgF2 at 0·1 GPa

% MgF2 T (°C) Time Products 
650 16 NC + L 
700 16 NC + L 
750 16 NC + L 
10 600 16 NC + L 
10 650 16 NC + L 
10 700 NC + L 
10 800 
15 600 16 NC + L 
15 650 
20 450 36 NC + NY + PV + EIT + DOL 
20 550 24 NC + NY + PV + EIT 
20 600 16 
20 700 
20 800 
25 650 PV + L 
25 700 
30 600 16 PV + L 
30 700 PV + L 
30 800 
% MgF2 T (°C) Time Products 
650 16 NC + L 
700 16 NC + L 
750 16 NC + L 
10 600 16 NC + L 
10 650 16 NC + L 
10 700 NC + L 
10 800 
15 600 16 NC + L 
15 650 
20 450 36 NC + NY + PV + EIT + DOL 
20 550 24 NC + NY + PV + EIT 
20 600 16 
20 700 
20 800 
25 650 PV + L 
25 700 
30 600 16 PV + L 
30 700 PV + L 
30 800 

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.

Table 3:

Experimental data for the join (NC70CC30)–MgF2 at 0·1 GPa

% MgF2 T (°C) Time Products 
650 16 NC + L 
700 NC + L 
750 
10 500 24 NC + PV + NY + EIT 
10 550 24 NC + L 
10 600 16 NC + L 
10 625 NC + L 
10 650 NC + L 
10 800 
20 450 36 NC + PV + NY + EIT 
20 500 36 NC + PV + NY + EIT 
20 550 24 NC + L 
20 600 
25 550 24 PV + L 
25 600 PV + L 
25 650 PV + L 
25 700 
30 500 16 NC + PV + NY + EIT 
30 550 PV + L 
30 600 PV + L 
30 700 PV + L 
% MgF2 T (°C) Time Products 
650 16 NC + L 
700 NC + L 
750 
10 500 24 NC + PV + NY + EIT 
10 550 24 NC + L 
10 600 16 NC + L 
10 625 NC + L 
10 650 NC + L 
10 800 
20 450 36 NC + PV + NY + EIT 
20 500 36 NC + PV + NY + EIT 
20 550 24 NC + L 
20 600 
25 550 24 PV + L 
25 600 PV + L 
25 650 PV + L 
25 700 
30 500 16 NC + PV + NY + EIT 
30 550 PV + L 
30 600 PV + L 
30 700 PV + L 

L, liquid; NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; Time, duration of the experiment in hours.

Table 4:

Experimental data for the join (NC60CC40 )–MgF2 at 0·1 GPa

% MgF2 T (°C) Time Products 
650 NY + L 
750 NY + L 
800 NY + L 
10 650 NY + L 
10 750 NY + L 
15 600 NY + L 
15 650 NY + L 
20 600 16 CC + L 
20 650 
25 600 16 CC + L 
25 650 
30 600 16 PV + DOL + L 
30 650 PV + DOL + L 
30 700 PV + L 
% MgF2 T (°C) Time Products 
650 NY + L 
750 NY + L 
800 NY + L 
10 650 NY + L 
10 750 NY + L 
15 600 NY + L 
15 650 NY + L 
20 600 16 CC + L 
20 650 
25 600 16 CC + L 
25 650 
30 600 16 PV + DOL + L 
30 650 PV + DOL + L 
30 700 PV + L 

L, liquid; NY, Na2Ca(CO3)2; CC, CaCO3; PV, NaMgF3; DOL, CaMg(CO3)2; Time, duration of the experiment in hours.

Table 5:

Experimental data for the join NY–MgF2 at 0·1 GPa

% MgF2 T (°C) Time Products 
700 CC + L 
750 CC + L 
800 CC + L 
850 
10 500 16 CC + PV + DOL + EIT 
10 600 CC + L 
10 650 CC + L 
10 700 
10 800 
20 500 16 CC + PV + DOL + EIT 
20 600 CC + L 
20 650 
20 700 
20 800 
25 600 CC + L 
30 600 CC + L 
30 650 
30 700 
40 600 
40 650 
45 700 PV + L 
50 500 16 CC + PV + DOL + EIT 
50 550 16 CC + PV + DOL 
50 600 PV + L 
50 650 PV + L 
50 700 PV + L 
% MgF2 T (°C) Time Products 
700 CC + L 
750 CC + L 
800 CC + L 
850 
10 500 16 CC + PV + DOL + EIT 
10 600 CC + L 
10 650 CC + L 
10 700 
10 800 
20 500 16 CC + PV + DOL + EIT 
20 600 CC + L 
20 650 
20 700 
20 800 
25 600 CC + L 
30 600 CC + L 
30 650 
30 700 
40 600 
40 650 
45 700 PV + L 
50 500 16 CC + PV + DOL + EIT 
50 550 16 CC + PV + DOL 
50 600 PV + L 
50 650 PV + L 
50 700 PV + L 

L, liquid; PV, NaMgF3; EIT, Na2Mg(CO3)2; DOL, CaMg(CO3)2; CC, CaCO3. Time, duration of the experiment in hours.

Table 6:

Experimental data for the miscellaneous compositions in the system NC–CC–MgF2 at 0·1 GPa

MF wt % T (°C) Time Products 
5:68:27 700 16 NC + L 
 5:68:27 800 
5:65:30 750 NY + L 
 5:65:30 800 
15:55:30 650 16 NY + L 
 15:55:30 700 16 NY + L 
20:50:30 550 NC + NY + PV + EIT + DOL 
 20:50:30 600 12 
20:45:30 550 16 NY + PV + EIT + DOL 
 20:45:30 600 12 CC + L 
 20:45:30 550 16 
25:48:27 600 16 CC + L 
25:40:35 600 16 CC + L 
30:40:30 600 16 PV + DOL + CC + L 
 30:40:30 650 
10 35:35:30 600 PV + DOL + L 
 30:35:30 650 PV + DOL + L 
 30:35:30 700 PV + L 
11 45:30:25 650 PV + DOL + L 
 45:30:25 700 PV + L 
 45:30:25 750 
MF wt % T (°C) Time Products 
5:68:27 700 16 NC + L 
 5:68:27 800 
5:65:30 750 NY + L 
 5:65:30 800 
15:55:30 650 16 NY + L 
 15:55:30 700 16 NY + L 
20:50:30 550 NC + NY + PV + EIT + DOL 
 20:50:30 600 12 
20:45:30 550 16 NY + PV + EIT + DOL 
 20:45:30 600 12 CC + L 
 20:45:30 550 16 
25:48:27 600 16 CC + L 
25:40:35 600 16 CC + L 
30:40:30 600 16 PV + DOL + CC + L 
 30:40:30 650 
10 35:35:30 600 PV + DOL + L 
 30:35:30 650 PV + DOL + L 
 30:35:30 700 PV + L 
11 45:30:25 650 PV + DOL + L 
 45:30:25 700 PV + L 
 45:30:25 750 

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.

All starting compositions were prepared by mixing dried (140°C) 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 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 arc-welding. Experiments were performed at a pressure of 0·1 GPa, over the temperature range from 800 to 450°C. 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 ±5°C. 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 300°C min−1 for the first minute. For the rapid-quench vessels, the samples are quenched to 15°C in less than 1 min. 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 (Fig. 6), and subsolidus assemblages of NC + PV +  eitelite (Fig. 7) and PV + DOL (Fig. 8).

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 650°C. BSE image.

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 650°C. 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 600°C. 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 600°C. BSE image.

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 700°C. BSE image.

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 700°C. 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 700°C. 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 700°C. BSE image.

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 600°C. BSE image.

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 600°C. BSE image.

Fig. 7.

Subsolidus assemblage of Na2CO3 (NC), NaMgF3 (PV) and Na2Mg(CO3)2 (EIT). Composition (NC)75(MF)25 at 550°C. BSE image.

Fig. 7.

Subsolidus assemblage of Na2CO3 (NC), NaMgF3 (PV) and Na2Mg(CO3)2 (EIT). Composition (NC)75(MF)25 at 550°C. BSE image.

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 650°C. Back-scattered electron image.

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 650°C. Back-scattered electron image.

EXPERIMENTAL RESULTS

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 (872°C) towards a pseudobinary eutectic occurring at a composition of NC80MF20 at ∼600°C. NaMgF3 is the primary liquidus phase (Fig. 5) for compositions richer in MgF2. NaMgF3 melts congruently at 1030°C (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 (<550°C) showed the coexistence of Na2CO3 + NaMgF3 + 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 + NaMgF3 + Na2Mg(CO3)2, + liquid exists between 550 and 600°C (Fig. 9), between the Na2CO3 + NaMgF3 + 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.

Fig. 9.

Phase relationships along the pseudobinary join Na2CO3–MgF2 (NC–MF) at 0·1 GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; L, liquid.

Fig. 9.

Phase relationships along the pseudobinary join Na2CO3–MgF2 (NC–MF) at 0·1 GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; L, liquid.

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 ∼590°C. Interpreted subliquidus phase fields include Na2CO3 + Na2Ca(CO3)2 + NaMgF3 + liquid and Na2CO3 + Na2Ca(CO3)2 + NaMgF3 + Na2Mg(CO3)2 + liquid (Fig. 10). Subsolidus assemblages consist of Na2CO3 + Na2 Ca(CO3)2 + NaMgF3 + Na2Mg(CO3)2, from ∼525°C to ∼475°C. This subsolidus assemblage is joined by dolomite at temperatures below ∼450°C (Fig. 10).

Fig. 10.

Phase relationships along the pseudobinary join (A–MF) at 0·1 GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; NY, Na2Ca(CO3)2; DOL, CaMg(CO3)2; L, liquid.

Fig. 10.

Phase relationships along the pseudobinary join (A–MF) at 0·1 GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; NY, Na2Ca(CO3)2; DOL, CaMg(CO3)2; L, liquid.

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 of nyerereite crystallization for bulk compositions with <5 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 + L from that of NC + 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 ∼550°C. Subliquidus phase fields consist of Na2CO3 + NaMgF3 + liquid and Na2CO3 + NaMgF3 + Na2Mg(CO3)2, + liquid. Subsolidus assemblages consist of Na2CO3 + NaMgF3 + Na2Mg(CO3)2, at temperatures below ∼510°C.

Fig. 11.

Phase relationships along the pseudobinary join (B–MF) at 0·1 GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; NY, Na2Ca(CO3)2; L, liquid.

Fig. 11.

Phase relationships along the pseudobinary join (B–MF) at 0·1 GPa pressure. NC, Na2CO3; PV, NaMgF3; EIT, Na2Mg(CO3)2; NY, Na2Ca(CO3)2; L, liquid.

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 Na2Ca(CO3)2 (NY) and CaCO3 (CC) at ∼C17MF83 and ∼625°C; (2) between the primary phase fields of CaCO3 (CC) and NaMgF3 (PV) at ∼C28MF72 and ∼590°C. For compositions containing approximately >25 wt % MgF2 at temperatures below 650°C, CaMg(CO3)2 (dolomite; DOL) joins PV as a liquidus phase. Interpreted subliquidus phase fields include CaCO3 + Na2Ca(CO3)2 + liquid, CaCO3 + CaMg(CO3)2 + liquid, and NaMgF3 + CaCO3 + CaMg(CO3)2 + 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 + PV + DOL + EIT + CC.

Fig. 12.

Phase relationships along the pseudobinary join (C–MF) at 0·1 GPa pressure. NY, Na2Ca(CO3)2; PV, NaMgF3; CC, CaCO3; DOL, CaMg(CO3)2; L, liquid.

Fig. 12.

Phase relationships along the pseudobinary join (C–MF) at 0·1 GPa pressure. NY, Na2Ca(CO3)2; PV, NaMgF3; CC, CaCO3; DOL, CaMg(CO3)2; L, liquid.

Fig. 13.

Phase relationships along the pseudobinary join (NY–MF) at 0·1 GPa pressure. NY, Na2Ca(CO3)2; PV, NaMgF3; EIT, Na2Mg(CO3)2; CC, CaCO3; DOL, CaMg(CO3)2; L, liquid.

Fig. 13.

Phase relationships along the pseudobinary join (NY–MF) at 0·1 GPa pressure. NY, Na2Ca(CO3)2; PV, NaMgF3; EIT, Na2Mg(CO3)2; CC, CaCO3; DOL, CaMg(CO3)2; L, liquid.

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 ∼600°C. Interpreted subliquidus phase fields include CaCO3 + CaMg(CO3)2 + liquid, CaCO3 + MgCa(CO3)2 + NaMgF3 + liquid, and NaMgF3 + MgCa (CO3)2 + liquid (Fig. 13). Subsolidus assemblages below ∼550°C consist of CC + PV + DOL, and of CC + PV + DOL + EIT below ∼525°C. The join includes a very small field of NY crystallization for bulk compositions with <5 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 PV crystallize together. This pseudocotectic terminates at a pseudoternary eutectic (E) involving NC, NY and PV, with the approximate composition [NC49CC28(MgF2)23] at ∼525°C.

Fig. 14.

Liquidus phase relationships in the pseudoternary system Na2CO3–CaCO3–MgF2 at 0·1 GPa 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.

Fig. 14.

Liquidus phase relationships in the pseudoternary system Na2CO3–CaCO3–MgF2 at 0·1 GPa 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.

Along the NC–CC join the eutectic E1 (NC + NY) extends into the ternary system as an odd reaction curve (NY + L ⇒ NC + L) at low MF concentrations (< 5 wt % MgF2). At higher MF concentrations NY + NC co-crystallize along the pseudocotectic, which terminates at the pseudoternary eutectic (E). At more calcium-rich compositions on the NC–CC join the eutectic E2 (NY + CC) extends into the ternary system as a phase boundary separating the primary liquidus phase fields of NY + L and CC + 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 + CC and PV + CC pseudocotectics intersect at a reaction point R [∼575°C; ∼NY46CC30(MgF2)24], where CC is consumed in the reaction PV + CC + NY + L ⇒ PV + NY + 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 + PV.

APPLICATIONS TO THE CRYSTALLIZATION OF NATROCARBONATITE MAGMA

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 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 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 (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 + NY + PV pseudoeutectic and the NC + 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 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 (∼500–600°C; 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 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.

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.

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.

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 coarse-grained 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 Na- and 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.

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.

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.

CONCLUSIONS

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 (∼525°C) 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 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.

FUNDING

Our research is supported by the Natural Sciences and Engineering Research Council of Canada, the Geological Survey of Canada, and Lakehead University.

ACKNOWLEDGEMENTS

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.

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