Abstract

Mantle-derived carbonatites are igneous rocks dominated by carbonate minerals. Intrusive carbonatites typically contain calcite and, less commonly, dolomite and siderite as the only carbonate minerals. In contrast, lavas erupted by the only active carbonatite volcano on Earth, Oldoinyo Lengai, Tanzania, are enriched in Na-rich carbonate phenocrysts (nyerereite and gregoryite) and Na-K halides in the groundmass. The apparent paradox between the compositions of intrusive and extrusive carbonatites has not been satisfactorily resolved. This study records the fortuitous preservation of halite in the intrusive dolomitic carbonatite of the St.-Honoré carbonatite complex (Québec, Canada), more than 490 m below the present surface. Halite occurs intergrown with, and included in, magmatic minerals typical of intrusive carbonatites; i.e., dolomite, calcite, apatite, rare earth element fluorocarbonates, pyrochlore, fluorite, and phlogopite. Halite is also a major daughter phase of melt inclusions hosted in early magmatic minerals, apatite and pyrochlore. The carbon isotope composition of dolomite (δ13C = −5.2‰) and Sr-Nd isotope compositions of individual minerals (87Sr/86Sri = 0.70287 in apatite, to 0.70443 in halite; εNd = +3.2 to +4.0) indicate a mantle origin for the St.-Honoré carbonatite parental melt. More radiogenic Sr compositions of dolomite and dolomite-hosted halite and heavy oxygen isotope composition of dolomite (δ18O = +23‰) suggest their formation at some time after magma emplacement by recrystallization of original magmatic components in the presence of ambient fluids. Our observations indicate that water-soluble chloride minerals, common in the modern natrocarbonatite lavas, can be significant but ephemeral components of intrusive carbonatite complexes. We therefore infer that the parental magmas that produce primary carbonatite melts might be enriched in Na and Cl. This conclusion affects existing models for mantle source compositions, melting scenarios, temperature, rheological properties, and crystallization path of carbonatite melts.

INTRODUCTION

Magmatic carbonatites, composed of >30 vol% carbonate minerals (Mitchell, 2005), represent a rare but widespread rock type, with more than 500 examples worldwide. Most occur within old continental crust, where they are associated with large-scale faulting, and many are part of lithologically complex alkaline igneous provinces (Woolley and Kjarsgaard, 2008). The discovery of modern carbonatitic lavas at Oldoinyo Lengai volcano, Tanzania (Dawson, 1962), was a turning point in carbonatite research. Low-temperature (<600 °C) natrocarbonatitic melts at Oldoinyo Lengai are strongly enriched in Na2O and K2O (total of 38–40 wt%) and halogens (to 4.5 wt% F and 5.7 wt% Cl) and have low Ca (15 wt%) and insignificant Mg-Fe contents (<1 wt%) (Keller and Krafft, 1990; Keller and Zaitsev, 2012), unlike the Ca-Mg–dominated compositions typical of intrusive carbonatites. Such unusual compositions at the only active carbonatite volcano, coupled with petrographic and geochemical evidence of alkali-rich compositions in prehistoric extrusive carbonatites (Clarke and Roberts, 1986; Deans and Roberts, 1984; Guzmics et al., 2011; Hay, 1983; Zaitsev, 2010; Zaitsev et al., 2013) and some intrusive carbonatites (Andreeva et al., 2006; Chen et al., 2013; Kogarko et al., 1991; Sharygin et al., 2011; Zaitsev et al., 2002), suggest that the much more common Ca-Mg–dominated compositions may not represent primary liquid compositions. Calcitic and dolomitic carbonatites may instead represent cumulates and/or residues and alteration products of originally alkali-rich magmatic liquids.

If mantle-derived alkali elements and halogens play an important role in carbonatite petrogenesis, then at least some intrusive carbonatites should contain halides. Here we describe an occurrence of halite within dolomite and phenocryst-hosted melt inclusions of the St.-Honoré syenite-carbonatite complex (Québec, Canada) and discuss petrological implications for intrusive carbonatites.

HALITE-BEARING ST.-HONORÉ CARBONATITE

The 571 Ma St.-Honoré syenite-carbonatite complex (Fournier, 1993; McCausland et al., 2009; Thivierge et al., 1983) occurs in the Saguenay graben of the Iapetan rift system, and is entirely covered by the ca. 470 Ma Trenton limestone. The complex comprises ring-like diorites and syenites surrounding a 12 km2 core of calcitic, dolomitic to ankeritic carbonatites. The studied samples are from the deep 16th and 21st levels (490 m and 640 m, respectively) of the Niobec niobium mine (Magris Resources, Toronto, Canada). Here strongly foliated Fe-bearing dolomite carbonatites contain discontinuous, sinusoidal veins (to 20 cm), elliptical units, and discrete patches of halite-bearing dolomite carbonatite with distinct layers enriched in phlogopite and/or chlorite, apatite, pyrochlore, and pyrite (Fig. 1B). Individual veins have no obvious connection with adjacent veins or with the discrete halite-rich patches in the host dolomites, and cannot be traced between different levels of the mine.

Halite occurs as large crystals or crystal aggregates (to 5 cm; Figs. 1A and 1B) within dolomitic druses where halite can reach 25 vol%. More commonly, halite (<1 mm) fills interstices between euhedral dolomite grains (Fig. 1E), averaging 4–6 vol% and accounting for unusually high sodium contents of bulk samples (1.8 wt%; Table DR1 in the GSA Data Repository1). Polished surfaces of dolomite are pitted because of numerous inclusions of water-soluble halite. Fine-grained halite also occurs in veinlets through dolomite (Fig. DR1) and along cleavage planes in prismatic phlogopite crystals (Fig. 1F). Calcite, pyrite, and rarely quartz occur at the margins of the halite-rich patches. Other minerals [apatite, pyrochlore, bastnaesite-(Ce) and synchysite-(Ce), magnetite, barite, rutile, fluorite, and chlorite replacing phlogopite] are typically enclosed in and/or intergrown with halite (Figs. 1D and 1G; Figs. DR1–DR3). Net-textured halite appears as a cement to subhedral-anhedral apatite grains in lenses and patches dominated by apatite with minor pyrochlore and synchysite (Figs. 1B–1D; Figs. DR1D and DR2). Another common association is represented by coarse-grained coatings of halite around euhedral pyrochlore crystals (Fig. 1G; Fig. DR3).

Halite occurs as a major daughter phase in melt and/or fluid inclusions in apatite, pyrochlore, and pyrite (Fig. 2; Fig. DR4). Halite appears to be cogenetic with pyrite, apatite, baddeleyite, fluorite, and Nb-rutile that also occur in pyrochlore and pyrochlore-hosted melt inclusions.

ISOTOPE COMPOSITIONS

The halite-bearing rock is enriched in light rare earth elements and depleted in heavy rare earth elements (La/Sm = 5.4, Gd/Yb = 3.9; Table DR1), and thus is within the compositional range of other intrusive Mg-rich carbonatites worldwide (Fig. 3A; e.g., Jones et al., 2013).

Methods and extended results of radiogenic (Sr-Nd) isotope analyses are given in the Data Repository. Rb-Sr isotope data for a leachate-residue pair of unaltered phlogopite yield a 2-point Rb-Sr model age of 564 ± 8 (2σ), within error of a 571 ± 5 Ma Ar-Ar age for phlogopite (McCausland et al., 2009). Initial 87Sr/86Sr in minerals ranges from 0.70287 in apatite to 0.70443 in halite, showing marked intermineral differences. For example, phlogopite yields 0.70382, distinctly lower than coexisting dolomite (0.70431, 0.70401), but higher than dolomite (0.70336) in the halite-rich portion of the same specimen (Table 1). Three halite fractions in this halite-rich chip have higher 87Sr/86Sri (0.70443, 0.70437, and 0.70422). Dolomite and halite in another rock chip are similarly diverse (0.70360 versus 0.70436). In contrast, initial εNd is very homogeneous (+3.2 to +4.0; Table 1). Apatite has the lowest 87Sr/86Sri and its Sr-Nd isotope signature resembles that previously reported for St.-Honoré carbonatite (0.70289, +3.7; Bell and Blenkinsop, 1987). All Sr-Nd isotope ratios reported here are within the range of carbonatites globally (Fig. 3B; e.g., Jones et al., 2013).

The halite-bearing dolomite in this study has δ13C = −5.2‰ and δ18O = +23‰ that are typical compositions of dolomite-rich rocks in the St.-Honoré complex (Deines, 1989), indicating mantle origin of carbon and surface source of oxygen.

DISCUSSION

Textural relationships, mineral parageneses, and isotopic data indicate that halite in the St.-Honoré dolomite carbonatite is a late-stage mineral. The discontinuous nature of halite-rich segregations and halite intergrowths with dolomite, apatite, pyrochlore, and phlogopite (Fig. 1) strongly suggest that halite is a part of the carbonatite mineral assemblage and does not result from replacement or pore filling by halite crystallizing from externally derived fluids. However, the high δ18O of the halite-bearing dolomite, compared to St.-Honoré calcite carbonatite (δ18O = +7‰ to +10‰; Deines, 1989), suggests that the dolomite is not a primary magmatic mineral. This is supported by the Sr isotope results, which indicate (1) isotopic disequilibrium between dolomite and dolomite-hosted halite, and (2) higher 87Sr/86Sri in dolomite halite than in magmatic apatite (Table 1). We suggest that dolomite and, by inference, halite are products of recrystallization of the carbonatite at some time after emplacement and incorporation of radiogenic crustal Sr from the host rocks. However, variable 87Sr/86Sr within small (<5 g) volumes of carbonatite can reflect local leakage of in situ–produced 87Sr from phlogopite after partial replacement by chlorite (Figs. 1B and 1F).

While Sr-O isotope systematics are controlled by partial recrystallization of original minerals, the presence of halite as a major daughter phase in melt and fluid inclusions in apatite and pyrochlore (Fig. 2) that are proposed to represent an early crystallization stage (Fournier, 1993) indicates that a chloride component was initially present in the parental melt of the St.-Honoré carbonatite and was not sourced from external fluids.

Although other carbonatite complexes are known to have halite-bearing melt inclusions in a variety of phenocrysts (apatite, carbonates, fluorite, magnetite, perovskite; e.g., Andreeva et al., 2006; Chen et al., 2013; Panina, 2005; Sharygin et al., 2011; Zaitsev et al., 2002), none are reported to contain macroscopic segregations and/or veins with significant modal halite such as those in the St.-Honoré carbonatite. The only known exception is the natrocarbonatite lavas of the Oldoinyo Lengai volcano, where groundmass chlorides (halite and sodian sylvite), fluorite, and alkali carbonate phenocrysts (gregoryite and nyerereite) are primary magmatic minerals (Keller and Krafft, 1990; Mitchell, 1997, 2006). The occurrence of primary melt inclusions with chloride- and alkali carbonate–rich compositions in several intrusive carbonatites (Fig. DR5; also see Andreeva et al., 2006; Chen et al., 2013; Panina, 2005; Sharygin et al., 2011; Zaitsev et al., 2002) suggests that the current dolomitic-calcitic mineralogy of these carbonatites is not representative of their parental melts. This is supported by experiments that indicate that primary, mantle-derived carbonatitic melts have sodic dolomite compositions (Litasov et al., 2013; Wallace and Green, 1988) and generate alkali carbonate-chloride liquids by immiscibility at mantle conditions (Safonov et al., 2007).

The discovery of the chloride component in the St.-Honoré carbonatite magma indicates that chlorine may play a role in the petrogenesis of extrusive and intrusive carbonatites.

Experimental phase relationships in the system Na2Ca(CO3)2-NaCl-KCl at crustal conditions show a significant expansion of the liquidus field for calcite in Cl-rich bulk compositions, suggesting an important link between chlorides and natrocarbonatitic melts (Mitchell and Kjarsgaard, 2008). It is important that the other major halogen element fluorine exerts similar effects on phase equilibria (e.g., Jago and Gittins, 1991). In such halogen-rich compositions, calcite can precipitate over a wide range of temperatures, potentially producing calcitic rocks (Mitchell and Kjarsgaard, 2008), i.e., the most common carbonatite type (Woolley and Kjarsgaard, 2008), and complementary alkali carbonate-chloride residual liquids (Mitchell, 1997; Mitchell and Kjarsgaard, 2008), which are displaced toward the top and margins of intrusive magmatic bodies. These residual liquids may (1) erupt to form volcanic rocks similar to the salty natrocarbonatite lavas of Oldoinyo Lengai, (2) react with surrounding rocks to form the well-known fenitization halos surrounding carbonatite intrusions (von Eckermann, 1948), and/or (3) form rocks that undergo rapid degradation through interaction with meteoric fluids.

IMPLICATIONS AND CONCLUSIONS

This study reports on the occurrence of halite, both included in and intergrown with primary magmatic phases in the St.-Honoré carbonatite complex. The abundance of halite preserved, compared to other intrusive carbonatites, is accounted for by the recovery of samples from deep mining activities and the use of water-free lubricants during sample preparation. Whereas the presence of halite as an intergrowth might be attributed to postemplacement recrystallization, documentation of this phase trapped within primary magmatic minerals argues for an igneous origin.

A proposed chlorine-rich carbonatite melt has a number of implications. First, if halite is a common liquidus phase in carbonatites, then its absence from most intrusive carbonatites may reflect removal in deuteric and postmagmatic fluids. Macroscopic halite-rich segregations are absent in carbonatites where phenocryst-hosted melt inclusions are enriched in chlorides (Fig. DR5). Second, Cl-bearing primary carbonatite magmas imply Cl-bearing carbonated mantle sources, suggesting that chlorine is more abundant in the source mantle than currently thought. This is consistent with chlorine- and alkali-rich compositions of minerals and fluid and/or melt inclusions in mantle xenoliths and kimberlites (e.g., Giuliani et al., 2012; Kamenetsky et al., 2014), and with suggestions that an alkali-chloride component is critical in the formation of diamond in carbonate melts (Palyanov et al., 2007; Tomlinson et al., 2004). Third, chlorine reduces mantle solidus temperatures (Litasov et al., 2013; Safonov et al., 2007), potentially allowing carbonatite and carbonate-silicate melts to form at lower temperature. Once formed, chloride-bearing carbonatite liquids, such as the one identified here, are highly mobile, reactive, and capable of pervasively percolating and wetting ambient peridotite (Hammouda and Laporte, 2000). Fourth, expansion of the calcite liquidus field (Mitchell and Kjarsgaard, 2008) might explain why intrusive carbonatites are dominantly calcitic and poor in Cl, whereas extrusive carbonatite lavas are Cl-rich natrocarbonatites. The St.-Honoré halite-bearing carbonatites are mineralogically distinct from the Oldoinyo Lengai natrocarbonatites and, on the basis of current evidence, appear to be a unique variety of dolomite carbonatite. However, chlorine in both occurrences is of mantle origin.

We thank Franco Pirajno, James Brenan, five anonymous reviewers, and editor Brendan Murphy, who pointed out various omissions and possible misinterpretations. This work was funded by Australian Research Council Discovery Grant DP130100257 and by the Natural Sciences and Engineering Research Council of Canada.

1GSA Data Repository item 2015239, Tables DR1–DR3 (full analytical results) and Figures DR1–DR5 (backscattered electron images of rocks, minerals, and melt inclusions, is available online at www.geosociety.org/pubs/ft2015.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.